| Author | Message | ||
     Michael Pledger (michaelpledger) Username: michaelpledger Registered: 02-2014 |
Hi all, I've just got hold of a 112-rd from a friend which is making a popping sound every couple of seconds, it's not passing any guitar signal. Popping gets louder if I turn up the first volume which would seem to indicate a problem with the preamp early in the signal chain... chopsticking didn't reveal anything... wondering if anyone had had a similar problem before I start properly debugging? Thanks in advance. | ||
| Michael Pledger (michaelpledger) Username: michaelpledger Registered: 02-2014 |
schematic is 2165 rd, gd-1 | ||
| Lars Verholt (lmv) Username: lmv Registered: 11-2009 |
Hello Michael, will the amp pass sound inserted at the Hi or Lo insertion points? Is the symptom the same on both distortion settings? There is at least half a dozen coupling capacitors in the amp that could fail and cause little to no sound to pass through. The electrolytics are the most likely culprits. We'll get you through this one. Cheers, Lars | ||
| Michael Pledger (michaelpledger) Username: michaelpledger Registered: 02-2014 |
My initial suspicion was that one of the ICs had died causing oscillation every second or so... That turned out to be correct as the amp now passes guitar signal after replacing the ICs with tl072s. On that subject does anyone have any recommendation for ICs in this amp? But... I've now discovered that the reverb doesn't work and there's a fair bit of crackling/noise. Still some more work to do then. | ||
| Mike Kaus (mm210) Username: mm210 Registered: 05-2006 |
They use IC's for reverb drivers in that amp so that's where I would start. Looks like they use two different op amps in there as well. I assume you have checked to make sure reverb foot switch is not grounded. | ||
| Michael Pledger (michaelpledger) Username: michaelpledger Registered: 02-2014 |
All ICs have been replaced with tl072s which should be ok as sub for both the 4558s and lm1548s that we're in there. So think it must be something else. Reverb doesn't work with footswitch removed so I will need to do some more debugging, first step will be to check out the tank. | ||
| Mike Kaus (mm210) Username: mm210 Registered: 05-2006 |
THe original tanks were pretty flimsy. I don't know too many MM people who HAVEN'T had to replace a tank. Had to steal one from my MM head(now deceased) to get my 2-10-65 running. And now I don't reallt use 'verb. Figures. | ||
| Rob Flindall (flinly) Username: flinly Registered: 12-2012 |
I replaced all of the TL072 with 4558 in my RD112-100 with the GD-2A board (1981), and I believe this was a good switch. My amp also calls for LM1458 in places. The reverb send and recovery was stock TL072, but the 4558 works there, and sounds good. Seems like you have moved past the ICs now, but I am intereste to see how the reverb resolves. | ||
| Michael Pledger (michaelpledger) Username: michaelpledger Registered: 02-2014 |
Reverb fixed. Tested input and recovery amps by directly connecting RCAs on the board, they worked so it had to be issue with the pan. Pried the pan cover open and discovered loose wire on the input RCA connector which I re-soldered and that fixed the problem. Chopsticking around seems to have fixed the intermittent signal on the foot switch channel too, would like to have known what was causing that... Possibly bad pot contact | ||
| Mike Kaus (mm210) Username: mm210 Registered: 05-2006 |
Glad you've made progress. Mike | ||
| Michael Pledger (michaelpledger) Username: michaelpledger Registered: 02-2014 |
Thanks for the help. One more question... I was wondering if anyone could tell me what speakers these shipped with in 1978… I suspect the celestion in there isn't original. | ||
| Mike Kaus (mm210) Username: mm210 Registered: 05-2006 |
Mostly eminence with a smattering of EV in the good stuff. I've heard people say that jbl was an option but I've never found any. Mike. | ||
| Lars Verholt (lmv) Username: lmv Registered: 11-2009 |
If you look at the 1978 MM price list, it states that the replacement part for the speaker in the RD-112 65 is A-12G, AlNiCo - the same as in 112 65 Reverb (and RP-112 65). A-12G and the later ceramic C-12G are both made by Eminence (mfg code 67). The A-12G with the 18oz magnet has the old 'horseshoe' rear part where the C-12 has a 'hockey puck'. Whether all GD-1 amps had the A-12G, I do not know. The change may have come before or after the switch from GD-1 to GD-2 revisions. I'm fairly certain that all GD-2A amps (i.e. with the 6L6 tubes replacing 6CA7/EL34) had the later ceramic C-12G. Cheers, Lars Verholt |