buzz / hiss in SS stereo power amp

[marc9889]

I have a noisy / buzzy / hissy sound coming from my Randall RRM 2-80 power amp. It's loud enough to be obnoxious, and definitely didn't come that way from the factory, so I'm assuming something is failing. One thing that is unique about this series (or that is my understanding) is that they have a switch in the back that provides a way to select between constant voltage and constant current. I'm not sure what the goal in that would be, but I can tell you that for guitar rig purposes, it sounds better on constant current. Let's assume the buzzing noise is consistent regardless of the switch position. What's the best way to diagnose the issue? Is there a "typical" component that starts failing after, say, 25 years? (Obvious semi-newby question).

[loudthud]

If you can find a schematic, either post it or a link to it here so everyone is on the same page.

This is just a WAG, Wild Ass Guess. I'm assuming the circuit is basically the same as the RG-100 guitar amp that has been around for like forever. The input transistors of a typical solid state power amp can be degraded if they are over driven too hard or too long. It's usually reverse base to emitter breakdown. Many power amps have no protection on the inputs, driving them with a tube preamp can kill them. The transistors will become noisy before they fail outright. So carefully remove the existing transistors and try to find exact replacements. If possible, I would install sockets.

[nickb]

It's stereo. Do both channels have the identical level of hiss?

Suggest you measure and report the AC Vrms at the speaker terminals using a DVM with just the hiss present.

[Jazz P Bass]

Here is the schematic: RRM2-80 - Schematic.zip

Good luck finding that 2N8098.

Possible substitute?
2N2060.pdf

[nosaj]

Here is the schematic: [ATTACH]54157[/ATTACH]

Good luck finding that 2N8098.

[ATTACH]54158[/ATTACH]

Schematic not working.
Datasheet does.

nosaj

[nosaj]

Here is the schematic: [ATTACH]54157[/ATTACH]

Good luck finding that 2N8098.

[ATTACH]54158[/ATTACH]

Looks to be still available.https://www.google.com/search?q=2n20...w=1366&bih=662
nosaj

[Jazz P Bass]

Here is the OM.
RRM2-80-OwnersManual.pdf

There is a description of the differences in the Constant Current vs the Constant Voltage Selector Switch.
Interesting.

[Jazz P Bass]

Schematic not working.

Reloaded it as a zip file.
https://music-electronics-forum.com/...2&d=1561943639

[nickb]

Here is the schematic: [ATTACH]54162[/ATTACH]

Good luck finding that 2N8098.

Possible substitute?
[ATTACH]54158[/ATTACH]

Well spotted. Those are expensive at $30 a pop.

You can substitute a pair of transistors such as KSC1845 that are inexpensive. For best results super glue them with the flats touching each other.

[nevetslab]

A

Well spotted. Those are expensive at $30 a pop.

You can substitute a pair of transistors such as KSC1845 that are inexpensive. For best results super glue them with the flats touching each other.

NICE NPN part! If you do change to these, you'll need to reform the leads, as the pinout is different. Facing the flat, left pin is E, middle pin is C, right pin is base