New project: MC502 refurb

This fall I restored three different Sansui units - an AU-717, AU-317, and AU-505. All three were torn down, recapped, cleaned, lubed, and dialed in. All came out very well and sound fantastic.

Since finishing these three amps I've been looking for another project and decided to purchase an MC502 with a smashed faceplate. The FedEx man dropped it off this morning and overall it is in very nice condition, aside from the faceplate which I'll be ordering from McIntosh parts soon.

I popped the cover just now and this thing is beautifully built. It also looks like it's going to be very easy to work on. I counted about 20 electrolytic caps, all of which are easy to get at.

So my plan for this unit is:

1. Order new glass and bulbs (maybe get LEDs?)
2. Replace all electrolytics
3. Clean and lube all controls
4. Clean chassis
5. New sil-pads on outputs
6. Test and dial in

This project should be *much* easier than the AU-717.

Are there any other things particular to the MC502 that I should be aware of?

Thanks guys!
-Matt

Initial parts list

After reading through the service manual and inspecting the amp itself, here's the initial list of capacitors I plan on replacing:


COMPONENT ___________ VALUE ________ MOUSER
C101,C102 NP ________ 2.2u / 25V ___ 647-UES1H2R2MDM (Nichicon ES; replacements are 50V and may be big)
C305,C306 NP ________ 10u / 35V ____ 647-UES1H100MPM (Nichicon ES)
C205,C206 AXIAL _____ 2200u / 16V __ 647-TVX1C222MCD (Nichicon axial)
C207 ________________ 470u / 6.3V __ 647-UFG0J471MPM (Nichicon FG)
C202,C204,C208 ______ 47u / 16V ____ 647-UFG1C470MEM (Nichicon FG)
C301,C302,C303,C304 _ 0.47u / 50V __ 555-RFS50VR47ME3#5 (Elna Silmic II)
C307,C308 ___________ 22u / 25V ____ 555-RFS35V220MG3#5 (Elna Silmic II; 35V caps were in amp)
C313,C314 ___________ 10u / 50V ____ 647-UFG1H100MDM (Nichicon FG)
C315,C316 ___________ 100u / 16V ___ 647-UFG1C101MPM (Nichicon FG)
C319,C320,C321,C322 _ 47u / 50V ____ 647-UFG1H470MPM (Nichicon FG)


I'm trying to use Silmic II for the signal path and then Nichicon FG and ES series for everything else. The brown ROE caps will be replaced by Nichicon ES. I'm also planning on swapping out the op-amps for brand new 5534AN On-Semis from Mouser.

I'll order glass, bulbs, and possibly a new relay from McIntosh parts.

-Matt

This sound like a reasonable plan. Please post few pictures.
Regards

C101,C102 ___________ 2.2u / 25V ____TBD (Muse ES?) < rca input ? Bi-polar ? WIMA film on my restore >
C301,C302,C303,C304 _ 0.47u / 50V __ 647-UFG1HR47MDM < film caps are available that fit nicely, I used Silmac II >
C305,C306 ___________ 10u / 40V ____ 647-UES1H100MPM < bi-polar ? >

I used WIMA audio grade film caps < 2.2 ufd same physical size >, and Elna Silmac II audio grade for these on my recent MC2205 adventure. 10 ufd film caps are unsightly large so Muse or Silmac may be best choice for any of those in audio pathways due to size consideration only. < unless you can find smaller then normal WIMA's or similar film caps that fit the board areas >
Since these ufd sizes are most probably in the audio pathways by their values you might find the film caps a positive and enjoyable experience. Most tube folks do since film caps are used thru out their tube amps for interstage coupling...just my take on this, Your mileage may vary...

Have you looked at Panasonic FC series, they have extended lifespans in most cases at 105C temps even. I have used these with good success in power supply areas and such. Their lifespans in some cases are as high as 5000 hours. A tough to beat lifespan rating I have found. < Hope some of this is helpful

I Just looked and their is no MC502 service manual uploaded into digital docs here, perhaps you might consider sharing your service manual with all of us by an upload ??? pretty Please ?

