HELP! Rescuing Dad's old PL400 S2 (pics)

[kl0neMan]

Problem persists at 16 ohms on left, or right, and an open circuit on the other output.

 

[Gepetto]

Problem persists at 16 ohms on left, or right, and an open circuit on the other output.

Have you tried it with the 1.8K ballast resistors you took out? 2 of them in parallel for a 900 ohm load?

 

[kl0neMan]

I will try the parallel 1.8 Ks in a few minutes. I have to determine where I stored them after removal. I also have those 7.5 K carbon resistors but they are only a handful of watts…

 

[Gepetto]

I will try the parallel 1.8 Ks in a few minutes. I have to determine where I stored them after removal. I also have those 7.5 K carbon resistors but they are only a handful of watts…

If it latches up with those in parallel, you should be safe to debug without blowing things up.

 

[kl0neMan]

900 ohms does NOT trigger the latchup, on the left OR the right.

 

[Gepetto]

Will it persist with the 900 ohm load if you get it to latch up with 16 ohms initially?

Put the 900 ohm load on the output solidly. Attach your 16 ohm load and then detach it after it latches up on power up. See if it then persists.

 

[kl0neMan]

I am wondering if it is something amiss in the protection mechanism versus something amiss with the op-amp front end. The fact that the latch up is not triggered by 8-ohm loads applied after power is stable nor by a 900 ohm load, but a 16 ohm load applied to either side DOES TRIGGER latchup is interesting.
I would like to believe that the first and second items rule out the large transistors. Also whatever the failure mechanism is, it is common to both sides.

 

[kl0neMan]

Will it persist with the 900 ohm load if you get it to latch up with 16 ohms initially?

Put the 900 ohm load on the output solidly. Attach your 16 ohm load and then detach it after it latches up on power up. See if it then persists.

The DBT extinguishes when the 16 load is removed.

 

[kl0neMan]

If the 16 ohms is reconnected afterwards, the lamp stays off

 

[kl0neMan]

Going to put a scope on board contacts 9 and 5. Those appear to be the controls for turning on the transistors Q102 and Q104. If they were on, wouldn’t that steal drive from the pre drivers Q101 and Q105?

 

[kl0neMan]

R114 and R126 both go positive by about 20 VDC when the power up latching occurs. When the 16 ohms is removed, one of them goes negative very quickly, then both transition to zero…hmmm.

Something similar occurs to the load removal occurs during power up with the 16 ohm load removed.

 

[Gepetto]

R114 and R126 both go positive by about 20 VDC when the power up latching occurs. When the 16 ohms is removed, one of them goes negative very quickly, then both transition to zero…hmmm.

Something similar occurs to the load removal occurs during power up with the 16 ohm load removed.

Well the output is probably doing the same thing. Those 2 points are tied to the output with a 0.33 ohm resistor.

 

[kl0neMan]

Yep. I originally observed the output doing some strange transitions when I first started troubleshooting. The new wrinkle is having that small bit of load resistance.

The differences between halves of the circuit due to it using the same polarity devices for both side appear to be minor. These include a handful of resistors, a diode and a capacitor or two on the drive board. I can see that would would have been emitter resistors arr connected to the collectors in the chassis harness. It is a bit confusing for me.

I do not want to give up on the board. I bet the fault is something simple.