Help with DC on volume pot

[Mooly]

Originally Posted by mkusan

Have we just confirmed that the pot is the indeed the cause of the noise or a problem in amp innards is beyond me.

Yes we have confirmed the pot is now the problem. It was the caps for the noisy switches (leakage) and now we have this problem.

Don't lose sight of the fact that the original pot works OK.

Is the new pot still "scratchy" when turned or has it just the hum problem ?

I suggested a plastic pot to eliminate any possibility of there being leakage from the pot body to the shaft (for the scratch noise). So is that scratchy noise still present.

[mkusan]

Originally Posted by Mooly

Yes we have confirmed the pot is now the problem. It was the caps for the noisy switches (leakage) and now we have this problem.

Don't lose sight of the fact that the original pot works OK.

Is the new pot still "scratchy" when turned or has it just the hum problem ?

I suggested a plastic pot to eliminate any possibility of there being leakage from the pot body to the shaft (for the scratch noise). So is that scratchy noise still present.

The noise of the plastic pot while turning is not "scratchy" as in TKD pot, totally different kind of noise, it reminded me of the noise when we had one leg of the C506 hanging in the air, not tied to the ground, interference kind of noise.

I am aware that original pot works OK, I did not expect it to be TBH.
What puzzles me, and same story was with caps, TKD was silent for months and it turned noisy abruptly. I would understand that TKD was noisy from the first day of install, either because it arrived noisy from the factory, either because too much heat while soldering damaged it or because it was unsuitable for this application.

[Mooly]

The noise on the new pot sounds like it is picking up stray hum.

I really don't know what to suggest to you other than that the pot is the problem and you need one to match the spec of the original.

The one that you have been using and that has gone noisy... again that is down to the pot. It has worm prematurely in some way.

I know I keep saying this but all these problems come down to the strange circuit layout of the amp.

Maybe something like an expensive ALPS pot (and they are the best) would be suitable... if it would physically fit as they are bigger than most.

Is there no way of opening the old pot up and cleaning it and movint the wiper slightly to a different part of the track.

I suppose another option is to see if they are available as a spares item from Rotel.

[mkusan]

Originally Posted by Mooly

The noise on the new pot sounds like it is picking up stray hum.

I really don't know what to suggest to you other than that the pot is the problem and you need one to match the spec of the original.

The one that you have been using and that has gone noisy... again that is down to the pot. It has worm prematurely in some way.

I know I keep saying this but all these problems come down to the strange circuit layout of the amp.

Maybe something like an expensive ALPS pot (and they are the best) would be suitable... if it would physically fit as they are bigger than most.

Is there no way of opening the old pot up and cleaning it and movint the wiper slightly to a different part of the track.

I suppose another option is to see if they are available as a spares item from Rotel.

I have absolute confidence in you so I will do as you told me.

I suppose spares from Rotel for amp this old are unobtainable, but I know a man from local diyaudio forum who claims the old pot can be restored by the means of cleaning with isopropyl alcohol and fine sanding paper.

Worst case scenario I can continue to use TKD, it is completely silent in operation with volume knob attached, and since I plan to use volume knob...

So are we finished now, or shall we connect all wires to the original pot, remove jumper wire from C505 to the ground and test once more with everything in place?

[Mooly]

Thanks for your kind words... I haven't got all the answers though

Not sure about sandpaper. The resistive track in a pot is mega delicate, it's just a layer of "carbon" or other resistive material deposited on an insulated backing.

If you can physically open the pot by bending any tabs etc to remove the covers then the track is cleaned with a cotton bud and iso. The wiper is cleaned the same way perhaps using a piece of ordinary paper on the wiper as "sandpaper" to just clean it it. Then the wiper is retensioned if necessary and more importantly bending the contacts slightly so they run on a different part of the carbon track. Then it's all put back together. Some pots have a clear grease in them and others don't. Best left dry probably.

If there is nothing to lose it might be worth giving the TKD pot a little squirt of something.

Would your new pot be noisy (hum) I wonder when the amp was all boxed up and the pot shielded. Dunno

[mkusan]

Originally Posted by Mooly

Thanks for your kind words... I haven't got all the answers though

Not sure about sandpaper. The resistive track in a pot is mega delicate, it's just a layer of "carbon" or other resistive material deposited on an insulated backing.

If you can physically open the pot by bending any tabs etc to remove the covers then the track is cleaned with a cotton bud and iso. The wiper is cleaned the same way perhaps using a piece of ordinary paper on the wiper as "sandpaper" to just clean it it. Then the wiper is retensioned if necessary and more importantly bending the contacts slightly so they run on a different part of the carbon track. Then it's all put back together. Some pots have a clear grease in them and others don't. Best left dry probably.

