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Interesting A-4 head [message #28432]

Tue, 01 February 2022 01:14

JDinPA17603
Messages: 69
Registered: May 2018
Location: Lancaster, PA

Member

Friend got an A4 amp the other day and wanted to know the values on some of the caps on the boards...wanted to replace the white mallory caps and a couple that were showing signs of cracking. So I had him get me his board numbers and send me a pic of the layout. Hmmm... no driver transformer. Board complement in the beast...
102 preamps (2)
302A/B reverb/tremolo/vibrato
402A/B Harmonic clipper/Selective boost
702 driver
502 plus 23 volt regulator
602 minus 23 volt regulator
When I checked the model number it says A4 but the serial number indicated it was built at the tail end of the A series and start of the B series.
Only issue is .47 volts DC across the speaker terminals and a slight amount of 120 hz hum.
He wants to change out the Mallory beasties and the filter caps to try and address the hum. The Mallory caps on the 702 are showing degradation along with the filters on the 502 and 602 regulator boards.
Mainly posted here in comments because of the odd board lineup.
If I need help with the DC offset I will open a post in Repair.
Have a great day, and it is good to see the board still up.

Old Us Navy Vietnam Vet
Playing for my own enjoyment and irritation of the neighbors.

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Re: Interesting A-4 head [message #28433 is a reply to message #28432]

Tue, 01 February 2022 06:29

stevem
Messages: 4773
Registered: June 2004
Location: NY

Senior Member

That's definitely a transitional amp between the second A design and the B series with that 502 and 602 board!

I have never seen such a Kustom in all the ones I have worked on!

It must be the in between step before they put both preamp regulators on the later 703 board.

If the filters don't help with that + .47 volts on the output jack then it might simply be mismatched output Transistors, or drifting resistors on the output Transistors shifting the bias on one or more of them.

Have any been changed?

Keep us posted!

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Re: Interesting A-4 head [message #28434 is a reply to message #28433]

Tue, 01 February 2022 13:39

JDinPA17603
Messages: 69
Registered: May 2018
Location: Lancaster, PA

Member

Steve, I had him recheck the DC and it is holding at 0.075 volts. Cleaned up and reseated the thermal diode and added some heat sink grease. Going to change out the remaining Mallory caps ( only 4 in the rig) and replacing the filter caps on the 502 and 602 boards that are cracked. Parts should be in by mid to late February.
By the way what are you using to replace the tantalum tear drops with? Just Aluminum electrolytic or something else? Can yoou point me at a reasonable source? Thanks.
John

Old Us Navy Vietnam Vet
Playing for my own enjoyment and irritation of the neighbors.

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Re: Interesting A-4 head [message #28435 is a reply to message #28432]

Wed, 02 February 2022 07:19

stevem
Messages: 4773
Registered: June 2004
Location: NY

Senior Member

I just replaced those with new electrolytics since there not in the audio stream anyway!
I use axial types and just stand them on end .

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