Replacing 2N2147 Germanium Power Transistors In A JBL SE401 Transducer Energizer

[NVGrampy]

Punch Line First: I am starting this thread to elicit input from interested members, if there are any, about the 'best' option for replacing and/or updating output transistors on vintage Germanium based audio products.


I was given a JBL SE401 as a 'gift.' I spent a lot of time during 2019 'restoring' it. I was fortunate to have had a lot of help learning how to do that from several gurus. I heartily thank them. It may lead to another thread.


I started out 'lucky' that all of the original transistors tested 'good'. When I finished, a month ago, it worked, sounded and tested OK. I listed the SE401 on eBay.


Sadly, I continued testing with Ch B of the SE401. The original 'protection circuit' consisted of four self-resetting circuit breakers ['CB'] and a fuse. When two of the CB 'lit up' (who knew?) I guessed that I had a problem.


It turns out that the CB don't protect the 2N2147s. Two of the four on Ch B now have a dead short from collector to emitter.


I have several choices:


1) Give up - chalk up the losses - 'A Good Choice.'


2) It seems like there are no new 2N2147 actual replacements available at any believable prices. Perhaps, it might be possible to buy some old, expensive ones. They could 'go bad', too. This is 'A Sketchy Choice.'


3) Replace the 2N2147s with modern, current production, Silicon TO-3 output transistors. Revise the bias network to make that work. For example, ON Semiconductor MJ21193G TO-3 PNP are $6.71 each on DigiKey. They should do the job, and only two per Ch are actually needed since the MJ21193G seem to be a lot more powerful. This is a 'Decent Choice.'


4) Convert the output stage to 'Full Complementary' with a revised bias network and some re-wiring. ON Semiconductor offers the MJ21194G TO-3 NPN for this purpose - those are $7.06 each on DigiKey. This is another 'Good Choice' with some upside potential.


Since 3 and 4 will will require another total disassemble, this is a tough decision.


I will appreciate all responses.

[Don C]

I haven't done all of the research, but did you look at using NTE 121s? People say bad things about NTE, but they've saved my butt a couple of times.

[NVGrampy]

I haven't done all of the research, but did you look at using NTE 121s? People say bad things about NTE, but they've saved my butt a couple of times.

I have been 'warned' that the NTE 121s, even if they are purportedly 'new out of the box', may have leakage problems. To insure functionality, I would need to replace at least an entire four transistor side at $30 each. In addition, they are still not 'originals.'

I was also warned that the same can be true of 'NOS' originals, if you can find any, from RCA and the other 'name brand' manufacturers.

It seems that Silicon does not have that problem. On the flip side, the higher base-emitter voltage, with related thermal and crossover distortion issues, will need to be dealt with.

I have used several NTE components (capacitors, diodes, LEDs, resistors, etc.) with good results. They used to be readily available at the local Fry's here in Las Vegas - nowadays, not so much.

[Kay Pirinha]

I don't know these NTE 121's, but if they're germanium, increased leakage in comparison with silicon devices of similar power capability isn't that unusual. Anyway, you're right, 30 bux each, while still not restoring to it's original state, sounds somewhat weird.

Do you have the unit's schematics handy?

Best regards!

[NVGrampy]

"Do you have the unit's schematics handy?"

I attach my latest revision of the schematic. I have been working on the color coding of the wires - truly a 'rainbow' - not done yet. Ch B wires have white stripes on them. I also took a lot of photos along the way - maybe for the 'book'? This has been, so far, a 6 month waste.

FYI, my SE401 does not have the 'dual primary export power transformer.'

The actual values of the new replacement electrolytic capacitors are generally higher than the originals (which are in this schematic) to correspond to modern availability.

The 1N2321 Si diodes, as it turns out, are for 'clamping' - not thermal stabilization - they only have 0.25 Vdc across each of them.

[Kay Pirinha]

I assume you wish to get this unit working again? Honestly, due to the vertiginous prizes of more or less suitable substitutes and the brittleness of germanium power devices in general, I'd consider to replace anything between both input and output terminals by a pair of chipamp boards. At supply rails of just ±25 Vdc, chips like LM3886 or TDA7293 and the like come to mind. Al of them include reliable safety protection against overload, over temperature etc.

I'm also a contributor to DIYAudio forum. Bob Cordell's Compact 3886 and Super Gainclone LM3886 boards are being discussed there extensively and built in large numbers, also Tom Christiansen's Neurochrome Modulus boards. PCB's to all of them are available through the DIYAudio store (or from Tom in Canada, resp.), same with complete component kits. And no, I'd never recommend any of these cheap Chinese crap boards on ePay, AliExpress etc. Most of them contain fake chips!!!

Best regards!