WIMA 2.2 replacements picture uploaded

Thanks!

1moreamp said:

C101,C102 ___________ 2.2u / 25V ____TBD (Muse ES?) < rca input ? Bi-polar ? WIMA film on my restore >
C301,C302,C303,C304 _ 0.47u / 50V __ 647-UFG1HR47MDM < film caps are available that fit nicely, I used Silmac II >
C305,C306 ___________ 10u / 40V ____ 647-UES1H100MPM < bi-polar ? >

I used WIMA audio grade film caps < 2.2 ufd same physical size >, and Elna Silmac II audio grade for these on my recent MC2205 adventure. 10 ufd film caps are unsightly large so Muse or Silmac may be best choice for any of those in audio pathways due to size consideration only. < unless you can find smaller then normal WIMA's or similar film caps that fit the board areas >
Since these ufd sizes are most probably in the audio pathways by their values you might find the film caps a positive and enjoyable experience. Most tube folks do since film caps are used thru out their tube amps for interstage coupling...just my take on this, Your mileage may vary...

Have you looked at Panasonic FC series, they have extended lifespans in most cases at 105C temps even. I have used these with good success in power supply areas and such. Their lifespans in some cases are as high as 5000 hours. A tough to beat lifespan rating I have found. < Hope some of this is helpful

I Just looked and their is no MC502 service manual uploaded into digital docs here, perhaps you might consider sharing your service manual with all of us by an upload ??? pretty Please ?

WIMA 2.2 replacements picture uploaded

Click to expand...

Thanks for all of the feedback!

I opted for the Nichicon FG/ES based on some of the feedback from other McIntosh experts in various posts here on AK. I'm sure the Panasonics would be more than adequate as well.

C101,C102,C305,C306 are all bipolar which is why I'm planning to use Nichicon ES series caps.

I hadn't thought of using WIMAs for C101,C102 and C301-C304. I'll have to investigate this and see what Mouser has available.

I found the MC502 service manual on another site free of charge. I downloaded a copy and can upload one here if I can figure out how to do it.

-Matt

I had an MC502 a few years ago and enjoyed it. Pretty cool compact amp.

Murray

Some pictures

TR6man98 said:

This sound like a reasonable plan. Please post few pictures.
Regards

Click to expand...

These pictures are a little blurry (I have to play around with my camera's macro settings I think).

Aside from the faceplate, this amp is in very nice condition. Someone has been inside it though .. the ribbon cable that connects to the power guard lamp board has been replaced with individual wires. The axial caps on the power supply boards have also been replaced with radials. Things look to have been done pretty well, but I'll clean all of this up when I recap it.

McIntosh used slightly different caps on the driver boards for the C301-C304 (0.47uF / 50V) and C307-C308 (22uF / 25V) positions as they have an orange jacket. I'll have to look at the schematic again, but these may be in the signal path. If so I'll try to use Silmic II for these.

-Matt

UPDATE: Looks like only C303,C304,C307,C308 are in the signal path. C301 and C302 are not in the signal path.

Mouser has the WIMA films and the Elna's audio grade also, and very reasonable also, and the FC's also, and they will likely be able to supply the very large rail caps also in Nippon Chemi-con.

You can also find a large selection of WIMAs on fleabay also, I just picked up some 4.7 ufd about the same size as those 2.2's I have pictured. The 4.7's could be stacked to make a reasonable sized 9.4ufd to replace any 10.0 ufd wet caps and still be withing 10% value where as most wet caps are -20% to +50% in value tolerance typically.

I don't buy wet caps off fleabay, they tend to be outdated end of shelf live dumps and no one needs that sort of part in their work. A lot of folks don't know that wet caps have shelf life and after a certain time of sitting all distributors dump them to salvage wholesalers and they tend to end up on fleabay.
Buy only fresh ones from Mouser, or Digi-Key etc.. But film caps last forever so no worries about where you get them from, just value check them just like any part and use them since they pretty much have a unlimited lifespan in most cases...

