Valuable thoughts, Ian.

I have seen emitter resistors burn out in power amps, though I must admit It's a long time and I can't rememebr whether they went low or high.

AFAICR - only carbon composition ever go low, but I'm not considering nonlinear like thermistors and varistors etc.

MOVs usually crack or shatter - so they also go O/C.

 

OK with the transistors in the right position the amp powered on OK and I was able to play thru it for about 30 seconds when there came a small "pop" thru the speaker and once again the fuse has blown. So I'm better off than I was however things are not quite perfect yet. Going to bed and letting this rest for a day or two.

Can you please post the schematic as an image file (GIF or PNG etc) and not a PDF?

If so I can check it out. One technique is to separate the final power stage from the driver stages, and inject a signal there.

That has saved my bacon on a few occasions where I had done extensive replacements on the power stage just to have it blow parts again later when the amp was in use.

 

OK so I have removed R96 and R118 out of the circuit with these results:
R96 = .3
R118 = .2
Also to mention that my meter is off by .9 and I have already subtracted that from the original result.

I have also found that once again TR8 and TR10 have shorted across the collector and emitter. I removed them and read across the solder pads and no short there. The outputs read short in and out of the circuit.

 

Stand by RB

 

Here it is RB. Let me know if it came out OK? Never really did a file conversion before.

 

Hello,

You might be interested in this old thread:
http://forum.allaboutcircuits.com/showthread.php?t=28235

In there there is the manual of the valvestat and a diagnose by wolf and me.
THE_RB has also some posts in the thread.

Bertus

 

Hello,

You might be interested in this old thread:
http://forum.allaboutcircuits.com/showthread.php?t=28235

In there there is the manual of the valvestat and a diagnose by wolf and me.
THE_RB has also some posts in the thread.

Bertus

Thanks Bertus.....as a matter of fact it was that very post that led me to this forum in the first place. I had skimmed thru it originally and I use the term skimmed due to my attention span.....lol. My attention span is why I only dig in so far into something and know when to pause it for a little while. I guess I'm going to have to force myself to really study that thread and hope it sheds some light on what my issue might be. I know in time and thru this forum my troubleshooting skills will get better.

 

Can you please post the schematic as an image file (GIF or PNG etc) and not a PDF?

Sweaty has posted page 1 of the schematic, which refers to the mixer 7 effects section only in post#65

Page 2 - the power amp appears on post #27

You will also find some photos on post#37 that show parts of the board. Unfortunately they show how difficult it is to isolate sections. There is also a table of voltage measurements in post#33

 

Wow that GIF did not turn out well did it..........told ya it was my 1st time doing that.....lol

 

OK new question guys. When I had TR8 and TR10 swapped, what other components could I have damaged from doing this?

 

Bertus I read the thread from Wolf last night till I was cross eyed. From what I gathered and correct me if I'm wrong that TR8 and TR10 was his problem all along. Now after replacing mine (and putting them in correctly) they still short after about 20 or 30 seconds of light low volume playing. I only have one more new pair to play with then I will have reorder and wait weeks. So if there was any particular part of that thread that I should follow please point it out to me.


Thanks!

 

Hello,

In the posts #11 to #15 I am talking about the current limiting and biasing of the output transistors.
Here is the schematic of the output stage:

from post #11:

There are parts directly related to the current protection of the TR8 and TR10.
For TR8 the resistors R116, 117, 118 and transistor TR12 and D5 make a current limiter.
For TR10 the resistors R96, 97, 115 and transistor TR11 and D4 make a current limitter.
The "quiesing current" is set by the resistors R73, 74 and transistor TR9.

from post # 13:

The transistors TR6 and TR7 make a level translator for the positive side,
as the output of the opamp can only go ±15 Volts and you want to have a output swing of ± 40 Volts.
The transistors TR5 and TR4 do the same for the negative side.

As for transistor TR9 the voltage accross C-E should be about ( (R73 + R74) / R73 * 0.7 ) = (1800 + 680) / 680 * 0.7 = 2.55 Volts.

