Vox Berkeley III SS amp, need schematic or advice

[sonicmojo]

I've got a Vox Berkeley III SS amp in for repair and I'm a bit grasping at straws on it as there is no schematic. The main issue I am troubleshooting right now is no sound on Channel 2. I've been probing the signal and sound is getting into preamp, tone stack, etc, but not moving through to the power amp. Does anyone have a schematic for a V1083?

I did find this helpful site with a board layout someone spent time on but I can't really tell the signal flow based on it. Maybe someone with more familiarity with these amps knows or can point me to where the two channels are spliced into the downstream parts of the amp so I can work back from there to find the fault.

http://www.voxshowroom.com/us/amp/v1083_hood.html

Thanks!

[sonicmojo]

Yes! I figured out that the dotted lines in that layout drawing converge at the power amp so that gave me a clue. I worked backward and found the faulty 1uF cap between the tone stack and the Q16 transistor. Now on to the reverb which is also broken....so still could use a schematic. I see I can "buy" a schematic from that linked site but would rather not.

[sonicmojo]

Reverb fixed. If you ever work on one of these amps you should suspect the red and black, dual positive caps first. Several in the preamp and reverb circuits are 1uF, a couple of other values also. I think they just die over time and are the weak link. The amp sounds pretty decent now for a SS amp. Funky looking.

[Stevem]

If you still have it on the bench you should do as I did on mine and replace all of the electrolytic coupling caps with non polarized ones because once one fail the others do a conga line behind it!

[sonicmojo]

If you still have it on the bench you should do as I did on mine and replace all of the electrolytic coupling caps with non polarized ones because once one fail the others do a conga line behind it!

These guys, right? I think they are all gone flaky or bad in this amp and another Pacesmaker I'll be opening next. Due to the nature of the construction of these amps, I usually snip and splice on top. I don't want to go under this board...

[drew]

Reverb fixed. If you ever work on one of these amps you should suspect the red and black, dual positive caps first. Several in the preamp and reverb circuits are 1uF, a couple of other values also. I think they just die over time and are the weak link. The amp sounds pretty decent now for a SS amp. Funky looking.

So cool looking. It deserves some new grille cloth.

Doesn’t RG Keene make a complete replacement circuit board for these?

[AtomCap]

Just reworked a Berkeley II about 2 months ago and sold it to a friend with a note saying (when/if this amp dies - it burns in my yard- no questions. .)
What a pain in the butt, the pcb's in these old vox just disintegrate when you apply heat to desolder components and thats only if you get the main board out without it snapping in half or the pcb mounts..
These amps were surely not built with the intent of being around as long as they have... Anyway, they're quite quirky circuits - the best suggestion I can make, Rebuild the power supply, recap all the coupling caps and electrolytic caps, change all the voltage dropping resistors on the psu caps- with the power supply and coupling stabilized you can at least feed a signal in, scope the transistor outputs looking for audio and distortion. New transistors all around isn't a bad idea- with the power restored back to 100% it would only be a matter of time before the 50+ year old transistors fail.. Replacing the coaxial cables to the reverb tank, add a grounded plug (remove death cap) all helps too. In the case of your 2nd channel not working - have you verified all grounds are present? the Berkeley 2 I was working on had lifted/failed grounding in the tremolo and reverb- the solder joints on the board looked fine but there was no connection made, also questionable @ reverb recovery. Ended up replacing and re-flowing most of the connection points which managed to restore operation.. Schematics are tough because VOX in their infinite wisdom revised these amps all the time and the documentation was shoddy at best!! Especially transitional VOX.. My project went south fast as the Schematic only reflected maybe 50% of what was inside the chassis.. Your likely going to have to swing at this blind folded =/

[Stevem]

Yup, that's what happens, they pop there ends out !
Just as a side note on these amp, I went through the very time consuming process on mine of trying out silicon output transistors, and after getting them biased in pretty well ended up going back with the Germainiums as the higher current load to bias in the silicons right only cut down on the amps overall output power.

I may get a higher current PT and go back to the silicons over this coming Winter if I am really board for a project to do!

[R.G.]

Sorry I missed this when it started. I can only plead that I was submerged into the Dallas Guitar Show prep and personal recovery.

The correct answer for any amp that hasn't had new electrolytic caps in the last decade or two is to replace them all. Electro caps have a built in decay mechanism. They rot away, in place. They rot faster if the amp isn't in use, but eventually they rot away anyway. Replace them all because if you fix only the one actually failing one, it was just the first, and your customer will be back complaining about your shoddy workmanship since the amp failed again a week/month/etc after you fixed it. And it was only the next worst one failing that killed the amp again. Replace them all.

This is hard to do in Thomas Vox gear. They're hard to work on. But they need the love.

These and other difficulties are why I assembled all the original factory schematics and field repair info I could find and wrote "The Vox Owner's Safety Net", that being a description of the individual circuits in Thomas Vox amps and how they're strung together in the various amplifiers. It also includes general how to fix it advice and where to get replacement parts. Beyond that, I wrote specific repair booklets per individual model. The Berk III is the latest one. You can find these as well as tech tips and resources for Thomas Vox amps at thomas-vox-repair.com

I did design and have prototyped PCBs to replace the innards of the Thomas Vox line. There is a replace-it-all PCB for the Berk III, as well as all of the Beatle, Guardsman, Buckingham, Viscount, Berkeley II, Cambridge, Pacemaker, Pathfinder, Westminister and Sovereign. That sounds like a gigantic task until you realize that I did that the same way Thomas did theirs: Two PCBs cover the Beatle, Guardsman, Buck, Viscount, Westminister and Sovereign; one more does the Cambridge, Berk II, Pacemaker and Pathfinder and one more does the Berk III.

Here's another of my little forays into fixing Thomas Vox:

[sonicmojo]

Thanks for the info R.G.! I agree wholeheartedly about replacing all the caps. For a solid state amp, it seems there are as many or more caps as a tube amp!

[Stevem]

I think there are more caps then the over sea's tube version of this model!

[R.G.]

Transistor amps do in general have more caps than tube amps. My personal impression is that transistor amps simply have more active devices than tube amps. Tubes were very, very expensive back in the 50s and 60s, so as few tubes were used as possible in each amp design. Transistors were merely expensive instead of very, very expensive, so more of them could be used to get more function. Each active device in the signal path tends to need an input cap, an output cap, and perhaps a cathode/emitter cap, so more active devices = more caps.

[dbeasley]

I acquired one of these awhile back that was in real rough shape.

I decided to ditch the chassis and build a single channel AC30 top boost into it. Sounds great now [emoji23][emoji23][emoji23]

Sent from my Moto G (4) using Tapatalk

[Stevem]

RJ I have to disagree with you a bit on the cost difference between tubes and transistors back then, at least in regards to those parts here in the states!

Here are some prices for both out of my 1958, 2000 plus page Allied supply catalog out of New York City.
The two pages of prices without the Tung sol heading are RCA tubes.

[R.G.]

I completely agree with you - for 1958.

Transistors were hugely expensive in 1958. They were just barely becoming available to commercial uses, and all had to be essentially hand made. They were just transitioning from metal cased military experimental stuff. In the Vox amps, all of the preamp transistors excepting one or two were the new plastic packaged types that had vastly lower prices. It would be very interesting to see a series of those catalog pages from, say, 1956 to 1970 to track the moving price of transistors. The cost of a transistor plummeted during that time, especially for low power non-specialized devices used for audio. Metal packaged power devices stayed high for longer as solutions were found for how to make plastic packaged power devices.