SS Amp repair questions

[shay1510]

Hey guys,
I've been building some effects pedals, but now I feel like moving to the next level, which is repairing/building amps. Fortunately, I just got a simple SS amp from a friend to work on (Matrix ma30c)
and I have a few questions:

1. I get radio frequencies and a lot of humming when I connect my guitars to it. Pretty sure it's a grounding issue. I believe I need to replace the power cord to a standard european power cord with a 3d grounding pin. What do you think?
2. I know that SS amps are safer to work on and that the filter caps drain faster. What's the best way to drain them? (safety question)
3. I took a photo of the filter caps and there is some glueish stuff hanging from one of them. It doesn't seem like a leakage from a cap. again what do you think.
4. I don't have a good access to the soldered part of the board. therefore I cannot reach the leads of the caps in order to drain them. Do u have an idea how to do that without disassembling all of the pots and jacks?






Thank you for your kind help.

[Roly]

Hi shay1510 and welcome.

1. this is actually two different ground issues.

Firstly, by its nature a guitar amp isn't double-insulated, the exposed metal parts of the guitar are connected to the common rail of the amplifier, and will normally be connected to the metal chassis of the amp.  For electrical safety it is important that all exposed metal parts are connected to the mains safety earth, so yes, a three-core mains lead is a very good idea.

Secondly, the input to the amplifier is between the common rail and the input active.  If you have a lot of hum, and in particular radio signals, this generally means that the connection for the common rail is not being carried through to the screening on your guitar cable.  This may only be due to dirty jack plugs and sockets, but it can also be caused by a break in the braid connection to the guitar lead plugs internally.  You can check this with the ohms range of your multimeter, body-to-body and tip-to-tip should both test as short/very low resistance.

But fit a proper earthed mains lead first and see if that improves/cures your problem.

2. SS amps are safer to work on than valve amps because the supply voltages used are (normally/mostly) low enough not to be shock hazard (this isn't true in some high power and rack amps), but can still produce some mighty splats (and possible damage) if shorted due to the high current capacity available when charged.

You can use exactly the same mains lamp trick that we use on valve amps.  This will quickly and safely discharge the power supply capacitors (see 4).

Something that is common to all amplifiers is some power mains wiring.  This isn't always as well insulated as in your picture, and represents the major shock danger inside any mains operated amplifier.

3. The "gluish stuff" is in fact glue.  It is often used under large electrolytic caps to provide some mechanical support apart from the solder joints - and it is often applied in the rather hurried manner shown.  This brown stuff is some sort of fairly-hard setting rubber-like glue, but it is more common these days to see blobs of white hot-melt glue.

4. In your third above-board pic you can see four diodes to the right of the power supply electrolytic caps, where the two red wires from the transformer arrive.  These are the main bridge rectifier.



This has four nodes; two are a diode anode and cathode, each connected to one of the transformer AC wires, and the other two nodes are two diode cathodes (silver or white band ends) connected together and to the +ve supply, the other node being a pair of anodes (non-band ends) connected together and to the -ve supply.

Where you can get to the copper side you can identify the +ve and -ve nodes on the rectifier and use those points as connection to the +ve and -ve rails.

These can be traced when the amp is cold using our ohm meter, and the +ve and -ve supply rails confirmed with the amp on as having typically between 35 and 50 volts steady +ve and -ve to ground/amp common.  I normally mark these two points with a blob of red felt-tip pen, and blue, respectively (black being for common/ground when needed).

HTH

[shay1510]

First of all,  thank you for the detailed answers, You've really helped me.

I understood everyhting you've talked about except the Cap discharging issue. It was a bit too techincal for me.
You talk about bridge rectifier that takes an AC signal, converts it to DC signal using 4 diodes and smoothens it using 2 filter caps. And u showed me where it is. That's ok.
I've never discharged caps before but I know u could put a screwdriver between the two leads of a cap (not the right way to do it) and u can put like a 10k resistor between them (better way of doing it). I'm assuming u've tried to explain how to identify using a Multimeter if there's still a current held in those caps.

Assuming I can't reach the copper side of the pcb, what exactly do u suggest I'd identify with the ohm-meter and what should I do with it?

* I just went through section 4 of your answer again, and maybe I understand it a bit better. You want me to identify the +ve and -ve nodes of the rectifier in order to short them using a resistor? and in order to do that I should use a multimeter and connect each of the four cathodes (of the diodes) to the ground connection and see if there's a voltage read?

Thanks.

[Roly]

Ahh... Okay.

You've got four diodes with eight ends, however these ends are connected in pairs, two for the AC input, and one pair each for the +ve and -ve supply nodes (the same point as the top and bottom of your respective filter caps).

With the amp on use your multimeter to identify the pair (with band) that gives you the steady +ve rail (red dot), and the pair (without band) that give you the -ve rail (blue dot).

By bridging the red dot to the blue dot with a resistance you discharge the filter caps.  {I have to say that this is generally much more of an issue in the back of a valve amp where several hundred volts may be lurking, and not so much of an issue with SS amps, however since it is good practice anyway...}

You can use a suitable resistor, but another way to harmlessly discharge just about any cap is simply a light globe (40W, conventional) in a holder with a couple of flying leads terminated in small insulated croc clips.

Before diving in; always unplug the amp and put the plug up on the bench where you and see it (don't assume a switch opens the mains active - it may not); clip your discharging lamp across the supply(s), red dot to blue dot.  Now you're good to go.  If you forget to unclip it the lamp may light a bit and remind you when you power up again, but no harm will be done.

I have a lamp I use for discharging caps, testing earth leakage circuit breakers, and as a limiting lamp for sick amps under test;



http://www.ozvalveamps.org/mains.htm#killer

Also see;

http://www.ssguitar.com/index.php?topic=2093.0

HTH