V4 (1971 Magnavox Model) Rev E. Schematic

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V4 (1971 Magnavox Model) Rev E. Schematic

Postby rciancia on Mon Dec 21, 2015 2:38 am

Hello all,

Working on an old V4 and have already replaced :

70/70/40 E.C - times 2
40/40.40 E.C.

Unit powers on, all tubes light and heat. Taking unit off of Standby after warm up bring a loud crackling/popping sound that goes away when putting back on Standby.
I have De-Oxed all tube sockets, tried wiggling tubes and cracking does not stop.
I have De-Oxed the OHM selector and make sure its not the issue
I have also tried a completely new set of tubes.
During a tube pull test, the crackling stops when I pull out the 12DW7 tube on the preamp board. Replacing it with another does not help.

I have not replaced the 10uf/100V and 10uf/25v E.Caps yet, but that is next.

Any ideas where else to look ?
Thanks
Ron
rciancia
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Re: V4 (1971 Magnavox Model) Rev E. Schematic

Postby danb435 on Thu Jun 30, 2016 8:10 pm

Not sure if you got this resolved, I suggest you re-flow the solder on all the tube sockets in the PCB's. Not a bad idea to re-flow everything on the boards, the expansion/contraction from heating/cooling cycles can cause hairline cracks in the solder joints, hard to see even if you're looking for them, not an uncommon problem. Unless you are very familiar regarding safety measures with the high voltages in these amps, DO NOT MESS WITH IT, REFER IT TO A PROFESSIONAL! Some older Ampegs run around 600VDC, significantly higher than a lot of other amps out there. THIS CAN KILL YOU! Another topic, I'm a bit confused, as I recall Magnavox did not get into the Ampeg business until about the mid-70's. There may have been some "transition" amps that looked like the earlier stuff (metal knobs and silkscreen metal faceplate) but I would reckon a '71 would have been made in Linden NJ. Magnavox later Ampeg V4's had a plastic faceplate insert with plastic knobs. Regarding schematics, ampegV4.com has some pretty good ones, as does vintageampeg.com. Good luck, nothing else in the world sounds like a well-maintained vintage Ampeg V4 guitar amp (IMO of course), these things have a unique character that intentionally veers off (in a good way) from the Fenders and Marshalls of time, amazing cleans, tone for days, loves pedals.
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Re: V4 (1971 Magnavox Model) Rev E. Schematic

Postby rciancia on Sat Jul 02, 2016 2:57 pm

I did get it working , but thanks for your reply. The issue was caused some of the smaller Electrolytic caps on the board ( I forget the value) but I have it working like a brand new amp, and yes... those things can be extremely lethal.

Thank you for replying.
Ron
rciancia
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Joined: Mon Dec 21, 2015 2:14 am


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