Seeking guidance on a 4 pin 24v regulator repair

[Parttimesavant]

First I'd like to assure anyone that would like to assist me here that I have some basic experience working on electronics and am informed well enough to work safely on these components. I have referenced the schematics and repair vids from Vintage Vibe and have tested all points usually associated with 24volt regulator issues. These include correct readings on the regulator connectors......+24 volts on the middle -36 and +36 to the left and right on both connectors. I have also gotten a good reading from the 5 Ohm 10 Watt resistor connected to the regulator. I have a good functioning preamp that sounds great going through two other 4 pin suitcase bottom power supplies, but when I connect the same prepamp  to the supply in question it shows no activity even though there is 24 volts across pins 1 and 4 right up to the pre amp end of the cable. The power modules are fine as I have also tested them separately. So, with a good preamp and 24 volts coming right up to it, what could it be? Should the 24 volts at least be able to power the prepamp enough to see the vibrato lights come on or hear a signal through the headphone jack without having the supply connected to the amp modules? Could there still be a problem with the 24v power transistor even though it shows a 24v reading at the module connectors? I have searched for similar instances of this problem in as many places as I can. Vintage Vibe does not currently have an electronics tech available so I reach out to this knowledgeable forum. Thanks for any assistance and happy holidays. -Mike         

[The Real MC]

The RCA connector on the harp assembly has been known to go flaky.  Run a cable direct from harp to another amp to see if the harp works. Do this on your known working piano first, then on the patient.

[Parttimesavant]

Thanks for the reply. I had already checked the harp RCA connector at an earlier time in the troubleshoot though. 

[Max Brink]

If you're getting all the voltages I would go back and double check all of the contact points and fuses to make sure that the signal isn't getting lost somewhere. It's common to find that the solder tracks of the Peterson's boards were accidentally crossed with one another by someone else that previously worked on the amplifier and it's important to double check every point and track.

Here's the first two things that I would double check that will help point us map out the amplifier and its issues:

1) Disconnect the power modules from the power regulator. Double check all of the voltages for the six nodes of the plugs that run to the power module boards. If these voltages check out, we should hopefully be able to move beyond the power regulator assembly to the module boards (after test 2 below). --From here we should be able to hone in on the module or the regulator depending on what's occurring.

2) Can you run the external amp outputs and get any signal? --If so, the problem is going to be on the power module and we can start to take a close look at that.

From there we should have a better foundation to work from and figure out where to look next.


Warning for anyone else who may be working on these amplifiers: The voltages stored even after being turned off or unplugged are dangerous and potentially lethal. Make sure that you are using all of the proper precautions to drain the circuit before working on it. Any electrical troubleshooting advice I am offering is under the assumption that you are aware of all of the risks and dangers of working on electrical equipment and know how to keep yourself safe from harm.

If you are interested in having it sent to my workshop I would be more than happy to service the amplifier for you. I can even send you a shipping label for you to drop off the amplifier at any UPS drop-off location. You can find my contact information in my signature and I am available at all reasonable hours of the day (central time zone).

[Parttimesavant]

Solved! Thank you Max for a well thought out, careful, and accurate response.  I would also like to thank you for your note of safety. It benefits a forum very much when people make an effort to take care with potentially dangerous things. 

I have poured over this issue for some time testing components individually and so on with all sign pointing to good regulator function. After a short break and some time away from the project, it appeared that the factory at Fullerton revealed some unfinished business. One of the leads to a 470 Ohm resistor that sits on the regulator board was never soldered to the board! It had the characteristic squashed\cut leads that the factory used to hold the components to the board better (making removal difficult) and holding this resistor in place with no solder since 1972. After all this time going over and over all other points on the unit, I need to stress the importance of prioritizing diagnosis. I had checked all solder points for cracks and such as I would do in any electronics examination, but, it goes to show that it's important to take time with the simplest possibilities first as one can spend much more time on the complex ones. Such was the case here. I encourage anyone with a similar 24 volt regulator issue to THOROUGHLY check your solder points!

