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

[DPM309]

I am new to this forum and came across it while trying to repair a Rick TR35B. I changed out the volume pot and everything was working fine. A few days later, it had low, distorted output. I suspected the power transistors and was able to find them on Amazon. The schematic shows them as 2N5988 (PNP) and 2N5991 (NPN). The markings on the transistors are TIP42B (PNP) and TIP41B (NPN). The ones I purchased were TIP42B and TIP41B which are good replacements. I am still having issues after I installed them and one of them popped and cracked when I tried to fire it up. I am still having some kind of shorting issue with this amp that I am still trying to fix.

[g1]

The schematic shows them as 2N5988 (PNP) and 2N5991 (NPN). The markings on the transistors are TIP42B (PNP) and TIP41B (NPN). The ones I purchased were TIP42B and TIP41B which are good replacements.

I responded to your post over at MEF forum, but will include it here for others who may be working on these amps.
The TIP41 and 42 have BCE pinout.  The original 2N5988 and 5991 have ECB pinout, so the TIP's will not work as replacements.
There are fake datasheets out there showing ON Semi 2N5988 with TO3P case and incorrect pinout, but those transistors do not exist.
The real 2N5988 and 5991 were TO-225 case with ECB pinout.

Edit:  received response that this amp had been working and came with TIP41 and TIP42 outputs.
So can only conclude there are more than one version and be sure to check where the traces of the output devices are going, as there seems to be only 1 version of schematic available, and it does not match the later version.

[J M Fahey]

The schematic shows them as 2N5988 (PNP) and 2N5991 (NPN). The markings on the transistors are TIP42B (PNP) and TIP41B (NPN). The ones I purchased were TIP42B and TIP41B which are good replacements.

I responded to your post over at MEF forum, but will include it here for others who may be working on these amps.
The TIP41 and 42 have BCE pinout.  The original 2N5988 and 5991 have ECB pinout, so the TIP's will not work as replacements.
There are fake datasheets out there showing ON Semi 2N5988 with TO3P case and incorrect pinout, but those transistors do not exist.
The real 2N5988 and 5991 were TO-225 case with ECB pinout.

Edit:  received response that this amp had been working and came with TIP41 and TIP42 outputs.
So can only conclude there are more than one version and be sure to check where the traces of the output devices are going, as there seems to be only 1 version of schematic available, and it does not match the later version.

Guess you are right.

Back in the day Motorola salesmen were VERY active and lured many amp manufacturers to use their stuff.
One of them was Marshall and apparently Rickenbacker fell for it too.

This is a Marshall 100W SS amp made between 1975 and 1978.

This comes straight from the 1973 Motorola Power Amp design handbook.

It does not use generic transistors such as 2N3055, TIPs, etc. but specific models made only by Motorola, so end user is not tempted to switch later to cheaper suppliers.

Another dirty trick (as seen on the Rickenbacker amp) was to use non standard case sizes and pinout, so repairs HAD to be done with the original Motorola stuff.

Only Marshall "guitar" mods were to add a Presence control and a tapped autotransformer so user could have 4-8-16 ohm options, both features present in Tube amps and which Guitar players expected and were used to:



These were among the earliest Marshall SS amps, they had little to zero experience with them, so they went through with a respected suplier suggestions to play it safe.

Funny thing, I am quite certain Motorola sold them A TON of transistors, or they signed a contract to buy a ton per year for "X" years, so they ended up WAY overstocked.

So they used these anywhere, including a puny 12W amplifier: 5005/Lead 12 , where it is nonsense to use a pair Metallic TO3 10 Ampere 80V 150W transistors which eeasily power a 100W amp.
I bet they were SICK of having them for breakfast, lunch and dinner so they designed in inexpensive beginner amp to move them out fast.

I see no other explanation.