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S'men A2502 trouble, help please.

Mazeppa

Mazeppa

Active Member
Acquired a very nice looking Soundcraftsmen A2502 a couple of months ago. Just about the time I really started liking the way it sounded (almost immediately) I noticed some background hiss from B channel. Tried swapping speakers, preamps, speakers wires, interconnects; everything I could think of before checking the DC offset and found what I was hoping I wouldn't.
Ch/A=-79mv, Ch/B=264mv :tears:

Based on what I've read, those combined symptoms on the same channel seem to indicate troubled MOSFET(s). Amp uses Hitachi K135/J50 which don't seem to be readily available. While I'm confident in my ability to effect repairs, my electronics troubleshooting skills are limited and I have no confidence in in the only shop around. Please save from having to decide whether to throw parts at it, or write it off. :sadwave:

Help!
 
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Do you have the schematic for this amp? The info below is for the following models PA5001 MA5002 EA5003 PA5001A MA5002A RA75XX and RA6501
To correct DC offset issues on these amps you need to get a bunch of ZTX694B transistors mouser sells them.
A bunch is defined as enough of them to find 2 pairs of matched transistors measuring the hfe should be matched within a couple of percent.
Then replace Q1 and Q2 with these matched pairs.
Q1 and Q2 are MPSL01 originally to find them trace the input to the board it will go to an axial cap 22uf BP C1 this is the only axial cap on the board.
C1 connects to R2 2.2K other side of R2 goes to R4 1K other side of R4 goes to the base of Q1. Q1 and Q2 have the emitters connected together.
Recap the boards while you are at it to get ride of the hiss
Hope this helps
 
Schematic was ordered this morning, should be in my inbox at home. Won't be until later in the week before I can get back into the amp and pull those boards to locate Q1 & Q2, and inventory the cap.s to be replaced. All the electrolytics, right?

Meanwhile, I'm studying how to determine HFE for matching. Ib/Ic=HFE sez google. Hmmmm, do I assume correctly that these currents have to be measured for each transistor? If so, how?

I had a 1967 GTO back in the day, any connection between the vehicle and your user name?

Many thanks.
 
Good thing on the Schematic I do not have one so I can not be sure on the info I posted about Q1 and Q2. Could I get a copy to verify? If so I can put together a cap list for you too.

To measure the HFE you will need to get a Digital Multi Meter that has this functionality. I got one from Parts Express

Yes goat67 is directly related to the 1967 GTO that is under restoration in the garage, not getting to far since I got back into audio.
 
Q1 & Q2 are indeed "joined at the emitter" and they are MPSL01. Will pursue acquisition of HFE measurement capable multimeter.

Would you like to provide me an e-amil address to send the schematic pdf to?
By PM, perhaps?

A 1967 GTO is not what I'd call a subtle ride, but can be very entertaining. American muscle at it's best, especially a 4 speed. Mine was an automatic. Good luck with your restoration.
 
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Got the schematic thanks, Can you provide the dimensions of the main filter caps?
If they are 2.5 inches X 4 inches these will work

2 X 18,000 75v
75-36DY183F075BC2A

Here is a list of the other caps from mouser.

2 x 1uf 250v film bypass caps for filter caps if you want to bypass them
539-150105J250JE

2x 22uf 25v BP on these I use a radial instead of the axial
so you will need to be creative on the mounting
647-UES1E220MEM

Here is an axial if you like
140-NPAL50V22-RC

2 X 10uf 100v
647-UPW2A100MED
Visually check how many of these 10uf 100v caps you have.
If the boards are built like other amps one will be on each board
these are the secondary rail filter caps

2 x 4.7uf 35v
647-UPW1H4R7MDD

2 X 220uf 16v
647-UPW1C221MPD

4 x 47uf 35v
647-UPM1V470MED

diff pair transistors order a bunch if you can not find or get a HFE meter
PM me and maybe we can work something out on getting you two pairs of these.
522-ZTX694B

Very few caps and a very simply designed amp
 
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goat67 you are a prince of a fellow. An ordering list right down to qty, wow.

10uf/100v qty for my amp is 2 per board, C6,11.

May I confirm a couple of things?

