Fisher 170 Distortion - Printable Version
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RE: Fisher 170 Distortion - morzh - 03-22-2015
Ron
One thing I can say: you have a scope - use it.
If the distortion persists after awhile, this is as simpe as scoping from the output back and seeing where it starts and what kind it is.
We were able to guess your power was bad when we saw voltage drooping last time.
Same thing; we need to know the location(s) and the manner of distorion to guess as to its origin.
Your scope is quite adequate.
RE: Fisher 170 Distortion - PhilcoMike - 03-23-2015
Ron,
I don't mean this to be a step by step tutorial but a few suggestions having been through similar problems. In older receivers like this one your fingers or a IR thermometer can be your friend. When a channel is biased wrong is will run hotter, especially the later stages output transistors.
Another area to check out is some type of thermal-component issue. While transistor testers are good they sometimes don't pick up on thermal stresses. If care is used a hair dryer and a can of cool spray can be your friend. By thermally heating and cooling some areas you can narrow down the search. I had a speaker protection transistor in a JVC that gave me fits. The only way I found the problem was alternating heat/cool until I hit this transistor. Bingo!
Your description of fine and then distortion returning sounds thermally related. Assuming that voltages are staying stable.
Good luck,
Mike
RE: Fisher 170 Distortion - Ron Ramirez - 03-23-2015
PhilcoMike - thank you for the thoughtful reply.
I've put it back on the shelf for now, but I'll keep this in mind when I decide to try working on it again.
RE: Fisher 170 Distortion - Ron Ramirez - 01-23-2016
To wrap up this thread, in case anyone's wondering whatever happened to the 170:
I took it to Kutztown last May and left it on the DVHRC donation pile.
[Video: https://www.youtube.com/watch?v=_VrFV5r8cs0]