Electronics > Repair

Repair of Optimus MD-1150 Keyboard & Strange Power Supply Failure


Greetings EEVBees:

--This Optimus MD-1150 Electronic Keyboard, from the dumpster, was in perfect shape, and missing only the music stand and the power supply. It needed a 9 Volt, 1 Amp, 5.5 mm barrel plug with center negative. I took a center positive 12V supply and switched the leads to make it center negative. I then plugged it in and as expected, no joy. A DMM showed that the power input jack had broken a trace on the board. After resoldering and reinforcing the power input jack pads, the keyboard was working in all particulars.
--While recapping I measured the ESR in and out, of each cap, and what oh my daddy-o, they are nearly all either out of spec or about to be out of spec. All were replaced with good name brands. The 22, 33 and 47 at 50 Volts, and the rest at 25 Volts. The contemporaneous notes on the caps are included below. Almost none of the caps were good but the unit functioned normally, but I was afraid to leave 85C Elna's in place.
--I needed the 12V PS adapter to drive  a universal CCFL to power the lamps on a small TV. So, I bought 9V center positive and switched the leads, IE cut the power cable with the red and black wires and soldered red to black and visa versa. I plugged it in and slid the power switch to on. The display lit up very brightly, there was a popping noise, and the display went off. I thought I had put an electrolytic in backwards, and had caused an explosion, but checking the PS adapter showed zero volts. Opening the adapter soon revealed the problem. These 9V 1A PSs are made from a repurposed USB PS, and the stubs from the USB circuits were left on the board. One stub is very near the Line prong when the board is put in the case. The board was also loose. So the stub touched the prong creating an arc and a tiny explosion  melting the Line wire off of its prong and scorching the resistor.
--The weird part is one of leads of the 9V zener was cut as if by laser, and with no scorching or signs of heat. The resistor measured up to spec, and so did the zener, so I soldered the zener leads back together with a blob of solder and Bob's your uncle. Of course I replace the PS adapter caps as SOP. When I reassembled the adapter, I put some extra rubber insulation to guard against shorts.
--There do seem to be a lot of these Two Hung Lo adapters out there, and they do seem to be a fire risk, so I recommend, inspection and repair before use.
--Even though this is a Radios Hack product, it sounds quite good and is a very usable, and rugged student instrument. I am learning how to play "If I Only Had a Brain".

Optimus MD-1150 Electronic Keyboard
Name   Brand   T     Value   Volts   ESRin   ESRout   Cond
C202   Elna      85   22   50      0.79      0.78         Bad            
C204   Elna      85   22   50      0.84      0.82         Bad
C207   Elna      85   22   50      0.80      0.78         Bad
C213   Elna      85   22   50      0.80      0.81         Bad
C214   Elna      85   22   50      0.80      0.80         Bad
C227   Elna      85   22   50      10.0      10.0         BadBad
C233   Elna      85   22   50      10.0      10.0         BadBad
C206   Elna      85   33   50      0.68      0.62         Bad
C223   Elna      85   47   25      0.66      0.67         
C224   Elna      85   47   25      0.10      0.62
C239   Elna      85   47   25      0.69      0.69   
C220   Elna      85   100   16      0.68      0.68         Bad
C221   Elna      85   100   16      0.67      0.64         Bd
C222   Elna      85    100   16      0.62      0.62         Bad
C237   Elna      85    100   16      0.60      0.59         Bad
C218   Elna      85    100   16      0.60      0.68         Bad
C203   Elna      85    470   6.3      0.27      0.27         Bad
C219   Elna      85     470   16      0.03      0.14         ?
C205   Elna      85   1000   6.3      0.13      0.14         Bad
C212   Elna      85   1000   10      0.12      0.12         Bad
C215   Elna      85   1000   10      0.09      0.11         Bad
C201   Elna      85   2200   16      0.03      0.05

"In the heat of battle, my father wove a tapestry of obscenity that as far as we know, is still hanging in space over Lake Michigan."
Ralphie Parker

Best Regards
Clear Ether

Good job fixing it but I would say most of those capacitors were ok.

Dispose of that 9v adaptor it is too risky. For instance, there is a 10v capacitor on a 9v rail not much of a margin. Also I wouldn't think it was originally designed to supply 5v at 1A let alone 9v at 1A.
Buy yourself a 9v 1A adaptor from a reputable supplier.

This, like most Radio Shack products is a rebrand of a major brand.  In this case a Casio CTK 541


Several years back I purchased a pair of Yamaha speakers from Incredible Universe (remember them?) and about 4 years into ownership, one of the cones started rattling.  They had a 5 year warranty on them through Yamaha so I took them in to Incredible Universe and they sent them off only for them to completely disappear with no word for several months.

Finally Incredible Universe issued me a credit for them but they had nothing comparable in that price range, so they gave me a full refund.  I went to a Radio Shack and found that same exact set of speakers, for the same exact price I spent, but sold under the Optimus brand.  So I bought them and they've been just as good as the Yamahas and been working fine for at least 15 years now.

Anyway, here's the service manual for you:


Is it just me or does anyone else read all of SgtRock's posts in the voice of Sgt. Hartman from Full Metal Jacket?


--- Quote from: Stonent on June 30, 2014, 08:29:13 pm ---Is it just me or does anyone else read all of SgtRock's posts in the voice of Sgt. Hartman from Full Metal Jacket?

--- End quote ---
I read them for the great quotes. They are a nice touch.  :-+


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