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Old 19th February 2013, 05:18 PM   #241
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Default Nigel

Quote:
Originally Posted by nigelwright7557 View Post
If the heat-sink is getting to hot then the output transistor are passing too much current.
The first few stages of the amp are about voltage amplification so shouldn't affect it.
Then the bias stage gives the correct bias voltage to the output transistors.

You could try changing r95 and r96 to 3k3 and see if the problem goes away.
This would decrease the bias voltage so you might get some crossover distortion but it would prove where the problem is or isn't.
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Old 19th February 2013, 06:25 PM   #242
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Quote:
Originally Posted by chris661 View Post
PREamp out!!

Using the power amp output in its current faulty state could destroy any subsequent equipment.

I think that's what you meant, but I feel we should be clear on this point.

Chris

PS - to the OP, have a look on eBay for Samson Servo. There's a few bargains to be had, if you can get one to fit nicely in the cabinet.
Sorry, was almost asleep
Yes, I meant to write "the to Power Amp signal" , which is present at the "Preamp Out" jack.
Good eye.
Unfortunately badraven is so desperate by now (this said with a good meaning, I would too in the same situation ) that he does whatever we write here, even if it was meant as an example or a comparison and not an actual repair tip.
My "Plan B" is to replace the full power amp with something new and proven.

I suggested in that case removing al Power Amp parts and cleaning the PCB because unfortunately this is an "everything in a huge PCB".
In another amp, as in, say, a Laney one with separate PCBs it's easy to remove the Power one and fit a new one.

In my own amps, which I have been making for 43 years now, for the last 30 years I have been using different Power schematics, from 2N3055 to TIP142/147 to Mos Fets but *all* of them are mountable on the same back panel/heatsink with the same 2" separation screws, and have the same connection wires (transformer/speaker out/ground/audio in) so you can easily pull a dead Power unit, junk it, and install a new one in the same place, same wiring.

In fact one of mine will work there ; I'm not offering you one simply because I'm 6000 Miles away.

I have used them to repair hopelessly burnt Marshalls, Peaveys, Laneys, Gallien Krueger, Acoustic, Crate, etc. , typically after they went through a couple ham fisted "Techs" who think a jackhammer, a chainsaw and a welding torch are proper tools for amp repair.
Oh well.

By the way, Sessionnette 75 amps use a similar idea.
For many years they concentrated on improving the Preamp sound , where the real work is, and used a generic ILP 100W OEM Power Module .
There are both MosFet and Bipolar versions.
Since ILP disappeared some years ago, they became unobtainable, so they designed and made a new module, different technology but which can be mounted instead of the original one, with the same footprint and wiring.
End result? : those wonderful 35 Y.O. SS amps, preferred by Pete Townshend and Jan Akkerman (among others) are still alive and kicking, how's that?

NOTE: *if* you decide to mount a new power amp to the underside of that chassis, I suggest this one:
Velleman, Inc.
Velleman, Inc.
http://www.vellemanusa.com/downloads...k8060_rev1.pdf
The way it was designed, it's a physically "flat" amp, ideal to build a "Powered cabinet" Plate Amp or what I suggest, on the down side of the chassis.

Although, of course, maybe you can find a used complete working amp for less than that.
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Old 19th February 2013, 06:56 PM   #243
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Default R95 R96

I changed R95 R96 from 2.2k to 3k and didn't get much of a difference. Still overheated and it didn't make much of a change time wise.

I changed them to 4.6k and it took about 5 minutes longer for the outputs to overheat. Now if I continue raising this value there would come a point where the transistor would shut off and kill all current in the C-E terminals, thereby shutting the entire circuit off by virtue of no power flow. Since the overheating is a byproduct of too much current flow at the output transistors, the trick here is to figure out why there is too much current flow (I know, that's obvious huh? I'm thinking out loud)

Taking Q15 into consideration: Between E-B we have R96 2.2k Between B-C we have R97 2.2k and R98 10 ohm and D31 BAT85. A diode drops voltage not current, is that correct?

I don't understand what D31 is doing in this circuit. I am attaching the Datasheet for it because I can't find a value for it. Can someone help me with this please.

On a different note: Since Q19 and 21 do not appear to have bias circuits, Q15 appears to set the bias for the entire circuit on the -42V side. Since the overheating is a result of too much current flow, this really shouldn't be this difficult to diagnose should it? On the other hand, this amp worked just fine (for the most part) for 5 + years. So it seems strange that there would be a need to change the bias as much as I did last night to get the change in the time it took to overheat with the new bias resistors.

BR

Quote:
Originally Posted by nigelwright7557 View Post
If the heat-sink is getting to hot then the output transistor are passing too much current.
The first few stages of the amp are about voltage amplification so shouldn't affect it.
Then the bias stage gives the correct bias voltage to the output transistors.

