help me find a transformer for my amp??

benmurray85

WARNING. THIS COMMENT CONTAINS EXTREME LEVELS OF NUMPTY-NESS

erm ok so this is the rear of my amp. particularly bad pics though.

So I unsolder one end of the smaller blue cap (the one thats slightly tucked under the board)

Then i use my multimeter either side of the LEFT-most, BIGGER BLUE CAP?

Again massive thanks guys

ICBM

No - first measure the voltage across the resistor that's in parallel with the small blue cap with the amp running. (Should be about 25V). Then unsolder one end of the small cap and measure the voltage across the resistor again.

Keep away from the ends of the large blue caps, they will have about 350-400V on them.

If the voltage across the resistor is higher with the cap disconnected, the cap is leaking.

knuckleberryfinn

No.  Measure the DC voltage across (i.e. one DMM lead on one lead of the big blue cap on the left, and the other DMM lead on the other lad of the cap) BEFORE doing anything, with the amp unplugged.  If the cap has discharged it will read between 0 and 5 volts.  If it reads 360V or something like that, it is not discharged and you need to discharge it before working on the amp.  It should discharge very slowly if you leave your multimeter in place for a while, so do that.  (I'm not going to tell you to make a discharging lead thing because it's more complicated than removing the bias cap, and you probably don't have the components!)  

After you've made sure the amp is safe to work on, yes, unsolder one of the bias cap leads. After doing this, measure the resistance across the bias resistor - it should be 250 ohms or thereabouts.  If it is significantly lower or reading no resistance, then this is the culprit.  If it is 250ohms, then test the amp by turning it on and playing through it (though of course you will need to remove all the DMM leads from it, and make sure you don't touch anything inside, and then also make sure you discharge the filter caps as above before you start working on it again).  If it works, then the bias cap was bad, and you can simply replace it.

If it doesn't work, then it's something else.

Good luck.  And be careful.

EDIT: Do what ICBM says as he knows much more than me.

ICBM

I should have given more advice in discharging the HT caps though...

This is a very easy amp to do it on - there is a simple method: pull the rectifier valve and turn the amp back on again for a minute.

knuckleberryfinn

ICBM said:

This is a very easy amp to do it on - there is a simple method: pull the rectifier valve and turn the amp back on again for a minute.

Well I didn't know that.  Very handy.

ICBM

knuckleberryfinn said:

ICBM said:

This is a very easy amp to do it on - there is a simple method: pull the rectifier valve and turn the amp back on again for a minute.

Well I didn't know that.  Very handy.

And true on any valve-rectified amp* - you're just allowing the caps to discharge via the other valves, and with no rectifier in they can't re-charge.

(*Except the Mesa Blue Angel - which has a separate solid-state rectifier for the preamp power supply. I don't know of any others designed like that.)

benmurray85

Ah man I'm really not capable (and frankly s**t scared) of sound this. Think I'll just get a second opinion off another amp tech before I go further. Anyone fancy it or know anyone in east lancs area?

martinw

benmurray85 said:

Anyone fancy it or know anyone in east lancs area?

Happy to help if I can. I'm south of Manchester, but go up to Oldham and Middleton regularly.