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200a transistor testing

Started by juniornoodle, July 27, 2014, 04:01:50 PM

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juniornoodle

Hi everyone,

I have been working on restoring a 200a the playing condition. Until recently, my work has been focused on cleaning and lubricating everything and regulating the action. However, I just turned the piano on, and am getting no output sound (though the piano powers on just fine). This was not the case when I acquired the piano, though I have been pretty gentle with everything since I've had it. I've consulted the manual already and see that, aside from checking the power source, the next suggested aspect to troubleshot is voltage values on the various transistors. Is checking these values the best course of action I should take?

If so, could I please get some instructions on the step-by-steps for testing the voltage on the transisitors. I have already begun to try this and am pretty confused as I've had no experience with troubleshooting electronics in this way in the past. Answers to all of the following would really help:


  • which lead on the multimeter (red vs. black) do i need to touch to which lead of the transistors (emitter, base, collecor)?
  • which transistor lead is which? i don't know the difference between the emitter and the collector.
  • how do i determine which voltage value shown on the schematic i should be reading? for instance, for tr-1, i'm seeing 3 values on different sides of the transistor, 2.45, 1.95 and 4.1.

Thanks for putting up with this question. As I'm sure you can tell, I am a bit out of my element here and could really use the assistance. I will also need to follow up on questions about testing filters and diodes, as per the service manual, if there don't appear to be issues with any transistors. By the way, I've already checked for bad soldering joints and can't find any.

cinnanon

#1
Before you do that, perform a reality check on the piano. I bought a 200A about 3 months ago that supposedly had a blown amp and was very distorted. No sound except static was coming through the speakers. The line out was heavily distorted and had low volume. I got it home, opened it up and found numerous things wrong. The "original" power cord was actually wired backwards (meaning the hot was the neutral, and visa versa). The piano was wired correctly though. Someone just replaced it with the wrong cord at one time (original oval socket kind too). The speakers were disconnected (and they were wired in series instead of parallel!). So then I checked the on-board fuse. BLOWN! No wonder. Replaced the fuse and fuse holder with the radioshack one. Rewired the speakers, got the right cord, checked to make sure my reed bar wasn't shorted out to ground, turned it on and voila!

Perfect!

Make sure you work on it with power disconnected too. No real need at this point to do anything with power connected. I thought for sure the transistors would be blown, but they are perfectly fine. Good luck!

pianotuner steveo

#2
I would be sure the fuses are not blown first. I have seen the transistors go, but it is usually the fuses on the amp board.
1960 Wurlitzer model 700 EP
1968 Gibson G101 Combo organ
1975 Rhodes Piano Bass
1979 Wurlitzer 206A EP
1980 Wurlitzer 270 Butterfly Grand
2009 73A Rhodes Mark 7
2009 Korg SV-1 73
2017 Yamaha P255
2020 Kawai CA99
....and a few guitars...

juniornoodle

Thanks for the replies guys. I actually did look at the fuses already and they do not appear to be blown. By this I mean that there is no black residue on the inside of the glass or other similar signs. Is this a sufficient test of whether there is a blown fuse? Should I also be testing the fuse in another way? As I said, when it comes to electronics, things may need to be spelled out for me a bit. If just looking at the fuse as I described above is sufficient, what should I look at next?

Thanks again for the help.

pianotuner steveo

With the power off, check the fuses with an ohm meter. Good fuses will read "0" ohms (I.E. shorted) , bad fuses will read open (no reading at all)
1960 Wurlitzer model 700 EP
1968 Gibson G101 Combo organ
1975 Rhodes Piano Bass
1979 Wurlitzer 206A EP
1980 Wurlitzer 270 Butterfly Grand
2009 73A Rhodes Mark 7
2009 Korg SV-1 73
2017 Yamaha P255
2020 Kawai CA99
....and a few guitars...