I found some Elna's Silmac II's tend to be much physically larger when you get into the larger ufd values so be forewarned anything above 10 ufd they tend to be larger size wise and can end up being a very snug fit in most cases. Something to do with much wider spaced plates internally like most high end audiophile grade caps.

I like Nichicon also and use them from time to time when I have to. But I second choice them back in the early 90's after taking into account lifespan ratings in my cap selection for mobile applications.
The FC's have some of the longest lifespan ratings of any wet cap available especially at elevated temps of 105C which most of my work in car audio lead me to need taking into account hot car trunk temps in summer of +140F and above.

I would love to see the schematic if possible, please let me know if you get it uploaded here or in digital docs where it is missing??? Thanks and have a great restoration....:thmbsp:

1moreamp said:

Mouser has the WIMA films and the Elna's audio grade also, and very reasonable also, and the FC's also, and they will likely be able to supply the very large rail caps also in Nippon Chemi-con.

You can also find a large selection of WIMAs on fleabay also, I just picked up some 4.7 ufd about the same size as those 2.2's I have pictured. The 4.7's could be stacked to make a reasonable sized 9.4ufd to replace any 10.0 ufd wet caps and still be withing 10% value where as most wet caps are -20% to +50% in value tolerance typically.

I don't buy wet caps off fleabay, they tend to be outdated end of shelf live dumps and no one needs that sort of part in their work. A lot of folks don't know that wet caps have shelf life and after a certain time of sitting all distributors dump them to salvage wholesalers and they tend to end up on fleabay.
Buy only fresh ones from Mouser, or Digi-Key etc.. But film caps last forever so no worries about where you get them from, just value check them just like any part and use them since they pretty much have a unlimited lifespan in most cases...

I found some Elna's Silmac II's tend to be much physically larger when you get into the larger ufd values so be forewarned anything above 10 ufd they tend to be larger size wise and can end up being a very snug fit in most cases. Something to do with much wider spaced plates internally like most high end audiophile grade caps.

I like Nichicon also and use them from time to time when I have to. But I second choice them back in the early 90's after taking into account lifespan ratings in my cap selection for mobile applications.
The FC's have some of the longest lifespan ratings of any wet cap available especially at elevated temps of 105C which most of my work in car audio lead me to need taking into account hot car trunk temps in summer of +140F and above.

I would love to see the schematic if possible, please let me know if you get it uploaded here or in digital docs where it is missing??? Thanks and have a great restoration....:thmbsp:

Click to expand...

1moreamp, you seem extremely knowledgable about electronics repair in general so I have kind of a dumb question...

Are film caps a recent innovation? Because if they last forever, why is it that all these manufacturers of the Japanese, American and Danish electronics I love didn't use them in the first place!

Sorry... threadjack /off

playdrv4me said:

1moreamp, you seem extremely knowledgable about electronics repair in general so I have kind of a dumb question...

Are film caps a recent innovation? Because if they last forever, why is it that all these manufacturers of the Japanese, American and Danish electronics I love didn't use them in the first place!

Sorry... threadjack /off

Click to expand...

Film caps have been around since tubes were invented. Tube amps have them in between the tube stages to block the 300 to 600 DC volts plate voltages present but to allow audio to pass to the next stage....
I have seen many amp makers use film caps explicitly just to render higher SQ similar to tube amp SQ. I can point you to many amp makers internal pics and they all use film caps whenever possible in audio pathways.

Film caps are:
1 more stable
2 live longer more trouble free lifespans
3 have tighter tolerances
4 do not dry out and leak like wet caps
5 less temperature sensitive in most cases
6 more expensive cost wise
7 are usually slightly larger then wet caps nowadays except for larger ufd capacitive values.


Wet caps are:
1 CHEAP !
2 SMALL
3 just reliable enough to survive most warranty periods by a fair margin
4 A usable band-aid in any emergency LOL LOL LOL...


Please bear in mind wet caps in any amps audio pathways serve ONE main purpose, and that is to Block errant DC voltage offsets associated with gain stage interconnections and between amp inputs from preamps and such.. There are other electronic reasons but this one listed above is the main reason. Other reasons may be low frequency roll off and inhibition of excessive low frequency information and DC voltage input to the amps next stage. basically frequency compensation of some sort..

