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-   -   Fisher 101D (http://www.diyaudio.com/forums/tubes-valves/195521-fisher-101d.html)

eeplok 28th August 2011 11:25 PM

Fisher 101D
 
Hi every body,
I have this Fisher 101D for a while and I love the sound. While we listen music one evening and the sound pop from the amp and everything quiet.......the fuse blow .Next day buy new fuses box of five (2 Amp the same as the old one) put it in, turn power on and pop again. I try 3 times ( pull all tubes out) but it's keeping blow fuse.
Any Idea why ? What should I look ?

thaumaturge 29th August 2011 12:44 AM

If you feel comfortable opening it up and taking some measurements, go straight to any big filter capacitors. Check them power off for low resistance. My best guess.
Doc

HollowState 29th August 2011 12:49 AM

Assuming your Fisher 101D is similar to the 101C, there is a solid state voltage doubler in use to power the amplifier. Since the fuse pops even when the tubes are removed, the most likely failure is a shorted diode. Get your multimeter out and start checking the diodes in the power supply. These will be very near the secondary wires that come out of the power transformer. And while you're in there, also check the electrolytic capacitors associated with the doubler and subsequent filter. Don't have a multimeter? Go buy one if you want to do-it-yourself.

Also, instead of wasting fuses, connect a 60 or 100 watt incandescent light bulb in it's place. If the bulb lights brightly, you still have a short. When things are correct, it will hardly glow at all.

eeplok 30th August 2011 11:38 AM

I'm very handy man for car, house hold and woodwork, but I just very dummy on electronic. I do have multimeter ,how you measure those caps, what its look like ? I will take pictures and post later.
I bought this amp on eBay because I love the sound and try to put my hands on if something went wrong.

thaumaturge 31st August 2011 04:25 AM

1. Make sure the amp has been off and unplugged at least 24 hours.
2. The round things that stick out the top that aren't tubes are caps (electrolytic capacitors).
3. Take off bottom.
4. On underside of the round things that aren't tubes will be some terminals. Those on the outside are the negative terminals. Those inside circle of outside are positive terminals. With tubes unplugged one of those caps might read pretty much a dead short between one of the positive and negative terminals. Find an exact replacement if you can or refer back here for possible substitutes. A good cap should slowly climb in resistance when measured to basically over range on your ohms scale.
5. If one is bad, odds are the others are due for replacement as well. Old electrolytic caps dry out and lose efficiency even if they don't short outright.

Be aware that high voltages may still exist on the big caps or other small ones even after the circuit has been removed from power for 24 hours. Start measurements on high DC voltage range and carefully measure for presence of voltage before making resistance readings. I once had a bleeder resistor open up on a high power transmitter and lost about 3/8" off the end of a heavy aluminum hotstick (safety shorting bar). Extra caution with high voltage is my only devout religion.

Also keep track of exactly which tubues you remove from where. They need to get put back in same place. I seem to recall some of the Fishers had balance pots but no meters. Don't adjust anything without knowing what you're doing.

Doc


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