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#1 2013-07-22 04:29:57

jlewis184
New member
Registered: 2013-07-22
Posts: 4

V-22 Combo Repair

Hello all,

I'm trying to get my friends V-22 combo amp working and I need some help.  I'm using this schematic,
http://www.ampegv4.com/images/schematics/V4.jpg because it matches what the large board is populated with.

So, the problem is no power output to the speakers.  I turned up the volume and saw at least one power tube arc near the socket.

I've run the preamp output into a separate power amp, and the preamp is working fine.  I've also checked pins 3 and 4 on the power tubes, and they are each around 520-530 volts.  I've checked the large 10 ohm/5W resistors and the 470ohm/2 watt's feeding the power tubes and they are all good. I can see the heater coils on each power tube light up.

I suspect one or more of the power tubes are bad, but I was told they are new.  They are 7027's.  The board has some obvious signs of previous repair from blowed up resistors.

My question is, how can I determine which tubes need to be replaced.  And, what else can I do to ensure everything is working before I replace the power tubes.

Thanks for the help!

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#2 2013-07-26 15:45:18

jlewis184
New member
Registered: 2013-07-22
Posts: 4

Re: V-22 Combo Repair

An update:

I borrowed a couple 6L6GC's from a friend and tried running the amp off just two of them.  Still nothing out of the speakers.  There is not even a hum or hiss.  I tried another cabinet to no available.

As I increase the volume, I can hear the audio vibrating from what seems like the output transformer.  And then a tube will start to redplate, so it seems like the power is being reflected back from the transformer?? 

I probed the OT the other day and had convinced myself it was OK, but I'm suspecting it could be bad. 

What sort of troubleshooting can I do before I think output replacing the OT?  My bench is fully stocked with all sorts of test equipment...scope, meter, power supply.

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#3 2013-07-26 22:14:16

greengoblin
Member
From: FL
Registered: 2009-05-14
Posts: 54

Re: V-22 Combo Repair

One tool I found that was very useful for troubleshooting signals was a signal tracer.. If you don't have a scope. It was recommended to me and saves a lot of time:

BillM signal tracer

Then input a signal or tone through the input via a laptop or a smartphone ( I use my iPhone with a 3.5mm cable to a 1/4" adapter and use a signal generator app similar to Bills video..

Sounds as though there is an interuption between the preamp and output..

Last edited by greengoblin (2013-07-26 22:15:03)

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#4 2013-07-28 23:26:53

jlewis184
New member
Registered: 2013-07-22
Posts: 4

Re: V-22 Combo Repair

Thanks, for the tip.  If I probe the C11 and C13 (the coupling caps to the power stage), will that be a very large AC signal?

I've noticed R53 and R52 are getting very hot when I take the amp out of standby.  This is with and without the power-tubes in the amp and the volume turned down.  Should this be expected?  I've measured both resistors at 3.0k when the amp is off.

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#5 2013-08-04 04:07:23

AEB-1
VIP
Registered: 2011-12-02
Posts: 26

Re: V-22 Combo Repair

after you saw the arcing, did you check the tube socket to be sure it doesn't have a carbon short? when you use one pair of tubes make sure you use the inner pair or outer pair of output sockets and not the pair with the arced socket.
Other things to check: If pin 3&4 are correct on all 4 output sockets, check the negative 52 volt bias voltage pin 5 on all four sockets. If the is a large difference c11 or c13 could be leaky.
did you measure all the leads (unconnected) of the transformer to its frame to check for shorts? the primary red should have similar ohms o the brown and blue wires. Has anybody messed with the wiring to impedance switch/output jacks?

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#6 2013-08-04 16:43:34

jlewis184
New member
Registered: 2013-07-22
Posts: 4

Re: V-22 Combo Repair

Thanks for troubleshooting suggestions. The output impedance switch turned out to be faulty.  I hard wired it to 4 ohms and voila, the amp is working fine.

smile

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