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Scott 130 Preamp Sound Issues - Are Ceracaps to Blame?

BrianC7927

BrianC7927

Active Member
Over the past two years I have had intermittent sounds issues on my Scott 130 preamp that have become so frequent now the unit is un-listenable. Following is a description of the issues, feedback from another AK member, and pics of the unit. At this point I want to diagnose the issue, then either I or a qualified tech can do the work. I've brought it in before but they are unable to locate the issue. Any feedback or assistance from the member community would be greatly appreciated. - Brian

Sound Issue - I have already cleaned all the sockets, switches, ports. When i start the unit usually it sounds fine, balanced b/t left/right and with normal tone. After an undetermined amount of time (it has gotten worse lately and happens during each side of an album) the right channel gets lower in volume and clarity and the left gets higher and get piercing on the highs. If i flip the phase switch on the 130, the right channel goes much louder and left much quieter. If i turn the unit on and off quickly (bad i know), it resolves the issue sometimes. Much of the time when listening the sound is pretty balanced b/t the two channels, but i hear piercing on the left channel. I can here the piercing go in and out intermittently. I have tried switching the speakers black/red if it was a phase issue and it did seem to reset it, but it went back out 2 mins later.

From Patrick_W
Have you had the 130 recapped?

I am no expert in Scott amps and preamps. There are many others here on this forum who know way more than me. However, I have had 3 different Scott components, a 299 D, a 233, and an LC-21 which is the kit version of the 130 preamp. In my experience with those units all the signal tubes tested good. Now they may have been treated conservatively their whole life, but I think that the Scott is generally fairly gentle on the pre amp tubes.

Talking with one well-respected AK tech about the Scott LC-21, I was told that the caps can fail in these. Scott used a lot of Ceracaps which are stable ceramic caps, until they are not. They do not fail by going out of spec, but by going intermittent. This tech explained to me that he had fixed a Scott that had previously been "restored" but the owner was being driven crazy because his Scott would suddenly go overly loud or soft in one channel at times and also change timbre when it did. What had not been changed out in the previous restoration was the caps that are connected to the front panel controls. One of these started going intermittent. Once the caps on the front panel were changed, the problem disappeared. These are hard to get to and change out and thus are often left as is, because it is assumed the Ceracaps do not go out of spec. In fact they can even measure in spec and still be going intermittent. This sounds similar to what you describe.
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I just cleaned up a Citation I preamp for a friend. I initially put it on a scope and got terrible readings from one channel. The same suggestion was made to me, and cleaning the volume pot and contour switches with DeOxit resolved the problem. There was still a slight difference in channels but I think drifting components accounted for that. So try cleaning all pots and switches.
 
Do you work on these things yourself? Basically, this is all it can be aside from needing replacement. Maybe when it acts up again, try gently wiggling the tubes in their sockets and flipping, pushing, or turning the pots and switches and see if it doesn't help.

Resistors, capacitors, and tubes USUALLY don't act intermittent but it could be possible but in my experience, it is usually something else pretty simple. Thinking as I type, it could also be a loose connection or cold solder joint. Could also be bad phono interconnects. Try wiggling them when it acts up again.
 
Unfortunately all of this discussion was already covered in the other thread. I can't actually recall if those ceracaps are still in mine, I need to look at some point. Which caps on the schematic are they?
 
my understanding of those Ceracaps is they are film, but can fail mechanically. The lead separates from the foil inside and it can make no or intermittent connections. Seems to me that lightly tapping the caps would prove that out.
 
gadget73 said:
my understanding of those Ceracaps is they are film, but can fail mechanically. The lead separates from the foil inside and it can make no or intermittent connections. Seems to me that lightly tapping the caps would prove that out.
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Thanks i will try
 
cademan said:
Do you work on these things yourself? Basically, this is all it can be aside from needing replacement. Maybe when it acts up again, try gently wiggling the tubes in their sockets and flipping, pushing, or turning the pots and switches and see if it doesn't help.

Resistors, capacitors, and tubes USUALLY don't act intermittent but it could be possible but in my experience, it is usually something else pretty simple. Thinking as I type, it could also be a loose connection or cold solder joint. Could also be bad phono interconnects. Try wiggling them when it acts up again.
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Thanks good points, I will try that, it did work on my Dynaco ST70 when i had problems.
 
Just a guess, but possibly a carbon comp resistor is increasing resistance when it warms up. Usually the offending resistor measures off value cold, usually...
 
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