Peterson 4 pin preamp issues.

[justinmroth81]

G'day all.  I have had problems with hum and buzz since I got my 73 suitcase.  I bought a preamp rebuild kit a few months ago with intentions of, well, rebuilding in hopes that it solved my buzz issue as well as I had a bulb out.  I kept putting it off and putting it off until, of course, a few days ago, a month before our record release show, I started experiencing some more issues.  It started sounding distorted for no reason.  I turned it off to check a few things, and after turning it back on it sounded ok for a few minutes.  Then it started distorting again.
  My set-up is harp board-out to pedal board, back to preamp input, 4 pin to power amp, 1/4" from ext. amp out to tube preamp, to external power amp, to 4X12" trapezoid cabinet.  I should mention I play pretty loud, but not pushing much through the suitcase cabinet, I let the external cab do most of the work.  I get the hum at any volume, as long as the power is on, even after the rebuild, but this is no longer my main concern.  I don't hear the distortion anymore, however the output is super low.  Before rebuild I usually maxed volume out around  9-11 o'clock (3-4 per Rhodes marking).  Now i have to crank it to almost all the way up, and it still is pretty quiet through the suitcase cab.  I can compensate with the external cab but I experience more noise issues.  I noticed in the VV kit there were NP caps replacing caps marked as polarized, I guess this is ok, maybe even mentioned somewhere.  I checked cap polarities and for loose connections and could find none.  I took care to switch the base and collector leads of the two transistors I replaced (shown below circled in yellow, these are what I saw to replace per the schematic).  I do not have a scope, however I have been an electronics tech. most of my life, with a degree.  Thanks for any help.

[sean]

Justin,

Using non-polarized electrolytic caps is great.  They are sometimes more expensive and sometimes physically larger than standard electrolytics.  But modern caps are probably much smaller (physically) than twentieth-century capacitors, and the price difference is insignificant for small quantities.  So I buy non-polarized electrolytics and multi-layer ceramic capacitors for use in the audio path or nearby.  You will notice that the Peterson schematic specifies 5uF NP caps in six locations.


Sudden loss in volume is suspicious.  Get a flashlight and magnifying glass and make sure you didn't put one of those transistors in "backwards" - meaning the collector and emitter are reversed.  At low voltages, the transistor will survive, but the gain (hFE) will be much lower.  Take are really good look at the schematic and make sure you are clear about exactly what solder pad should be the emitter, base, and collector.  (It would not be insane to mark them on the circuit board with a pencil or sharpie.)  Peterson preamp schematic:  http://www.fenderrhodes.com/org/ch11/fig11-8.jpg

Also, you might simply be unlucky and one of the new transistors was born with low gain (poor nutrition, didn't go to the gym, silicon boule smoked during pregnancy).  In that case, replace the transistor again, and hope for the best.  Or maybe the old transistor had higher than average gain, and the perfectly-good replacement looks weak in comparison.  But differences in hFE across responsibly-sourced transistors shouldn't be so bad that you have to crank the volume to maximum.


Since you are an electronics tech, you should get an inexpensive oscilloscope.  I recently got a PicoScope 2204A, and I love it.  You need a laptop to plug it into, but it is a great little device for $140.  I don't know why Amazon doesn't have them bundled with the probes currently.  You can get the bundle from PicoTech directly.  There is only one time that I wished I bought the model up (2205A), and that was because the 2205A has more sample memory. 
See https://www.picotech.com/oscilloscope/2000/picoscope-2000-overview
Their software is good.  The signal generator is great (I bought a BNC-to-RCA adapter to inject audio into circuits), and the FFT spectrum analyzer is sweet.  You can't use the scope on high-voltage circuits (I think the voltage limit is +/-20V), and I don't know if the x10 probe increases that range of safety.  But at the same time, they list the over-voltage protection limits at +/-100V.  If you do any automotive work, or digital signal work, they have other models better suited to multi-channel work.

Sean

[sean]

On your hum issue, your complex setup may be creating ground loops, high-impedance paths, or simpy have dirty connectors that create high-impedance paths. 
Have you isolated the hum down to the preamp itself? 
Sort out the hum issue on the least-complex setup as possible.  No extra preamp, no effects loop, no external amplifier.

There could also be slightly different AC potentials between one power mains circuit and the next, so try plugging all the gear into power plugs that are very close to each other.  Use ground lifts if you dare.


The distortion problem could be related to an overheating transistor.  The outputs in the suitcase bottom would be the primary suspects.  I think that also means checking the 1/2Ω resistors (and maybe the 820Ω and 2.7Ω resistors too).  You don't get distortion directly out of the Peterson preamp, do you?

Sean

[justinmroth81]

"Sudden loss in volume is suspicious.  Get a flashlight and magnifying glass and make sure you didn't put one of those transistors in "backwards" - meaning the collector and emitter are reversed.  At low voltages, the transistor will survive, but the gain (hFE) will be much lower.  Take are really good look at the schematic and make sure you are clear about exactly what solder pad should be the emitter, base, and collector.  (It would not be insane to mark them on the circuit board with a pencil or sharpie.)  Peterson preamp schematic:  http://www.fenderrhodes.com/org/ch11/fig11-8.jpg"

I switched around the base and collector as per the VV video here https://www.youtube.com/watch?v=ab7s0HMQpuo , the emitter remains in the same spot.

"On your hum issue, your complex setup may be creating ground loops, high-impedance paths, or simpy have dirty connectors that create high-impedance paths.
Have you isolated the hum down to the preamp itself?
Sort out the hum issue on the least-complex setup as possible.  No extra preamp, no effects loop, no external amplifier."
 It hums with nothing in the loop (1/4" patch to complete path).  It also hums without the preamp 4 pin cable plugged in, I just noticed this but I guess this would point to the PS or power amps, though I don't think it's the amps...here's the thing, the hum is present in the external amp output and in my external cab, which would lead me to conclude that I have a dirty out-of-tolerance power supply, probably needs new filter caps at the very least.

"The distortion problem could be related to an overheating transistor.  The outputs in the suitcase bottom would be the primary suspects.  I think that also means checking the 1/2Ω resistors (and maybe the 820Ω and 2.7Ω resistors too).  You don't get distortion directly out of the Peterson preamp, do you?"
The distortion is present at both the suitcase cabinet and the external cabinet, my guess would be it would have to be coming from the preamp. (I tested with no effects, with and without external cab, and also straight from the harp board which of course, sounds fine)

[Ben Bove]

Good suggestions by Sean, and am I correct that the distortion started before you rebuilt the preamp?

It's very hard in these amp systems to nail it down to the preamp, power amp, or amp modules because they're so interconnected. 

Does the vibrato work, and at slow speed / full intensity, does the distortion happen on both amp channels?  This would help rule out the amp modules which usually need a rebuild, but rarely fail together.  it's also good to check that both channels are in fact working.  Headphones help to hear things clearly.

If the distortion is present on both channels, and you've had a preexisting buzz before the distortion, it could either be the preamp or the main power amplifier.