Fisher 450T fails dim bulb test after warm-up

[satellite]

I tried this twice with the same results(definition of insanity?).

First time right after I got it I plugged it in to the DBT, the bulb went bright and then dim and all seemed to be going well. Was getting sound from both channels, was fiddling with the various functions when....Wham!...the bulb went full bright and got a terrible hum through the test speakers.

A couple weeks later I gave the receiver and its controls a good cleaning and visual inspection. No obvious signs of trauma or physically leaking caps, or anything that stood out, so I decided to try it again. This time it took quite a bit longer. I was actually listening to music starting to dig it, thinking the first time might have been a fluke, when Wham!...it happened again.

So, where to start looking? What components would fail short after warm-up?

 

[awillia6]

That's usually something thermal. Could be at the component level (i.e. runaway transistor) or physical problem due to expansion (cracked tracked, bad solder joint, etc.) Get out your freeze spray and go to it. Good luck.

 

[Tom B]

DBT is used typically for testing, but I don't really use it long term, because you are typically running the unit "under voltage" and some units don't like that. I typically use a DBT tester to initially fire up a unit after repair, and do a quick check of DC bias and DC balance. It they look OK or I can adjust them OK, I then plug into mains power and reset the DC bias and DC balance (which are typically different than the setting done on dim bulb because of the "under voltage".
Have you ever just plugged it in?

 

[awillia6]

DBT is used typically for testing, but I don't really use it long term, because you are typically running the unit "under voltage" and some units don't like that. I typically use a DBT tester to initially fire up a unit after repair, and do a quick check of DC bias and DC balance.

DBT can definitely bite you in the ass. The B&K gear I've been working on won't play nicely with DBT's current limitation. The units simply aren't getting enough juice to fire up all their circuitry. Direct mains connection and they fire right up, oops!, make that turn right on.

 

[satellite]

Have you ever just plugged it in?

No I haven't. I was hesitant to do so after what happened the first time. What you're saying seems logical to me. Maybe I over used the DBT and the unit just couldn't stand being operated in an "under voltage" state?