1. C1 is 25uf/25v on schematic & board. 22uf/(1E in PN=25v?) Muse on your
list is an acceptable variation of capacitance value in this application?

2. 1uf/250v film bypass cap.s on your list are to be connected in parallel to
C15,16 main filter cap.s? I've read much on how effective that technique
is in improving sound quality and would certainly try it.

Concerning differential transistors, I'll order 25ea. May enlist your aid in matching them if I can't find some help locally, or bring myself to buy an HFE capable meter. I already have a multimeter and the thriftiness imparted to me by my dear German/Irish granny rebels at the thought of buying another meter for just one function.
 
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1. C1 is 25uf/25v on schematic & board. 22uf/(1E in PN=25v?) Muse on your
list is an acceptable variation of capacitance value in this application?

I have done 5 amps and a couple of pres with these so far. So yes the value is acceptable.

2. 1uf/250v film bypass cap.s on your list are to be connected in parallel to
C15,16 main filter cap.s? I've read much on how effective that technique
is in improving sound quality and would certainly try it.

Yes I solder eye connectors to the leads I put shrink wrapping on the leads and attach them to the posts.
 
goat67 said:
To correct DC offset issues on these amps you need to get a bunch of ZTX694B transistors mouser sells them.
A bunch is defined as enough of them to find 2 pairs of matched transistors measuring the hfe should be matched within a couple of percent.
Click to expand...

25ea ZTX694B transistors (and all cap.s you recommended) are in hand, Hfe capable multimeter is on the way.

Now for a "No such thing as a dumb question." question.

2% of what exactly? The average value of a given pair? The lower value of a given pair? Am I even close?
 
I generally will measure the HFE of each transistor - keeping track of each transistor by laying them out on a sheet of paper with there corresponding Hfe values. Once you have 4 that are within 1 Hfe of each other your done. I try and shoot for the 4 at the highest values measured.The closer you can match'm the better. Don't try to measure the HFE with a ceiling fan right above you...It will cause your readings to flucuate.Transistors are temperature sensitive.
 
What I meant by 2% is for example if the HFE for a transistor is 800
and these will be in this range then you have a range of 16 so 784 to 816
looking at this I would say 1% is really required so 8 HFE tolerance 792 to 808.
As Avionic states I try to get them to be within a HFE or 2

Even if you get a perfect match you will end up with some DC Offset less then 10 mv generally
 
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Progress report: B&K 2704C is in the inventory and measured its first batch of transistors. Remeasured approx. 25% and repeatability was excellent.

Many thanks to avionic for the tip about temperature sensitivity w/o which I'd probably still be trying to read the first one or sending a good meter back to mouser.

? for goat67 and/or avionic: When selecting which 2 pair to use; Is greater importance to be attached to using the highest Hfe value pairs, or the pairs more closely matched to each other?

743/742-716/719 pairs or 642/642-644/644 pairs the better choice?

Thanks again, fellas. Y'all have been terrific.
 
Mazeppa said:
Progress report: B&K 2704C is in the inventory and measured its first batch of transistors. Remeasured approx. 25% and repeatability was excellent.

Many thanks to avionic for the tip about temperature sensitivity w/o which I'd probably still be trying to read the first one or sending a good meter back to mouser.

? for goat67 and/or avionic: When selecting which 2 pair to use; Is greater importance to be attached to using the highest Hfe value pairs, or the pairs more closely matched to each other?

743/742-716/719 pairs or 642/642-644/644 pairs the better choice?

Thanks again, fellas. Y'all have been terrific.
Click to expand...

Matched gain is most important.
But if a choice between matched pairs is available use the pair with the highest gain.

Tough call but use the ones that match, the reason is the DC Offset will be lower if the gain is the same. But if all you had found was the first set they would be OK also.
 
goat67 said:
Matched gain is most important.
But if a choice between matched pairs is available use the pair with the highest gain.

Tough call but use the ones that match, the reason is the DC Offset will be lower if the gain is the same. But if all you had found was the first set they would be OK also.
Click to expand...

Check, that confirms my inexpert inclination. Now all I need is a block of uninterrupted time to work.
 
Finally had some time to do some soldering. Electrolytic cap.s and transistor pairs replaced per goat67's recommendations. No cap. explosions on power up. DC offset 15mv and 17.8mv (was -79mv and 264mv). Not <10mv as was hoped for, but I'm very pleased nonetheless.

