[Kenwood] TL-922A blowing fuse, May have found the problem???

[Kenneth G. Gordon]

On 16 Jun 2010 at 9:12, Dutch Maurer WB7DYW wrote:

> Well gang here is the latest on the unfolding saga of troubleshooting
> the TL-922A, I removed the pack of 200uf @ 500V caps

According to the manual, those are supposed to be 200 uF @ 550 VDC 
SURGE, which pretty much accurately describes modern electrolyitcs of 
450 VDC WORKING voltage.

> and measured them
> on a cap meter, I got readings all the way from 256uf to "0" uf so
> back to the trusty Simpson 260 and measured them with it, again I got
> a lot of different measurements but there was still that one that
> measured "0"ohms on the RX-1 scale, I believe it may be VERY
> bad????

That indicates a shorted....something.... Either the cap itself or a resistor 
across it, or a shorted diode.

In any case, this would NOT cause your problems with a blown fuse. All 
that would do is make the voltage across the remaining caps and/or 
diodes a bit higher.

Of course, the bad one should be replaced. (In fact, I would replace ALL 
of them), but the shorted whatever is NOT causing the blown fuse.

> Now on to a source to replace them with, any suggestions???

Yes. I buy good 450 VDC electrolytics from Mouser or Newark or Digikey 
or off eBay. Nichicons are excellent, as are Rubycons (unless you get a 
counterfeit).

As far as capacitance values are concerned, you may do what you wish. 

However, remember several things: 1) the value tolerance of the older 
caps was often +100%, - 20 to 80%. In other words, the values are not 
terribly critical (!), and 2) due to the much HIGHER internal resistance of 
modern caps, you can increase the values considerably with no adverse 
effect on the amp's power supply. 

In fact, after doing some experimentation recently, I determined that one 
could increase the values of those filter caps by from 100% to 500% with 
no adverse effect at all.

In my own SB-200s (I have two) I have installed 470 uFd caps in one and 
560 mFd caps in the other. The original caps were 125 uFd.

The advantage to doing this is that the DYNAMIC regulation of the power 
supply is vastly improved, reducing distortion at the amp, and other good 
things.

I have NOT had to add a "soft start" circuit to the power supplies either.

Also, modern electrolytics are drastically smaller than the originals.

In any case, I am about 90% certain that the original problem you are 
having with blowing the left fuse has to do with a problem with the wiring 
at the INPUT to the transformer, and neither the transformer nor the 
power supply filter are causing this.

Ken Gordon W7EKB