Need a detective

[yalesmith]

Okay. So I have a Carvin Valve master that is not working properly. There is little volume on the first channel and no volume on the second channel. I took voltages at the plate of tube (pin1) and it was 85 volts and should be 185. The resistors are good.

I traced back to the filter caps and I tried to measure the voltages there and it burned out a resistor and began to smoke. I tried again and it sparked. Am I crazy to think my filter caps are bad? The Amp is 25 yrs. old, so it is not a stretch to think they have dried up.
Interestingly enough I was able to get full volume using a cheap guitar that had a distortion built into it.

Carvin Valve Master Schematic:
http://www.carvinmuseum.com/pdf/amps/ValveMaster.pdf

Thanks

[selloutrr]

Filter caps have a life of about ~ 10 years. At 25yrs it's wise to replace. as well as any burned components you find.

If it's a tube amp look at the preamp tubes for the loss of volume. If it's solid state, look at the opamp driving the input signal.

If you are causing sparks or burning while testing with a volt meter you are shorting a connection. Make sure you are metering the proper place and your meter is set correctly for the voltage you are trying to read.

[tubeswell]

If your B+ voltages are low, look for things that are pulling it down, like partially shorted filter caps etc. Burnt supply resistors are a sign of too much current - meaning that the current is probably going where it shouldn't be going (i.e.: through a filter cap instead of through a tube)

[yalesmith]

Yes... and I have been guilty of looking at a filter caps and saying "there are no lumps, i am sure they are good". Funny the supply resistors were okay until I tried to measure the voltages on the filter caps. They fried when I put the voltmeter to the filter caps

[Zippy]

On what setting was your voltmeter?

That just shouldn't happen.

[yalesmith]

yes I know, first time that has happened to me....think they might be a short in the filter cap.

I have 16UF caps lying around but they call for 22uf...you think it will make much of a difference?

Thanks

[Reeltarded]

Not much. Try it. You might like it

[Phil_S]

If 16u isn't enough, you can always parallel two of them for 32u.

[yalesmith]

how do you think it would effect the sound qualities? less or more bass, more touch responsive or not at all

[Reeltarded]

Lower is looser. 16uf should be a bit more 'vintage' in the lows than a 22, but not so much. That amp will probably benefit from the softening? Probably.

[yalesmith]

thanks! thats exactly what the amp needs.

[rdjones]

Yes... and I have been guilty of looking at a filter caps and saying "there are no lumps, i am sure they are good". Funny the supply resistors were okay until I tried to measure the voltages on the filter caps. They fried when I put the voltmeter to the filter caps

You have a problem with your meter.
Measuring voltage is done with a very high impedance, and should not load the circuit at all.

Reading current will place a nearly short circuit and can only be done in series with the measured part of the circuit.
Reading ohms will place a (low) voltage of it's own in the circuit, but does have some resistance between the probes.
You have likely damaged your meter by measuring a high voltage on the wrong range.

rd

[yalesmith]

when you say "be done in series with the measured part of the circuit" you mean you ground to chassis and measure the filter cap and not putting and not placing both probes on the filter caps. If that's what you mean i get it.... will test caps and see if they are good.

[yalesmith]

i think i have it...the power transformer is toast. it is only putting out 75 to 80 volts...wonder if there is an easy way to test an power transformer.

thanks

[Cygnus X1]

when you say "be done in series with the measured part of the circuit" you mean you ground to chassis and measure the filter cap and not putting and not placing both probes on the filter caps. If that's what you mean i get it.... will test caps and see if they are good.

Series is placing the leads in line.
You have to disconnect one leg of the component (capacitor for example) and place one probe on the cap and the other to where the cap normally connects.

Meters are cheap in the long run...even high priced ones.
Get some spares!