Rhodes student model transistor swap - power supply

[Coyote Circle Studio]

Hi all- first off, howdy from Colorado.  This is my first post!  Excited to be a part of it all.

Anyhow, I have been looking at swapping the germanium transistors in a 1969(?) student model with Peterson amp to silicon.  I have done homework and have seen the previous discussions here and on other forums regarding the two output transistors (currently Delco 801522, swapping with MJ15016G) and how to adjust for the different bias needed for silicon (2.7 ohm to 12 ohm resistor).  I am pretty clear on what needs to be done there.

Problem is there is a third germanium transistor mounted to the back of the chassis, no heatsink, just bolted to the wall, that feeds a power supply board mounted vertically on the inside wall of the amp.  Currently it is a Delco DTG-110B, and I would like to also swap it with a silicon.

The one available schematic for the student model, Rhodes service manual figure 11-19, unfortunately, does not contain the power supply.  It references a separate drawing #6803-5 that I have been unable to find.  One helpful tech suggested I look at the 80 watt Peterson power supply schematic, and while it does seem to be close, it is significantly different enough that I cannot really get anywhere with it.  I admit that is probably more a function of my limitations than lack of information contained, but I am stumped nonetheless.

I am looking for any knowledge anyone might have regarding the rebuild of the power supply, what would be an appropriate swap (would another MJ15016G work there also?) and what would need to be changed to accommodate for the swap.  If any of you have any insight or info that you could share I would be quite grateful.

Thanks in advance for any help anyone might have, and again, psyched to be here!

[Mike Borish]

First off, why do you need to replace the Germanium transistors?  If they blew up, there's also a good chance that the impedance matching transformer blew up too. 

The power supply isn't too complicated.  It's the same as the 80W version, but designed to reduce wattage.  Instead of +-38v unreg and 24v reg you have +-30 unreg and 20v regulated supply.  If your power amp is blown, I have modern replacements that are more stable. 

If anyone is interested in sending me their student piano guts, I'll design a modern replacement with all of the suitcase features. 

-Mike

[Coyote Circle Studio]

Hi Mike- thanks for the reply.  Well, the germanium output transistors are notoriously unreliable.  The amp already was exhibiting inconsistent volume levels, background hum, pops and clicks, an occasional low frequency oscillation.....just lots of bad behavior that indicated it needed attention.

I was planning on changing the two output transistors to silicon, as the owner wanted me to set it up for long term reliability.  When I plugged in the unit on the bench to get some before/after voltage measurements to ascertain the health of the circuit, the third transistor (power supply/regulation) failed and a resistor tied to it blew up. 

I now need to replace all three transistors, because one blew, and I don't want to just wait for the other two to follow.  I am swapping to silicon because I see no point in swapping in an obsolete part that is destined for repeat failure.  I am swapping to silicon for long term reliability and significantly lower cost. 

The two transistors in the output I can find documentation regarding the swap and what needs to be adjusted to maintain stability, I am good to go on that front.

I am unfamiliar enough with how this stuff woks to know what, if anything, needs to be changed to accommodate a silicon transistor in the power supply.  That's why I am looking for someone who has actually done this work and has knowledge of the actual circuit.

Unfortunately, while the power regulator is very similar to the 80 watt model, it is not identical.  It has enough differences in part counts and values to not really be helpful for me.  Also, the widely available schematic for the student model isn't the same model.  It is first version, this one must be "second" or something because it isn't a perfect match.  The schematic for the student model (which unfortunately does not include the power supply) states it wants to see +-30 and +20, while the actual board in the amp has a +35 and +25 solder terminal and no -35 (or any negative voltage) terminal, so I am not sure how to interpret the difference.

That's why I was hoping to reach out and maybe find someone who had first hand experience and/or knowledge they would be willing to share regarding this.

I don't want to install a replacement power supply (if that was the case, I could just go buy a cheap Chinese power supply online and save myself the trouble here), my friend the owner wants to make it reliable while maintaining as much of the original vintage circuit as possible.  So I am stuck trying to repair/modify it.

