VJr Sudden Power Loss - Need Help

[Auherre756]

Well folks, I can't seem to resolve my problem: When my VJr head's volume is pushed to 'round 2 o'clock, it has a sudden loss of power leaving the amp all but useless at any volume setting -- distorted lo-fi. Now, based on past recommendations received from a number of very knowledgeable individuals at this forum=d> , here're some of the things I've done: 1. changed tubes, 2. reflowed solder connections, and 3. switched out R10 and R14 resistors that brought the dissipation down to 12W (it was running a lot hotter than that before I switched out the R14 and R10 220Rs with 240R-2W and 1KR-3W resistors, respectively. With these changes, the amp sounded oh so sweet. Then, as mentioned earlier, I tweaked the dial up to about 2:00 and POOF here we go again. Back to the lo-fi ghetto.

 

Somewhere, I read that I might have a faulty bypass cap. First, which cap(s) are bypass caps? Second, how do I test the functionality of these caps (my multimeter doesn't have a capacitance function)? Do I test the voltage drop across the caps? If so, what should the voltage readings be? Maybe there's a more basic question should be asking...am I chasing my tail? All suggestions are welcome. I miss playin' my VJr!

 

Thanks.

Auherre

[lpfan]

I would suggest a repair shop. It would probably be a lot quicker and easier.

[Eracer_Team]

if you need to you can remove C3&c4 bypass caps' but doesn't sound like you issue.

Don't forget the orange fuse holders need to have their contacts cleaned from time to time too.. Just open the fuse holders and push the white pastic back a little and scrub with a screwdriver tip of a little sand paper (remember power off).

 

also did you replace the volume pot or add a gain pot on R7?

I never did it but I have read that a number of the guys had a stray strand of wire that they really couldn't see shorting out something.

 

Also watch out for lifted pads on the board. too much heat or you pulling on a part or wire could damage the board. Just cause you reflowed the solder doesn't mean the pad isn't broken away from the rest of the 'land'.

You may need to meter the connection for continuity

[Auherre756]

Thanks for the info, E_T. It can't hurt to clean off the fuse contacts (and pin contacts) again. I'll also perform a continuity check. Interestingly, while doing some component checks under power, I found R11 (ref value 220K) to show erratic readings, then indicated no resistance at all. Whether this is THE cause of the problem or a result of the problem I'm not sure. As I continue to get more experience under my belt with this stuff, issues like this will likely become easier to understand. But, until then...

 

I'll be replacing bad components and performing continuity checks.

 

Cheers,

[Eracer_Team]

remember that R11 is the drain resistor for the B+ filter caps. so what you're seeing is the charge/discharge cycle of the caps.

only true way to test a resistor is to remove it from the circuit.

When I say check the lands, you're really not checking across something; just the point a to point b type of thing.