JL 250/1v2 factory defect

[miniman82]

Should I drive a signal into it and check?

 

[Perry Babin]

It won't do anything with the 1-2 jumper on the servo op-amp.

You can install the outputs and check the drive. If you do, you can either remove the rectifier or just power up for a few seconds and then remove remote. The low-voltage supply will keep running for about 15 seconds during which time you can check the drive signals.

 

[miniman82]

They were never out...

That’s what I was saying in #59, with the servo shorted I have equal drives so what is the next step? I’m assuming that since that opamp has nothing on its input that likely means it’s bad correct?

 

[Perry Babin]

If one is grounded, it won't have anything.

If the other is the input and it's shorted to the output, you won't see a signal.

Everything is likely OK. Power it up after removing the jumper and see if it's OK. Of course, be careful as you would powering up any amp after a repair.

 

[miniman82]

No joy, output still pegged. That servo only has like .6v B- and like 1.3v B+ feeding it though, and that can’t be right. I checked the opamps up on the pre card, same story there. Must still be an issue with the standby supply dammit.

 

[Perry Babin]

What do you mean output pegged?

 

[miniman82]

100% duty

I think something is still wrong with the standby supply, I’m not getting proper B+/B- voltages.

 

[Perry Babin]

Is the driver transistors for the low voltage supply producing pulses to the transformer?

What do you mean by proper?

What was the drive like without the main rail voltage?

 

[miniman82]

Isnt there just a single TO-220 case fet driving the LV standby supply? It doesn’t appear to be doing anything, and like I said I would expect a bipolar supply on the various opamps in this chassis but I have next to nothing. That tells me it’s not actually running.

Drive for what, the outputs?

 

[Perry Babin]

The drain of the FET driving the LV transformer should be swinging about 24v. It drives pin 1 of the transformer. Pin 10 of the transformer goes to 12v.

Does the 5v regulator have output?

I wouldn't expect there to be drive voltage without the LV operating but I don't know the 250 very well and there may even be different revisions like in the 500.

 

[miniman82]

The FET driving the LV transformer is driven by a 38C40 device. I verified it has power and ground, and the RT/CT pin is making pulses but there’s no gate drive coming out of it. I think it might be bad.

 

[Perry Babin]

There is a low-value resistor connected to its source leg. If it's out of tolerance, the IC will go into over-current protection.

 

[miniman82]

Doesn’t it have to at least attempt to start up to go into OC protect though? I can’t even get a single pulse out of the thing, but I’ll check that resistor when I get back home.

I looked into the COMP pin as well since it can be used for shutdown (either at ground or 5v), it’s right at .6v does that sound right?

 

[Perry Babin]

I don't know what the comp pin should be but I don't see where it can shut down the IC with 5v.

0.6v may stop it from producing output. I 'think' you can lift this pin to eliminate other circuitry in the amp.

 

[miniman82]

Me either, I just know that it’s a design feature of that chip from reading the data sheet.

 

[miniman82]

There is a low-value resistor connected to its source leg. If it's out of tolerance, the IC will go into over-current protection.

That 1-ohm resistor on the standby FET read infinite in the circuit. Attempted to remove it, and one of the legs fell clean out. I swear, this amp has more failure modes than I’ve ever seen.

 

[Perry Babin]

In other amps, those resistors are 0.1 ohms.

 

[miniman82]

I'll have to look at it again, could have swore it said 1 on it. If it's .1, I likely have a replacement laying around the shop.

 

[miniman82]

Put a new .1 in it, and it finally booted all the way up without exploding!

Hopefully this is the end of work for this amplifier, thank you again Perry.