Help troubleshooting WOPL 400

[rosseau]

I'm in over my head here and am hoping someone can help me. I have a PL400 that I successfully WOPL'd a couple of years ago. However, I made a mistake with a subwoofer a few months ago, heard a pop, and the amp no longer worked correctly. I may have made it worse by my own troubleshooting. This is now the situation:

- The bulb on a DBT glows brightly and does not dim after the capacitors charge up
- No signal comes through either channel (although when it first happened I had some signal in both channels)
- I currently have all the output transistors removed for testing and further troubleshooting
- The left output (6L on the WOPL circuit board) is nearly grounded (reads 0.5 ohms to ground on the DMM), which I think indicates a problem
- D9-13L test okay in circuit, as do Q7-8L

I tested the resistors and diodes on the backplane and they all seem okay. I removed and tested most of the transistors on the left channel earlier, however it was in this process that I lost all signal and the DBT started glowing brighter, so I may have ruined one or made a mistake in reinstalling.

Can anyone help me with some next steps in troubleshooting?

 

[mlucitt]

Do you have a Variac to limit the voltage to the amplifier? All the trouble shooting can be done at a supply of 30VAC or so, to reduce the chance of damage to you or the amplifier.
You mentioned that the capacitors in the Power Supply are charging, so presumably the Power Supply is OK. You did not mention that any fuses were blown, so I assume they are OK. The DBT indicates a short circuit. You can eliminate half of the circuitry by disconnecting the Red wire at the Positive Terminal on the Main Power Supply Capacitor to see if you still get a bright DBT. If you do, then you can disconnect the Black wire at the Negative Terminal on the other Main Power Supply Capacitor. That will tell you if the short circuit is somewhere on the Positive Rail or the Negative Rail.

On the Control Board, with regard to 6L:
Do you have +75VDC at 7L and 11L?
Do you have -75VDC at 3L and 12L?
Do you have the proper voltages at +15VDC and -15VDC?
What is the Bias Voltage on both Left and Right channels?

Do you lose the Ground indication at 6L when you disconnect the wire at Terminal 6L? If not, the issue is likely on the Control Board. If you do lose the Ground indication, I would look at the second column of Output Transistor Sockets (looking inside the amplifier).
Do you have the Watts Abundant DC Protection Relay/Delay Board installed? If so, pull the relay and see if Terminal 6L is still grounded.

Check back with us here and let us know how you are getting on.

 

[rosseau]

Do you have a Variac to limit the voltage to the amplifier? All the trouble shooting can be done at a supply of 30VAC or so, to reduce the chance of damage to you or the amplifier.
You mentioned that the capacitors in the Power Supply are charging, so presumably the Power Supply is OK.
You did not mention that any fuses were blown, so I assume they are OK.

The DBT indicates a short circuit. You can eliminate half of the circuitry by disconnecting the Red wire at the Positive Terminal on the Main Power Supply Capacitor to see if you still get a bright DBT. If you do, then you can disconnect the Black wire at the Negative Terminal on the other Main Power Supply Capacitor. That will tell you if the short circuit is somewhere on the Positive Rail or the Negative Rail.

On the Control Board, with regard to 6L:
Do you have +75VDC at 7L and 11L?
Do you have -75VDC at 3L and 12L?
Do you have the proper voltages at +15VDC and -15VDC?
What is the Bias Voltage on both Left and Right channels?

Do you lose the Ground indication at 6L when you disconnect the wire at Terminal 6L? If not, the issue is likely on the Control Board. If you do lose the Ground indication, I would look at the second column of Output Transistor Sockets (looking inside the amplifier).
Do you have the Watts Abundant DC Protection Relay/Delay Board installed? If so, pull the relay and see if Terminal 6L is still grounded.

Check back with us here and let us know how you are getting on.

I don't have a Variac, unfortunately.

I do not know if the power supply is okay, I just meant it doesn't dim down again like it normally would after the caps charge.

Yes, fuses are okay.

I don't have the Watts Abundant Protection Relay installed (yet).

I did remove the wire from 6L and it is no longer grounded, which I think you're saying indicates the problem is in the backplane or power supply, correct?

Before following your other instructions, I will need to reinstall the output transistors (which are currently all out of circuit), correct? If I understand correctly, I can bring it up with only the bottom two rows installed? Are there any other steps I should take before I put those back in the circuit?

 

[Gepetto]

I don't have a Variac, unfortunately.

I do not know if the power supply is okay, I just meant it doesn't dim down again like it normally would after the caps charge.

Yes, fuses are okay.

I don't have the Watts Abundant Protection Relay installed (yet).

I did remove the wire from 6L and it is no longer grounded, which I think you're saying indicates the problem is in the backplane or power supply, correct?

Before following your other instructions, I will need to reinstall the output transistors (which are currently all out of circuit), correct? If I understand correctly, I can bring it up with only the bottom two rows installed? Are there any other steps I should take before I put those back in the circuit?

pin 6L is the output of the amp left channel meaning that you have a short from output to ground somewhere.

 

[rosseau]

pin 6L is the output of the amp left channel meaning that you have a short from output to ground somewhere.

Okay, so what should I look for?

 

[Gepetto]

Okay, so what should I look for?

After you disconnect (with power off of course) the gray wire going from the control board 6L point to the backplane board pin 6 so you have isolated each board from one another, do you have a short on the control board from 6L to ground? Or do you have a short on the backplane board pin 6 to ground? With a wire connecting them, you do not know which is the culprit. Determine which is shorted to start with.

 

[mlucitt]

If you can send us some pictures of the Left Channel circuitry under the Control Board, that would help. Also, check to make sure your Left Channel (White) Speaker Binding Post is not touching the chassis.

 

[rosseau]

After you disconnect (with power off of course) the gray wire going from the control board 6L point to the backplane board pin 6 so you have isolated each board from one another, do you have a short on the control board from 6L to ground? Or do you have a short on the backplane board pin 6 to ground? With a wire connecting them, you do not know which is the culprit. Determine which is shorted to start with.

The ground is on the backplane side, not the control board side.

 

[rosseau]

If you can send us some pictures of the Left Channel circuitry under the Control Board, that would help. Also, check to make sure your Left Channel (White) Speaker Binding Post is not touching the chassis.

 

[wattsabundant]

The 400 manual has a good troubleshooting section. It's attached here. The section on testing outputs is very helpful. A broken lead on the bias transistor will give the same symptoms of blown outputs.

 

[mlucitt]

I do not like the Speaker Binding Posts installation. It does not look Phase Linear original to me.
I can send you a new set of original Phase Linear Speaker Binding Posts from Mouser, at my cost which is $28.00, if you are interested.

 

[rosseau]

I do not like the Speaker Binding Posts installation. It does not look Phase Linear original to me.
I can send you a new set of original Phase Linear Speaker Binding Posts from Mouser, at my cost which is $28.00, if you are interested.

You are right about the speaker binding posts. Before reading this, I had checked last night and one had shorted against the chassis, and was causing the issue. It’s been removed and at least preliminary tests seem to indicate the amp is back to working again. I’m in the process of installing the Watts Abundant speaker protector, which involves new binding posts anyway. After that I will put it through its paces, but this appears to solve the problem. Thank you very much for your help. The (rather obvious it seems now) suggestion to remove the wire and narrow down which side the short was on was a game changer for me.

 

[laatsch55]

Yep, it's a good day when you can eliminate 1/2 the potential problem area...