Transistor Selection

[jaymanaa]

Hi all, I'm sitting here typing this with a headache the size of Cleaveland. Transistors are way harder to self teach than tubes. I'm working on a 1964 Fisher model 59 hybrid which I can't find a schematic or service manual for anywhere. To make a long story short, I'm looking for some transistors. It has 4 per channel and two of them are shorted thanks to a mucked up variable resistor (bias pot). I'm planning on keeping this forever due to sentimental reasons, so I figured I would just replace them all (recap and retube are complete). Word has it that these amps had a tendency to blow up channels, which this one had apparently done at one time or another. One side has had the transistors and interstage TX replaced. My problem is that I don't understand how to order the transistors or cross the numbers. A search here got me to the NTE site where the RCA# TR35144 crossed to an NTE121, and the RCA# 35524 crossed to a NTE181. I would have thought that I would have needed 180s and 181s. Can anyone help an old tube guy give this old Fisher a few more years of singing. Thanks, Jay

[anatech]

Hi Jay,
They are probably Germanium transistors, and you do need a complimentary pair. It may be possible to rework it to use silicon transistors.

-Chris

[jaymanaa]

Yes, I beleive you are correct. I remember something about geranium on one of the data sheets I looked at, at PE. Seems as though the part numbers reflected matched pair with a MP suffix, and Complimentary pair with MCP. Would I be correct in assuming that a complimentary pair would be one NPN and one PNP which have been matched? I apologize for the rookie nature of my questions, but I have really only dealt with tubes, capacitors, and resitors in the past. These transitors and associated "lingo" are pretty much Greek to me. They are apparently much less forgiving to the novice as well. At least tubes "red plate" for a few seconds before self destructing. I really appreciate the help, Jay

[anatech]

Hi Jay,
It would be nice if the complimentary pairs were matched. Don't expect that to be the case. It's not terribly harmful if they aren't.

What you need to do is figure out what the total supply voltage is. If you have positive and negative supplies, add them together. If it's a single supply with an output capacitor, the higest potential is the supply voltage. Your new part has to be rated for this voltage plus a safety margin. It's common to use 120 V devices when the total supply voltage is 80 VDC.

What is the model number of your set? There are good places to buy manuals and reproduction manuals.

-Chris

[jaymanaa]

Yep, I measure 35v and -35v, so 120 would sound about right with 50v safety margin. From my research so far, it's looking as though these are Geranium and about 25 bux a pop for NTEs at mouser. It's a Fisher model 59A which uses 1 12AX7 and two 6HU8s in front of 8 transistors (2 pair per channel) for outputs. I've searched everywhere I can think of for a service manual and or schematic, with no luck. I'm thinking they didn't make these for very long. I wonder if some of the very early Fisher SS stuff would have a similar output topology? Any ideas on where to buy a service manual would surely be appreciated too. Thanks, Jay

[anatech]

Hi Jay,
I have purchased manuals from A.G. Tannenbaum in the past. Click here. I'm pretty sure there must be others.

I did a quick search and couldn't find any reference to your model. Is that the complete number?

-Chris

[jaymanaa]

Anatech, Just wanted to follow up with a thank you. I think I have the deal figured out now. I even found a nice looking lot of ECG 121s (NOS) on epay for 10 bux. This has, and will continue to be a good learning experience for me, thanks to folks like you, who are willing to help. Thanks Again, Jay

BTW, I'll be sure to let you know how she turns out.

[anatech]

Hi Jay,
Good job.

These are one of those times when ECG or NTE parts are about the only thing you can use without getting very lucky or wasting a great deal of time.

Make sure you measure the leakage on these before you install them. Germanium transistors are expected to be a little leaky. Silicon parts have extremely low leakage, none for our purposes here.

-Chris

[Tom Albrecht]

Hi,

I'm working on fixing this same model, and likewise have not been able to find any information on it. In one channel, all four output transistors are blown, and I would like to find replacements.

In tracing the circuitry and testing the remaining good parts in the other channel, I find that all four transistors are PNP and appear to be germanium.

If I use the NTE cross reference, type 35524 gets cross referenced to NTE181, which is a silicon NPN power transistor, whch doesn't seem correct. Type 35144 cross references to NTE121, which is a germanium power transistor, so it may be correct.

Does anyone know what these transistors should be? Does anyone know of a source for a schematic or parts list for this unit? As mentioned above, I wonder if this was in production for only a very short time. I see virtually no information anywhere on this model R-590 (amp chassis 59A).

[Netlist]

Tom, either there's a fault in your or in my NTE/ECG book.
TR35144 and TR35524 both point to ECG121.