Rhodes Suitcase Dual 50W amp

[melvin]

Hi everybody. Not being terribly experienced with solid-state repair, I would greatly appreciate any tips on keeping this thing running and reliable.

I have a Rhodes Suitcase Dual 50 W amp in my care that has been in for a couple of things:

First, it had distorted output due to cracked solder at the connector pins.
It then came back about a month later with one burnt resistor and a dead transistor: R16 and Q6.

I only replaced R16, R13 and Q6, rebiased it (incl. offset) and gave it back. I used a TIP31C I think- it's an NTE291.

It made it through several shows. But I'm pretty sure I should have replaced more because it did the same thing a week later.

So, here's my questions, what else should I replace when a driver in a complimentary pair burns up? The other part of the pair? Anything else I should check? Did I use the totally wrong transistor? (I just looked up an available replacement at a local parts store). The .5 ohm resistors, by the way, are pieces of resistive wire. And this thing is being gigged a lot, at full volume, has never had a cap job, and has all the original parts aside from what I replaced. (Although, I think the other 50W amp has a Q6 that is unoriginal but no burns. A TIP29B, I think..)

Thanks for looking at this...

[Enzo]

First off, Q6 is an RCA29B, not a TIP29. RCA29B is a 4A 80v part. TIP29 is a 1A part. Even TIP31C is only a 3A part. And I am unclear on what you used. An NTE anything is not a TIP anything, the NTE is what the NTE people feel is their best part to use in place of the TIP part. SO did you use a TIP31 or the NTE?

How healthy are Q8,Q9? And the .5 ohm R18,R21? They may be resistance wire, but are the conections 100%? Are the Qs in sockets or soldered? Any chance a loose socket pin?

Look at the circuit. If R16 burns up, the only route for the exdcess current to take is through Q8. Either Q8 is faulty or perhaps R13 was open, leaving ONLY Q8 for a current path.


If this whole circuit is being unreliable, I wouold probably start with a pair of new drivers. Good luck finding RCA29B and RCA30B. I would probably use TIP41C and TIP42C instead of TIP31, myself. Or really I might use one of many Asian number TO220 I have in stock. I would also replace the three 100 ohm R13,14,15. Verify R16,17 as 4.7 ohms.

Outputs 2N6254 are 150W 15A 80v parts. If they are at all suspiciousw, replace them. If I can;t find them, I replace BOTH with something similar. I'd probably replace them with something a little more modern like a couple MJ15003 from my stock.

How are you biasing this amp?

[melvin]

Enzo, thanks for the help. Sorry for the confusion. I have only NTE parts available to me locally. I used the NTE recommendation to replace an RCA29B which is the NTE291, a 4A part.

Funny, there's a actual TIP29 still going in the other amp that someone must have put there in place of the RCA29B. I suppose that would be unreliable if it's a 1A part and the original was 4A.

Q8 and Q9 are not fried, but I am not sure how healthy they are otherwise. Is there a way to find out?
The .5 ohm R18 and R21 connections are good, are you thinking they may have taken a lot of current and be questionable, or at least their connections?
The Qs are soldered in, no sockets. I biased for about 30mA through R18 and 21.

I guess Q8 is suspect, but I'm at a loss as to replacements. If I ordered the MJ15003, do I need to match a pair? I will replace R13, 14, 15, 16 and verify 17. Can something faulty before Q6 cause this to happen?

[Steve Conner]

I suggest checking the connections between the PC board and the output transistors. For instance, if P2-4 were loose, Q8 collector would receive no B+. The driver transistor Q6 would then try to supply all of the speaker current through Q8's base-emitter junction and R16. This would explain the smoked Q6 and R16. It may also have degraded Q8 through excessive base current.

Another possible replacement for Q8, Q9 would be the 2N3773. Don't think matching is too important.

[Enzo]

Most any output transistor would work there if it meets the basic specs. Nothing to match here, only one xstr per side.

[gbono]

Don't know how old this amp is but I would check/replace C1/C2 in the power supplies. Don't have data sheets for RCA 30b/29b - do they have integrated heat sinks like the 40409's?

[melvin]

Thanks for your help guys. It looks like Q8 was definitely not seeing B+ at some point because current draw liquefied the solder on P2-4 and disconnected it. I suppose once it melted, the current continued through R16. Or vice-versa maybe? The traces are okay, surprisingly.

The ballast resistor wires test good, but I wonder if I should find something with which to replace them. Another schem shows R18 more accurately as two .2 ohm 10W resistors in series. (R21 as well) Seems like they'd be tough to recreate.

I did manage to find a surplus of 2N6254's. Any tips on mounting these properly? I'm really not sure what type of thermal grease is best, whether I should replace the mica (?) insulators, or how tight to get the mounting screws. I just expect it's to be a thin layer and even pressure. I really don't want my inexperience to put any more black marks on this board.

gbono: The power supply checks out, although I may have put a strain on it. RCA29/30b are TO-220 packages but they are not attached to heat sinks; they are mounted flat against the circuit board.

[gbono]

AN1040-D.pdf

Check out this application note on mounting semicondutor packages.

[soundmasterg]

Makes me glad my 1975 Mk1 Suitcase 88 key Rhodes is still working well. I don't know enough about the guts to work on it either.......hope you're able to solve your issues.....


Greg