Biamp SR-240 Reverb Rebuild Question

[Deflator]

I have a Biamp SR-240 spring reverb that is my go too for anything requiring that type of thing. However, it's always had a bad 60cycle (and associated harmonics) hum. I usually filter it out with a noise reduction plugin but want to start using it outside the box with my tape machine and I'd love to get that hum down.

Not sure where to start.

I have very little use for the EQ, wet dry blend and input limiting functions. I can achieve all of this with ouboard gear and plugins. Also, I never plan on selling it so I'm all for function over form in thus case. I've considering nuking all of the extra circuitry and just rebuilding the drive and recovery circuit based on the schematic and adding balanced I/O but that just seams mean.

I should probably try and rule out a simple fix first.

My gut says (in this order):

1. Replace power supply electrolytics and try a different power supply. Try moving the power supply to external enclosure.

2. Replace all electrolytics.

3. Play with tank input wiring, shield grounding, routing etc...

4. Try bypassing the EQ and Input Limiting sections.

5. Replacing pots and/or opamps

Does anyone have any insight into what else I might try?

Schematic and Manual attached.
Any help is mucho appreciated

 

[tinn]

If you short the secondary of T3.
is ist quiet then?

 

[rock soderstrom]

However, it's always had a bad 60cycle (and associated harmonics) hum.

A 60Hz hum indicates direct magnetic interference of the mains transformer into the recovery transducer of the spring. (Edit: and/or T3)

Extend the send and return signal lines of the tank and increase the distance to the mains transformer as a test. Also rotate the position of the spring tank and observe the hum level.

Problems with the PSU filter caps tend to lead to 120Hz hum as they come after the rectifier.

 

[abbey road d enfer]

Main cause of hum in spring reverbs is magnetic field from power transformer into the reverb tank. Extend the wires and place the tank in a quiet place far from any other equipment.

 

[Whoops]

adding also some help to the already great advice Abbey and Rock gave,
I would do what they told plus replacing the original Mains Transformer with a Toroidal transformer (Less EMI) and stick one of this shielding sheets around the Toroid:
https://www.don-audio.com/trafo-shielding
I'm sure if you follow this advises the noise will be greatly reduced, if the problem is Electro Magnetic Interference, which probably is

 

[Deflator]

Tried some stuff here, thought I'd report back with my results.

Moved springs to separate enclosure. -> helped a little
Moved Reverb out of rack. -> helped a ton

It seems like the proximity of the unit itself to other devices is where the majority of the hum is coming from. Not exactly what that means or what the solution is but for now I just put the whole unit away from everything and it's very quiet.

 

[rock soderstrom]

It seems like the proximity of the unit itself to other devices

Distance is king. The receiver transducer of the springs is a super-sensitive sensor for magnetic interference. Therefore, position it as far away as possible from its own or other power transformers (or chokes).

You can also remedy this a bit with a lot of shielding ($$$) or a second identical spring pan in humbucker position.

The easiest way, however, is to isolate the springs. The control unit, the amplifier electronics and the PSU can stay in the rack or elsewhere.

With a little effort and thought you can leave everything in one box (it serms to work for you), often the problems come from external transformers on which you have less influence in terms of position and orientation in the rack.

Separating the springs from the rest allows you to operate the device directly at the workplace without the need to stand up and running around.

Thanks for feedback.

 

[Ike Zimbel]

Tried some stuff here, thought I'd report back with my results.

Moved springs to separate enclosure. -> helped a little
Moved Reverb out of rack. -> helped a ton

It seems like the proximity of the unit itself to other devices is where the majority of the hum is coming from. Not exactly what that means or what the solution is but for now I just put the whole unit away from everything and it's very quiet.

This sounds like the power transformers in your other rack gear is radiating hum into your device. If your best scenario is keeping it in the rack, you can try re-arranging gear to put the reverb near newer units which are more likely to have SMPS (and no big transformers). If that's not an option, steel is your friend.
In the PA world, I had an application where I had 2 x 2U amplifiers and 2 x 1U processors in 6U racks. Originally, this was with lightweight amps with switching power supplies (Yamaha PD-2500) but after those died we had to replace them with linear amps (QSC MX-1500a) and when we did that we got hum in the processors. As an experiment, I jammed the lid from a 9" x 9" electrical back-box in between the processors and the amps and the hum went away. After that, we cut the lids down to the actual size of the processor bottom panel and attached them permanently. Problem solved.