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Messages - Coyote Circle Studio

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1
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.

2
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. 

3
Please correct me if I am wrong, I have a different model (1977 88 suitcase) and don't want to assume, but I thought the rca direct off the harp was before any of the tone (or bass boost) or volume  controls. 

If that is correct, then if you were to split the signal at the rca direct out of the harp, you would have one signal going as normal to the tone or volume controls then going to the tip terminal, and a second direct signal straight off the harp going to the ring terminal.  That way you could use the TRS y-cable (insert cable) and have access to the normal signal path (tip) and/or the direct unaffected signal path (ring) as you choose.

You would bypass the tone/bass boost/volume by only sending the ring terminal signal to whatever amplification/recording device.

I may have confused things by previously saying "preamp in" and "preamp out" instead of "tone/volume in and out" as I was looking at my 88 suitcase with facerail preamp.  I apologize for that.  Nonetheless, it would be the same idea even with passive tone or volume controls.

Adding that push/pull pot still leaves a component in the audio path, allowing some amount of insertion loss.  My method takes a direct tap off the harp, no switches or other components in the signal path, which as I understand, was the o.p.'s original goal for recording.  Less potential for signal degradation.

4
It might be a tad bit convoluted, but you could wire up a TRS jack that replaces the standard TS jack.  That way no modifications like drilling holes.  Split the signal send from the back of the harp, and send one signal as normal to the preamp and the other to the TRS jack ring terminal.  Wire the jack with ground to sleeve, preamp out to tip and direct from harp to ring. 

Then make yourself a few custom cables.  Make one that has a TRS plug on one end and TS on the other.  Only wire the tip and sleeve.  Plug the TRS end into the output jack of the piano and it will not send the direct output anywhere, only the regular preamp out to an amp.  If you use a regular TS instrument cable, it will short the signal from the harp to ground, you don't want that.  You need that ring connection on the output jack side to float with no connection out or to ground.

For recording, get a TRS Y-cable, send the tip (preamp out) to your amplifier, and the ring (direct from harp) to a di box.

It does require making at least one custom cable, and a few minutes of soldering, but it would be completely reversible, and not screw things up by drilling extra holes.

5
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.

6
Here is the board in question.  The 820 ohm resistor blew up and has been removed.

7
Wow- beautiful instrument.  Unfortunately, no, The one I am looking at looks pretty different in the guts.

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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

9
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!

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