Uneven/unequal panning vibrato

[garagebandking41]

I finally fixed the power modules for my 74 rhodes (4-pin). But it seems that the panning vibrato pans uneven in terms of time. it seems that the panning is probably 70/30 instead of a true stereo sound being 50/50. How do I adjust to fix this uneven pan? I tried to find a similar problem on this forum with no luck.

[Ben Bove]

I've heard of this problem before, but haven't seen it first hand.  I think someone called it "syncopated" vibrato or... trying to think of the term.  But it has happened before to a couple of people from my recollection, not sure the proper fix.

[Max Brink]

I haven't come across this exact issue so bear with me here while I "think out loud..."

I'm not an electrical engineer so my background isn't strong enough to know all of the theory behind points within the circuit but my first inclination troubleshooting this issue would be that something is out of spec from one channel of the vibrato in relation to the other. That's the area of the circuit that I would be looking at first. I know that changing out the bi-polar caps in the vibrato for other values will change the speed of the vibrato but I have always swapped them out in matching pairs so I would not know how they would react if one of them were to drift in value due to aging... Could it be that one of the channel's is creating a faster wave form than the other because of these caps? I think it sounds reasonable? If that's not the case then I would move on to measuring other specs of the two channels. 

[ianmgull]

I only briefly glanced at the schematic but I would start by checking the bulb/ldr pair.

Each of the L/R channels has its own bulb/ldr combo. If one channel is louder or on longer than another there is likely some asymmetry here. Visually check to see if each bulb seems to light up for the same length of time and approximate brightness.

Some LDR's have very slow response times (especially older ones). Was it always this way? There are more modern optoisolator retrofits that should give you more consistent results if you decide to do some mods.

[Tim Hodges]

There are more modern optoisolator retrofits that should give you more consistent results if you decide to do some mods.

VV do a LED retrofit which I'm planning to install in a customer's 4 pin. I've had problems with mine thumping so hopefully that'll help fix that as well.

[8675309]

I'd recap the entire preamp- all the electrolytic caps have likely drifted out of spec.. Visually check bulb intensity, verify the optos are evenly spaced to the bulbs. If the issues remain at that point..
(Avoid drilling holes / modding a peterson at all costs)  You might need to send the preamp and amps to a tech  for a rebuild rather than mod it which could potentially make it worse.  your preamp could have another issue not even related to the opto/bulb setup. With that being said you could mod the board, and still have the same issue -  anythings possible.

Peterson pre-amps are scarce, commanding a $300+ price tag, difficult to source, even harder to source an original which hasn't been goobed up with 30+ years of hap-hazard repair work. That's whats Leary for me about modding one.. They sound great when rebuilt properly.

When you repaired the modules, did you recap as well? 
Its safe to say any suitcase regardless of production year at this point would require a total recap all the way round, amps, preamp.

[Max Brink]

Peterson pre-amps are scarce, commanding a $300+ price tag, difficult to source, even harder to source an original which hasn't been goobed up with 30+ years of hap-hazard repair work. That's whats Leary for me about modding one.. They sound great when rebuilt properly.

I agree. And I don't know why anyone would want to pay that $300 price tag for something that doesn't have the classic vibrato circuit. The vibrato is the best part! If anything the only retrofit that I would like to see would be a better sweep of the EQ controls. The vibrato circuit is what makes the amp the Peterson.

If anyone is in need of a bulb just shoot me a call or email and I'd be happy to help you out.