| Author | Message | ||
     Dirk Wacker (doc_holiday) Username: doc_holiday Registered: 02-2007 |
Hey guys, I bought another 112RD50 to run it stereo with my first 112RD50 and you will not believe it ... this amp has also problems with the reverb ! The reverb effect itself is working, but when I turn the reverb up, it starts to hum at around 3, and at 5 or upwards this humming noise is very loud - when I turn it to zero, the hum is gone. Here is what I did or tried without any success: 1. Change the reverb cable 2. Change the complete reverb tank 3. Take the tank out of the combo, the noise did not change 4. Shield the tank with a layer of MU metal 5. Tried a tank without ground connection 6. Disconnect ground on the AC plug 7. Changed the two transistors and the IC in the reverb section on the PCB All this did not make the smallest change, any more suggestions ? Maybe itīs the OT, sending the noise to the reverb tank ? | ||
| Jim Wainwright (av8or3) Username: av8or3 Registered: 10-2007 |
I had a similar prob with my 65/210. Loud hum with reverb on. Went away following new tubes and bias adjustment | ||
| Dirk Wacker (doc_holiday) Username: doc_holiday Registered: 02-2007 |
Hi Jim, thanks for your answer .... should it really be that easy ? I got the amp without tubes, so I put a new pair of matched TAD 6L6 in and set the Bias to 12mA for each tube, identical to my first 112RD50. I will put in the tubes from my other MM amp and give it a try. I replaced the "fixed bias" resistors with trim pots, so re-adjusting the Bias is not a big thing. | ||
| Jim Wainwright (av8or3) Username: av8or3 Registered: 10-2007 |
The problem you describe is exactly the prob I was having. I too replaced the tubes with new when I first got it. (don't remember if it hummed before I changed the tubes) However,following the bias adjustment (per the instructions found on this forum), I was pleasantly surprised when the hum was gone. Don't know why, don't care...but it's gone. Best of luck to you. PS I just scored a blonde RD112 Fifty. My first Fifty, and am told it's pretty rare. | ||
| Dirk Wacker (doc_holiday) Username: doc_holiday Registered: 02-2007 |
I changed the 6L6 tubes with a pair that is working perfectly in my Fender Bassman amp and lowered and raised the BIAS for both tubes. I had the BIAS set to 12mA for each tube and raised it to 16 without any change. Afterwards I lowered it step by step, down to 3mA and this also does not change the problem. Any more suggestions ? | ||
| Mike Kaus (mm210) Username: mm210 Registered: 05-2006 |
Sounds crazy. Hmmmmm? Did you look at all the ground points and re-check them. It almost sounds like a ground with a bad solder joint or star washer. You're going too have to probe with the amp ON and with a plastic or wooden(NOT a pencil) probe and see if you can alter it. Does this amp use a driver tube? That would be my next place to try. Unusual but possible. Also, you MIGHT try shielding the PS transformer with some shielding and see if it's putting out some wave forms that are being picked up by the reverb circuit, which, if I remember correctly, is pretty damned close. Let me know what else you try. Sounds interesting. MIke. | ||
| Dirk Wacker (doc_holiday) Username: doc_holiday Registered: 02-2007 |
Hey Mike, thanks for taking care, I will try probing inside the amp as you described and I can try to put in the PS transformer from the other amp I have to see if it makes a difference. I will let you know ! | ||
| Mike Kaus (mm210) Username: mm210 Registered: 05-2006 |
I wouldn't bother to swap the transformer, maybe just wrap some shielding aroung it or even sit a piece of sheetmetal between it and the rest of the amp to see if it changes anything. It's a long shot anyway. Also, wire routing can cause hum if the preamp wiring is unshielded and running too close to the AC imput source or too close to a transformer. Also, a bad cap could be the problem. It could be close enough to the reverb receive to pick up some junk. | ||
| Ed Goforth (ed_goforth) Username: ed_goforth Registered: 06-2006 |
Hi, have you replaced IC-3 the reverb recovery IC? The hum only is audible when the reverb control is up, right? So if you disconnect the reverb cables from the chassis, there is still hum. I would try another IC opamp (TLO72, 4558, NE5532, LF 353 etc will work). It has happened before that these opamps can go bad. Also, make sure all connections to do with the reverb circuit are good, like solder joints and tight (not overly tight) star ground connections like Mike said, could be a cap or solder connection on a component. Does it hum if the footswitches are removed? I have in the past had problems with the 4016 mosfet switching chip going static causing hum and noises from the limit channel. So I would check those areas and maybe swap the IC's carefully to see if it makes any difference, the chips are not very expensive. If all solder joints are good, I suspect either IC-3 (TLO72 or LF353) or IC-2 (4016 switching chip). Ed | ||
| Ed Goforth (ed_goforth) Username: ed_goforth Registered: 06-2006 |
Be sure you know the precautions for replacing the 4016 C-mos chip as it is static sensitive and can be destroyed from static electricity that can build up in just handling the device, I usually (if the amp is plugged into a known well grounded wall outlet) ground myself by touching the amp chassis to discharge any static electricity that may be present, and carefully handle the IC. You can maitain being grounded while installing the IC by resting your palm over the front edge of the chassis while carefully pressing the IC into the socket, just be careful not to bend the legs out or under the body of the IC. I usually take a pair of needle nose pliers and square the IC legs a little so they go in easier. Good Luck! | ||
| Dirk Wacker (doc_holiday) Username: doc_holiday Registered: 02-2007 |
Hi Ed, thanks for your answer. You are absolutely right, the hum only is audible when the reverb control is up and there is no hum if the footswitches are removed. Does this info help to trace the error ? | ||
| Ed Goforth (ed_goforth) Username: ed_goforth Registered: 06-2006 |
Hi Dirk, yes I think it helps narrow the possibilities. have you tried different IC's? I have had similar problems with the opamps going bad once in a great while they have been known to go bad. | ||
| Ed Goforth (ed_goforth) Username: ed_goforth Registered: 06-2006 |
You could (with footswitches removed) use aligator clip methods to jump the rca jacks to see if the switching circuits are functioning. Its basically tip to ground on those jacks for switching. If the hum persists, I would try replacing those opamps IC's I mentioned earlier. Good Luck! Ed |