All electronic amp gear uses caps between preamps and main amps inputs, and their main purpose is to block any errant DC fundamentals from being sent inside of the amp stage, and to also limit low frequency inputs also if designed with that intent in place. Mac uses these also, so don't shoot me for telling the truth its standard engineering since the beginning of time and small, cheap wet caps fit back then for many reasons explained below I hope....

With the advent of DC wide bandwidth amps came the possibilities of amplifying DC voltage also.
Now on Mac amps with their auto transformers this isn't as bad of deal as with regular DC < Direct Coupled > amps that have no transformer coupled output.
DC voltage presented to many amps inputs will be even larger DC output to a speakers passive crossover and or the low bass driver directly which will offset the drivers cone from its true center resting position...This also will cause excessive heating of the voice coils, and burn out of drivers, along with muddied up sounding bass and damaged cones and spiders possibly...

So DC voltage output is a bad thing if your woofer is standing a half inch off center of its X max home position.
Plus any DC voltage present will rob real power from the amps driving ability.

SO amp makers use DC blocking electrolytic caps on preamp outputs and on main amp inputs. Mac used these also, and due to Film caps of that Time era being rated for 400 to 600 volts for tube use, well they were just to dang big to place inside of these new solid state wonders.

Newer, smaller footprint film caps with 50 and 60 volts ratings are nothing really new but. But cost wise they have been an impediment of manufactures because bean counters get into the picture of many amps being brought to market and they cut back on money expenditures like film caps to save money and to use smaller wet caps < which fail with age and what not >

Wet capacitors are cheap, small, and will usually out last most base warranty consideration for electronics manufacture, except Chinese wet caps < they don't have the chemistry down yet, but they are trying to steal it from the Japanese, Europeans, and the Americans >.
So no no to Chinese wet caps always, just google bad caps and see their entire web site of bad Chinese wet caps...I will let you be the judge after seeing what i already know...

Film caps are dry caps, so no wet electrolyte to dry out or leak out. dried out caps short out internally and wet caps that leak well lets just say they destroy PC boards when they leak.
I have seen Wet caps leak and burn wholes right thru fiberglass PC boards.

Most film caps are considered to be Self healing as they tend to clear internal shorts in most cases. I have seen a few melt down so they are not fool proof, but IMHO they are a better choice for all the best of reasons...

My MC4000 had this problem in its power supply, < nice quarter sized hole in its PC board in it power supply from a leaking wet cap > so YES even Mac amps have bad wet caps in them < Now please don't hit me for saying that !!! lol lol lol... > Its just the plain true about cheap, small wet electrolytic caps....:yes:

So for better SQ and more reliable service work I use Film caps when and where possible, and to date have never had any bad feedback about it and the amps SQ.
I own dozens of amps, and one particular set of amps I own made by Phoenix Gold exclusively uses nothing but film caps in any of the audio circuitry. They do sound better then the rest of their gear they made IMHO..

PPI used a big 4.7 ufd film cap inside of each channel between the preamp sections and the main amp input and they sold millions of car amps because they sounded good to most folks.
Tube SQ had been characterized electronically speaking and a lot of it has been tracked back to the use of FILM type caps...

You be the judge, I did not invent this stuff. I just kept up with the technology so I would be a good engineering tech to all of my clients. Which many come to me just because I will upgrade all of their gear internally where others will not hassle with it...Hope some of this was useful and helps some of you in some way..........
Oh and dump all tantalum caps you ever run into on any gear. They have been the death of a many a piece of gear in my service history experiences...They never really lived up to engineering expectations for them.

Please forgive the thread-jack OP...

Tantalum caps?

I've attached a few closeups of the right side driver board. I think I have a good handle on all of the electrolytics, but at those orange caps with green paint tantalums? If so, I should probably replace them as well, correct?

I'll order some new TO-220 silpads for the transistors with heatsinks also.

Thanks!
-Matt

eedork said:

I've attached a few closeups of the right side driver board. I think I have a good handle on all of the electrolytics, but at those orange caps with green paint tantalums? If so, I should probably replace them as well, correct?

