Help with solid state power amp and driver

[dax]

Sorry for the vague post...can not give too many specifics...
I have a solid state guitar amp with an issue and wondering if anyone can help me analyze this circuit.  I am a little unsure of whats going on here., specifically the "bias" and "trigger" circuit. 

I have good signal from the pre-amplifier section (no schematics attached) into the "driver circuit"  and then signal drops feeding the power amplifier.  The amplifier is rated at 300 watt into 4 ohm but really gets about 80 watts.  As far as I can determine the amplifier and power devices are functioning.  I think there is an issue in the dirver circuit.  Signal up to sencond half of opamp U1-b is ok but then i loose it.  Op amp is ok. 
Bias is plus and minus 1 volt DC  sine wave on speaker output jack shows on all connections: CT, B+, T+, T-, B-

See next post for power amp

 

[dax]

Power amplifier

 

[PRR]

How OLD is that thing??

I'm going to take a flying-leap and say the schematic has major errors.

In Any Case: DC readings at key points should be the start of diagnosis. Is the output near-Zero V DC? How about both ends of the bias-thingie Q2?

 

[JohnRoberts]

;D ;D Yeah, I think the draftsman who drew that schematic got fired before finishing it... 

The amplifier schematic looks pretty straightforward, Vbe multiplier driving standard complementary emitter followers.

The front end schematic is drawn rather confusingly... but contains a relatively high impedance drive circuit and protection current limiting and the like.

The Bias+ and  bias- is the high impedance input to amp stage... the whole power amp is just a simple buffer to take those high impedance inputs and deliver a low impedance output.

The T+ and T-  are sense lines coming back from the emitter degeneration resistors to use for current limiting. It looks like a combination V-I limiting, so it will fold back and deliver less current into 0V, and more current into higher output voltages.

CT is the output connected to negative feedback.

That front end schematic is giving me a head ache and I'm not getting paid to figure this out, so I won't. Sorry I can't give more specific advice.

You mention low power... does it clip symmetrically, or is it limiting perhaps prematurely?

Good luck... the good thing about troubleshooting a production product is you know it worked before. 

JR

 

[dax]

I am guessing this amplifier is close to 20 year old ... I appreciate the help as this diagram is most certainly confusion for me ....
Bias+ read apporx 1 vdc, Bias - read -1 vdc .  Very low Dc on output.  I am thinking the Power amplifier section is ok as the output waveform is symmetrical and

I suspect that there IS an issue with premature current limiting.  I traced a sine wave through the opamp and get lost at Pin7 ....

Thanks again, I will examine more tomorrow and report back

 

[JohnRoberts]

While a long shot, a fault like C4 (output trap) shorted could increase load on amp.

Does it put out full voltage with no load?

JR

 

[dax]

Update*  C4 while just a cheap film resistor was ok.  I  removed the power supply / driver circuit board to check some transistors out of circuit .  Q20 was faulty - reading error: device unknown on my atlas dc-55. 

Also, The resistors feeding the low voltage regulators had bad solder connections and I contact cleaned the 15 multi-pin connectors before re-assembly.    burning into 4 ohm load for a while 300 watts+

I appreciate the help!