| Author | Message | ||
| Chris Metcalfe (chris_m) Username: chris_m Registered: 10-2006 |
I recently acquired an RD50 off ebay for �250,( makes you think, when fender champs routinely fetch �300!!) and on getting it back home was reminded just how great these amps are in terms of contruction and sound. I played MM in the 80's, but had forgotten since i migrated to polytone/ archtop sound. However my enjoyment was slightly clouded by the fact that the clean channel had an intermittent distortion at all volumes - not heat related, since it was there initially, but then came and went. I have recapped the amp, cleaned everything I could clean, especially jacks/pots, and removed the ics and cleaned the sockets. So far, this has got rid of the distortion, but bearing in mind that recapping will only restore the voltages to original specs, I'm wondering if this has actually fixed anything? I know nothing about ics ( although I do know about tubes) and don't really know if an ic operating at sub-optimal voltage could produce an intermittent distortion? Can any of you experts help? On recapping, I have done this a lot on other amps, but this time made a near-fatal error - Like others on this site, i attached the new caps to the cut-off leads of the old caps, above the board. Unfortunately, one of them heated up too quickly and slipped right down though the pc board!Fortunately i was able to patch the cap in to a nearby contact. The reason for mentioning this ( duh!) is that the obvious hadn't occurred to me, which ( just in case anyone hasn't thought of this) is to bend the cut-off cap lead at right angles to the board, so that if it does overheat and melt the solder, it doesn't fall though the board. Anyway, having rediscovered MM after 25 years, i'd really like to fix this intermittent fault if i haven't already, so any comments /advice from any readers /experts would be most welcome. PS no chance of finding a real MM service expert in London is my guess; not that many were imported here anyway, and it has been over 20 years.... thanks Chris | ||
| Mike Kaus (mm210) Username: mm210 Registered: 05-2006 |
I think I would have to suspect the driver section since it comes and goes. Don't have a schematic right here but I'd check the drivers which would be the JE1692's and their local components, like the caps aorund the drivers. They will all be on the driver board(if the RD's have a separate driver board). Also, you might want to check and see if the distortion IS in the final, as it could be front end distortion, I guess. You may have to scope it and follow it through. The RD's use a relay(I think) to switch the drive section in and out so if it's stuck in drive, it could be a problem. You're just going to have to figure out which side of the power/preamp the distortion is on first and then go from there unless you just want to start throwing parts at it. Mike. | ||
| Chris Metcalfe (chris_m) Username: chris_m Registered: 10-2006 |
Thanks Mike Tip about the relay is useful, but it's not stuck in drive; it switches OK. Do you know about the specific point of whether running ics undervoltage, due to ( probably) excess leakage current through old caps could cause an ic to distort? How do they react to undervoltage? It's actually ocurred to me that this symptom could be caused by the splitter malfunctioning on one side intermittently - what do you think; are they prone to this fault? I see they still use a paraphase splitter, which was abandoned by fender in the 50s!! thanks for your help so far. | ||
| Mike Kaus (mm210) Username: mm210 Registered: 05-2006 |
If the drivers are set up wrong, they will distort. If you look at bias stuf on the site, I think you'll find the adjustment procedure for the drivers but they are notorious for being bad and most of the complaints I see here seem to be driver transistor related. That's where I would start and the caps srrpounding them. I would honestly think that the IC would be more distorted with OVER voltage but that's just a guess at this point. I think you'll find the driver's are the culprit. |
Tue, 03/25/2014 - 09:29
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