I'll order some new TO-220 silpads for the transistors with heatsinks also.

Thanks!
-Matt

Click to expand...

I don't see any tantalum's on the board. But please see attached painted photo.

The item circled in green is a bi-polar cap 10 ufd 40 volt.

Yellow caps circled in blue are just regular electrolytic's I can't see their values or temp spec, and I need the schematic and or parts list or hands on to make any judgement call on them.

Caps in red circle are film caps, so Mac did use then in certain places in this and other amps like the 2205 I recently did.

Heat sinks appear to not be electrically isolated by insulators, this is better for heat transfer. I think I would refresh the silicon paste and call it a day on those because Mac wanted them to have the best possible heat transfer and direct contact < electrically hot heat sinks was a common practice by Mac >

Adding Sil-pads will require that you use much higher contact pressure for the Sil-pads to be effective as a heat transfer agent.
Most Sil-pad makers list the pressure required to achieve good thermal efficiency and it is in foot pounds of pressure so its a lot higher then any other contact transfer method.
Please see sil-pads manufacturers web page for mounting pressure requirements so if you do use them so you adhere to their installation spec's.

They work, but they just require a much higher mounting pressure then any other method...

hope some of this helps, having the service manual would be handy for some of your questions you asked... I did my best visual guess though...

Thanks! I've sent the service manual off to the person who should upload it to the AK database. It should be there in a day or two.

I'll have to order some thermal grease from Mouser - I don't have any on hand. Is this the right stuff?:

http://www.mouser.com/ProductDetail/Aavid-Thermalloy/249G/?qs=sGAEpiMZZMsG1k5vdNM/c3OPMeAxBM%2bE

-Matt

eedork said:

Thanks! I've sent the service manual off to the person who should upload it to the AK database. It should be there in a day or two.

I'll have to order some thermal grease from Mouser - I don't have any on hand. Is this the right stuff?:

http://www.mouser.com/ProductDetail/Aavid-Thermalloy/249G/?qs=sGAEpiMZZMsG1k5vdNM/c3OPMeAxBM%2bE

-Matt

Click to expand...

Sure that will work. You could even use some of the super heat sink compounds like the CPU's use, they have copper and silver in them for added thermal transfer capabilities. Since the sinks are at live rail potentials either way it does not matter if the compound is electrically conductive or not.. < kind of pricey but they are touted and being better by many for there thermal transfer ratings> Either way your good :thmbsp:

Thanks again

1moreamp said:

Sure that will work. You could even use some of the super heat sink compounds like the CPU's use, they have copper and silver in them for added thermal transfer capabilities. Since the sinks are at live rail potentials either way it does not matter if the compound is electrically conductive or not.. < kind of pricey but they are touted and being better by many for there thermal transfer ratings> Either way your good :thmbsp:

Click to expand...

Do you think it is even necessary to apply new grease? The outputs actually look fine, so I'm tempted to leave them alone. These TO-220 transistors on the driver boards don't look that bad either.

Another issue I will have to address is the ribbon cable that connects from the lamp board to the power supply board. The original cable is long gone and someone used individual wires instead. I suspect I'll have to re-do this when I pull the power supply board and am thinking about using Category 5E networking cable for the replacements wires. Cat 5E is solid core 22 AWG with teflon insulation so should work well in this application.

Last question - I need to build up a dummy load to bias and test this unit when I'm done with the recap. I'm thinking about using two 50 Watt 16 Ohm wirewound resistors in parallel per channel as dummy loads. I won't leave the amp cranking for long periods into this, but for the two minute bias procedure called for in the manual I think this will be fine.

-Matt

eedork said:

Do you think it is even necessary to apply new grease? The outputs actually look fine, so I'm tempted to leave them alone. These TO-220 transistors on the driver boards don't look that bad either.

Another issue I will have to address is the ribbon cable that connects from the lamp board to the power supply board. The original cable is long gone and someone used individual wires instead. I suspect I'll have to re-do this when I pull the power supply board and am thinking about using Category 5E networking cable for the replacements wires. Cat 5E is solid core 22 AWG with teflon insulation so should work well in this application.

