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#1 2008-04-07 05:12:12

emmanuelxgoldstein
New member
From: Vancouver
Registered: 2008-02-22
Posts: 7

Measuring transformer impedance.

So, this is the second in a recent string of problems. I originally had the standby switch blow on my V4 a couple months ago, and after a replacement of it and the polarity switch the amp returned to normal. Everything was working great, I threw two KT88's in, and it was sounding awesome.

The other weekend I turned it on at practice and it just died instantly. I thought I saw a bit of an arc across the top of the power switch and that was it. Nothing more. I thought the power switch must have gone, so I opened it up this afternoon, got out the multimeter, and the switches and fuses were all fine.

I'm thinking it might have been a transformer that blew, as I was running a 16 ohm and an 8 ohm cab with the head set on 4 ohms. So now I'm looking to measure the PT and the OT to see if either is shot. I've never done this before so I can't remember what it entails. Any help or advice?

Thanks.

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#2 2008-04-07 08:56:20

hangman
Banned
From: Seattle Washington
Registered: 2006-09-04
Posts: 1848

Re: Measuring transformer impedance.

I may be wrong, so I won't say its an impossibility,  but that just doesn't sound like blown transformer.

you've checked both the internal and external fuse? 

take your multimeter and Measure what you see on the primary of the power transformer.   measure off the actual primary wires (so on either side of the power light)  that will tell you if you're getting 120volts to the primary. 

if you are,  then see if you are getting anything off the secondary.

let me know what you measure.
-steve

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#3 2008-04-28 03:37:12

emmanuelxgoldstein
New member
From: Vancouver
Registered: 2008-02-22
Posts: 7

Re: Measuring transformer impedance.

Thanks for the advice, Steve.

It turned out to be the internal fuse. I used it for three shows last weekend and it was soarin' hot. We were so loud we blew a breaker at our first show. However, when I went to turn it on at practice today it was dead.

When I opened it up tonight it was the internal fuse once again. This time it was totally fried.

Anyway, the external fuse on the amp is the one that was in there when I bought it. In there it has a 32v 20 amp Buss. The internal one I put in there as a replacement was a 10 amp, and it blew as the one before did.

What has me curious is if this is a problem with components, or if this has to do with the fact that I have two KT88's in there along with the 7027A's and this is drawing too much current when I've got it cranked wide open. What do you think?

Thanks for all the help. Who knows, maybe if I decide to overhaul this thing and get all the caps done at some point I might have to take it down to Olympia!

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#4 2008-04-28 06:39:04

hangman
Banned
From: Seattle Washington
Registered: 2006-09-04
Posts: 1848

Re: Measuring transformer impedance.

20Amp Fuse??!!
that is no good.  the external fuse should be a 6amp.

there is likely an issue with the tubes.   Did you have someone work on the amp before you put the KT88s in there?   
Kt88s and 7027s require different bias voltage,  and have very different characteristics in general.

I really feel like you should decide what kind of tubes you want in there rather than try to run two different kinds.   its not a Mesa.
trust me, your amp will sound and run better.   

Anyway, I'd be happy to work on your V4 if you'd like.  call me, my number is in the tech section.  I just rebuilt two V4s last week. why not one more?  I could install my Tone circuit mod for you too,  which will blow you away!

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