VintageKustom.com: Repairing Kustom Amps » Tantalum Capacitors sub w/ Aluminum Electrolytics?

Re: Tantalum Capacitors sub w/ Aluminum Electrolytics? [message #26437 is a reply to message #26435]

Wed, 13 December 2017 12:14

polaris26
Messages: 6
Registered: December 2017
Location: PA, USA

Junior Member

Hello chicagobill,

A little history about this amp - Recently, I was approached by a friend of mine who has a co-worker who wanted someone to look at this amp to see if it could be put into working order. I was not entirely sure why he wanted it fixed or what he wanted it for. In fact, it was originally told to me that it was a guitar amp, but after I got it, it became clear that it was designed to be a PA head.

After having recently had success with a repair on a somewhat similar amp, I decided to take on the project. I always am careful to take plenty of pictures and notes as I examine something I am working on for someone else, both for my own convenience and to protect myself from potential liabilities. I document any visible/cosmetic issues as well as any electrical issues as I come upon them. The work goes in spurts because my free time is limited.

In this case I just got the chassis out of the wood case, got the reverb 'subframe' out (there was a spun nutsert/pemnut that I had to deal with), and started looking the innards over for signs of catastrophic failure or previous repair work. I have no idea about the usage history of the amp but it showed signs of storage in less-than-idea conditions, with dinge and corrosion here and there.

Having not seen any signs of obvious disaster, I brought the amp up from essentially 0 VAC with the variac. I believe the bulb was actually not part of the setup at this time, so I was actually running straight from the Variac while monitoring the voltages on the main caps. I don't know how long it's been since they've been at working voltage, so I bring them up very slowly. As I was doing this, there was a DC offset on the speaker jacks that grew larger in proportion to the main rails. I understand that with DC-feedback designs, you need to be at a minimum working voltage on the circuit for the voltage offsets to come into range due to some devices in the loop not being biased properly at very low voltages. In this case, the offset just kept getting bigger as I brought up the rails. The offset never hit the rail voltage, being always some fraction of it. I didn't leave the amp powered on in this condition very long but again there was no signs of smoke or drama.

I didn't put much more time into the diagnostic process at that point, because I had not gotten a firm commitment from the "customer" regarding what he wanted done, and how much he was willing to spend. I felt confident that I could resolve the issues with the power amp board for a certain cost and didn't want to put any more of my time into the amp until I got a more firm idea of what the owner expected from this repair, and what he was willing to spend. I set the project aside and began working on some other projects I have in the 'queue'.

In the mean time, I was taking a cursory look at the board and the schematics and noticed the use of the tantalum caps, which prompted me to pose the original question, since I didn't think it was necessary to use tantalum caps in such a design, but just wanted to check with the more Kustom-aware techs regarding thoughts on replacing them.

It seems the consensus is that there is little chance of the caps being bad and wholesale replacement is not necessary. I will take that into account while trying to separate the desire I have to see this thing leave with shiny new parts stuffed into it (probably my OCD) from the practical approach to just getting the thing fixed at a reasonable cost.

One thing I should note is that I am also desirous to replace the line cord with a three-pronger. I hope that's not too controversial, but I really don't see using two prong cords in the 21st century. But that's a topic for another thread...

Regards,
Dave

Report message to a moderator