Hi,
Reposting an issue I have with my 1981 RD100.
When I put the amp into standby...it takes a good 3-4 seconds to get into the standby mode. And there is a hiss or squeal that gets louder before cutting out.
Shouldn't the amp go into Standby (dead quiet) instantaneously?
I see a 470 pf capacitor across the tube pins, and in the schematic, that appears to be just upstream of the output transformer.
Is this possibly the source of the slowly dissipating signal to the speaker?
Sun, 08/23/2020 - 13:53
#1
RD112
Check Bleeder Resistors
It probably is not the 470pf caps causing the problem. Their job is to shunt hi frequency noise on the HV line to ground. It sounds like the HV Power Supply is slow to discharge when the amp is put in Standby Mode. The job of resistors R49 and R50 (150kOhm 2W) are to drain the HV Supply when de-energized. If they are malfunctioning, the HV PS tries to discharge through the tubes, which might explain the squealing noises. Check to see if one or both of those resistors have cracked or gotten burnt. Good Luck and Be Careful. High Voltage. -mgriffin
Capacitor and Bleeder Resistor Replacement
Thanks mgriffin115!
I re-capped back in June 2018. I did not pull the resistors at that time...I will pull the resistors and replace them along with the two diodes under the can. That was where I was thinking of going next. Makes sense if they are cooked.
The photos from June 2018 recap are in a previous post in the Capacitor Replacement section. The parts look old but not damaged... time has gone by however.
Bleeder Resistor Repalcement in HV Supply
OK...I replaced R49 and R50 along with the two diodes D10 an dD11 in the HV Supply (under the can). The originals all checked pretty well within Spec' (around 155K ohm) but I replaced anyway since I had them pulled. The original MR510 also tested OK.
However, there is still no change in the issue with the amp going into Standby slowly and the squeal or loud hiss at the end.
Also, I noticed that that once the amp was in the so-called "Standby" mode....I can still get a pretty loud sound out of the amp by touching the tip of the guitar cable. There is no sound from the guitar strings when it is plugged in and in "standby" but I get the noise by touching the cable tip.
In comparison, on my 1965 Fender Band Master...once it is in stand-by there is absolutely no sound if I touch the cable tip. The Bandmaster Standby circuit is simliar to the GR-2A standby circiruit except for a small Transformer TR2 after the Standby Switch in the Fender AB763
In the GR-2A schematic for the RD-100 I see R29 (also 150K ohm) on the piece of coaxial cable coming Pin #2 of IC 7...the Co-axial cable terminates on the speaker impedence switch. I should have checked that line while I had it opened up...but I just noticed it now. Could R29 be the culprit?
Any other suggestions?
HV PS Discharge Time Constant.
R29 and the coax are part of the power amp feedback loop. It is difficult to see how they would affect the squacky standby feature. I did the discharge time constant math for the HV supply and it should take about 15 seconds for the HV PS Caps to discharge through the 150kOhm resistors. 100e-6 x 150e+3 = 15 seconds. Perhaps this is a feature and not a problem. My RD65 sometimes makes a peep or two for a few seconds after switching to standby. If the amp sounds good when being played, maybe all is OK. -mgriffin
HV PS Discharge Time Constant
Yes...makes sense. With tau at 15 seconds to get to 37% discharged, then tau at 0.2 = 3 second which is the observed time to go “quiet” and that takes the power down by about 20% of total which I guess is enough to equal a standby.
I’ll experiment with how long it takes to silence the “wet finger on cable tip” input noise...but if it is around 4-5 tau...like one minute...then it is all good I suppose.
I think the Hi-Standby-Low power setup on the RD112 is not really a cutoff like my Fender...but that is OK. Just a feature as you suggest.
Also I cleaned up all the solder in the HV PS including cleaning up all the crud between the two plastic eyelet slabs. That seemed to help with reducing the noise as it goes into standby.