problem with a Fender Rhodes amplifier

[spy]

I have tried an other speaker, the problem is still there. Also, the original speakers have been tested with another Rhodes amplifier and they sound great.

Let me know if you have anything else in mind. Power supply caps are still the originals, I think it's time to replace them!

[52 Bill]

Power supply caps are still the originals, I think it's time to replace them!

When in doubt, change them out.

I'd go back to the new drivers. If someone had replaced them before, there may be an issue there that was never solved or was created by the new parts. What voltage reading do you get from base to emitter on the two driver transistors?

[Enzo]

Spy, my "sustain" comment was merely a joke. It had absolutely no technical value, and nothing to add to your situation, sorry. It is a sort of shop joke here, refering to a stereotype young kid coming into the repair shop complaining he can't get enough sustain from his amp. Up there on our list with the kid who got a Strat Pack three months ago and proclaims "Music is my life."

[J M Fahey]

Not forgetting the guy who asks " I have XXXXXX problem (which may be *anything*, including bad breath) ... it's the transformer, isn't it?... "

[spy]

Enzo, ok! Now I got it!

52 Bill, you may be right but I don't know what were the originals.
Does anybody know?
I did the measurements and Vbe on both transistors is .54-.57V, they are MJ4101.

[52 Bill]

I don't know what the original generic part numbers were either. Your Vbe voltage readings are in the ballpark though.

Are these transistors in the TO-66 cases, like mini power transistors?

[spy]

Yes, you're right, MJ4101 are in the TO-66 cases! Let me know what do you suggest!

[52 Bill]

Yes, you're right, MJ4101 are in the TO-66 cases! Let me know what do you suggest!

With all that you have already done, there's not much left to suggest.

I remember once working on an old stereo amp that did something similar. Similar case, as it had been repaired once before I got it. Pulled out what little hair I had left on my head until I went back and checked what was done before it came to me. While there was nothing that I could see that was wrong with the earlier repair, I decided to replace the newly replaced transistors. That fixed the problem. The SK series replacement transistors that were used apparently didn't have enough gain to work in the circuit as designed.

Perhaps if you go back and try replacing the drivers with a different type it may clear up your problem output. TO-220 transistors were designed to fit the TO-66 pin and screw spacing, so you could try a TIP series part in there. Just watch the voltage and current specs of the replacements.

[J M Fahey]

Fully agree with 52Bill and add: all the schematic says about the drivers is "1A 60V"; apparently those 2N4101 fit the bill.
Exact match (per schematic) would be TIP29A but I would splurge and use TIP31C , specially because it's the "preferred part".
You can , as noted above, mount TO220 instead of the TO66 case ones, by carefully bending B/E legs down into original PCB holes. Use the proper rectangular mica, of course.
As of the Collector, how is it connected?
Electrical contact through the screws? Insulated screws? Post a couple pictures.
If contact through screws, you can cut the center collector leg; if not, you may bend it upwards, cit it leaving a 1/8" stub and solder some wire to it, if necessary.
I am replacing TO3 metallic 2N3055 in my old amps with TO218 or TO247 transistors with similar small mods; so I know it works.
Only problem is that *sometimes* the metallic case is used as a "bridge" between 2 pads, so if you use a 1 screw TO220 later the amp does not work.
Easy to solve with a small piece of wire ... if you are aware of the possibility.
Good luck.

[oc disorder]

Juan and others there is an unrelated You Tube clip that will give you a visual idea of whats going on.
He had a problem with the differential pair having it's earth via a panel mounted jack.

Fender Rhodes Jordan Amplifier Tech Note - YouTube

[spy]

Just replaced the drivers with a pair of TIP31C, thanks Juan for the suggestion, and I'm feeling that the problem is gone. I haven't heard the amp with the piano but with a guitar preamp connected and a guitar plugged in, the notes are clean and separated as they should be! To be 100% sure, I'll test it with the Rhodes and I'll let you know! Hope everything is fine and don't come back disappointed.

Edit: Forgot to mention that on new drivers I have Vbe around 0.63V on both of them!

[52 Bill]

That's great to hear, I hope it works right with the piano as well.

Did you leave the earlier bias modifications in place as well?

[spy]

So, it was the drivers! It works great with the piano and the amp is clean. Thanks for your help guys!

52 Bill, yes I left the bias mod in place, it's a great mod. I rebiased the amp after the drivers replacement.

[spy]

So, it was the drivers! It works great with the piano and the amp is clean. Thanks for your help guys!

52 Bill, yes I left the bias mod in place, it's a great mod. I rebiased the amp after the drivers replacement.