Kenwood 700M vs. the newbie

DrumminDaddy

Hit it, Baby !!
Before I get into describing the situation with the 700m, just a brief bio on myself.
I am a machinist by trade with some years spent as a CNC repair tech. Most machine tool electrical faults involve switches and relays. The modus operandi for troubleshooting board level faults is to determine correct inputs/outputs. If the correct readings aren't seen, the board gets replaced. We aren't trained to look at discrete components on the boards. If the chips ain't flyin', the machine's not making money. The cost of a whole new board pales in comparison to the down time. Get in / get out. $$$. Obviously, that's not an option here. I have never done a repair on an amplifier before other than to clean some scratchy switches. So I decide that the 700m is going to be my first project (somebody stop me !! ).

That being said, my goals are simply:
1) Do no more harm.
2) Learn, have fun at it.
3) Achieve a working amp. (Perhaps the full monty by EchoWars at a later time...)

The scenario:
Upon recieving my 'new' amp, in my zeal to listen to this new toy, I turn it on and a couple seconds later hear the speaker relays click in. Titties!
Well, not so fast. Just wanting to hear some music through it, I plug in my headphones, and poof! Out comes a whisp of the magic smoke. Nuts!! (Well, that's not quite how I put it at the time.)

So off comes the cover and looking around, I do believe that I may be the first one inside (feeling a little naughty!).

I see R114 and R113 to the headphone jack L/R are roasty well done. Put a meter on the outputs and read 70vdc on both channels.

Lesson learned:
Check the DC offset on any unknown amp BEFORE hooking a load to it!

On EW's recommendation, I purchased the service manual from www.stereomanuals.com. Also had a brief exchange with him in another thread regarding dbt and the operation of the soft start circuit. He cautioned against using less than 100w bulb as to inhibit the soft start relay coil from operating.

I've been studying the schematic and I've got some idea what's happening in certain aspects, in other areas, quite ignorant.

What I've done/seen so far:

Fired up on dbt. Bulb lights bright for a second then dulls down.
Panel lights up, speaker output relays engage.
Disconnect meter board, no change. Still 70vdc on speaker outputs.
Disconnect output boards (leaving STV-3H varistors connected per EW), no change.
At one point I realize that the soft start relay (RL3) isn't even engaging. At first I'm a bit perplexed by seeing line voltage on the NO contacts. Studying this circuit, I've come to the conclusion that the NO contacts have voltage because the amp must be running all through the power resistor R301, which runs in parallel with the contacts. I suspect the reason R301 hasn't gone open is because whatever is shorting voltage to the outputs isn't drawing a load. (Unless, of course, I give it one. Like my headphones! )
I've removed the protection board from the chassis and determined that the coil for RL3 reads open and it shows heat stress.
There is also signs of heat stress on the PCB at Rp1, and on connector A-2 pin14. I suspect the heat mark on the pin connector was caused by the resistor. It was pressed tight against the connector. (Why wouldn't it be?). I conclude this because the heat mark is only on the side the resistor was touching, but I may be missing something. Rp1 (12k resistor) measures 1248 ohms.

I am perplexed as to why the protection circuit is engaging the speaker relays with this kind of voltage present on the outputs. Could the source of this voltage be the protection circuit itself?
I have a very cursory understanding of the operation of the driver boards, but it can be summed up as: signal in<gain out.
Can this amp be placed under power with the driver boards removed without doing any harm?

Any suggestions at this point which could help in isolating the cause of this output voltage would be greatly appreciated.

Regards,
Terry
 

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I'm a former board changer too.. When working in AK's north slope oil fields, when a piece of gear went down all BP wanted was to get back in production, the money wheels must be kept spinning 24/7 in the oil patch as every minute down could mean tens of thousands of $$. So locate the problem, replace the culprit card and get the well pad back online. I've come to realize this type of experience does me very little good for working on audio :no:.

Anyway I have decided I'm staying the hell out of my 700's! To my good fortune my 700M is working real good, the sound is sweet to my ears, so much so that I have taken back up a new evening activity, listening to music! Of course every time I switch it on I cross my fingers that this isn't the time this 40 year old amp is going to smoke like my woodstove, but so far so good. The DC offset is still in the acceptable MV range TG, not like brand new but probably the distortion is beyond my 59 year old ears, which is good enough for now..

