Acoustic 370 - troubleshooting power amp

[Matt C]

I'm working on repairing an old Acoustic 370 bass amp. I am stumped and here to seek advice. The problem seems to be within the power amp section (I've attached a schematic.) When I turn it on, two resistors burn (they are highlighted in the schematic.) This has happened a few different times. At this point I've replaced almost every component in the power amp circuit, and still get the same results. The most recent time, the amp seemed to be stable for a few minutes and the power amp circuit seemed to be biased appropriately, but as soon as I applied signal, these same two resistors burned.

While there could be other issues hiding, I'm fairly sure the power supply and the preamp circuit are working fine.

My understanding of solid state circuits like this is pretty limited, so I'm a bit stumped here. Anyone have a clue for me?

 

[CJ]

Look for a possible solder bridge between traces, I see no logical reason why those two R's would have cooked due to shorted transistor or diode but you might have bridged something when replacing parts.

 

[Khron]

Is the output capacitor still ok? And do the voltages in the spots mentioned on the schematic, check out in the real circuit?

 

[Matt C]

the output capacitor is new, so I'm pretty sure it's okay. For the couple minutes I had it powered on and stable, the DC bias voltages matched what the schematic shows (Q301-302 both had 11Vdc on their bases, Q304 had a 2 volt drop between emitter and collector)

Also worth noting, after another unsuccessful power up just now, I made the following resistance measurements:
B+ to ground = 3 ohms
output (pre-cap) to ground = 1.3 ohms
I know in-circuit resistance measurements can be deceptive, but certainly these numbers must indicate a problem??

I'm starting to hunt for accidental solder bridges, etc. Before it came to me, another tech was trying unsuccessfully to get it working, so it's possible they goofed something up too. But like I said, I tried the shotgun approach and replaced just about every component on the schematic above, and still it smokes every time.

I should say too, the last few times it's been those two resistors burning that I indicated on the schematic, but earlier on in the process it was a couple different resistors that kept dying. I'm baffled.

 

[JohnRoberts]

If you can note DC voltages with no audio playing of all the power devices and drivers on a schematic. Checking those device terminal voltages can reveal what is misbehaving or not. That looks like a pretty basic class AB amp.

JR

 

[Matt C]

Right now, with the burned resistors still sitting there, basically all the terminals of all the output and driver transistors are sitting at the B+ voltage (85Vdc). Only exception is the Q312-314 (lower half output devices), those emitters are sitting at 0V. Everything else is at 85V.

 

[s2udio]

Starting with transistor/diode checks would be a first call IMHO, forget the power up for now !
Actual component measurment verification,
Current measurments next.
Parts cannons do not generaly reveal results in these circumstances.
A Variac, or Tungsten LB limiter is advantageous

 

[Audio1Man]

From the limited data, i would say one of the lower 48-15 (special selected RCA 2N3055 Hometaxial) Q12, Q13 or Q14 is shorted.
When I designed the 270/370 family we did 100% incoming testing of all TO3's
As with all power amplifiers use a current limiter for the FIRST POWER ON TEST. 50-100 watt incondestant light bulb.
Duke

 

[Matt C]

Thanks for the input everyone. I do have a light bulb limiter I can use, but is it helpful to take voltage measurements if the bulb limiter is definitely on and preventing the amp from blowing? I kind of assumed everything would be out of whack in that situation.

At this point I guess I'll replace the burned resistors, check the transistors for shorts/damage, then power it up with the bulb limiter in circuit.