There is on last thing. Now that it is working through the speakers it seems one channel is favoring more tone. Not so much as to say that one side is louder than the other, but when the vibrato is at max intensity and slowest speed with bass notes played it is more "tonally" pronounced in one of the channels. It is as if one channel has been EQ'd to bring out more low mid. How can we balance these channels? Could this be a case for a preamp recap or amp module rebuild?

     

[Max Brink]

You should be on the right track with the caps for the EQ. Unfortunately, while I'm strong with trouble shooting I still have some work to do when it comes down to amplifier circuit theory and couldn't tell you exactly where to start there. When I get into that territory I have a couple close friends that have designed their own amplifiers and I would probably take one of them out for a happy hour to pick their brains on the subject. But I might be able to get you started on the right track and hopefully you could share what you find.

First, I would determine if the EQ is coming from the preamp by checking the external outputs.

Next, I would be to check the voltages channel by channel to see if they are identical. Since a lot of the resistors may vary by +/-20% you'll find that two amps of the same exact specs may sound different from one another side by side with the same settings. If you start to notice a difference in voltages, you may find that a couple of the resistors were off and that variance is causing the shift. After checking voltages and resistors within the circuit, I would grab a digital capacitor reader and test the caps to see what the values were on the side that was preferred. In both cases, if you end up grabbing parts from Mouser or another electronics wholesaler order a few extra so that you can test them for perfectly matched channels.

[Tim W]

I actually think you are experiencing the magic of the suitcase speaker cabinet.

Many folks think that the stereo pan from the Rhodes preamp circuit is the whole Rhodes stereo effect, but that is really only 50% of the story.

That is not to say I haven't seen preamps or power amps with significant differences on the outputs, but if all appears to be working and measuring right, especially after you swap amp modules, inputs, outputs, check with external amps from the line outs, etc...

The Rhodes suitcase speaker cabinet is ported on only one side (the left as you sit at the piano).  The speakers on the left side are very close to these ports where as the speakers on the right side are far from them.  The left speakers virtually or almost run in an 'open back' configuration because of the port openings- the speaker cabinet's resonance has little effect on the sound from the left speakers.  The right speakers, however, are far from the ports and there is a significant resonant chamber created between these speakers and the ports.  As a result, you get more bass response from the right set of speakers.  Now, as the sound pans from right to left, you not only get the direct pan effect, but also a shift in tone and EQ.  There are also some effects from the room, reflections, and the like as the sound moves-- but the cabinet porting is more dominant.  Rhodes Suitcase magic!

Some of you already know I am working on my own Rhodes preamp (called the PeterPan) that EXACTLY captures the Peterson panning envelope.  In addition there will be a cabinet simulator added to the circuit to mimic the suitcase cabinet effect (which can be disabled if you don't want it).  Using a hybrid of analog and digital techniques, this can be accomplished.  No optos, LEDs, or light bulbs to burn out, drift, or match.  Audio path remains entirely analog.  The proto works, I hope to have the product design finished relatively soon.  I spend more time trying to work out the physical interface and control layout than the circuit itself!

I used to work in industry designing high performance analog and mixed signal ICs for medical and automotive apps.  I will always be envious of Doug Curtis (RIP) of CEM fame who was lucky enough to be in the right place at the right time and have the opportunity to design ICs for synths.  Even he had to move into designing for commerical and industrial markets when the analog polysynths started dying off in the 80s.  I left industry to start Retrolinear, thinking I'd have time to design lots of cool electronic products.  Well, starting the business and being inundated with service work has still taken significant time away from the design process!  But it is coming....

In any case, here is a video of the 1st proto in action.  Please excuse the poor audio quality (due to camera and editing) and mismatched amps, etc.

http://www.youtube.com/watch?v=uy2opjzG8T4

Comments/suggestions are welcome!

Best,
Tim
retrolinear.com

[Max Brink]

PeterPan! --What a great name! It sounds like you're more than on the right track capturing the Peterson. With the camera's sound capture I wouldn't have bet against it being the Peterson on a blind test. I'd love to hear a better capture once you get one to share.

