Fender Rhodes germanium to silicon transistor swap

[MichaelNuzum]

Hi all-

I was wondering if anyone here has actually swapped the transistors in a Fender Rhodes amplifier from germanium to silicon? If so, would you be willing to discuss some of the procedure with me? I have done some homework and am aware of how to change the output transistors (currently Delco 801522 to mj15016g(?)) and about changing the 2.7 ohm resistors to 12 ohm to change the bias.

EDIT- model I am working on is a 1969(?) student model with Peterson amp -

Problem is, there is a third transistor (currently Delco DTG-110B), part of the power supply section, that would also need to be replaced. If anyone could help enlighten me as to what its role is in that section (amp, switch?) and what I would need to look at to make the circuit accept a silicon substitute, I would be eternally grateful.

As far as available documentation, there is one schematic from the Rhodes service manual for student model, first version, figure 11-19. It, unfortunately, does not include the power supply section with this particular transistor. I posted a request in the schematic request section of the forum. Hopefully that yields some results. One tech suggested I should check out the power supply schematic for the Peterson 80 watt. It is quite similar, but I am afraid it isn't quite the same and my skills just aren't good enough to interpret the difference.

Would another mj15016g work in the power supply section? And what other component values would need to be changed to accommodate the swap?

If anyone has the ability to explain any of the math or theory behind what I would swap and why (on a low-to-mid tech comprehension level), you will become my new personal hero. Thanks in advance for any help anyone can give.

[MarkusBass]

I cannot help you but it seems that the issue was discussed on this forum few years ago. Maybe someone will be able to find a link to the discussion. It would also help if you post the schematic.

Mark

[MichaelNuzum]

Hi MarkusBass- sort of, but not really.

As I mentioned in my first post, I have seen most all of the info about the two output transistors, they are not the ones I am looking for help with. I have done exhaustive research, and while the two output transistors and the effects on that part of the circuit are mentioned in a few different posts, not one mentions the third transistor in the power supply.

The info regarding the third transistor in the power supply is the specific info I am looking for, and trust me it hasn't been discussed here before. I spent a good four to five hours with the search function here looking through previous threads to make sure I wasn't wasting time posting a redundant question.

I'm afraid I need someone who understands this subject much better than I do to take it a little further than it has gone in previous threads. Thanks for your reply, though.

EDIT- also if you read my first post, I already explained I have the one available schematic for the preamp and power amp, but it doesn't have the power supply. That schematic doesn't seem to exist.

[MarkusBass]

I assume that you know the fenderrhodes.com website. Could this one: http://www.fenderrhodes.com/org/ch11/fig11-10.jpg be similar to what you looking for?

Mark

[MichaelNuzum]

Hi Mark- yep. I have spent plenty of hours at that site, excellent resource. It has helped me in restoring my 1977 suitcase 88.

That schematic you've linked to is the 80 watt Peterson schematic that another tech mentioned I look at. Unfortunately, it is somewhat different from the amp section I am looking at, both in component part count, and values. Both are Peterson, but they unfortunately aren't quite the same circuit.

I am afraid I just don't know enough about how to look at the 80 watt and identify the similarities and differences. They are enough unalike that I am unable to get much use out of the 80 schematic.

[Enzo]

My documents are stored away.

But if you already have it covered to go silicon on the outputs, why must we change the third one now? If it is a pass transistor for a power supply, one day it might fail, and we can adjust. If it is a driver, well, convert like the others. IS there a small transformer driving the outputs, thewhat drives the primary of the transformer? Could it be this device?

[MichaelNuzum]

Funny you should ask, Enzo. I was hoping to only swap the two output transistors, notorious unreliability (flames at a gig type stuff). There is a pretty clear explanation of what to do on the electric piano forum (I think that's where it was). I figured I would replace the outputs and test the one on the power supply (regulator?, switch?, amp? I don't know, over my head on that one) to see how voltages checked. It seemed to be part of the power supply/ power regulation circuit, so if I got a good and steady +-35 and +25, I would let it go.

Well, I cranked up the voltage on the variac to do some before/after voltage checks before swapping any components, and one of the resistors on the board fed by the Delco DTG-110B power transistor exploded! I powered down, unsoldered the transistor, and got full continuity on all legs, checking with ohms resistance and a diode checker. So blown transistor, yes?

Again, I am trying to punch so far above my weight with this one, so I apologize in advance if this is some basic stuff I should already know. But I will try to communicate what I think I know as best I can.

Anyhow, I am pretty sure the transistor fried and needs replaced. I can understand that when swapping out the output transistors from Ge to Si, you need to switch some of the resistors with it to change the bias, Ge wanting to see a different voltage than Si(yes?). Problem is, the surrounding circuit that the power tran references to is different enough topology from the one the output references to, I am unable to suss out which parts need to be adjusted to accommodate a Si tran, or if the swap can even be done.

This is where I was hoping to find some guidance.

[MichaelNuzum]

Also, there is a small transformer driving the outputs. The primary seems to be driven by two other transistors, an 1129 and a 1212(?) I believe from looking at the schematic for the power amp.



This is the only schematic for the amp I have been able to find, and it does not contain the power supply. It references another drawing #6803-5 that I have been unable to find anywhere.

The two output transistors are the 1069's at the right side. The two 2.7 ohm resistors are changed to 12 ohm to work with Si.

The only close Peterson power supply schematic is linked in another post above (80 watt version), but it is not the same as the board I am looking at.

I will try later to draw out the circuit to the best of my ability and see if I can come up with something approximating a schematic for that board and power transistor.

