Recent Posts

51

Solid State / Re: Copicat IC400 Vari Speed, speed stuck on full...

« Last post by Rontone on July 22, 2026, 11:25:09 am »

Thanks for the replies, I went for some lunch and then sat down with this again,

I was about to order some 741 IC's but then realised I did have one in an fx pedal so tried that and its working fine!

Getting a smooth change of DC, with the internal trim pot you can set it real slow

So whatever took out the rectifier may have damaged the IC, or the other way round

Motor now needs a bit of a clean but its working pretty stable even with the original drive belt, I've recapped most of it, just crackly pots left to clean

52

Solid State / Re: Copicat IC400 Vari Speed, speed stuck on full...

« Last post by sluckey on July 22, 2026, 09:03:58 am »

Shorted TR101 needs to be high in the suspect list. Remove from circuit and check for low resistance reading between emitter and collector.

53

Solid State / Re: Copicat IC400 Vari Speed, speed stuck on full...

« Last post by shooter on July 22, 2026, 07:49:41 am »

looks like pin 3 should have a vdc that changes with the vari-pot,  use chassis as ground ref for measuring.


you've verified the 15vdc that feeds the vari-pot?

54

Solid State / Copicat IC400 Vari Speed, speed stuck on full...

« Last post by Rontone on July 22, 2026, 04:30:38 am »

Just repairing a spare Copicat I picked up, was dead so went through the power supply and the rectifier was blown and the internal fuse, sorted that out and got it running

But the motor [in this case its the model with a DC motor] is stuck on full speed, potentiometer is fine and the internal trim pot doesn't alter it

There is a transistor on the incoming power [TR101= RCA 40871 or BD303 in my case] and the IC101 741 chip seems to be adjusting the DC to the motor?

I'm presuming its the 741 IC as full power is coming through to the motor so I think the transistor is working

Which points on the schematic attached would be good to test what happening here, or not happening!

55

Tube Amp Building - Tweaks - Repairs / Re: Bias Help for Rob Robinette JCM800 6V6

« Last post by pdf64 on July 22, 2026, 04:26:33 am »

... Amp is done, the Bias setup worked great... BUT my amperage draw is a bit low when maxed out at 18ma. I might drop the 47K resistor by10K. ...

Correct, try 39k or 33k, that will allow a warmer idle.

56

Tube Amp Building - Tweaks - Repairs / Re: Princeton Reverb circuit refinements

« Last post by acheld on July 21, 2026, 12:05:01 pm »

You're unlikely to get the deep drip of an old Gibbs pan with a new one from any manufacturer. That's just the way things are. I don't futz with old broken reverb pans anymore. The juice is not worth the squeeze.

I agree with this statement.  At least in my hands, I can't seem to get what I want out of a reverb unit built into my amp builds.  It may well be my own lack of expertise, but I've given up on the idea of true old school reverb in favor of using pedals to enhance my tone. 

57

Tube Amp Building - Tweaks - Repairs / Re: Anyone care to help me finish my build ?

« Last post by mountainhick on July 21, 2026, 07:29:14 am »

Maybe it's as simple as reversing the OT primaries.

First thing I'd check. Odds are 50%.

58

Tube Amp Building - Tweaks - Repairs / Re: Anyone care to help me finish my build ?

« Last post by Rontone on July 21, 2026, 07:28:23 am »

Try moving the Output Transformer away from the ECC99.

Possibly angling it 90 deg at the same time, those brown and blue OT primary wires could be a lot more direct to the power tube then

The NFB wire also has to travel past the OT, the Power Tube and then right next to the heaters, it may be more direct going over the top past V3 instead of the power tube? Or along the chassis between the alloy tube mounts?

59

Tube Amp Building - Tweaks - Repairs / Re: Anyone care to help me finish my build ?

« Last post by elams1894 on July 21, 2026, 01:38:09 am »

Try moving the Output Transformer away from the ECC99. You could be getting a heap of induced positive feedback from the OT as it looks very close indeed.

60

Tube Amp Building - Tweaks - Repairs / Re: Anyone care to help me finish my build ?

« Last post by antolc37 on July 21, 2026, 01:31:26 am »

The presence potentiometer reduces the hiss indeed so maybe I could add some more NFB (I've already tried disconnecting it completely), I will try adding a stereo potentiometer after the PI if it doesn't do the trick. You're probably right, I remember that when the power tube was biased cooler (I had only 630R at my disposal) I didn't have the squeal.