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reverb tank replacement / troubleshooting?

Started by oxbowlake, January 27, 2024, 05:28:07 AM

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oxbowlake

Hello,

I have a Univox U65Rn from the mid-70s which I've managed to get working except for the reverb. 

http://vintageunivox.com/pics/schematics/u65rn.jpg

It looks like an appropriate signal gets is getting sent to the tank, and when I tap the springs I get spring noise out the other end. But, none of the signal being sent to the tank makes it out the other side.

It appears to be a genuine Accutronics tank with two ~3" springs in it, but I have been unable to find any replacement tanks that look like it. I was going to attach photos but can't figure out how to upload them? The input DCR is 166R, and the output is 700R. I would honestly prefer to replace the tank with a larger one, i.e. a 9" one, and based on the schematic it looks like a unit with relatively high input/output impedance, and grounded input/isolated output would be appropriate to try - maybe something like an 8FB2B1B? I'm not sure how close I really need the impedances to be.

Any suggestions on troubleshooting or tank replacement generally would be much appreciated!

~O

phatt

You obviously have continuity on both ends of the tank so either the cable is open or shorted, or the drive circuit is dead.
The drive circuit has just 2 transistors so it should not be hard to find replacements.
Phil.

g1

Agree with Phatt, tank should be ok.  Fix the reverb first, then change tanks if you still want to.
Sometimes there is signal but not enough.  The tank takes quite a bi to drive it.  You said you thought it was appropriate but what is the level you are getting in to it?
Also you should be able to get very loud crashing from shaking the tank if reverb and volume are both turned up.

oxbowlake

Thank you for your responses! I will focus on making sure the drive circuit is working correctly first. I pulled out the two transistors in the reverb drive circuit and attempted to test them with a multimeter.

The 2SC536F (silicon) gives me 0.6V drop going from base to collector or base to emitter, and open circuit in any other configuration. Seems right.

The 2SB405GN (germanium) gives me 0.15V going B-E or B-C, open circuits when reversed. But, I get 1V going E-C, open circuit when reversed. I am not particularly familiar with germanium transistors, is that 1V going from emitter to collector abnormal? Is there any other testing I can easily do with just a multimeter before spending $10 on a replacement transistor?

Thanks again!

p.s. Apparently I now have the option to attach files, so for anyone who's curious here are photos of the tank.

g1

When you are getting the 1V from E-C, short the base to the emitter and see if the reading goes to open circuit.  If so, it should be ok.

oxbowlake

Shorting the emitter and base got me to open circuit, thanks!

I soldered the transistors back to the board, and now the reverb seems to work - bad solder joint? However, once the reverb knob is turned up past about halfway, it oscillates. I checked all the electrolytics in the reverb circuit (which look original) with an ESR meter and they seem okay, so I disconnected the tank and wired in an 8EB2C1B and that seems to work fine, without any oscillation.

So I think my long-term plan is to install RCA connectors to a larger tank housed outside of the chassis. Which is fine since the stock reverb sounds kind of like playing into a metal toilet.

Is there any further debugging to be done, or is it fair to say the reverb tank is at fault here?

~O

phatt

Obviously a very early design tank, maybe not double sprung.
If you notice most fender amps have the internal spring assy connected to the outer case via springs but then the whole unit is also slipped into a soft bag to reduce any tendency to feedback.
But if the other tanks works better then easy fix.

Those of us who are old enough to remember how hard it was to stop vinyl record players from feedback at high volumes in disco teks, we found many inventive ways to stop that problem.

I built a few DJ rigs for DJs and some turntables were harder than other to fix.
Phil.