Sansui 2000a F-1180 - Two unpopulated Capacitors and Trimmer Question

[akadrhouse]

Good evening,
I was rebuilding the driver.amp board (F1180) on my Sansui 2000a and I noticed that C810 and C820 were never populated. No solder old or new in back and just empty holes. See Yellow Circles. The service manual says these are a ceramic 68pf/50v capacitor.

1) Should I populate?
2) Would either of these be acceptable? (Choice One or Choice Two). Are there better choices?

Unrelated. If I were to replace VR802/804 with new trimmers (large objects below the yellow circles), they are quite large, and the normal 25 turn bourns are too small. These are rated at 1k Ohm.

1) The service manual doesn't list wattage. Is the standard 1/2 watt the right choice?
2) I couldn't find a replacement at the right size. What do some of the restorers here use?

I would be fine not replacing, but I have some serious hum in this unit despite rebuilding it with all new electrolytics and replacing the 2SC871/2SC458 with KSC1845. I've done the service manual repair with the ground, but it didn't make it better or worse. Neither did the new electrolytics/transistors mentioned above.

With the pre-amp/amp jumpers pulled, it is dead silent. So I figured I would replace the VR trimmers here in the amp board , and also start checking resistor values. I was going to work on the F1253EQ board resistors next. I'm open to other ideas.

Thanks in advance!

 

[AnalogueBen]

I'm not sure for the large trimmers, I left mine and hadn't seen what ppl are using, which is a little surprising for such a popular model so hopefully someone has found a goid sub. 1/2 Watt will be fine. Mine also is missing C810/C820 and normally you would just replace what you see. Hums can vary from unit to unit, try tightening earthing screws etc or perhaps you have a scope to diagnose the hum. Good luck

 

[larryderouin]

I've done 4 or 5 2000A's(the last 4-5 years ago) and haven't yet come across a populated C810 or 820. Trimmers use a 1/2w. I can't remember what series Bourns' I used. But they were all side control.

Most likely the noise is the 2sc458's in the tone control amp, or the EQ board. Or you installed a transistor / cap wrong. If it's in both channels it could be the power supply (Suggest you rebuild both the P.S. and the diode board 1st).

 

[akadrhouse]

Thanks @larryderouin and @AnalogueBen .

I will leave C810 and C820 unpopulated. I've already replaced the 2SC458s through the unit so it isn't those. I typically find that those will give a popcore crackling instead of a fixed hum when they are bad, but they have been replaced none the less.

I do have a scope, and a signal generator but was unsure how to use a scope to diagnose this. Based on the frequency of the hum, it seems like it is probably a ground found hum, but I'm new to this.

With regard to the 1/2 watt bourns trimmers, these are about twice the distance for the 3 legs, and the normal ones don't fit. I've not found a larger trimmer that would.

 

[AE Bob]

C810 and C820 are in the negative feedback part of the amplifier. They would increase negative feedback at very high (RF) frequencies. This would prevent high (RF) frequency oscillation in the amplifier. The 2SD202 (low frequency) output transistors likely help keep the frequency response down to a reasonable audio range. If the transistors were replaced with 2SC devices, the capacitors would likely be necessary to prevent oscillation. I remember many years ago installing the missing capacitors on some on F-1040 boards in a Sansui 5000, after repairing both output stages using ECG 130 replacement output transistors (decades ago when I used ECG semiconductors). I couldn't get the amps to stop oscillating until I installed 50 pF capacitors in similar locations (un-installed capacitor locations on the boards), although the 5000 schematic didn't show the capacitors at all.

 

[AnalogueBen]

With regard to the 1/2 watt bourns trimmers, these are about twice the distance for the 3 legs, and the normal ones don't fit. I've not found a larger trimmer that would.

Nor have I found a fit for those large trimmers, I once modified a Bourns 3296 with longer outer leads to fit, it was tricky but it can be done.
For my 2000A I removed the originals and used deoxit and a fine brush to get then clean and like new again, then a coat of the deoxit Gold protection. I tested them on my meter and they work full range fine so I'm confident to reuse them.
I'm also new to scope testing but think It's a matter of following the circuit and finding the point where the distortion starts, if that makes sense. Hope someone can give a little better explanation than myself.. Best.