Now to finish getting the 700C boxed up and sent to EW! I hope it isn't beyond bringing back from the dead..
Good luck on your project, I sure hope you can bring the old girl back! :yes: You are a braver man than I.
 
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You are a braver man than I.

Yeah, sometimes I've got more balls than brains. That's why I'm hangin' here on AK. This is where the brains are! :D

Seriously, I wouldn't even consider working on any gear that I didn't own. If I muck it up, I've got nobody to blame but myself. Let's hope it doesn't come to that. :tears:

And speaking of aging ears, you don't wanna live inside my head having worked around screaming high freq spindles and whining pumps for decades. Not to mention as a drummer whaling away and music blasting. I've got a jet engine running through my head 24/7. But that's another thread in itself.
 
Rp1 is 1.2K, so yours reads fine. It gets hot, which is why it's rated for 2W. The scorching on the connector is a little odd, but not likely the root cause of any failure or indication of an issue.

If the soft-start relay has failed, of course it needs replacement. Should be this one.

You measure the same +70V offset on both channels? The Kenwood shares a erratum with many protection circuits, in that if one channel fails with a positive offset of, say +70V and the other channel fails with a negative-but-equal-magnitude offset of -70V, the protection thinks everything is hunky-dory and the speaker relay will close. If that's not the case, then I'd have to assume that the protection circuit has major problems, or the signal from the output of the two channels is getting lost on the way to the protection circuit. In any event, I'd be repairing the two blown channels, and replacing lots 'O parts in the protection circuit to make sure that it works as it should.
 
I'm a former board changer too.. When working in AK's north slope oil fields, when a piece of gear went down all BP wanted was to get back in production, the money wheels must be kept spinning 24/7 in the oil patch as every minute down could mean tens of thousands of $$. So locate the problem, replace the culprit card and get the well pad back online. I've come to realize this type of experience does me very little good for working on audio :no:.

Anyway I have decided I'm staying the hell out of my 700's! To my good fortune my 700M is working real good, the sound is sweet to my ears, so much so that I have taken back up a new evening activity, listening to music! Of course every time I switch it on I cross my fingers that this isn't the time this 40 year old amp is going to smoke like my woodstove, but so far so good. The DC offset is still in the acceptable MV range TG, not like brand new but probably the distortion is beyond my 59 year old ears, which is good enough for now..

Now to finish getting the 700C boxed up and sent to EW! I hope it isn't beyond bringing back from the dead..
Good luck on your project, I sure hope you can bring the old girl back! :yes: You are a braver man than I.

Exact same situation in the semiconductor manufacturing biz. I worked in a wafer fab for almost 20 years. Diffusion, Epi, Ion Implant, Wet Benches, Laser Scribes. Just trace the problem to the board and replace the board. I've had production supervisors standing over me while working, telling me how much it costs the company per hour to have a machine down. And your right, that type of experience is pretty much worthless when trying to work on audio equipment.
 
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You measure the same +70V offset on both channels?

Yes, I was sure to take note of the polarity, readings taken with the common lead referenced to chassis ground.

I also took readings on all the connection points of the protection board before removing it. One set of readings is standing out to me that I'd like to run by you. With the output boards disconnected, the only connection I see between the driver boards and the protection board is thru the +/- 70v pins on A2, which read accordingly, and pins A2,19 and A6,25. It is these latter two that I'm concerned with. Both read +70v. Both share a common node thru Cap6 and the speaker outs on the protection board. Can I safely power this amp with the driver boards disconnected? If so, and I still see this voltage on these nodes, I'll concentrate on the protection board first. Of course, I'm resigned to the possibility that there may be issues on the drivers as well, but I'd sure like to disconnect them and see if it effects a change.
 
And your right, that type of experience is pretty much worthless when trying to work on audio equipment.

In understanding board level circuitry, agreed. But there are aspects that help build a foundation. For example, using a dmm, reading technical drawings and schematics, tracing circuits, skill with a soldering iron. We gotta start somewhere.
My take is, if I muck something up, I know not to do THAT again! :D

Believe me, I've got some old POS boards laying around that I'm gonna sharpen my soldering skills on before I touch the Kenwood. I may be a newb, but I'm not a hack. :cool:
 
Powering up with the driver boards out isn't a problem. Just powering up with the heatsinks and their associated transistors/bias diode disconnected (while the driver boards are still in place) is what is a bad idea, since the removal of the bias diodes may lead to smoked parts on the driver board.