Thanks-
Michael

[Tim Hodges]

Michael is it like this one:

https://flickr.com/photos/85013700@N00/sets/72157672373285100

[Coyote Circle Studio]

Wow- beautiful instrument.  Unfortunately, no, The one I am looking at looks pretty different in the guts.

[Coyote Circle Studio]

Here is the board in question.  The 820 ohm resistor blew up and has been removed.

[Coyote Circle Studio]

UPDATE - I have tried to draw out the circuit and can now see the similarities to the 80 watt schematic some of you have mentioned.  I humbly apologize that I was unable to see the connections between the two and denied they were alike.  Sorry about that.  I still can't, however, figure out what needs to be done (if anything) to swap the power transistor from germanium to silicon.  Still trying to figure that part out.

[The Real MC]

I'm an EE with years of experience restoring and maintaining vintage keyboard.  Don't waste your time trying to restore that power supply to original.  Just buy a modern replacement.  It's only a power supply, nothing that impacts the tone or the value.

PNP Germanium power transistors are very unreliable.  There's a REASON why they haven't been made in decades.

[Coyote Circle Studio]

Hey all-  I realize this is a total zombie thread, but I myself strongly dislike unfinished threads that lack resolution.  So, to feed my compulsion for completeness, here's where everything ended up.

Firstly, don't listen to self professed experts who say it can't or shouldn't be done.  It is do-able, and very straightforward!! (On a side note, you lose points telling me not to try to rebuild using germanium transistors, when the freaking thread is titled "transistor swap" and starts with me asking how to replace the germaniums with silicons!  Work on those reading comprehension skills people!) 

While I realize it would be cheaper and easier to just buy a cheap generic modern amp and replace it with that, it kills the vintage value.  That's lame and the lazy way out.  As a collector of vintage instruments, I find that approach unacceptable if avoidable.  I men, heck, if you're gonna be lazy, why not just gut the thing and install a midi keyboard where the original keybed was (I actually did sound for a band where the keys player had really done that!).   

It might be arguable that the power supply has little effect on the tone (although as an amp repairman I don't fully agree with that statement).  Regardless, it certainly affects the vintage value having as much original as possible.  The transistor swap/update can totally be done while retaining a large amount of originality.

My whole goal was to keep as much of the power amp as original while replacing the germanium transistors with a more reliable silicon unit.  There is some info out there about the two output transistors, but I had the third germanium transistor, the voltage regulator, go out and couldn't find any info regarding swapping that one.

Turns out you can use MJ15016G silicon transistor for not only the output drivers, but it works just fine as a swap for the voltage regulator, too.  The output drivers need a resistor swap (from 2.7 ohm to 12 ohm) to bias correctly, but the power regulator is a direct swap.  There is a variable potentiometer on the power supply board that adjusted the regulated power rail into range, no other components needed swapped.

I decided to replace the electrolytic capacitors with new ones for safety and reliability, and I replaced a few resistors that had drifted, plus one pot that had died.  Other than that, I was able to maintain originality in the rest of the circuit.  The amp is now dead quiet and very powerful. 

[sean]

This is pretty awesome.  That means for less than $30 (transistors, resistors, caps, and shipping), you can get a dead amp module working again, right?

You are talking about swapping the pair of "120725" transistors in http://www.fenderrhodes.com/org/ch11/fig11-9.jpg, and replacing the 2.7Ω resistors with 12Ω resistors.  Did you just use one-watt resistors?

And the MJ15016G is a drop-in replacement for the "020725" transistor in http://www.fenderrhodes.com/org/ch11/fig11-10.jpg?

Cool.

Sean

[Coyote Circle Studio]

Sean-

yes sir, you are absolutely correct.  I used 1 watt metal film resistors, but I think that might have been overkill.  From what I understand, they are bias resistors and aren't responsible for much current draw, but better safe than sorry.

And yes, the MJ15016G seems to work just fine for both driver and regulator positions.

Nice to know old units can potentially be easily rehabbed.