Last question - I need to build up a dummy load to bias and test this unit when I'm done with the recap. I'm thinking about using two 50 Watt 16 Ohm wirewound resistors in parallel per channel as dummy loads. I won't leave the amp cranking for long periods into this, but for the two minute bias procedure called for in the manual I think this will be fine.

-Matt

Click to expand...

As for not servicing heat transfer mediums like silicon paste I have found and feel that over time the silicon oil base often creeps away under heat load. So I address this issue by at least disassembling and looking at the condition of heat sink compounds physically. I have often found silicon based compounds to have dried out completely leaving behind a dry cake of filler material typically alumina powder but they do have other materials.

When I find the compound dried up I generally feel its a worthwhile consideration and effort to clean and replace the compound as needed to restore proper heat transfer characteristics.
Sil-pads do not require this but if you take apart any Sil-pad mounted devices you must use new Sil-pads as the old ones tend to be ruined and are not considered to be reusable.

Cabling issue:
I need to visualize the cable your talking about to make any firm recommendation. Cat 5E cabling while perfect for telecommunications and networking may not be of suitable gauge to work properly. I will look up this problem when the service document gets posted and get back with you on this with my thoughts, and or concerns.
On my recent MC2205 adventure I did away with the flex circuit ribbon cable up front and hard wired in something that will never fail and need buying from Mac at any price. The original flex cabling should have never failed in the first place if it was designed and manufactured correctly by Mac's supplier. But I have some background in flex circuitry manufacture so when I see them fail I know why and how this happened.
My suggestion to Mac is find another supplier of flex cabling as the one they have had for the last 40 years is a joke. I saw how and why the cabling failed and it just rubbish in my opinion, just plain rubbish...None of these flex cables should be failing apart like they are, and turning green inside from moisture contamination??? Like WTF!!! Just rubbish IMHO...

Dummy load considerations:
I would measure on first power up any DC voltage present across the Wires connecting to the auto transformer primary with the auto transformer disconnected from the amps output.
This DC voltage is called DC offset, and the output transformer will hide this if you leave it connected in circuit. So I test without the output transformer connected for any signs of errant DC voltage presence.
If this test yield very low to little DC offsets then You can shut down the amp and reconnect the output transformer and you can use real speaker to play musical content thru to warm up the amp if you like.

I am familiar with Macs typical realignment procedures concerning bias and offset adjustments where applicable. They don't really do anything any different then most other amp makers do.
Most manufactures actually calibrate bias without any speakers or loads connected and generally they don't have any input to the amp either to do this sort of alignment, and some even ask that the amps input be shorted using shorting RCA's so there will be no drive to the amp while its being base calibrated.
I will look at your service manual just to be sure about Mac's call out before recommending anything further. But in most cases no input and no load are the typical setups for base adjustments. Then warm up and retest and recal if needed due to tracking abnormalities and such.

Most modern amps track thermal rise situations so well now that the bias will track properly even from base calibration. But it is wise to make sure thermal tracking is working properly, and there are several ways of doing this besides Mac's procedures.
This is why I keep thermal cool spay on my bench and a hot air reflow wand to supply heat in areas of tracking devices and thermal sensors so all of these can be sequentially tested before the amp is released to final burn in testing, and subsequent release back into a clients hands for normal usage.

Dummy loads are nice to test sine wave power into non inductive resistive loads. The key being that the resistors should be of a NON Inductive variety. So although your load resistances look OK the resistors inductive qualities are in question in my mind, not their rated load values.
Although real world speaker loads are a combination of inductive, capacitive and resistive < LCR> for base testing purposes most makers call out simple non inductive purely resistive test loads for sine wave testing. anything else will probably skew test results somewhat.
This amp is capable of less then 100 watts per so overheating of test loads will be minimal except while full power testing with sine waves. This will tend to heat up the amps sinks rather quickly in many cases and can cause thermal overloads to trip if done incorrectly or excessively. Sine wave testing of any amp is meant to push the amps abilities, that is why it was design and called out for way back when by even Mac and the FTC and IHF.

