Replacing 2N2147 Germanium Power Transistors In A JBL SE401 Transducer Energizer
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.
Long Term Germanium Issues - 'Whiskers'
Quote:
Originally Posted by
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.
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.
THANK YOU I'm Not Certain Who Would Want A JBL SE401 That Was Actually Something Else
Quote:
Originally Posted by
Kay Pirinha
I assume you wish to get this unit working again?
Honestly ... I'd consider to replace anything between both input and output terminals by a pair of chipamp boards. ... Best regards!
I appreciate your suggestion. THANK YOU.
I already have several 'vintage' amplifiers that work reasonably to very well. So, I don't really need another one.
My objective was to 'restore' this one (which I did, with help from several 'gurus'), test it (which I did, with help from Audio Precision), listen to it (which I did while it worked - it was OK) and SELL IT.
I assume you are telling me to 'give up.' That is an honest and straightforward answer. You are one of the few people who respond in a sincere, forthright, direct and candid way. I value that. I wish more people were like you - I would save a lot of time and effort.
My SA 660.. Same problem.
I Have owned A JBL SA660 Black Integrated amplifier for over 30 years. It has the same problem with the output transistors. I do not want to modify this amplifier so I just use it as a showpeice.
The SE401 Project Died A Natural Death
Quote:
Originally Posted by
Gary Wolf
I Have owned A JBL SA660 Black Integrated amplifier for over 30 years. It has the same problem with the output transistors. I do not want to modify this amplifier so I just use it as a showpeice.
Thank you for your note. A friend if mine, Steve Hogan, spent an inordinate amount of time with me re-designing the output stage with Si Power Transistors. It might have worked well. Then, the pandemic ... This is now in a bankers box.