scott 299a bias caps replacement confusion

muscmp

Active Member
i have a hum that has developed in my scott. it used to be that i could flip the phase switch and it would go away but no longer. it has been indicated on this forum that it would probably not be the switch causing the hum. all electrolytics except for the bias caps have been replaced so i plan on replacing them.

on the side of the two caps it is indicated that there are two 100mfd, 75v and one 10mfd, 75 volt. however, the schematic shows c5 having two 100mfd with no indication of the 10mfd, while c6 shows three 100mfd with no indication of the 10mfd.

i'm enclosing the schematic as well as a blowup of the rectifier area.

thanks,
mikeB
 

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Does the 10 uf looks like a stock part or an add on by some tech? Either way an additional 10uf of filtering on the bias voltage supply circuit is not going to make much of a difference. Some guys like to bypass the electros with a film cap but that is not really necessary for the bias supply. Leave it in or take it out, there would not be any difference if it is just additional capacitance for the supply.
 
both are cap cans that appear stock. i was referring to the markings on the side of each. i would most likely replace with axials, however, if that is six axials to replace the two cap cans, i'm going to have to find some room. the confusing part is that the schematic doesn't match the side markings of the cap cans so i don't know how many of what i need. thanks,
 
Don't go by the schematic. A lot of times they are wrong. Go by the actual parts installed.
 
Don't go by the schematic. A lot of times they are wrong. Go by the actual parts installed.

great, thanks,

it is easy to find 100uf, 100 volt, but, difficult finding 10uf with 75 volt. i can easy find 50volt and 450volt(way too large sized). any recommendations where to get 10uf with about 75-100volt? i normally shop amplified parts/antique electronics.
thanks,
 
You can also go up in capacitance but don't go gung ho on values. Engineers designed them with at least 10% tolerences.
 
You've probably already checked, but it is possible that 10uf is either not in use or just tied to one of the 100uf in parallel (110uf would still be within tolerance).
In either of these cases, you could leave it out without issue.
If it's going someplace not indicated on the schematic, then you probably want to keep it.
 
Muscmp, how are you on reading schematics? Usually those multi-section electrolytic can capacitors are indicated as A, B, C, or D on the schematic and are not necessarily close to each other.
 
bias caps.jpg
You've probably already checked, but it is possible that 10uf is either not in use or just tied to one of the 100uf in parallel (110uf would still be within tolerance).
In either of these cases, you could leave it out without issue.
If it's going someplace not indicated on the schematic, then you probably want to keep it.

i'm enclosing a pic. you are right, the top bias cap can shows that the 10 is jumpered to the 100, while the bottom one is wired from the bias resistor network and goes to the bias pot. i've found i have four 100uf, 100 volt and one 10uf, 160volt. so i could use the new 10uf in place on the bottom cap where that wiring goes from the resistors to the bias pot and eliminate the 10uf for the top cap can. how does the 10uf, 25 volt perched above come into play? that isn't shown in the schematic either. should it be replaced also? thanks,
 
If its in one of the cans, a 10uF @ 25 volt cap is usually the cathode bypass and are usually indicated as "D" on the schematic. EG: C1D

I believe that all the capacitors are being used so you probably shouldn't be removing any of them.

I can't blow up the pics with the old dinosaur computer I am using at the moment. :(
 
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Muscmp, how are you on reading schematics? Usually those multi-section electrolytic can capacitors are indicated as A, B, C, or D on the schematic and are not necessarily close to each other.

hobbyist so i am not trained but i have repaired a few dynaco st70s, sansui 200sax, and about 20 guitar amps.
i do know that these are cap cans C5 and C6 in the schematic. C5a, C5b, C6a, b and c in the schematic. no indication of C5c.
 
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i wanted to double check that this 1046, 10uf,75v cap is red color=positive. i'm replacing it so wanted to make sure. thanks,
bias caps.jpg
 
i wanted to double check that this 1046, 10uf,75v cap is red color=positive. i'm replacing it so wanted to make sure. thanks,
Right you need to make sure that the positive end of the cap is grounded if this cap is part of the bias circuit. Electrolytics are always marked + and - on the terminals.
 
primosounds: thanks. that's the strange thing about this cap. it isn't on the schematic and the positive end doesn't go to ground. there are no markings so i initially thought it was non polar. i don't want to blow up the replacement.
 
i have a hum that has developed in my scott. it used to be that i could flip the phase switch and it would go away but no longer. it has been indicated on this forum that it would probably not be the switch causing the hum. all electrolytics except for the bias caps have been replaced so i plan on replacing them.

on the side of the two caps it is indicated that there are two 100mfd, 75v and one 10mfd, 75 volt. however, the schematic shows c5 having two 100mfd with no indication of the 10mfd, while c6 shows three 100mfd with no indication of the 10mfd.

i'm enclosing the schematic as well as a blowup of the rectifier area.

thanks,
mikeB
You should ALWAYS replace the bias caps. They are critical to the proper functioning of your amplifier. If they go, the power tubes could go, and take an output trans. with it.
 
You should ALWAYS replace the bias caps. They are critical to the proper functioning of your amplifier. If they go, the power tubes could go, and take an output trans. with it.
that is what i am doing-replacing the bias caps. however, even tho this 10uf75v cap appears original, it doesn't appear in the schematic and i'm being cautious about properly replacing it. it is the last piece and then i can check out the unit.
thanks,
 
that is what i am doing-replacing the bias caps. however, even tho this 10uf75v cap appears original, it doesn't appear in the schematic and i'm being cautious about properly replacing it. it is the last piece and then i can check out the unit.
thanks,
Someone might have just stuck that in there as a test or some kind of fix. If its not in the Scott schematic, I would leave it out.
 
Why would you debate the original HH Scott schematic? I find them to be accurate.

Dude, I'm not debating the Scott schematics as I own an LK-48 along with an accurate schematic. My experience, along with hundreds of people here, know that there are a lot with errors with schematics in general, even with Scott schematics.

There's even a recent thread going on now about a person with a 299d who says his schematic doesn't match his unit. I will always debate schematics, even Scotts.

EDIT: Actually it is the 399.
http://audiokarma.org/forums/index.php?threads/hh-scott-399-help-needed.823492/

I've also been a repair tech for over 40 years so what else have ya got? :whip:
 
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Someone might have just stuck that in there as a test or some kind of fix. If its not in the Scott schematic, I would leave it out.

It could have been a production change. They do it all the time. To the OP, I think I
would leave it be for now until you get more experienced.
 
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