Bertus

 

Hello sweatyk,
In post #37 your photo shows TR9 standing up.
It needs to be mounted to the heatsink for proper thermal stability of the bias circuit. You may be encountering thermal runaway if it is not attached during testing.

 

Hello sweatyk,
In post #37 your photo shows TR9 standing up.
It needs to be mounted to the heatsink for proper thermal stability of the bias circuit. You may be encountering thermal runaway if it is not attached during testing.

Thanks tubeguy......I had only had it like that temporarily for testing purposes. Once I had reinstalled the new outputs I had all 3 mounted properly.

 

...
Page 2 - the power amp appears on post #27

You will also find some photos on post#37 that show parts of the board. Unfortunately they show how difficult it is to isolate sections. There is also a table of voltage measurements in post#33

Thanks Studiot (and also Bertus for posting a more readable one in post #72).

That is the complete power stage, so to isolate it is very easy, just lift one leg of C18 (input to opamp).

Then connect a large 5W 1k resistor to be a "dummy load" loudspeaker.

Then power it up, and check there is no DC across the 1k load resistor.

Check the two voltages on pins 5 and 6 of the opamp, they should both be about 0v.

Also the voltage on the left side of R100 (after the opamp output) will tell you if the power amp biasing is roughly correct, it should be not too far from 0v.

R87 and 75 (220 ohm) are of a value that could be damaged if the output pair TR8/10 were shorted and TR4/7 were shorted or turned on hard by a biasing fault as that part of the circuit can allow significant current.

For the same reason, TR4/7 will be suspect (and shoudl be replaced!) if either of the the output pair was shorted.

Personally I would just replace all 9 transistors, and the two 4007 diodes, and the two 4148 diodes. And the TL072 too probably. It is often a good idea to spend $10 to $15 on parts rather than spend many hours pulling your hair out saying; "Darn, why does it keep blowing up?"...

 

The sad part is that I had already changed all of those parts you list once. And when I got confused and put TR8 & TR10 in the others place I probably damaged them all again.......ugggghhhhh.

So now I will place another order for the parts....again. A double order this time. Educational cost as I refer to it.

So RB your suggestions for testing should it be with the new parts in or as is? And should I have it plugged into a current limiter while testing?

 

For the current limiter a good idea is to replace the two fuses (the two main amp +ve and -ve rails) with small 10 ohm 1/4w resistors. They will work for testing, and if you have a shoot-through fault then one or both resistors will smoke up and blow. I used to have a heap of dead fuses, with 10 ohm resistors soldered on the outside of the fuse (so they can plug in).

The 5W 1k load resistor will be safe enough for the common type fault, where the amp output will pull hard to +ve or -ve rail. It will just get a bit warm.

I'm reluctant to give a specific procedure as I have only been half following the thread and don't know exactly what you have already done or what test equipment you have.

As a starting procedure from here;
1. two 10 ohm resistors instead of fuse
2. a 5W 1k resistor instead of a speaker
3. check resistor values R85 and R75 (220 ohm)
4. power it up with all the parts in, and check these DC voltages;
* output voltage (across 5W 1k resistor)
* pins 5,6 of the opamp
* left side of R100 (voltage to gnd)
* across R85, R75, R118, R96

That should give a good idea of what the amp is doing.

 

Forgive me for asking a dumb question but if a speaker is 8 ohms them why would I use a 1K resistor? I realize you know what you are talking about I guess I'm looking for the logical answer that I am not understanding.

 

Because the amp is a voltage amp. It develops the same output voltage regardless of the load resistance (above the nominal value).

The power drawn, however depends upon this resistance, given by P = V^2/R

(Hence the 5w resistor you don't really want to ouput 80 watts until you are sure)

So you can test the volume control, which will output an increasing voltage as you crank it up.

 

In addition to THE RB's recommendation's, I would replace the 0.33 ohm emitter resistors (R118 and R96) Even if they check OK they have been heavily stressed with the overloads. I say this from past experience.