Panasonic RE7070 Repair

[mfinch0983]

Hello all!

I am repairing an old solid state Panasonic RE7070 8 track stereo. It’s full of germanium transistors and some need replacing. I’m having trouble finding modern silicon equivalents. Can anyone out there help me? I’m specifically looking to replace a coulple of 2SB346.

Thank you!

 

[chazix]

silicon equivalents

I don't think anyone can advise whether silicon parts can be used without any way of knowing (as a first cut) whether the different Vbe matters.

If neither Vbe, nor case style, nor a much higher fT matter, then you could probably use something like KSA708. (Avoid "C" suffix, which wil have the collector in the middle.)

If you wanted to stay with germanium parts so as to not worry about the Vbe, you could probably use NTE158, which even have the original case style.

These are really spitball suggestions, since I have no idea about the circuit they're going into.

 

[mfinch0983]

Thanks so much for your reply!

Attached is the schematic for this part of the circuit. These transistors are the first audio amps after the tape heads. TR 9 and 10. I believe the Vbe is .6V according to this.

My assumption was that silicon would be better and quieter in this application, but I’m open to being wrong. I’m still learning.

Thanks!

 

[chazix]

schematic

Well, that calls my bluff. And I'm pretty much stumped. But until someone smarter (or/or handy with a circuit simulator) comes along, I'll offer some musings anyway.

First thing is that voltages given on schematics always need to be taken with a grain of salt. There are some real doozies in this one, like TR10's collector voltage being different (if I'm making out the numbers) from the base voltage of the following transistor, even though they are directly connected. And I simply don't believe that any Ge transistor in a linear amplifier circuit will ever run with a Vbe of 0.6V - more like 0.3V.

With the whole circuit running at such low voltages, and using DC feedback, I suspect that Vbe does matter - which would argue for sticking with Ge transistors, even though you're right to think that they tend to be noisier than Si.