My test loads are connected to very large aluminum heat sinks with silicon thermal compound used in their mating surfaces, But I test all sorts of gear even 2 KW amps so mine is sort of industrial level and meant to handle that much loading for extended periods of time. Yours will not likely need be so acutely designed to my thinking.

I mandatory do DC offset testing before doing anything else to any amps calibrations and load testing. Over the many years of my journey thru amps I have found this one base test to be all telling to me, and all warning or eminent failure also.
So I recommend you worry about any errant DC voltages present on your initial power on of your Mac amp or any amp and that voltage measurement In my mind is to be taken in front of the transformer output with the transformer disconnected. On many older amps its not unusual to see as high as 50 MVDC offset. I have seen higher and did my best to lower it to numbers I feel acceptable. Current transformer-less amp designs are capable of near zero DC offset brand new off the assembly line, but this is due to newer device being more truly complimentary to begin with then what was available when older amps were built.

I am a ZERO DC offset oriented service tech, but this is not possible on some of this older gear without re-engineering what you have in front of you. So you do the best you can and test the amp to see if the DC offset grows with input drive levels. If it does you got unresolved internal issues in the amp channel and I highly recommend you digress back into troubleshooting mode to find out why and how this happening.

This is running way too long, my apologies to all, OP Please feel free to PM me and directly email me for further discussions at this level. I will gladly guide you offline so we don't bother the forum so much and fill up their hard drives with pages of these instructions...lol..:thmbsp:

The painted items are selected transistors, graded that Mac culled out, and the paint identifies them.

I believe the bias is set without a load at idle on this vintage of Mac amp.....but need to pull the schematic to be sure.

Having spent the day recapping and tuning up a MR80 a evening of live jazz was a break needed. Local pianist Terry Lower is exceptional.

Had the schematic in the home office. This is the only amp of this generation that Mac specifies to set bias with the heat sinks hot, driving some low impedance speakers should do the trick.

Wow - thank you again for all of the wonderful info.

I'm going to place an order with Mouser soon and believe I have everything I need. I'll also take a picture of the flex cable replacement in my amp. The flex cable was basically replaced with individual solid core wires.

I'll post periodic updates once I get started.

-Matt

Thanks!

c_dk said:

The painted items are selected transistors, graded that Mac culled out, and the paint identifies them.

I believe the bias is set without a load at idle on this vintage of Mac amp.....but need to pull the schematic to be sure.

Click to expand...

The service manual should be available in the AK database shortly. Here's the bias procedure:

a. Operate amplifier at high power (approximately 50 watts) for about 2 minutes to warm the amplifier.

b. Remove the input signal and turn R351 and R352 controls full ccw. Bias must be adjusted soon after the input signal is removed while the amplifier is still hot.

c. Measure the line input power (approximately 25 watts) or current (approximately 0.2A).

d. Rotate R351 bias control cw until the line input increases by 3.5 watts o 0.03 amps.

e. Rotate R352 bias control cw until the line input increases by an additional 3.5 watts or 0.03 amps.

I assume to do this I will need to run the amp into a dummy load for a few minutes first (step a). I can easily make the power measurements using a Kill-a-watt meter (much easier than using my DMM).

-Matt

1moreamp said:

Film caps have been around since tubes were invented. Tube amps have them in between the tube stages to block the 300 to 600 DC volts plate voltages present but to allow audio to pass to the next stage....
I have seen many amp makers use film caps explicitly just to render higher SQ similar to tube amp SQ. I can point you to many amp makers internal pics and they all use film caps whenever possible in audio pathways.

Film caps are:
1 more stable
2 live longer more trouble free lifespans
3 have tighter tolerances
4 do not dry out and leak like wet caps
5 less temperature sensitive in most cases
6 more expensive cost wise
7 are usually slightly larger then wet caps nowadays except for larger ufd capacitive values.


Wet caps are:
1 CHEAP !
2 SMALL
3 just reliable enough to survive most warranty periods by a fair margin
4 A usable band-aid in any emergency LOL LOL LOL...


