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.

Hi,

My current project is a 1979 Rickenbacker TR35B (35W) bass amp. I figured I would share the schematic.

I have a Roland Blues Cube 60. The amp works perfectly when it is first turned on, but then as if by a function of how hard it is driven and how long it has been on it develops a ticking sound reminiscent of a playing card against the spokes of a bicycle wheel. The sound of the guitar can still be clearly heard as well as the ticking noise.

The cadence of this tick changes based up on the attack of notes played. It also occurs in both preamp channels as well as if the guitar is fed just to the power amp via the effects return.

The guaranteed way to cause the ticking is to hit a 6th string 5th fret or a 5th string 7th fret harmonic. As the note decays the ticking slows and then abruptly disappears.

Any and all help is appreciated.

Thanks,

- Grady