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 Post subject: Fisher F-590 (amp 59-A) power output transistors
PostPosted: Jun Sat 28, 2008 11:56 pm 
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Does anyone know what type of power transistors are used in the Fisher Futura F-590 stereo (type 59-A amplifier chassis)? I've got one here with all four transistors (Q1-Q4) blown on one channel. From the good channel they test as PNP transistors -- all four, which surpises me a little.

Q1 and Q3 are marked "35524" with RCA brand, and Q2 and Q4 are marked "TR-35144" with no brand.

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Tom K6VL


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PostPosted: Jun Sun 29, 2008 4:15 am 
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Should be complimentary. Probably germanium devices. Grab an NTE cross reference or even an old RCA SK crossreference, perhaps you can get the specs of the replacements there.

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PostPosted: Jun Sun 29, 2008 5:39 am 
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Are the 5 digit numbers I mention above SK or NTE cross-referenceable numbers? For whatever reason, I was thinking SK numbers were just 4 digit.

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PostPosted: Jun Sun 29, 2008 5:45 am 
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Tom, those are actual RCA part numbers. Should cross in an NTE or SK book.

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PostPosted: Jun Sun 29, 2008 6:42 am 
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Hmmm... getting interesting.

If I go to Moyer Electronics and use the RCA cross reference, these part numbers are not listed. If I type them into the ECG or NTE cross references, here's what I get:

35524 = NTE181 or ECG181 (NPN Si power transistor, TO-3)

35144 = NTE121 or ECG121 (PNP Ge power transistor, TO-3)

What's funny is that if I test some of the remaining good 35524 transistors (other amp channel), it looks like a PNP Ge power transistor.

And furthermore, this is NOT the usual complementary output stage. By inspection of the circuit, I can see they have 4 PNP power transistors in series (E-C-E-C-E-C-E-C) between the (+) and (-) power supply, with the speaker connected between the center of this string and ground (power supply is split polarity).

The base of one of the "top" pair of transistors is fed with a signal that is 180 deg out of phase (I assume) with the signal that feeds the base of one of the "bottom" pair of transistors. There is a dual secondary interstage transformer (from the previous tube stages) driving this output. Looks vaguely like a circuit that should in fact work...

So what's the deal with these cross reference numbers? The 35524 cross referencing to an NPN Si transistor can't be right.

I also don't see the Fisher F-590 or 59A in my SAMS index. Does anyone else have an index to check to see if we can find a schematic or parts list for this thing?

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PostPosted: Jun Sun 29, 2008 1:27 pm 
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Hey Tom,

This cat posted a similar issue at diyaudio. You might pick up some clues here:
http://www.diyaudio.com/forums/showthread/t-93671.html

Interesting how the op transistors are stacked. I'll bet the designer used a hi B+, then had to stack units to handle the typical tube B+ voltage. I think the transitional period from tube to transistor spawned some weird designs.

Good luck, Tim


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PostPosted: Jun Sun 29, 2008 3:21 pm 
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Tkilboy wrote:
....I'll bet the designer used a hi B+, then had to stack units to handle the typical tube B+ voltage. I think the transitional period from tube to transistor spawned some weird designs.

Good luck, Tim


And let one short; it takes out the entire string! What a nightmare...

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PostPosted: Jun Sun 29, 2008 3:34 pm 
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The design is a little strange, but it is not using the tube B+. It has a split supply of +/- 35 VDC.

I checked in over at the DIYaudio site, and a very helpful gentleman there looked up the appropriate ECG numbers in his hardcopy book. This confirmed that both the 35524 and 35144 are germanium power transistors, and both cross reference to ECG121.

The online cross reference at the NTE website in fact gave incorrect information for the 35524.

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