One channel of the power amp in my band's Studiomaster Powerhouse version 908E has blow up and I require some help, please?
Having spent several hours replacing almost all of the faulty transistors and diodes, I managed to get the power amp working, however the amp is still showing and imbalance with 0.5v dc across the speaker terminals and I can't seem to trace the problem.
I attach a copy of the circuit with DC voltage measurements taken from the working channel (in BLUE) and reading (in RED) taken from the faulty channel.
Any guidance would be appreciated.
Regards
Geoff.

 

Although it says preset, you may have to adjust the pot to set the bias of your new transistors.

 

Yes, make sure the quiescent current is correct first. Then I would note its a DC coupled amplifier, so the next thing is to make sure that the differential input voltage is zero volts DC. Add a temporary short jumper from IN+ at R9 to IN- at R11 and add a short across R11. If the first two stages are working right that should bring the quiescent output to the same voltage as 0VREF. If there is an offset consider that something in the first stage is out of balance. Look for resistors being correct and in tolerance, then TR4 and TR5 having nearly matched VBE voltages. Another thing might be TR9 is leaky collector to base and that is upsetting the stage balance there. Good luck!

 

What is the DC voltage at the base of TR5? Is the output stable (instability often results in a DC offset).
What is the DC input voltage on IN-?
It is an unusual amplifier with gain extending to DC. Good for bass phase shift but not for offset voltages. Also the gain is only 6 so it needs a big input signal (12V p/p)
Check the voltages across the 0.1Ω resistors to make sure you have the bias set right. A standing current of 50mA will give 2.5mV across each resistor, which you should be able to measure on the mV range of a decent multimeter.

What does the CLIP input do? Any current into or out of that pin will upset the offset.

 

Many thanks to all who submitted a response. Unfortunately not had a great deal of experience with the front end of these amps, most faults I've come across with DC coupled amps have been fixed my just replacing the power transistors. With this one all but 2 transistors had to be replace and a number of diodes. TR14 had completely split in half.
However, I was blinkered by just checking voltages from various points to earth and missed looking at the differential voltage between TR8 & 9, which was 1volt. Fault cleared by replacing R15 for the correct value, this reduced the speaker offset to 88mv. Happy days !
Thanks once again for all your help.
Regards
Geoff.

 

I'd consider 88mV a bit high, for a gain of 7 amplifier it means a bit over 12mV unbalance on the front end. That's about 4-5 times what a passable op-amp can do. It may be OK though, the DC power into 4 Ohms is just a couple milliwatts. Hopefully TR4 and TR5 are fastened together thermally so they track with temperature. Do R15 and R16 match within 1%, preferably 0.5%? If not I'd consider doing a fine trim by carefully shunting R15 or R16 with maybe 330K as needed to reduce the output DC. If the new R15 is 1% then you could be OK as is. I'm a bit more demanding, I'd want to see the output DC down in the 10mV range whether needed or not. With 1% R15-R16 parts I'd try using a 1 megohm shunt for test purposes. Or maybe a 1 meg trimmer plus 100K to determine the exact value needed for the fixed resistor to use as a permanent tweak.

 

It's high because most amplifiers have unity gain at DC, so 12mV offset on the input transistors is 12mV offset on the output.
A couple of non-polarised electrolytics in series with R9 and R12 will improve matters.

 

The DC offset at +/- in is only 1.4mV, all resistors are correct, the Vbe of both transistors are within 2mV yet I still have approx. 0.5v difference at the collectors of Tr 4 and Tr5. These transistors are just sitting in fresh are not thermally coupled in any way. It looks to me that R17 is feeding back a voltage from the output which and upsetting tr4/5 balance. However in desperation I shunted R20 which has reduced the speaker voltage to12mV and all looks OK. The audio is clean, no instability over the audio spectrum but the voltages are still different from the good chanel but it works !

 

Almost.
The reason it is out of balance is that the Voltage Amplifier Stage (TR6, TR8, TR9 and D8) is out of balance, because D8 and TR6 don't form a perfect current mirror, so adjusting a resistor in series with TR6 or D8 would be the way to balance it. A better way would have a transistor identical to TR6 instead of D8. You could try replacing D8 by an MPSA92 with its collector and base joined together.
It's also imperfect because one half of the differential amplifier (TR8/D8) is unloaded, but the other half (TR6/TR9) has the load of the output stage attached to it.
I still would have expected a better balance at the bases of TR8 and TR9.

 

Almost.
The reason it is out of balance is that the Voltage Amplifier Stage (TR6, TR8, TR9 and D8) is out of balance, because D8 and TR6 don't form a perfect current mirror, so adjusting a resistor in series with TR6 or D8 would be the way to balance it. A better way would have a transistor identical to TR6 instead of D8. You could try replacing D8 by an MPSA92 with its collector and base joined together.
It's also imperfect because one half of the differential amplifier (TR8/D8) is unloaded, but the other half (TR6/TR9) has the load of the output stage attached to it.
I still would have expected a better balance at the bases of TR8 and TR9.

 

With a DC coupled amplifier you need to set both the balance and the no-signal current to have it functioning correctly.

 

Many thanks for all your contributions gents. Not having a great deal of experience with DC coupled amps, can anyone recommend some additional reading material, please.?

 

Many thanks for all your contributions gents. Not having a great deal of experience with DC coupled amps, can anyone recommend some additional reading material, please.?

Doug Self's book http://www.douglas-self.com/ampins/books/apad.htm
John Linsley Hood's books (Art of Linear Electronics, Valve and Transistor Amplifiers, Audio Electronics)

 

Thank you all once again
Geoff.