Alan Lenhoff

When you were cleaning and  lubing everything did you do a lot of dis-assembly (like pulling the action out of the case, removing the reed shields, etc.)?  Several years ago, I did that, reassembled everything, turned on the piano --  and it was silent. Turns out that in re-assembling things, I had attached a preamp bracket to the wrong part, which shorted the reed bar to ground.  Corrected that, and I was back in business.

So, the first thing I would do would be to take a DC voltage reading from the reed bar. The piano won't work without voltage there.

Alan
Co-author, "Classic Keys: Keyboard Sounds That Launched Rock Music"

Learn about the book: http://www.classickeysbook.com/
Find it on Amazon.com: https://www.amazon.com/dp/1574417762/

1965 UK Vox Continental;1967 Gibson G101 organ; 1954 Hammond B2; Leslie 21H; Leslie 31H; 1974 Rhodes Mark I Stage 73; 1972 Rhodes Sparkletop Piano Bass; 1978 Hohner Clavinet D6; 1968 Hohner Pianet N II; 1966 Wurlitzer 140B; 1980 Moog Minimoog Model D; 1983 Roland JX-3P; 1977 Fender Twin Reverb; 1983 Roland JX-3P synth; Vox AC30CC2X amp.
(See the collection: https://vintagerockkeyboards.com/ )

juniornoodle

Thanks very much for the tips. Yes, I did do a lot of dis-assembly, including pulling out the action and removing the reed shields. At the time of my previous post, I didn't have the preamp screwed down to its mounting brackets, so I re-installed the preamp and am now getting some sound out of the speakers, though it cuts in and out and is accompanied by popping sounds. This sounds like maybe something is shorting out on my piano as well. Not sure what would be causing this, but perhaps if I experiment a bit, I'll bump into the issue. Any suggestions given this new information? 

Alan Lenhoff

This was my issue, as I recall:

There's a bracket that attaches the far side of the 200A preamp board to the reed bar. I had it attached on TOP of the reed shield. (Those shields are grounded, and doing this grounded the pickup rail.) The bracket was supposed to be attached UNDER the shield.  Making that change fixed my problem.

Alan

Co-author, "Classic Keys: Keyboard Sounds That Launched Rock Music"

Learn about the book: http://www.classickeysbook.com/
Find it on Amazon.com: https://www.amazon.com/dp/1574417762/

1965 UK Vox Continental;1967 Gibson G101 organ; 1954 Hammond B2; Leslie 21H; Leslie 31H; 1974 Rhodes Mark I Stage 73; 1972 Rhodes Sparkletop Piano Bass; 1978 Hohner Clavinet D6; 1968 Hohner Pianet N II; 1966 Wurlitzer 140B; 1980 Moog Minimoog Model D; 1983 Roland JX-3P; 1977 Fender Twin Reverb; 1983 Roland JX-3P synth; Vox AC30CC2X amp.
(See the collection: https://vintagerockkeyboards.com/ )

juniornoodle

Thanks Alan! I was able to solve my issue just by re-installing the preamp properly and making sure the ground wire was attached. I hope this will help others down the road who have as little knowledge about electronics as myself.

Alan Lenhoff

You're welcome!  I love it when you can find a simple solution to a problem.

Alan
Co-author, "Classic Keys: Keyboard Sounds That Launched Rock Music"

Learn about the book: http://www.classickeysbook.com/
Find it on Amazon.com: https://www.amazon.com/dp/1574417762/

1965 UK Vox Continental;1967 Gibson G101 organ; 1954 Hammond B2; Leslie 21H; Leslie 31H; 1974 Rhodes Mark I Stage 73; 1972 Rhodes Sparkletop Piano Bass; 1978 Hohner Clavinet D6; 1968 Hohner Pianet N II; 1966 Wurlitzer 140B; 1980 Moog Minimoog Model D; 1983 Roland JX-3P; 1977 Fender Twin Reverb; 1983 Roland JX-3P synth; Vox AC30CC2X amp.
(See the collection: https://vintagerockkeyboards.com/ )