BIAMP Spring reverb unit problems

[Steve Casas]

Hi all.
I have an old (1982 late version) Biamp brand spring reverb unit, model MR/140 that I LOVE! It has been the Yin to my Bassmans Yang for 15 years now. But recently it has become unusable due to feedback if any of the eq's are set above 1 or 2. There is hardly any reverb being mixed at these levels. This happens even with the drive all the way off. I used to use it with all controls at approx. 5 (12 o'clock). It has been gigged all over the place and the rack it was in was slammed around regularly, but there are no visual or mechanical issues that I can see. I have attached the schematic... There is a service bulletin regarding 120 cycle hum and the reverb tank but I don't think that's the problem. The feedback occurs at whatever band of eq that is turned up. If anyone has any ideas, I'd love to hear about it!

[52 Bill]

By feedback, do you mean an acoustic feedback from the reverb springs? Or do you mean that there is an oscillation or whistle from the circuitry?

[Steve Casas]

hi Bill. thanks for responding.

The feedback seems to be certain frequencies within each band defined by the 4 knobs. Say I turn all 4 knobs fully counter-clockwise, then turn the low knob up to about two, and play for a few seconds, very soon I will get a low freq oscillation that builds and overwhelms. The same happens with the other knobs, with the oscillation frequency getting higher according to which knob is up.

I don't know whether it is from the springs or the circuitry...

It sounds to me like the gain is off... each eq knob is supposed to dial in the gain for that band... it sounds like the knobs, at 1 or 2, are still giving too much gain and overdriving that band. Remember, I used to play with each knob at 5, and I don't like too much reverb. But now, at 1.5 or 2 on the knob, there is enough reverb but it oscillates and the knob maybe is not functioning in a linear fashion.

Does that help?

[Enzo]

Isolate the problem.

Is it acoustic feedback or electronic? Since this thing must feed an amplifier to be heard, turn the amp way down so you barely hear it. Is it still feeding back that way? Or does it only feed back over a certain volume? Move the speaker some distance away. ANy change?

If acoustic feedback, pull the reverb pan out and look inside it. Are all the long springs intact? Ther are four short springs at the corners, holding the innards suspended. Are all four intact or have any popped off? It is important that the spring assembly floats. If a support spring comes off, the inner spring assembly can lie against the floor of the unit and pick up vibrations.

Reverb pans are usually put into vinyl bags to isolate them acoustically. And regardless of that, it is important that the metal pan NOT be screwed down tight. It should float on the spongy rubber washers.


If none of that comes together, it may be electrical feedback. Disconnect the cables from the pan. By the way, are they connected to the correct ends? But with them off, can you turn up those EQs without feedback? Or short the return jack to ground to kill any possible signal. What happens?

Check the cables, any have an open ground or center conductor?

[chumus]

I have one of these with the same problem, just wondering if you figured out what was wrong?

[J M Fahey]

What was wrong on the analyzed reverb unit may not be the same problem you have.
3 or 4 different problems are mentioned, you test which one applies in your particular case.
Good luck and post results.