[MarkusBass]

It's still not clear what is the problem. Maybe, it's due to the fact that the attachment in the previous post is invalid . Can you correct it?
Can you make photo of the power supply board that you are talking about? Is it different from the board attached below? Even if the component values are different, this is a very simple circuit and must be very similar. And drawing schematic from the board is a very simple task .

Mark

[MichaelNuzum]

Hi mark- the attachment was for the schematic for the preamp and power amp. It does not contain the power supply as I've already mentioned a couple of times. I only included it to answer Enzo's question regarding the existence of a transformer driving the output transistors. It really includes no info for the power supply and isn't the primary source of info here, so don't focus too hard on that as a player in this situation.

The clear problem is that a germanium transformer in the power supply section has gone belly up. I need to know its purpose in life and how to go about replacing it with a properly adjusted circuit set up to accept a silicon diode to prevent another blow up. The schematic for that part of the circuit seems to not exist.

What's not clear is how to do the swap. The schematic I linked above will tell you pretty much nothing regarding that. I don't know why it didn't attach properly, but it is from the fender rhodes site/ rhodes service manual chapter 11 figure 11-19. But again, it would be of no service regarding the power supply portion of the circuit. I only linked it in reference to someone else's question.

As I mentioned before, I will try to draw out the circuit later, but I have a real life and I am already doing this job at a loss for a buddy, so it may a be a day or two before I find the block of time to draw it out. I am sure it is a "very simple task" for some who are used to doing it, but maybe not for everybody else. So it's gonna take me a few minutes.

I was hoping by coming here I would maybe find someone who had some experience and knowledge with this and save me a few steps and avoid some redundant work (like drawing out the circuit by hand if someone had already done it or had a more complete schematic). I am sorry if my terms and experience aren't as in depth and quick as others'. If they were stronger and I had all these "very simple" skills I wouldn't need to ask for assistance, but here I am.

If anyone has any real help and actually knows this circuit, I'd love to hear from them.

[MarkusBass]

It does not contain the power supply as I've already mentioned a couple of times. I only included it to answer Enzo's question regarding the existence of an output transformer. It really includes no info for the power supply and isn't the primary source of info here, so don't focus too hard on that as a player in this situation.

I think you are wrong. The schematic contains a lot of useful information (see attachment). For example, you can see that required voltages are +20V (low current) and +30V (higher current). This is almost sufficient to build the power supply from scratch (using even completely different schematic). Of course, you may also use previously posted schematic - assuming the voltages are adjusted to 20 and 30V.

I was hoping by coming here I would maybe find someone who had some experience and knowledge with this and save me a few steps and avoid some redundant work (like drawing out the circuit by hand if someone had already done it or had a more complete schematic). I am sorry if my terms and experience aren't as in depth and quick as others'. If they were stronger and I had all these "very simple" skills I wouldn't need to ask for assistance, but here I am.

You could really help yourself and all people reading your posts if you post a photo of the power supply board that you have. Come on, this is not a difficult task .
Bellow is the schematic of the preamp and the power amp.

[MichaelNuzum]

Mark- I appreciate your desire to help, but I don't think you are fully reading my posts.

Think about it, if I was able to design a power supply from scratch, I sure wouldn't need to be here asking for help, would I? So let's just get it very clear I am not an electrical engineer, I don't know how to design and modify circuits from scratch, if I did, I wouldn't be asking for help. I am trying to fix what I can on an already existing circuit, not re-invent the wheel by cooking up a new one. Plus keep in mind the vintage value of keeping as much of the circuit original as possible.

If you just want to chastise me for not having the "simple" skill set that you do, I would have to ask you from refraining from any future comments. Sure, maybe this all makes perfect sense to you, but not to me, which is why I am here for help.

Telling me I am wrong in stating a schematic doesn't contain the power supply section when it clearly doesn't is not helpful. The schematic clearly states "for power supply see diag #6803-5" right in the notes section. If your response is "well it shows the supply voltages, so the rest can be inferred", well, see above.

Again, I will try to sketch the circuit and try to draw the schematic plus take some pics when I have an opportunity. If anyone has any real info regarding this unit, please let me know.

[MarkusBass]

I do read you posts. Them problem is that most probably the schematic is not available (due to unknown reason). But it is easy to recreate from the PC board. If you post photos of the board (top and bottom) I will try to draw the schematic for you.

[MichaelNuzum]

I appreciate that, thank you. And I will try to post pics of the board and transistor tonight. Honestly, there's not much there, maybe a dozen components. I am sure several folks could just look at it and tell what each component does.

Problem is, that's only half the battle. The circuit is set up to accept a germanium transistor. The idea is, instead of replacing the blown one with another terribly expensive obsolete part that will probably fail, modify the circuit to accept a comparable silicon transistor which would provide more reliability at a significantly lower price.

That's where I need someone who understands the difference between germanium and silicon, and what would need to be adjusted in the surrounding circuit to make it stable, while maintaining as much of the original circuit as possible.

I will try to get pics and drawing up tonight if I can.

[Enzo]

OK, but you already converted the outputs to silicon. What did you have to do? Change one resistor for each transistor. We all (meaning experienced techs) know the difference between germanium and silicon, and if we can draw out the circuit, we can make whatever changes it might need. In the case of the power amp, if we didn't make the changes, we'd have audio distortion. In the case of a power supply transistor, distortion is not an issue. We might not have to change anything. But worst case is we'd have to change a resistor.

In post #9, Mark posted a picture of a board. From that we can easily draw the schematic of that board. SO we can do the same from photos of your circuit.