studiomaster powerhouse vision 908 & 912

[Eyefactor]

I'm trying to repair the above Studiomaster 908 and would appreciate some advice, please.
My friend in our band purchased said Studiomaster online (working ! ), when he returned home he decided to test his newly acquired mixer by injecting a tone in each input in turn and checking the signal lights worked. I should mention at this point that he did not connect any speakers. All went well until he tested channel 5 and the smallest movement of the channel slider pushed all the signal indicators to max and a loud internal bang was heard.
On inspection the bang had not only destroyed the four MJ15003/4 power transistors but also TR10,15, D1,4,5 and actually splitting the case of TR14.
Sorry for the pre-amble but here's where I need the advice. Having looked at the circuit it looks like the output voltage swing had enabled D11 (DIAC) and hence made TR21 (TRIAC) conduct and shorting the speaker output and blowing the transistors. Having never seen this type of overload circuit before it does seem ODD. Am I wrong in my assumption?
Regards Geoff.

[Dave H]

Schematics

Vision 908 schematics.zip

[Dave H]

Having looked at the circuit it looks like the output voltage swing had enabled D11 (DIAC) and hence made TR21 (TRIAC) conduct and shorting the speaker output and blowing the transistors. Having never seen this type of overload circuit before it does seem ODD. Am I wrong in my assumption?

It shouldn't trigger on AC signal swing as AC is filtered out by R43,C14,15. I think it triggers to protect the speakers if there's DC on the output i.e. it's the transistors that short first putting rail voltage on the output which causes the triac to trigger protecting the speaker.

[Eyefactor]

Hi Dave, Many thanks for your quick reply. I believe your correct in that it's there to protect the speakers, however I just can't understand how all 4 PA transistors were able to blow with no speaker connector, unless the failure/s was earlier in the audio chain. I've ordered all the parts, now it's finger crossed time!

[Jon Snell]

The triac and diac employed in this circuit is a typical idea from the mid 70s, widely used on many makes of power amplifier.
Consider a total overdriven signal. It clips between the rails, + and - supply voltages. If it becomes a square wave, that is basically DC voltage and will cause the diac to trigger the triac.
When it does that, there are two capacitors in series, shown at the bottom of the amplifier schematic page, these will store the DC voltage.
There will be more than 30volts of clipped signal. The diac reacts at about 30volts, clamps the voltage, which in turn switches the triac on with the dc voltage held within those two capacitors. In turn the triac places a short circuit across the loudspeaker, in an effort, (sometimes to no avail) to protect the loudspeaker. The triac and diac will not release until all of the energy has dispersed.
There is usually a fuse between the triac/speaker output and the amplifier. If the fuse is the wrong value or is missing, this can cause catastrophic failure.
Many older H&H power amplifiers use this system and if the output transistors were replaced by unskilled techies who picked the wrong or fake types, this is when they usually fail as per your description.
Nowadays, if this happens, a DC level is detected muting the amplifier and stopping the power supply before it becomes a complete failure.

All probably caused by a damaged volume slider.

[x-pro]

Was the Studiomaster 908 new? Has it been repaired before?
If so, the causes could be:
-faulty output transistors -thermal breakdown or Uec mismatch.
-bad filter capacitors in the power supply.
-polarized voltage at the input of the amplifier is higher than the maximum allowable (this will turn on the triac at the output).
-breakage of R21, R22 or TR7

[Eyefactor]

Many thank to X-pro and Jon Snell for you useful comments. Unfortunately, I have little knowledge of the amps history so I guess I'll have to start from a blank canvas.
Best regards
Geoff.