Probably have to resign yourself to the near certainty that you'll need new power output devices. On-Semi MJ21193G and MJ21194G, from a reliable seller such as Mouser or Digikey.

Also note that whenever output transistors get lunched, the V/I limiter transistors (Qe5,6,7, and 8) should be changed as well. And if you have the 'new' style driver board note that Qe5 is marked incorrectly both on the schematic and the silkscreen on the PC board (collector/emitter reversed).
 
Exact same situation in the semiconductor manufacturing biz. I worked in a wafer fab for almost 20 years. Diffusion, Epi, Ion Implant, Wet Benches, Laser Scribes. Just trace the problem to the board and replace the board. I've had production supervisors standing over me while working, telling me how much it costs the company per hour to have a machine down. And your right, that type of experience is pretty much worthless when trying to work on audio equipment.

The thing about fixing equipment on this level is after a while the extra boards get used up and there are no more to be had on the aging stuff. Then when something goes down and no parts, the whole machine gets yanked and cannibalized to feed machines still running. The stuff we worked on was already 20 years old and glitchy, so parts were getting pretty scarce. I have to wonder if this phenomena is or will start happening with the more modern audio stuff and become landfilled.

I would like to fix up a older amp however, just not one that is worth as much as the 700's are. I am thinking of an integrated amp, maybe a 60 wpc Marantz that has the preamp out option. That way when I get my 700C back and at the time sending the power amp off I would have something to play while that is away. I'll have to see as there never seems to be enough time during the day and I tend to run out of gas by evening time.. :boring:
 
... if you have the 'new' style driver board note that Qe5 is marked incorrectly both on the schematic and the silkscreen on the PC board (collector/emitter reversed).

Noted. I have the corrected schematic that you posted in another thread. Thanks for that. Looks like I have the newer style boards that you described as 'baby puke yellow". I must admit, when I first read about that error, I was a bit taken aback that such a quality control blunder would make it out the door. Particularly on their 'flagship' model.

As I'm looking over these driver boards and the schematic, am I seeing the screening for diodes D2 and D3 backwards also?
 

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Powering up with the driver boards out isn't a problem.

Got the driver boards out and the protection board back in. Under power and the speaker relays still kick in. The voltage on the outputs now reads -.002v on each channel. Yeah, you nailed it. Looks like I'm going to be replacing lots of parts. Just a cursory inspection and I'm seeing signs of bulging on some of the capacitors and one of the diodes reads shorted. Let the 'fun' begin.
 
polish the pin (is that what they're calling it now?)

EW, I read in another post how anal you are about getting those connector pins cleaned up (with good reason) and how you labor over it.
Check this out:

I'm using a machinist's pin vise, Starret .030 to .062" size.

Remove the nut from the vise and leave it off (for clearance between adjacent pins. Tweak the four collet fingers open to a size that gives you a bit of clearance over the connector pin diameter.

A dab of polish.

A gun cleaning patch placed over the pin. (I'm using Hoppes 1205 12ga. patches cut into fourths.)
Edit: actually cut them into 9ths, they dont need to be that big.
These are tough enough that when you push the pin vise over the pin/patch, the pin wont just poke through the patch.
If it does poke thru, just open the collet fingers a tad more.

Give it a twirl back and forth and in a few seconds it's shiney as a new penny.
I didn't even need to clean any excess polish up. Use it very sparingly and the patch does the rest.
 

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Got a couple of those for hand-drilling small holes. I'll have to try that.

We still have the issue that the protection failed with 70V on the output. I'd be replacing all the diodes and transistors in the protection circuit. And the three relays & the electrolytic caps. A snap-in cap will fit nicely where the 470uf 63V cap comes out.
 
I'd be replacing all the diodes and transistors in the protection circuit.

I pulled all the output transistors and was pleasantly surprised to find they all checked out ok. Also the emitter resistors and diodes on the output boards checked out ok as well.

I've been putting together a parts list. I've got most of the items sourced on the driver boards. What diodes do you recommend to replace d1,d5 YZ-140 and
d2,d6,d7 1S2076 on the drivers? On the protection board, I can't source 2SC1416 and 2SC1415.
 
