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HH Scott 299A channel imbalance

A

anti-doom

Well-Known Member
I have replaced rectifier, electrolytic caps (the two cans which close to the power transformer), coupling caps and some resistors on my Scott 299A. I have not replaced the 20/20/25/25 at 400/400/25/25 VDC yet.

The amp is working fine without hum, noise or hiss. However, it has channel imbalance which channel A is a little bit louder than channel B (I would say 9 on B and 10 on A).

I have swapped tubes, cleaned socket, cleaned switches, checked tubes.... but to no avail. I suspect the problem due to the capacitors at the tone control (the white ceramic 0.0068uf @ 400VDC four of them) but still need your help. So any recommendation or help would be appreciated.

Please help.

Thanks
 
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Did it have an imbalance before the re-cap?

I had to replace all those white capacitors in my LK-48 as they were all leaky after 40 some years.
 
Without knowing if they're was an imbalance before changing a few parts, your guess is as good as ours.

You really should perform a more in-depth rebuild since there's obviously an issue with something in there, somewhere.

If you still have the same issue afterwards, you can troubleshoot it knowing that the new parts have extended it's life for many years to come.

. . Falcon
 
Heya Doom,

Although those small ceramic/mica caps rarely go bad--doesn't mean they can't.

If there's a balance control, are you saying that in the neutral position of the pot you have an imbalance?

If so, could just be an inaccurate pot.

Do you know what your bias voltage is at the grid for each of your tubes? One of or both tubes in a channel with considerably lower than needed bias voltage could cause decreased output.

Best,

John
 
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You really need to replace that 20/20/25/25 cap.:yes: Someone had a thread here once, and actually found a replacement for that one (I think).
 
After 40 years of service, your volume control probably is the culprit. Clean it again with deoxit and forget about it...just use the balance control. Also make sure you clean all of those pesky slide switches- they can also cause issues like this.

I think Scott put a bigger priority on cosmetics than on high quality switches as pots.....
 
Another thing to check:

I had a similar channel imbalance after I rebuilt my Scott LK-72A. The culprit was simply that the resistors weren't matched enough (target is 1% matched) on each side.

I had a differential of 1VAC in favor of the right channel (measured at the speakers screws and input grids of the power tubes). I had to turn the balance control at 9 o'clock to achieve almost perfect balance...

Since I measured and replaced a lot of resistors (particularly in the tone control network and loudness/volume section), it's now between .02 - .05 VAC, always in favor of right side. This slight difference is probably caused by a little difference in the volume pot... Not a big deal now and it can't be heard anyway.

Once in there, you could anyway replace the tone control caps, just to be sure.

I had invaluable help from the member seta1 on this particular case...
 
Channel B has less bass and treble than channel A. I balance them by turning bass/treble knobs to +1 on channel B, then they match perfectly, and music sounds incredible. But I still want to figure out the problem and fix it. That is why we are here to help each others.

Couple more question about my Scott 299A

1. Do I need to replace 4-section can capacitor 75/75/57/75 @ 75VDC. I heard it rarely fail but want to make sure?

2. There are 4 capacitors value 0.0068uF@400VDC at tone control (bass/treble knobs) --> do I need to replace them all or just leave them there?

Any idea/recomendation would be appreciated.

Thanks.
 
Doom,

If memory serves from a few Scott 299 series restorations--I think that multi-section cap provides filtered DC for bias. I also read they rarely fail compared to the larger caps, but since I was restoring, I replaced it.

If your bias on the output tubes is the same for all four and within spec, the plate voltages are similar, you might check screen grid resistors...

It may also be an issue on the first gain stage. Do you notice the imbalance both when listening to a CD player AND an LP? Or just one and not the other?

Line and phono preamp use different tubes. If you hear a difference between one or the other that would rule out the output section and one preamp section as well as tone controls.

Best,

John
 
anti-doom said:
1. Do I need to replace 4-section can capacitor 75/75/57/75 @ 75VDC. I heard it rarely fail but want to make sure?

2. There are 4 capacitors value 0.0068uF@400VDC at tone control (bass/treble knobs) --> do I need to replace them all or just leave them there?

Any idea/recomendation would be appreciated.

Thanks.
Click to expand...

Replace the 0.0068uf caps. They are easy to get to and do (IIRC they are on one of the slide switches)

The 4 section can cap- I replaced mine with axials under the chassis and left the can. Maybe you don't have to....but if you are going through all the trouble of redoing the unit, it's not that much extra work. Space is tight but manageable if you don't go crazy upping the spec.
 
Thank you for all reply. I will be doing it tomorrow and post the result.

BTW, channel imbalance appears at all input, does not matter LP or TAPE or TUNER input.
 
heekma said:
It may also be an issue on the first gain stage. Do you notice the imbalance both when listening to a CD player AND an LP? Or just one and not the other?

Line and phono preamp use different tubes. If you hear a difference between one or the other that would rule out the output section and one preamp section as well as tone controls.

Best,

John
Click to expand...

anti-doom said:
Thank you for all reply. I will be doing it tomorrow and post the result.

BTW, channel imbalance appears at all input, does not matter LP or TAPE or TUNER input.
Click to expand...

Maybe somewhere in the tone control section, a bad pot or something in the output section too (resistors...?).
 
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