If there is nothing to lose it might be worth giving the TKD pot a little squirt of something.

Would your new pot be noisy (hum) I wonder when the amp was all boxed up and the pot shielded. Dunno

None of my questions were left unanswered by you that is for sure.
Besides great technical knowledge you posses rare qualities in this day and age, equally important to me, and that is modesty and patience. Your assessment of what, when and how much I can do was perfect.

TKD is sealed unit, so to squirt something in would require drilling a hole in the pot.

That reminded me of something I meant to ask you, if you remember that sticking right VU-meter, I tried to clean the plastic cover with anti-static glass cleaner. That had a very mild positive effect on needle movement, do you suggest a drop of contact cleaner+lube combo on a pivot needle?

[Mooly]

Not sure what to suggest on a sticky meter. Again they are fragile, the pivots are jewelled, in fact just like a watch balance wheel and hairspring. Any ordinary oil will be far too thick, probably something like a watch or clock oil would be good. I imagine you would have to take the movement out of the meter case to get at the lower bearing.

Something that might help... rig up a 555 timer as a low frequency oscillator to pulse the meter fully to 100% every second and leave it for a day or so. That might help free up a sticky bearing.

[mkusan]

Doumo arigatou gozaimasu Mooly-san, (that is thank you mr. Mooly in Rotel language)

I think our journey is nearing its end.

Jumper wire from C505 to E7 removed, all leads from original pot to the PCB soldered and Rotel is noise free while operating switches and turning volume.

Before I close amp for hopefully another 30 years, I do have a few questions

-for 555 timer what power source should I use, is 9V battery safe for vu-meter ?

-voltage regulator transistor Q904 (2SC1826) has one corner slightly chipped, black paint flaked off look, and I measured 54C (129F) on its heat sink, any reason for concern?

-headphone out on my amp is connected via 2W 330ohm fusible resistors to speaker A selector switch, can any sonic improvements be made by changing fusible to metal film resistors and if yes would some other value be more appropriate for my high impedance (600ohms) headphones

-C545, C546 should I revert to tantalums as it was, I bought new replacements

[Mooly]

That's good

A 9 volt battery is perfect for the 555 but you need a series resistor to the meter to limit current. If the meter is (guessing) 100uA for full deflection then you need (R=V/I) around 90K (Use a preset pot ) That's a guess, the meter might be 100uA or nearer 1 ma so experiment.

No problem with chipped transisitor. The die is right in the centre on the metal tab. Temperature is fine too.

Headphone feed using series resistor is the common way of doing things. Absolutely no sonic benefit changing to a different type. Where you can experiment is with different values to match sensitivity of phones (if you feel it needs it). You could also experimint with a voltage divider rather than series resistor to match the impedance better (you would need higher wattage resistors though).

C545/546 should be good quality low leakage caps. I don't think they need be tants though. At 30 year old I think best with something newer fitted there.

[mkusan]

Originally Posted by Mooly

That's good

A 9 volt battery is perfect for the 555 but you need a series resistor to the meter to limit current. If the meter is (guessing) 100uA for full deflection then you need (R=V/I) around 90K (Use a preset pot ) That's a guess, the meter might be 100uA or nearer 1 ma so experiment.

No problem with chipped transisitor. The die is right in the centre on the metal tab. Temperature is fine too.

Headphone feed using series resistor is the common way of doing things. Absolutely no sonic benefit changing to a different type. Where you can experiment is with different values to match sensitivity of phones (if you feel it needs it). You could also experimint with a voltage divider rather than series resistor to match the impedance better (you would need higher wattage resistors though).

C545/546 should be good quality low leakage caps. I don't think they need be tants though. At 30 year old I think best with something newer fitted there.

I will have no problem sourcing a pot in 100K value for 555, headphone resistors and C545/546 I will leave as they are then.

Speaking of pot, original one will have to go, I was even ready to endure poor tracking between the channels but the rotation of it and the noise it produces at it (mechanical noise, not from the speakers) is too much for me to bear, I guess I am spoiled by TKD.

I was thinking about this:
1pc PCB for ALPS RK27 RK16 Potentiometer Tin-Spraying | eBay

either in combination with new Alps RK27 or my TKD (I think it will fit).
It seems to me that it would be beneficially in two ways. It would mimic the looks of the original pot which also has the pcb with soldering points (perhaps the pcb could serve as a noise shield, that is an assumption). Second, it would provide better soldering connection to the pot than those tiny leads alone.

Your thoughts?