I am the proud owner of a 112 RP 65 that I have owned since new and used trouble free until now.
I turn on the power and after approx a minute turn on the standby (Hi or Lo) and the amp screeches loudly. Turning off the standby stops the noise.
I have tried new valves and unplugged all connections but problem persists. On a visual check, all appears fine except a 220 ohm resistor (R69) on one valve socket appears burnt. This resistor gives a reading of 1.2kohm. Can anyone help with this problem.
Tue, 04/26/2022 - 02:07
#1
112 RP 65 screeching
I have replaced R69, turned on power, seemed OK, set standby to lo, initially loud noise then no sound at all. Set standby to hi and R69 burst into flames, turned off and disconnected.
If anyone can advise me on this problem, I would be most grateful.
Check Power Supplies
A lot of current had to flow through R69 to catch fire. Did the tube "red plate" when R69 went thermal? If yes, check Q1 power transistor to see if it has shorted out. Also suggest to check all the power supply voltages with both tubes removed. Check +/-16V low voltage supplies (D10,D11). Also check the +22V zener diode(D9) that feeds R68 and 69. Repair R69. Now check the High voltage supplies. With the tubes out and standby switch in Hi mode, check for 700Vdc +/-10% at pin 3 of the tube socket and 350Vdc +/-10% at pin 4 of the tube socket. Fix any problems found with any power supply voltage. Question: has the amp ever had the power supplies re-capped? I'm just guessing but a leaky Hi voltage filter capacitor under the "dog house" may be the source of the screech. Does it screech with the tubes removed? Be careful. High Voltage. Good Luck. -mgriffin
Power Checks
Thanks mgriffin for the advice. I will update further after I have tested further
Power Update
Further to my previous posts:
I did not notice any red plating when R69 went thermal and the filter caps were the original Mallory caps.
I have now replaced the filter caps and replaced R69. On checking the voltages I found D9 to have gone short and have now replaced.
Voltages as follows:
D10 15.4V
D11 -15.4V
D9 22.2V
Pin 3 tube sockets 700v
Pin 4 tube sockets 369v
The amp now powers up OK but no sound apart from a slight hum from the speaker that increases slightly when gain and volume on max.
Any help would be much appreciated.
Speaker?
The voltages look good. Keep an eye on D10 and D11 as they're on the low side and might be getting tired. As for no sound, my RD65 had the same symptom when I first got it. Turns out the speaker voice coil had opened up. Unless you've already done so, check the voice coil resistance and make sure it's not 0 or infinite ohms. Unplug it from the amp and it should measure about 6 ohms on an "8ohm" speaker. If you can reach in and touch the speaker cone, give it a gentle push to see if it feels scratchy. The motion should feel smooth. Try connecting a different speaker if there is one handy. -mgriffin
112 RP 65 No Sound
Thanks mgriffin155 for the quick response. However, I have tried a known good speaker but the issue remains the same. Any other suggestions would be welcome.
Bpw
Check Bias?
Was the bias voltage checked at R60 & R68? -mgriffin
http://www.pacair.com/mmamps3/sites/default/files/docs/MM%20Service%20Bu...
Bias
I have checked the bias across R60 and R68 and get 24.5mV and 23.8mV respectively. I can also confirm I am getting no signal at the return of both inserts. Still just getting a low level hum and hiss with vol and gain on max.
Bpw
Try other input?
The power stage of your amp seems good. Have the 3V Hi input (J4) and 100mV Lo input (J3) been tried? There may be a failed IC somewhere in the signal path. See if you get any sound using J3 and J4 inputs. That might give us some clues for what to try next. -mgriffin
Signal Input
Thanks for your continued help with this issue. The amp is worth the effort.
I've fed a 400Hz signal into J3 and
J4. In both cases the signal can be heard faintly. In the case of J3 the signal is affected by both the gain and volume. J4 volume only.
IC Swap?
J4 has a direct path to IC7. Try swapping IC7 and test again. Pull IC5 and use it if you don't have a spare. -mgriffin
IC Troubleshooting
Swapping the IC's appears to have brought the power back to the signal.
However, on J1 and J2 I have the signal accompanied by a very loud crackly hissing but on J3 and J4 the signal sounds clean and healthy.
Thanks again Mike for your continued help.
Bpw
Success
Mike, based on your advice, the latest results and schematics, I have replaced IC1 and everything appears to be working fine apart from the phaser due to borrowing IC's. It seems that IC1 and IC7 had gone faulty/noisy.
Bpw