Fisher turntable and TX-300 - Printable Version

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RE: Fisher turntable and TX-300 - Ron Ramirez - 01-29-2016

Kirk

on both.

You want fast acting fuses for the four output transistor (speaker) fuses, but the manufacturer specifies slow-blow for the line (mains) fuse only.

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RE: Fisher turntable and TX-300 - OldRestorer - 01-29-2016

Thanks Ron,
She is killing me now.. She doesnt know how to replace them....
I am not even getting paid,

Me

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RE: Fisher turntable and TX-300 - morzh - 01-29-2016

Ron

Indeed if the transistor output oscillates it will try in no time (subsecond). Had it happened to me.
This said, most output amps, even those with one supply are designed as DC amps with DC blocking caps at input and output if one supply and none with dual supply (and differential amp for the input stage). They are also frequency-compensated to be stable and this in a well-defined amp does not depend on whether the load is there or not.

A not so well designed amp might go into excitation but .....bad designs are bad designs, this is it.

Here's a food for thought: many if not all transistor amps have the button that disconnect the speakers. Do you think they would do this if the amp were to oscillate?
My Kenwood has it, and once on a while they get disconnected, and I see that bar moving with no sound, and then I know the speakers are not on.

So....no, a good amp will never oscillate with no load.

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RE: Fisher turntable and TX-300 - morzh - 01-29-2016

Kirk


Haven't you learned yet? Fixing stuff for ladies you know you do not get paid. Ladies do know that very well

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RE: Fisher turntable and TX-300 - OldRestorer - 01-29-2016

Yeah,
She went from asking if I could repair it to getting me to do all her homework,

Sheesh!

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RE: Fisher turntable and TX-300 - morzh - 01-29-2016

Do you also carry her bag all the way to school?

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RE: Fisher turntable and TX-300 - Ron Ramirez - 01-29-2016

(01-29-2016, 08:09 PM)morzh Wrote:  ...most output amps, even those with one supply are designed as DC amps with DC blocking caps at input and output if one supply and none with dual supply (and differential amp for the input stage). They are also frequency-compensated to be stable and this in a well-defined amp does not depend on whether the load is there or not.

...yes, but...

We're talking about 1965 when high fidelity transistor amps were still in their infancy.

And to make matters worse, half of the eight output transistors used in the TX-300 are germanium! And there are no blocking caps between the output transistors and the speakers in this amp.

This was technology that was still developing.

Quote:Here's a food for thought: many if not all transistor amps have the button that disconnect the speakers.

None of my Fishers have such a button...and neither does Kirk's friend's TX-300.

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RE: Fisher turntable and TX-300 - morzh - 01-29-2016

Yeah...maybe back then.....(I did not imply DC cap saves it though, it does not even matter,simply allows unipolar DC amp to drive low ohms load) ....

I built two Ge transistor amps, the first being transformer output pushpull on Soviet P210
http://www.155la3.ru/p209.htm
(the largest Ge transistor at the time....well, the biggest of all times were P207-P208 but they were discontinued by that time)
http://www.155la3.ru/p207.htm
Behold. 74мм flange. The contacts were so this when one broke it was possible to drill in it, tap a thread and put a screw in. A 25A capable device, in those times.

Another was your regular pushpull with DC blocking cap on P215
http://www.radiolibrary.ru/reference/transistor/p215.html
These are smaller, about 10w dissip.

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