help with Carvin amp

[Doc Stitches]

Question. Carvin Vintage 33 is blowing R52... ( with and without tubes

in ) as soon as I turn it off of standby. R53 was bad so I replaced it and still

have the problem. T1 readings are good.

Thanks
Jim

[tubeswell]

If it happens without tubes in (but you think the PT is still alright? - Are you sure??) then it could be filter cap(s) shorted out (or something else shorting out the B+)

[Doc Stitches]

Thanks tubeswell

I think you may have nailed it.

I checked again and this is what I got :

schematic shows 260V on 2 brown wires to QC10 and QC11 ... I have ( 272V ) is that

the problem?

schematic shows 2.9V on 2 green wires to QC8 and QC9 ... I have 2.9V

[tubeswell]

Nah - 260VAC vs 272VAc for your HT windings is the same difference (if you know what mean)
Check to see whether you are getting (approximately) 325VDc on C38, 335VDc on C39, 355VDC on C40 and 360VDC on C41/42 (in that order). I would guess it is possibly C40 (if R52 is blowing out)

[Doc Stitches]

Thanks....will do tomorrow and let you know. Enough for one day.....

[Doc Stitches]

40, 41, and 42 are all 371VDc

but 38 and 39 read 0 VDc ...that is without R52,

doesn't R52 have to be in place and

good to get a reading on C38 and C39? If not, how do I check them?

I tried to read 38 with a new resistor as soon as I turned it off of standby but it blows 52

before I can get a reading.

[tubeswell]

Duh - Sorry I was looking at the power supply back-to-front! - C39 (Not C40) must be shorted causing too much load on that little 1/2W resistor. I bet if you replaced C39 with a new 450-500V 20-22uF cap it would kick it back into life.

[Doc Stitches]

Thanks.........will do it today

[Doc Stitches]

Tubeswell....Amp is now working and wow...does it sound GREAT .....but there is a hum. What I found...see picture...a very tiny thread-like piece of solder between 2 of the 500v caps. The first thing I do on amp repairs is a complete visual and somehow I missed it. Double DUH.

As for the hum...What I put in was a smaller 450v, 22mfd cap (1/2 the actual size) and a 47ohm 1/2 watt in R52. If the R52 is the problem would you give me the color code for the right resistor for 52? Thanks
Doc

[Doc Stitches]

the pictures

[Doc Stitches]

here is the right photo

[tubeswell]

Tubeswell....Amp is now working and wow...does it sound GREAT .....but there is a hum. What I found...see picture...a very tiny thread-like piece of solder between 2 of the 500v caps. The first thing I do on amp repairs is a complete visual and somehow I missed it. Double DUH.

As for the hum...What I put in was a smaller 450v, 22mfd cap (1/2 the actual size) and a 47ohm 1/2 watt in R52. If the R52 is the problem would you give me the color code for the right resistor for 52? Thanks
Doc

Well its good that its working again. The schematic I have states 1/2W 2k2 for R52. 47R won't do anything much there. Try putting in a 2k2 (red, red, red) first and see if the hum is reduced.

(PS resistor colour codes are explained in numerous places e.g.;
http://www.talkingelectronics.com/Fr...cts/Page7.html )

The other thing is how old is the amp? If one of the filter caps was bad, chances are you should replace them all - if they were about the same age. Could be that old caps aren't filtering properly - and that might be where the hum is coming from. (Although I'm only guessing where the hum could be coming from at this stage)

[Doc Stitches]

You were right. I found the last 2k2 in a parts amp and also put back the 500v cap I thought was bad ???...............and BINGO .....the hum is gone. I sure wish I had seen that small thread of solder on the bottom of board the first time. That was probably the only problem. Sounds like a new amp except for an occassional scratchey sound on channel 1. Thanks for your help and the resistor site.
Doc

[tubeswell]

Sounds like a new amp except for an occassional scratchey sound on channel 1.

Well that scratchiness could be just a dirty pot, or it could be DC on the pot from a leaky coupling cap. Or it could be a dodgy triode on that side or a dodgy plate resistor on that triode (I had a dodgy plate resistor in V1 do that to me once). Try swapping V1 first and see if that improves anything. Otherwise try cleaning the pot and then if that doesn't work - testing the prior coupling cap for DC.

[Doc Stitches]

Thanks.
I'll get to some of that tomorrow. Right now I am pumping water out of our basement.
Doc