Mosrite Amp Help Requested... Trying to Revive a 350...

[nategibson]

Howdy good group...

Does anybody know what kind of transistors went into the Mosrite Electronics amps? Apparently the ones in my non-functioning 350 amp are Westinghouse, transistor number 156053. But back in the 60's and 70's, electronic manufactures sometimes used "house numbers" on parts instead of the real number. This may be the case with these. Although it may be a common transistor, I have no way of knowing which one. Does anybody know for sure? Thanks in advance for the help!

-Nate

[zarfnober]

Mojo and Antique Radio Supply both have a good selection of transistors, give them a look.

Rocco

[nategibson]

Thank you kindly Rocco... We'll give 'em a call tomorrow and see what we can come up with. Hopefully I'll have some pix to post soon!

Best,
Nate

[Mr. Bill]

House numbered parts are a common problem in repairing electronics, especially vintage stuff where there is little or no information available from a factory source. Luckily, transistor designs are such that a wide variety of parts can be used to replace the originals with no problems.

My personal approach is to first figure out what the part is. In your case, a transistor can be bipolar, jfet, unijunction, etc. So what does the transistor do? What part of the circuit is it in? Are there any others in the amp that are not bad that can be tested to see what they are?

Once you have figured out what it does in the circuit, you can then look for a replacement with the right parameters to give you equal or better performance.

Now specifically, what transistors are you trying to replace?

[nategibson]

Howdy there Mr. Bill and thanks for the heads up! I'm having an amp repair guy here in Bloomington help me out as this is pretty far beyond my knowledge base at this point... I sent him your message and he responded;

"They are the output transistors. TO-3 case. Most likely bipolar.
Without a schematic, it is a bit in the dark."

Might you have any suggestions about what to do or where to pick up the parts? Or what the cross numbers might be for the transistor parts? It would be sad to think that we can't get it up and running, but that is what it is looking like thus far...

[Mr. Bill]

Output transistor, TO-3, bipolar. OK, from this you know what the purpose of the part is and what style case it has to have. And his guess about bipolar is right, as this was the main style used when this amp was made.

Here's the approach I take when I come across these problems.

First check the design of the power amp circuit. In the past 50 years of solid state amplifiers, there have been only a handful of designs used by guitar amp manufacturers. Most designs are based on designs created by the transistor companies trying to sell product. The design of this amp couldn't be all that different from that of one from Gibson or Kustom or Fender, etc. Once I know what type of circuit it is, I will know if all of the outputs are the same device or if half are NPN and half are PNP.

Next I check the power supply to see what the maximum voltages are. This will show me what the output transistor voltage ratings must be able to handle.

With this information I will pick a transistor that fits all of the parameters and try it out in circuit.

If you look at the eBay listing for the Mosrite amp that is there, there is a photo that shows a 2N3055 output transistor on the back. This may be a helpful clue.