VOX Essex bass preamp repair

[slipnote]

Amp came in, because it was "smoking". Imagine my surprise when I opened it up to find this. An unknown op-amp had vaporized. You can see the burn mark just below the A50k pot on the left. I don't really want to attempt to troubleshoot this dog's breakfast. I was reading some very old posts where R.G. Keen was working up some replacement preamp pcb's for Thomas/Vox amps of this era. I'm wondering if those ended happening. I don't see any other way to save this thing. Thanks for any help on this! Michael

[loudthud]

I don't see enough room for an opamp. Looks like discrete transistors or JFETs. Definitely a home brew or DIY project.

[slipnote]

Yep definitely a home brew, of unknown origin. And yes was a JFET that burned up, my mistake!

[J M Fahey]

Probably a perfboard version of the (very simple) original preamp, you might kludge your own, maybe somebody published a layout.

Just thinking aloud, I see a Zener diode almost touching the blown transistor (not sure why you think it is s FET), it might be an internal voltage regulator.
A load short would definitely blow it.

Can anybody post the original Essex schematic?

As a shortcut, there is a protoboard and PCB combo, where boards copy Proto layout.

That way you test and develop your circuit plugging in components, then when happy you transfer that to a soldered board.

Saves a lot of time and thinking, good idea for one-offs.and prototypes.

You design/develop on one of these:



Then build a real world usable prototype on this:



If "too much" there are half and quarter size versions.

https://www.adafruit.com/category/124

Prices are very reasonable and saves learning a PCB design software just for one project.ordering, etc.
.

[g1]

Can anybody post the original Essex schematic?

This is for the V1042 Essex Bass.

[loudthud]

Looks like it could be the whole thing except the power transistors... except where is the inductor ?

[slipnote]

Whoever did this made no attempt to copy the original circuit, so the inductor is long gone unfortunately.

[J M Fahey]

A small 4 Henry inductor, rated for filter/preamp use (not power supplyduty) will be hard to find.

Unless "something else" uses them, say a Wah, so they are available. .

IF I had to build this preamp in Argentina, where ordering from US is expensive, I would replace it by something sonically equivalent.

4 Hy + .22uF gives me a cutoff frequency of around 170 Hz, at 12 dB/octave.
Cavernous/muddy bass indeed, very old school.

Replacing inductor by a 4200 ohm resistor gives me same 170 Hz lowpass but at reduced 6dB/oct.
In my book, more usable (less muddy) than the original one., but I might cascade TWO such nets for a very close sound if important.
4200 - .22 - 4200 - .22
Resistors in series, capacitors to ground.

Would lose some signal, 1 or 2 dB, no big deal.

[R.G.]

In my Vox mania, I wrote up a repair booklet for the Essex series, so I dug into them enough to understand them some. The preamp is entirely hand-wired on terminal strips on the bottom of the control panel. The attached schematic is one of the various editions. The power amp is in its own box in the bottom of the cabinet (as you know by opening it up). The point is that it can all be re-built on terminal strips on the control panel. I think I have a few isolated pictures of the terminal strip from my research.

The inductor can be faked to some degree. Wah inductors are 0.5H, so you'd need a string of eight (!) of them to get to the original 4H value. Depending on how urgent the need for originality is, you could just drop in a wah inductor or two and see how good/bad it sounds. Or you could go to the extreme of winding one. There are wah inductor kits at effects/amps part places. You could buy one, and over-wind it. The kit will tell you the number of turns for a wah inductor; 2.83 (square root of 8) times the original number of turns gives eight times the inductance with no other change. You might have to find skinnier wire, depending on what ships with the kit. This is far less demanding than winding a pickup.

[slipnote]

Thanks everyone for the help with this. The power amp appears to be intact and functioning. This thing DID work for years apparently, before the current troubles. Mr. Fahey's ideas make good sense to me, and I think I can handle wiring a new preamp up on terminal strips. I've found a few good layout pics online. I've repaired thousands of tube amps over the years, but my solid-state skills are pretty terrible. The owner REALLY wants to save this amp, so I guess this is a good opportunity for me to improve those skills.

[g1]

Some pics in theses threads:
https://music-electronics-forum.com/...-some-pictures
https://music-electronics-forum.com/...matic-not-v-4a

[drewl]

Hey, it's R.G. !
Did you ever build those Vox preamp boards for sale?

[mozz]

I have some a little less than 3H if you are looking to rebuild close to stock.

[R.G.]

Hey, it's R.G. !
Did you ever build those Vox preamp boards for sale?

Yeah. Maybe more importantly, other people did too, and they worked.
I did boards for the Beatle/Guardsman/Buckingham/Viscount/Westminster/Sovereign (all the same board, like Thomas did); the Cambridge/Berkeley II, Pathfinder and Pacemaker (again, one PCB to rule them all, like Thomas); the Berkeley 3; and the UK Vox Series 700. In a fit of nostalgia, I also did a layout for the original copper pattern for the "Big Head" series, although I'm not sure why anyone would ever re-build a real original-pattern one.

I have been prevented from any bench time for quite a while by family issues, but the boards are from a time when I had more time.