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November 06, 2024, 06:54:38 PM

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Rickenbacker TR35b project

Started by agdrago, October 17, 2024, 10:38:56 PM

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agdrago

I am attempting to repair a 1979 Rickenbacker TR35B bass amp.

I would greatly appreciate help identifying replacement output transistors. This is not something I have tried to do before. I cannot find any 2N5988. I have several 2N5991 that diode test as OK. A Motorola X-ref doc stated 2N6490 (2N6487) as a replacement for the 2N5988, it appears using them would involve a significant redesign as does using 2N6491 and 2N6488 as a pair.


Here is what I know/verified so far:
  • There is minimal evidence the amp was worked on. There are no obviously burnt parts, wires, or traces.
  • The power cord was sketchy and I have replaced it.
  • The transformer tests as good (resistance and voltage). Produces 35VDC.
  • The rectifier diodes and all other diodes test ok, but I will still replace them later.
  • The power filter capacitors are cooked - one more than the other: ROE 4700uF 40V 85C electrolytic. I will be replacing all electrolytic caps.
  • Both output transistors are blown Motorola: 2N5991 & 2N5988 
  • The 4 ohm speaker is good.

agdrago

I have been able to scrounge up a handful of Motorola 2n5991 and the complimentary 2n5988 that pass a diode test with similar numbers. I don't have any gear (curve tracer, etc...) to help find matches (if there are any).

I am heading towards a solution similar to that discussed here unless there are other suggestions.
https://youtu.be/J6lUs1LI2QI?si=Wa7-t280fHH93jxl



Suggestions welcome, or PM me if there is anyone locally or otherwise willing to lend a hand in any way.


saturated

Nice looking amp  :dbtu:

Good luck with the repair  :tu:
I ask stupid questions
and make stupid mistakes

criticism, critique, derision, flaming, verbal abuse welcome

g1

Quote from: agdrago on October 29, 2024, 04:11:35 PMif there is anyone locally
Here in Tuktoyaktuk ?  ;) You have no location listed in your profile.
You said that in order to use a recommended substitute, you would have to do a major re-design, why is that?
Not everyone has time to watch an hour long youtube video, tell us what the 'similar solution' is.

joecool85

Life is what you make it.
Still rockin' the Dean Markley K-20X
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