Rickenbacker TR35b project

[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 

Good luck with the repair 

[g1]

if 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]

I wouldn't normally recommend UT Source, but in this case for the money I would try them out.

https://www.utsource.net/itm/p/1839737.html?srsltid=AfmBOorgbD-WK-__is7r9wtoO5nTRJq0o7_X9B2neeyT3ZtJHz5dYUhk

[J M Fahey]

In no particular order:

* that amp is very repairable, but is a weird design, which *can* bring lots of problems if not addressed.

* design is not *bad* but clumsy and uses weird transistors nobody else does.
Even worse, they do not match Industry Standard packages and pinouts which makes replacement harder, normally involving some surgery.
As a bonus, they are out of production for many years now and only remaining "suppliers" are VERY suspect.
IF at least they were available at Mouser, Digikey or Farnell, but no, only "mystery suppliers".

* above mentioned 2n6488 2n6491 are different case (TO220 instead of TO127) but much worse, opposite pinout, which will make amp explode if just replaced without important modifications (cutting and rerouting tracks, etc.)

* I can suggest modding and improving it by using Industry Standard TO218/TO247 case TIP142/147 transistors (what EVERYBODY uses: Fender Crate Marshall Laney Peavey Ampeg etc.) which can also replace drive transistors, BUT it requires mounting them "on their own" on heatsink and connecting them to different points on PCB using flying wires.
Most probably also rebiasing.


* For now do not start wholesale "improving" PCB, you will add more unknowns, just replace 1 or 2 obviously blown/leaking/dead supply capacitors, do not replace perfectly good diodes, what for?


AFTER amp is working you can get into details, not yet.

WHERE is "local"?

Post pictures showing board and heatsink closeups, to suggest new power transistor mounting.

Full size pictures please, not small unreadable thumbnails as on post 1