Pre-EB stingray preamp problem

[packrat]

I've got a pre-EB ('77) stingray which while playable, has an odd behaviour with the bass/treble knobs. The treble has relatively little effect, but the bass when rolled off actually cuts out everything but the highest frequencies. I haven't had a chance to capture an exact frequency curve, but that doesn't sound right.

Has anyone seen this, or is it in any way a common problem? I'm suspicious of an out-of-spec cap that would have been part of the frequency circuit.

 

[ddnidd1]

Electrolytic capacitors can go bad with age. That's one possibility.

The good news is that these preamps are easy to work on for an experienced electronic tech.

 

[packrat]

I'm assuming even experienced electronics techs are going to be pretty unimpressed to be handed an expoxied pre!

B>

 

[Geri O]

I've got a pre-EB ('77) stingray which while playable, has an odd behaviour with the bass/treble knobs. The treble has relatively little effect, but the bass when rolled off actually cuts out everything but the highest frequencies. I haven't had a chance to capture an exact frequency curve, but that doesn't sound right.

Has anyone seen this, or is it in any way a common problem? I'm suspicious of an out-of-spec cap that would have been part of the frequency circuit.

I worked on a 1979 Stingray for a buddy of mine. The bass had no output at all. I did the usual trouble-shooting, but after a half-hour, I called Music Man tech support, who informed me that I could get an OEM preamp for that bass (no epoxy version) for $53.00 once I send the original dead preamp to them (they don't do after-market-type sales). That was entirely too easy to do and well worth the money. I highly advise going that route resolve those issues with your bass.

(PS, I did this repair a year ago and I guess the owner has lost interest in the bass. I wasn't going to charge him for the repair, but after several calls to inform him it was finished, he shows no inclination to get his bass back. So I’m giving it a good home until he eventually decides he wants it back. It's a 4-string, so it doesn't get much attention, but it sits on a guitar stand along with my other basses and I try to plunk on it every once and a while. It has an awesome sound, no doubt.)

 

[nivagues]

I've got a pre-EB ('77) stingray which while playable, has an odd behaviour with the bass/treble knobs. The treble has relatively little effect, but the bass when rolled off actually cuts out everything but the highest frequencies. I haven't had a chance to capture an exact frequency curve, but that doesn't sound right.

Has anyone seen this, or is it in any way a common problem? I'm suspicious of an out-of-spec cap that would have been part of the frequency circuit.

The bass and treble controls are both cut and boost. The cut point for the bass is at the extreme anti-clockwise position of adjustment (as you have experienced). It then begins to boost from that point as the control is advanced. It has no centre flat position. In relation to the treble control the flat is approximately half way through adjustment although there's no centre detent. The boost can appear to have little effect until fully rotated but I find it depends on the venue.

 

[packrat]

I was surprised that the bass cut took out so much of all the frequencies although I haven't had a chance to put a frequency scope against it to measure it. This made me suspect the centre frequency (or filter Q) was wrong.