Please bear in mind wet caps in any amps audio pathways serve ONE main purpose, and that is to Block errant DC voltage offsets associated with gain stage interconnections and between amp inputs from preamps and such.. There are other electronic reasons but this one listed above is the main reason. Other reasons may be low frequency roll off and inhibition of excessive low frequency information and DC voltage input to the amps next stage. basically frequency compensation of some sort..

All electronic amp gear uses caps between preamps and main amps inputs, and their main purpose is to block any errant DC fundamentals from being sent inside of the amp stage, and to also limit low frequency inputs also if designed with that intent in place. Mac uses these also, so don't shoot me for telling the truth its standard engineering since the beginning of time and small, cheap wet caps fit back then for many reasons explained below I hope....

With the advent of DC wide bandwidth amps came the possibilities of amplifying DC voltage also.
Now on Mac amps with their auto transformers this isn't as bad of deal as with regular DC < Direct Coupled > amps that have no transformer coupled output.
DC voltage presented to many amps inputs will be even larger DC output to a speakers passive crossover and or the low bass driver directly which will offset the drivers cone from its true center resting position...This also will cause excessive heating of the voice coils, and burn out of drivers, along with muddied up sounding bass and damaged cones and spiders possibly...

So DC voltage output is a bad thing if your woofer is standing a half inch off center of its X max home position.
Plus any DC voltage present will rob real power from the amps driving ability.

SO amp makers use DC blocking electrolytic caps on preamp outputs and on main amp inputs. Mac used these also, and due to Film caps of that Time era being rated for 400 to 600 volts for tube use, well they were just to dang big to place inside of these new solid state wonders.

Newer, smaller footprint film caps with 50 and 60 volts ratings are nothing really new but. But cost wise they have been an impediment of manufactures because bean counters get into the picture of many amps being brought to market and they cut back on money expenditures like film caps to save money and to use smaller wet caps < which fail with age and what not >

Wet capacitors are cheap, small, and will usually out last most base warranty consideration for electronics manufacture, except Chinese wet caps < they don't have the chemistry down yet, but they are trying to steal it from the Japanese, Europeans, and the Americans >.
So no no to Chinese wet caps always, just google bad caps and see their entire web site of bad Chinese wet caps...I will let you be the judge after seeing what i already know...

Film caps are dry caps, so no wet electrolyte to dry out or leak out. dried out caps short out internally and wet caps that leak well lets just say they destroy PC boards when they leak.
I have seen Wet caps leak and burn wholes right thru fiberglass PC boards.

Most film caps are considered to be Self healing as they tend to clear internal shorts in most cases. I have seen a few melt down so they are not fool proof, but IMHO they are a better choice for all the best of reasons...

My MC4000 had this problem in its power supply, < nice quarter sized hole in its PC board in it power supply from a leaking wet cap > so YES even Mac amps have bad wet caps in them < Now please don't hit me for saying that !!! lol lol lol... > Its just the plain true about cheap, small wet electrolytic caps....:yes:

So for better SQ and more reliable service work I use Film caps when and where possible, and to date have never had any bad feedback about it and the amps SQ.
I own dozens of amps, and one particular set of amps I own made by Phoenix Gold exclusively uses nothing but film caps in any of the audio circuitry. They do sound better then the rest of their gear they made IMHO..

PPI used a big 4.7 ufd film cap inside of each channel between the preamp sections and the main amp input and they sold millions of car amps because they sounded good to most folks.
Tube SQ had been characterized electronically speaking and a lot of it has been tracked back to the use of FILM type caps...

You be the judge, I did not invent this stuff. I just kept up with the technology so I would be a good engineering tech to all of my clients. Which many come to me just because I will upgrade all of their gear internally where others will not hassle with it...Hope some of this was useful and helps some of you in some way..........
Oh and dump all tantalum caps you ever run into on any gear. They have been the death of a many a piece of gear in my service history experiences...They never really lived up to engineering expectations for them.

Please forgive the thread-jack OP...

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I love the attention to detail in your descriptions. Thank you for the explanation.