Know that a simple diode test of a transistor or diode is lucky to catch 7 out of 10 failures. But since the outputs are easily accessible even once the unit is reassembled, they can wait.

If you persist in searching for obsolete Japanese transistors, you'll either go insane or end up buying some remarked fakes from China. There are good modern equivalents for every transistor in that unit. The originals are long gone, and I'd view with suspicion anyone who is selling pretty much any part that is labeled with any number used in this amp. If I can't get the part from Mouser, Digikey, Newark, or B&D, I probably won't be buying it. If you're smart, you'll follow that lead.

Specifically, the 2SC1416 and the 2SC1451 are being used as switches on the protection board, so their replacement simply needs decent gain and low saturation voltage. A Zetex ZTX694B for Qp1 and Qp2, and an On-Semi 2SC3902T for Qp3. Diodes Dp5~Dp9 (1S2076) are replaced with plain-Jane 1N4148 signal diodes.

FYI, any 2SC1451/1452 or 2SA809/810 need to be replaced out of hand. Mouser has the 2SA1507T (to replace the 2SA809/810) and the 2SC3902T (to replace the 2SC1451/1452). De1 and De5 are just 14V 500mW zener diodes.
 
Know that a simple diode test of a transistor or diode is lucky to catch 7 out of 10 failures.

Understood. They can misbehave when under power in circuit. But glad to see at this point that I didn't find any that were outright shorted. Still hanging on to hope!!

If you persist in searching for obsolete Japanese transistors, you'll either go insane or end up buying some remarked fakes from China.
...If I can't get the part from Mouser, Digikey, Newark, or B&D, I probably won't be buying it. If you're smart, you'll follow that lead.

Yes, I realized in very short order that there are no original components available any longer. I've been culling through any thread I can find where there is mention of a replacement equivalent component. And some of them are years old and the replacement components are obsoleted as well. I referenced the original part numbers as being the only point of reference I can determine at this time. Comparing data sheets and giving a green light on an equivalent component is WAY above my pay grade at this time. This is an area where I need the most help. Trying to do as much homework as I can before asking but eventually run into a brick wall.

Specifically, the 2SC1416 and the 2SC1451 are being used as switches on the protection board, so their replacement simply needs decent gain and low saturation voltage. A Zetex ZTX694B for Qp1 and Qp2, and an On-Semi 2SC3902T for Qp3. Diodes Dp5~Dp9 (1S2076) are replaced with plain-Jane 1N4148 signal diodes.

FYI, any 2SC1451/1452 or 2SA809/810 need to be replaced out of hand. Mouser has the 2SA1507T (to replace the 2SA809/810) and the 2SC3902T (to replace the 2SC1451/1452). De1 and De5 are just 14V 500mW zener diodes.

Thanks. I'll be going over my parts list again. I'm sure I'll have more questions...
 
Got a couple of those for hand-drilling small holes. I'll have to try that.

If you don't have the right size, I'd be glad to send it to you. I've got a bunch of them rolling around in my toolbox. It's the least I can do for all your help. :thmbsp:
 
I like your pin cleaning tool! Purty slick...

I just take the whole assembly off the board, stick it in my bench clamp, and hit the pins with a fine wire brush. Knocks that crap right off, quick. Allot of times they need resoldered anyways, so why not.
 
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I like your pin cleaning tool! Purty slick...

I'm a machinist, so I guess I have a tendancy to look for a fast 'production' way of doing things. Especially tedious things.

I've been eyeing these bitchin' machined knobs on the 700 series and figured I could make a reproduction for the missing one on my 700T.

Sure comes in handy having a machine shop at your disposal. ( and the ability to use it ) :D

Photo of 1st operation. Stay tuned...

Edit: This knob production is a tangential issue to the 700m electronic repair. Because of this, I will continue this subject matter in its own thread.

http://www.audiokarma.org/forums/showthread.php?p=8108645#post8108645
 

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Specifically, the 2SC1416 and the 2SC1451 are being used as switches on the protection board...
FYI, any 2SC1451/1452 or 2SA809/810 need to be replaced out of hand

Understood. I've been studying on the modes of operation for transistors. I see these are used as switches to turn on the relay coils. They are full on for 100% duty cycle.
 
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