You could try changing r95 and r96 to 3k3 and see if the problem goes away.
This would decrease the bias voltage so you might get some crossover distortion but it would prove where the problem is or isn't.
Attached Files
File Type: pdf BAT85 Datasheet.pdf (126.0 KB, 0 views)
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Old 19th February 2013, 08:28 PM   #244
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Smile Looks like the amp is finally cured!!!!!

Gajanan Phadte

I don't have a way of testing caps other than to ohm them for a short. I just happened to have two .47uf 50v caps on hand so I replaced C40 and instant WONDERFUL!!!

No more buzzing, the outputs are running about 87 degrees after about 15 minutes of playing on all three channels. And it sounds wonderful! Did I mention that yet? lol Of course I still have the 4.6k resistors at R95 and 96. Is that going to be a problem? I ask because I really like the sound of the amp like this. It seems like it has a fuller sound. Although that could very well be because this amp has always sounds WAY better than my other amp. Maybe I should put the 2.2k resistors back in and see what it sounds like. Hmmm

Anyway I ohmed the old cap and its shorted. So I guess its no wonder the outputs were running hot.

Hopefully this is the end of problems with this amp for a while.
Of course I wish to thank Gajanan for his advice here because it led me to the immediate solution. But I wish to thank all of you for your tireless help. Sorry this went so long. I didn't give up though and I have learned so much from you guys and all the research I have had to do. Thank you thank you thank you! You guys are wonderful!

BR

Time for a beer!

Quote:
Originally Posted by gmphadte View Post
Have u checked C40 for leakage or short.

Sometimes, a diode shows good when cold tested but does not work/opens in the circuit.
Check voltage across inputs of each op amp.
You can also check for forward voltages of diodes including BE junctions with power ON.

Gajanan Phadte
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Last edited by badraven; 19th February 2013 at 08:33 PM.
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Old 19th February 2013, 10:32 PM   #245
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I changed R95 and 96 back to 2.2k and the sound definitely changed. It was noticeably thinner and there was some low level buzzing coming from the speakers. Also the output transistors got a lot hotter, like 145 degs in a short time.
Anyway, I put the 4.6k resistors back. I guess this means its not really repaired, it basically has a bandaid on it. It still gets up in the 140 deg range when I am cranking the volume but I think that should be expected shouldn't it? I think I am going to install a cooling fan inside like the kind used in a computer power supply. I have red plenty of articles where people say a fan is an excellent idea in any amp. I am putting the amp back together for now. I'm glad to be getting it off my bench.

BR
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Old 19th February 2013, 10:40 PM   #246
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So the fault was a s/c electrolytic ?

Had you followed my suggested plan then you would have found that pretty quickly.
Take out all SS and test them and while they are out check all passives.

Never mind I guess you were taking instruction from numerous sources.

If one electrolytic is gone then its likely the rest (due to age) are fading away.
I would replace all electrolytics including smoothing caps.
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Old 20th February 2013, 12:16 AM   #247
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Old 20th February 2013, 12:21 AM   #248
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Smile Nigel

Don't get your feelings hurt man, when I began I had no knowledge and it took until recently before things began making some sense. In the beginning a lot of what you guys were saying was more jargon that help for me. I don't remember who it was but one of you said not to worry about the caps because that is rarely the problem.
Didn't make sense to me but what did I know? Sorry if I took the wrong path, I did the best I could with what I had to work with.

I will get the caps ordered so I can replace them all. I still don't know what "s/c" electrolytic is.

I take it putting 4.6k resistors in R95 and 96 isn't a problem.

Thanks!

BR


Quote:
Originally Posted by nigelwright7557 View Post
So the fault was a s/c electrolytic ?

Had you followed my suggested plan then you would have found that pretty quickly.
Take out all SS and test them and while they are out check all passives.

Never mind I guess you were taking instruction from numerous sources.

If one electrolytic is gone then its likely the rest (due to age) are fading away.
I would replace all electrolytics including smoothing caps.
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Old 20th February 2013, 12:22 AM   #249
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Unless the amp case has lots of large vents then for a power amp you definitely need a fan or two.
I use pc cases for my amps and have 2 fans at opposite corners, one sucks air in while the other blows it out the opposite corner.

It sounds like your bias is at fault for the overheating problem. You need to find the setup procedure for the amp.
A simple approach would be to play a sine wave through the amp and look for crossover distortion on a scope. I was surprised just how little bias you can get away with.
However quite a few people prefer the sound if the bias is run a bit hot.
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Old 20th February 2013, 12:29 AM   #250
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Sorry s/c is short circuit.

Electrolytics dry out with age.
They also have a much shorter life than other types of capacitor.
Some smoothing capacitors can have lives of 2,000 hours or less so its not wise to leave the amp on permanently.
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