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Parts for model A-4 [message #26468]

Tue, 16 January 2018 13:41

Jonomega3
Messages: 81
Registered: November 2012

Member

Im looking for original size filter caps for my 67 Kustom A-4 4500uf 50volt, 2.5" diameter x 3.5" height Anyone where to get these? Thanks jon

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Re: Parts for model A-4 [message #26470 is a reply to message #26468]

Tue, 16 January 2018 19:27

chicagobill
Messages: 2006
Registered: April 2003

Senior Member

Do you have a time machine?

Finding the same size and rating caps will be difficult. Modern caps are much smaller then old ones so you will find the correct rated parts will be much smaller in size than the original ones.

What is wrong with the ones in the amp now?

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Re: Parts for model A-4 [message #26473 is a reply to message #26468]

Wed, 17 January 2018 14:09

stevem
Messages: 4775
Registered: June 2004
Location: NY

Senior Member

About the best your gonna do these days will be with a 2" OD filter ( 3.125" tall ) so you will need to rap some cardboard and tape around it to the tune of 1/4".
Newark electronics stocks a Mallory brand cap ,part number 69K1896 .
It's a 50 volter rated for 65 volt surge and it's 6800 uf ,which is better that stock to keep the amp punchy and there less then 20 bucks each.

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Re: Parts for model A-4 [message #26474 is a reply to message #26470]

Mon, 22 January 2018 15:18

Jonomega3
Messages: 81
Registered: November 2012

Member

Thanks for the responses! and understood, no original sized caps.
chicagobill I suspect filter caps due to loss of clarity and volume, distortion at low volume, some snapping, popping background noise across both channels, all the functions work otherwise. Havent pulled the caps yet but I figure they are due since theyre 50 yrs old and thought its a good place to start. Should I be looking elsewhere?

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Re: Parts for model A-4 [message #26475 is a reply to message #26474]

Tue, 23 January 2018 06:16

stevem
Messages: 4775
Registered: June 2004
Location: NY

Senior Member

these two main filters usally hang in there really well , especially if the amp was not just sitting around and not being used for years!
It's mostly all the other small electrolytic caps on the circuit boards that go bad and some can be a real pain in the Butt to change out if the amp is fully loaded with all the effects and features.

One test you can make on your 2 main filters is with a volt meter, set it for AC volts and hook it across each filter and report back here with what you find.
These filters being bad will produce a 120 hz hum / tone out of the speakers, not popping and snapping !

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Re: Parts for model A-4 [message #26477 is a reply to message #26468]

Tue, 23 January 2018 16:03

chicagobill
Messages: 2006
Registered: April 2003

Senior Member

I don't see the large filter caps go bad from age very often. I think I have had to replace them maybe six times in the last 40 years and mostly due to shorted conditions not loss of capacitance or high ESR.

If the noise happens across both channels, I'd start by looking at the power amp circuit. I'd try and fix the problems first, then if you want to go back and change out the caps or whatever parts you want to change due to age.

I've stated it here before, these amps were built to very high standards when they were new, using the best parts available at the time and most of them have aged very well compared to a lot of other amps out there from that time period.

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Re: Parts for model A-4 [message #26480 is a reply to message #26477]

Wed, 24 January 2018 10:28

Jonomega3
Messages: 81
Registered: November 2012

Member

The amp may go a year or so without use, its not an amp used live that much mostly for recordings. In any case one of the lrg filter caps was leaking, Tried to post a pic of it but no luck. Anyway they were replaced and most of the problems were solved with that, still a slight bit of distortion is present at a volume where there should be none but volume is better and a lot of the background popping and crackling is gone. Agreed, power circuit should be looked at, its had some poor work done there prior to my owning it. Is there a short explanation of how to post images?

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Re: Parts for model A-4 [message #26481 is a reply to message #26480]

Wed, 24 January 2018 10:45

Jonomega3
Messages: 81
Registered: November 2012

Member

Another question Ive been meaning to ask, is there any harm using the A-4's preamp out without a speaker load?

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Re: Parts for model A-4 [message #26482 is a reply to message #26468]

Wed, 24 January 2018 12:00

stevem
Messages: 4775
Registered: June 2004
Location: NY

Senior Member

To post pictures here you need to have made 10 post.
Make 8 post that say " test" and I will delete them .
It's fine to use the amp with no speaker load , no harm will take place.

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Re: Parts for model A-4 [message #26491 is a reply to message #26468]

Wed, 24 January 2018 15:23

Jonomega3
Messages: 81
Registered: November 2012

Member

Yea, not sure what Im doing wrong with the images here but here is the link to the Caps, posted on FB [ https://www.facebook.com/photo.php?fbid=436423483455908& set=pb.100012647792396.-2207520000.1516820570.&type=3&am p;size=2016%2C1512]

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Re: Parts for model A-4 [message #26492 is a reply to message #26491]

Wed, 24 January 2018 16:56

Jonomega3
Messages: 81
Registered: November 2012

Member

Well, the filter caps didnt solve the distortion problem and loss of clarity. The distortion slowly comes back after playing through it for more than 5 minutes or so. On a good note changing the Filter caps did clear up the background noises "popping and crackling", so far lol

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Re: Parts for model A-4 [message #26495 is a reply to message #26468]

Thu, 25 January 2018 06:13

stevem
Messages: 4775
Registered: June 2004
Location: NY

Senior Member

Your distortion showing up is likely due to a bias issue in the output stage if it's taking place on both channels.

If you look at the center of the metal output Transistor mounting strip you will see a metal clip and in this metal clip should be a metal cased 2 leaded diode, is it there and clipped in without its leads shorted to anything?

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Re: Parts for model A-4 [message #26496 is a reply to message #26495]

Thu, 25 January 2018 09:48

Jonomega3
Messages: 81
Registered: November 2012

Member

Thanks stevem, yes the distortion happens in both channels but the distortion is not present when through the preamp out. Yes, the diode is there and tightly in its metal clip, leads do not appear to be shorted to anything

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Re: Parts for model A-4 [message #26497 is a reply to message #26468]

Fri, 26 January 2018 06:01

stevem
Messages: 4775
Registered: June 2004
Location: NY

Senior Member

At What volume level does the distortion take place at, low, medium or high?

When the distortion starts to kick in is the amps pilot lamp dimming a lot?

What impedance speaker load is the amp being driven into?

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Re: Parts for model A-4 [message #26498 is a reply to message #26497]

Fri, 26 January 2018 09:37

Jonomega3
Messages: 81
Registered: November 2012

Member

distortion starts at around 9 o'clock on up and doesn't hang as the note descends, so the distortion is mostly on the attack of note and compresses as you turn the volume up. Not sure that helps just trying be descriptive as I can

Pilot light comes on bright and does not dim at all.

Im using an 8 0hm cab, Ive tried a couple different cabs as well as a 4 ohm with no change

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Re: Parts for model A-4 [message #26499 is a reply to message #26468]

Fri, 26 January 2018 14:59

stevem
Messages: 4775
Registered: June 2004
Location: NY

Senior Member

Are you playing Bass thru the amp when it distorts, or guitar?

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Re: Parts for model A-4 [message #26500 is a reply to message #26499]

Fri, 26 January 2018 15:35

Jonomega3
Messages: 81
Registered: November 2012

Member

guitar only

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Re: Parts for model A-4 [message #26501 is a reply to message #26468]

Sat, 27 January 2018 06:13

stevem
Messages: 4775
Registered: June 2004
Location: NY

Senior Member

what do you have for electronic test equipment?
When you respond to this question please do so in the repair section since now that's where it should go, thanks in advance!

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