Author Topic: Rhodes student model transistor swap - power supply  (Read 1269 times)

Offline Coyote Circle Studio

  • Pre-Piano
  • *
  • Posts: 5
    • View Profile
Rhodes student model transistor swap - power supply
« on: May 30, 2017, 04:45:22 PM »
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!

Offline Mike Borish

  • Vendor
  • Fiesta Red
  • *****
  • Posts: 20
    • View Profile
    • Borish Electronics
Re: Rhodes student model transistor swap - power supply
« Reply #1 on: May 31, 2017, 11:19:38 PM »
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
www.borishelectronics.com

We're a Chicago company that repairs amps, pro audio, DJ gear & synths!

Offline Coyote Circle Studio

  • Pre-Piano
  • *
  • Posts: 5
    • View Profile
Re: Rhodes student model transistor swap - power supply
« Reply #2 on: June 01, 2017, 01:00:57 PM »
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
« Last Edit: June 01, 2017, 08:08:03 PM by Coyote Circle Studio »

Offline Tim Hodges

  • Vendor
  • Mark II
  • *****
  • Posts: 806
  • Tim
    • View Profile
    • Facebook
Bristol Electric Piano
UK

Facebook
YouTube
Reverb.com

Offline Coyote Circle Studio

  • Pre-Piano
  • *
  • Posts: 5
    • View Profile
Re: Rhodes student model transistor swap - power supply
« Reply #4 on: June 01, 2017, 08:00:31 PM »
Wow- beautiful instrument.  Unfortunately, no, The one I am looking at looks pretty different in the guts.

Offline Coyote Circle Studio

  • Pre-Piano
  • *
  • Posts: 5
    • View Profile
Re: Rhodes student model transistor swap - power supply
« Reply #5 on: June 01, 2017, 08:02:09 PM »
Here is the board in question.  The 820 ohm resistor blew up and has been removed.

Offline Coyote Circle Studio

  • Pre-Piano
  • *
  • Posts: 5
    • View Profile
Re: Rhodes student model transistor swap - power supply
« Reply #6 on: June 01, 2017, 08:05:52 PM »
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.

Offline The Real MC

  • Mark II
  • *****
  • Posts: 751
    • View Profile
    • My '67 sparkletop Rhodes
Re: Rhodes student model transistor swap - power supply
« Reply #7 on: June 01, 2017, 11:50:11 PM »
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.