Memory can serve poorly in these cases but after approx. 3 hours of listening I've got to report a not drastic, but noticeable overall improvement in SQ at all frequency ranges. Actually that improvement is gravy because the amp sounded great making music before. It was the backgroud hiss and 60hz hum when preamp input was low that got me started on this project, which I considered to be more preventative maintenance than repair or upgrade (although higher value filter cap.s fall into that category). Speaking of the hiss & hum, both have been greatly reduced, but not eliminated.

I am very happy with the results achieved, and extend my gratitude to avionic and especially goat67. You went above and beyond, brother, may your audiokarma be increased greatly.

The 'ol A2502 sounds great and I look forward to years of enjoyment.

Now on to the next thing (It never really ends, does it?). Does anybody have any ideas about the remaining hiss & hum? I could certainly live with and love the amp the way it is, but I've got this thing about my stuff being the best it can be.

With everything hot, and preamp volume anywhere above 0, I can't detect it. With the amp disconnected from the preamp and powered up, the same hiss & hum increase in amplitude, not a lot, but noticeably. All this is happening with the amp input levels at 1 o'clock. A tired Marantz 1060 integrated sitting in the same spot as the A2502 that was used for a while w/the internal preamp and then w/the same Onkyo P304 preamp being used (now and before) w/the A2502 did not exhibit this behavior. One thing that did change while the A2502 was down and the 1060 in service was a speaker swap, but I'm calling that a wash as the hiss & hum have the same, easily identifiable "sonic signature".
 
Glad to hear the amp sounds good. A few questions. Did you adjust the bias?
Did you deoxit the gain controls?

Mazeppa said:
With everything hot, and preamp volume anywhere above 0, I can't detect it.
Click to expand...

Is this with an input present? Sounds like no problem with noise then right?

Is the hiss and hum on both channels?

Mazeppa said:
With the amp disconnected from the preamp and powered up, the same hiss & hum increase in amplitude, not a lot, but noticeably.
All this is happening with the amp input levels at 1 o'clock.
Click to expand...
Could be normal with no input connected.


Mazeppa said:
A tired Marantz 1060 integrated sitting in the same spot as the A2502 that was used for a while w/the internal preamp and then w/the same Onkyo P304 preamp being used (now and before) w/the A2502 did not exhibit this behavior.
Click to expand...
Are you saying that another amp with the same preamp does not have any hiss noise issues?

Mazeppa said:
One thing that did change while the A2502 was down and the 1060 in service was a speaker swap, but I'm calling that a wash as the hiss & hum have the same, easily identifiable "sonic signature".
Click to expand...

If the noise is on both channels and is the same it points to the power supply but there is not much there pretty simple.

One thing you could try is to disconnect the input AC to the led board and see if there is any difference. I doubt if this is the problem.

Check the routing on the input wires and the wires that run to the boards are they by the transformer? Can they be moved away so they do not pick up any noise?

For me if an amp does not have any noise issues when playing music and is silent between tracks I am happy.

I have never had an amp like this maybe it is as good as it gets?
 
goat67,

I'm almost embarrased to ask, but how does one sectionalize the quotes in a reply as you did? Makes these Q&A exchanges flow very well. The mini-quote option seemed like the deal, but I get no action when I select it. Can't figure it out.
Count computer literacy in with electronics as not being among my strong suits. I'm willing to learn, however.
 
Mazeppa said:
goat67,

I'm almost embarrased to ask, but how does one sectionalize the quotes in a reply as you did? Makes these Q&A exchanges flow very well. The mini-quote option seemed like the deal, but I get no action when I select it. Can't figure it out.
Count computer literacy in with electronics as not being among my strong suits. I'm willing to learn, however.
Click to expand...


Sorry to jump is but to quote stuff, just quote the whole thing and notice the square brackets "[" or "]" before and after the quoted message.

Your quoted message begins with "[QUO..=Mazepp..2754....] and it ends with "[/QU...]" I can't write it in or else it will quote. It's looking for these patterns to make a quoted text. Just wrap the message you want to quote with this format and preview it to make sure you've got it right...or write
 
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