Questions on OTL amp repair/tube safety
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MohawkUS

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I recently took ownership of a La Figaro 332C amp with the understanding it has a problem involving the headphone jack. It's unknown exactly what the fault is but it is effected by wiggling the headphone cable in the socket. It's a known issue that the jack on these may need to be resoldered. This will be my first time doing DIY/Repair. Pictures I took of the inside of my amp: https://imgur.com/a/q973Lny

Safety questions first:
1. It's my understanding I need to drain all filter capacitors to safely operate inside an amplifier. The purple caps are obviously the power supply filters, do I need to deal with the green or white(output capacitors?)
2. Are the resistors bridging the purple caps for self-discharge purposes?
3. Whats the proper technique when testing for voltage/draining capacitors such as the purples where I can't reach the terminals of each individual cap? Where exactly should I connect each multimeter probe?
4. I should work with the amplifier unplugged from the wall completely correct?

Diagnostics(Using multi-meter):
5. How can I determine if the fault is due to wiring, PCB, or a mechanical fault in the jack?
6. Will I need a pair of headphones plugged in when probing with the multi-meter?
7. Is trying to get a stable read of said headphones impedance between each channel/ground an acceptable way of testing for the location of the loose connection?
8. What are the various wires/solder points around the HP jack connected to? I see a red, blue, and yellow wire along with solder pads marked S, G, R, 2, 3, 1. Doesn't a HP jack only have 3 connections(Left, Right, Ground?)
9. Any advice in diagnosing this problem? Do I seem on track or am I missing anything important?
 
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Tjj226 Angel

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I think you are on the right path. IDK if this thing has any bleed resistors or not. But an easy way to check is to simple hook up your multimeter up to the positive and negative terminals of one of the purple capacitors and check for DC voltage. If you see low voltage then give it a minute or two and it should be fine. If it still has several hundred volts across the terminals 20+ minutes after you turn it off, then there are no bleed resistors and you will have to do it manually with some high value resistor and a pair of pliers. Fun fun fun.

And you only need to test 1 of the purple caps. Doesn't matter which since they are all part of the high voltage B+ rail.

And so long as you do not need to monitor active voltage or current levels, you should have it unplugged from the wall.

--------

Here is what I will say for the rest of your questions. All you want to do right now is perform a basic continuity test between your headphone jack and the solder terminals for the signal wire. Thats it. There are countless ways you can go about this. I would do it the red neck way and shove a probe into the headphone jack and run the other probe over the solder terminals.

The probe in the jack should make contact with all 3 connectors. If you run your probe over the solder points, you should get 3 individual beeps. Super simple.
 
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MohawkUS

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Thanks. That's one of the points I'm having trouble with, if you can see in the pictures the terminals of the purple caps are out of reach & really close together. They're under what I think are bleed resistors.

Am I overthinking this? If I'm exclusively working on the HP jack which can be pulled out the front(aka not working in the guts of the amp) do I need to be concerned with the filter caps? Without the tubes in there shouldn't be a connection between the power supply & jack, right?
 
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Tjj226 Angel

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Thanks. That's one of the points I'm having trouble with, if you can see in the pictures the terminals of the purple caps are out of reach & really close together. They're under what I think are bleed resistors.

Am I overthinking this? If I'm exclusively working on the HP jack which can be pulled out the front(aka not working in the guts of the amp) do I need to be concerned with the filter caps? Without the tubes in there shouldn't be a connection between the power supply & jack, right?
You are overthinking it IF you had previous experience. I think you are doing a great job for someone just starting out :beerchug:

Truth be told, the only real concern with the capacitors is if you stick your fingers somewhere they are not supposed to be. Once you get comfortable probing stuff, you could trouble shoot the amp with it turned on. Tons of people do it every day and live.

That being said...any time you work with high voltage you want to as safe as possible. So since the tests you want to do do not require being potentially exposed to high voltage, you ideally want to make sure the amp is safe.

Now I will tell you that you most certainly can get to the purple caps. I can see the terminals in your picture. It would just be kind of a pain in the butt is all.

However as an alternative you can measure the voltage between the little piece of wire that is coming off the purple caps and runs down between the green caps and the head of the screw near your IEC connector with the little yellow wire. It should measure the same thing.
 
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gimmeheadroom

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I seem to remember somebody using a lightbulb with alligator clips for this but I can't remember what kind of bulb will be ok with DC...
 
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MohawkUS

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I took the cautious approach and decided to stay away from the caps. The equipment I have here is worse than the bargain barrel from Amazon. My multi-meter has probes and un-insulated clips(only safe to connect to pre-confirmed neutral.) I don't trust myself to hold/maneuver probes around potentially stored voltage, I've played enough games of Operation to know my limits.

First thing I did was find a page on the jack in question which explains what all the pads are for. https://www.neutrik.com/en/product/ncj6fa-h
I don't know why a SE OTL amp has a combo SE/XLR jack but all the connectors are soldered and connect to their respective output type when probing with the multi-meter.
Next I re-soldered the L & R connection to the HP jack(labelled T, R) Would you believe it took two soldering irons and three rolls of solder to find a combo I could actually melt solder with? If I make a habit of this I need to buy something and get off the inherited gear.

With the amp powered on it has no signal to the right channel(suspect the previous owner's reports of inconsistency playing with the HP cable is due to the ease of aligning it so the the headphones are playing in mono from the left channel output.) If I'm understanding correctly this tells me that the fault could be anywhere in the effected channel and I need to follow the channel backward with probes until I find the disconnect. I'll start with one in the jack & one on the PCB just to make sure my soldering job isn't an additional problem.
The capacitors should be safe now, this amp is the type that keeps playing for a few seconds when you switch the power off so you can drain the powersupply of reserve energy.

The amp also hums like mad. Tried it with two different sources on two different outlets to the same effect, not sure how safe I feel using a cheater plug with this one. There any safe ways of eliminating a ground loop?
 
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