MDC Classic 42 - help with repairs

[edhorgan]

I have an allen mdc classic 42. When playing I heard a pop and the sound stopped. After unplugging and replugging it in, it worked for under a minute. I have checked all the fuses, voltages out of the power supplies are correct, the connections are connected. I called the allen repair man and he said it is either the mdc board (main controller) or the audio processor 2 board. I am a high school student with limited funds. Any suggestions on how to repair myself?

Moved to correct forum section where it should get more attention. Andy-Moderator.

[circa1949]

I have an allen mdc classic 42. When playing I heard a pop and the sound stopped. After unplugging and replugging it in, it worked for under a minute. I have checked all the fuses, voltages out of the power supplies are correct, the connections are connected. I called the allen repair man and he said it is either the mdc board (main controller) or the audio processor 2 board. I am a high school student with limited funds. Any suggestions on how to repair myself?

Moved to correct forum section where it should get more attention. Andy-Moderator.

I guess Andy moved this here from somewhere else?
Working for under a minute sounds like a bad power component overheating. (just my guess, people like don60 are vastly more qualified to answer this question.) Have you sent the audio signal from the MDC board to an totally external audio system (home amplifier for example) to see where its the amp or the MDC board? Isolate everything this way - isolate the amp, the audio processing board...you have to narrow down where to look. If it is the MDC board and you want to repair it yourself, I supposed you'd look for bad or bulging capacitors or transistors. Electrolytic capacitors have a finite lifespan, period, and with an old organ that might not have been used for years before you bought it, they will fail even faster because they depolarize or something. This is apparently not a myth: http://www.eaton.com/ecm/idcplg?IdcS...ILE&dID=204704

[indianajo]

I am a high school student with limited funds. Any suggestions on how to repair myself?

Just because the power supply is okay at the power board, doesn't mean it is okay at the IC or transistor. Power supplies are frequently "buffered" at the using IC to provide a local stable supply. These small electrolytic caps have even shorter life than the big ones in the power supply, due to the small ratio of volume containing water, to the rubber seal area. Your meter averages the power supply over about 4 seconds, whereas 1970's IC's need stable power one microsecond at a time. Later IC's are even faster.
Allen techs are trained to identify which board (PCB) is bad, and then swap that board with the factory. This is not a viable repair technique for an amateur on a budget. Nor does the advice they give, or that in the manual, help much with identifying bad components.
Components that fail in a 30 -40 year old consumer electronic device are in order, electrolytic capacitors, corroded connectors, toasted or out of value resistors, (particularly if non-US sourced) transistors (If early Japanese). Some power supply components like regulators or rectifiers tend to be toasted by out of value e-caps. High usage church organs have problems in the keyswitch and key mechanics areas, particularly after 1980 when rubber components were used in the keys by many vendors (not typically Allen). Allens have a particular problem with memory backup batteries leaking at end of life and corroding all sorts of connectors and PCB traces. Some PA equipment tends to toast output transistors of the power amplifier and all the driving parts, but organs tend to have permanently connected speakers and don't have problems with this unless the heat builds up due to excessive on time, excessive dust, or stuck fans. There is lots of help on the internet with repairing blown up power amplifiers, for example, which pertains more to PA and band use than organs.
Note some of the the professional organ repairmen contributing here, and the moderator, have entirely different opinions, which I don't share. New equipment and 20-40 year old consumer grade equipment have rather different failure patterns, IMHO.
The best path would be to buy a community college text like Thomas L. Floyd, Electronic Devices, the Electron Flow Version and learn something about the proper design and function of power supplies, and meters as test equipment. That text concentrates on problems occuring in prototype test, which is nothing like fixing 40 year old consumer electronics, but it is a good start. IC's other than voltage regulators are way down the list of things that fail, and their problems tend to be in the connection area anyway, because some brands used cheap (non-gold) connectors.
After you get the power supply possibilities out of the way, goes-into comes-out-of analysis is useful, as indicated by circa 1949. As he said, fading or improvement over a short time is a failure mode of either dried up electrolytic caps, or power semiconductors. Transistor devices take DC power and either oscillate, or amplify a previous oscillation, or change the character of it (frequency distribution) in some way, or mix several sources of oscillations. Finding an area where the right inputs are there, but the wrong things come out, is the basis of component level diagnosis and hopefully, repair.
As referenced already, don60 and jbird may have specific advice about the MDC that I don't share because I don't and won't own an Allen. I prefer brands with open source schematics, industry documented IC #'s on the schematic gold key contacts, and soldered joints like Hammond (pre 1975) and Wurlitzer. All the models in my tag line are typically available for <$100 plus the cost of moving.

[edhorgan]

I used a voltmeter to check the voltages and everything was right.