Thanks for the input on the ported cabinet too. That's a great consideration and something that I haven't thought about myself. The Peterson's tremolo still has a ton of charm recorded direct and through the Satellite sets but I never considered the affect of the port's location on the tone. I look forward to giving it a closer listen when I get back to the workshop.

[Abraham]

Besides the "ported" fx, one of my channels delivers more output than the other, not just on speakers but while using headphones also, where it becomes more evident.

I have accidentally noticed that whenever I disconnect all speakers from the main amps, output gain becomes balanced on the headphones... might be a clue for someone understanding how this works

I have swapped channels and the problem also got swapped, to my knowledge this points to the main amps not the power amp/speakers

I'd greatly appreciate any help getting this fixed, thank you

[Parttimesavant]

Well, after hearing what sounded like a greater bass response on the right side of the cab from the player side facing it, I considered the ported effect and just thought I would swap the speaker plugs on the amp inside to see what would happen. To my surprise, the bass response went to the left side of the cab! So I now know that while the acoustic effect of the porting is a reality, it is not enough in this case  to cause the one channel to sound with more tone to one side. I had to dig deeper.......On each of  the amp modules inside the cab is a variable resistor that that governs gain control. I found that I could perfectly balance my A and B channels to my liking just by making the gain adjustments from the amp modules! I would recommend that anyone having trouble with "uneven" vibrato or the like to consider a gain adjustment from these points once they can determine that all components on the modules and preamp are functioning normally. 

[Abraham]

hey, could you please point which component to tweak on the power amps in order to adjust gain?? this would fix my problem

thank you

[Max Brink]

@Abraham: According to the schematic all resistance values may range up to 10%. In my workshop I account for 20% variance before I usually consider replacing the part as long as the tone is acceptable for my client. This variance is one of the reasons that people will say that amplifiers are "hit or miss" or why you may be able to google the exact specifications of Clapton's Marshall amplifier that he used on Mayall's "Beano" album after that album made the guitar-tone history (just a hypothetical example, but I'm pretty sure you could find something analogous). Once amplifiers are mass produced with these sorts of variances throughout sometimes hundreds of components their tone will vary at what would be a notably wider margin than 10%.

So here's where my thought process begins as I work through the situation that you are describing:

1) Since you are experiencing the differences on your headphones, I still would go back to check whether or not the same difference is experienced through the external outputs(?). If you find the same difference on the external outs, then the problem lies in the preamplifier (or in a worst case scenario on both preamplifier and power module).

2) If it is in the headphones and not the external output, then you have narrowed down your issue to the power module boards. If so, then I would begin to compare your desired channel to your less favorable channel and find a way to emulate the desired channel.

3) There is only one variable resistor within the power module boards. It is the very first resistor from the RCA input that leads into the base of the first resistor, 2n2291A, and as long as you passed the two tests above you will probably find this variable resistor useful in matching the output of both of your channels.


Once again: The voltages stored in a circuit--even after being turned off or unplugged for a significant amount of time--are dangerous and potentially lethal. Please take all necessary precautions to drain the circuit before working on any components. Any electrical troubleshooting advice I am offering to this public forum is under the assumption that you are aware of all of the risks of working on electrical equipment and that you know how to keep yourself safe from harm.

If you are not 100% confident, I would love to service the amplifier for you and I will even help arrange shipping for you to and from my workshop. You can find my contact information in my signature and I am available at all reasonable hours of the day (central time zone).

[Abraham]

I have yet to figure out how to properly measure channels gain accurately, anyway I couldn't hear a difference while playing through speakers. Still have to make SURE but I think its just the headphone out and I will first try to locate such variable resistor and tweak a little. I had previously noticed this component but didn't dare to touch it before I could make sure this won't screw things up.

Thank you Max, I would consider shipping my amp but, living in Spain it doesn't seem like a real option... so that I have to work things out myself