Fisher Amp getting 56v output to speaker

[rustybeast]

Hi everyone. I recently was given an older fisher CA-65 amp and tuner. It had no sound from the left channel. After checking to make sure the amp was the problem I removed the cover and found the fuse blown. I replaced the fuse and got a loud POP from my speaker and a blown fuse. I replaced the fuse again and unplugged the speaker before turning it on. Checking with my volt meter I have found I have -56v at the left speaker terminal. Any suggestions on where to start? I am assuming one of the components is shorted and putting voltage there. I have Unhooked the A-B speaker switch and volume slider boards and confirmed still getting -56v output on the main board. I have begun to trace back from the output and have been removing each component and testing it but have not found anything yet. (starting with the transistors) Just thought I would try on here and see if anyone had any ideas or maybe a schematic for this thing. I have a fair knowledge of electronic components, what they do, and ho to test them. Thanks for any suggestions.

 

[Nigel Goodwin]

The power amplifier is blown - as a minimum the output transistors will need replacing, but it's VERY likely other components have been damaged as well - unless you change all the faulty components (and some may be intermittent!) the amp will blow again.

 

[rustybeast]

Thats where i'm at

Just going from here to there testing every component. I have some good power transistors for it, So as soon as I find the source of the problem I will replace them. ( Although they test good when removed I will replace them anyway.)

 

[Nigel Goodwin]

Individually testing components isn't really the way to do repairs, it's rarely effective.

Testing voltages is more helpful, do you have a circuit of the amp?.

 

[rustybeast]

No, I have been searching online with no luck for a schematic. Found a couple of service manuals for sale but was hoping to find a pdf so I don't have to wait for shipping.

 

[KeepItSimpleStupid]

-56 V is a good indication that an output transistor is bad. The problem is they usually take out other transistors in the process. The bias typically goes awry and starts taking out semiconductors. You will usually find small value resistoes, on the order 200-300 ohms in the output circuit. These may change value and need to be replaced. The bias will have to be reset.

 

[unclejed613]

it could also be a problem at the input stage, specifically with the DC blocking cap on the inverting input of the diff amp, or one of the diff amp transistors shorted. a missing supply rail at the diff amp or voltage amp (or both) could also cause this (and the output devices would possibly still be ok).

 

[leal]

fisher amp

Have a M# perhaps I could help. I use to work for fisher early 90s. I will do my best to help.

 

[Tann-44]

just to add...if one channel is ok use it to take readings to aid repair...this is how we used to do things....a balanced circuit will show low offset voltage in mv volts...looking at the post it's gone high to on of the power rail's.

At this point do not hook a speaker load or it will get fried!! Most of us use dummy load's such as high wattage resistors say 8 ohms connected to the o/p to test an amplifier...

there maybe a way to cut the feed to the faulty side via removal of say fuse's and or feeds..some tracing will need to be done..common faults are small long tail pair's followed by driver transistors bias circuit o/p's check for open resistors..

try this site for more helpful tips..Rod Elliot sound page... or we're try a help with your problem..

kind regards.