Casio DG-20, power supply

Guess what I bought last weekend.. :D

The instrument Darth Vader would use if he was into playing..

Yes, it’s the silly looking (and ridiculously overweight) guitar synth straight from the 80s, made famous by Flight of the Conchords.. the Casio DG-20! You probably now know what to expect to be reading in this blog over the coming weeks.. ;)

I got my DG-20 from Wolf Rami of Nightsatan. Apparently he had bought a few of these and used them to piece together a single working one for his own use.  So I knew exactly what I was getting into and actually bought the unit, if no good for anything else, just for the enclosure and string mech alone. Cheap “raw materials” and all that.. Should the synth indeed turn out unrepairable, alternative uses shouldn’t prove to difficult to come up with. Anyway, Wolf warned beforehand that the electronics have presumably seen a bit of overvoltage and that the battery compartment cover and one headpiece screw are missing.

Arrival check and indeed, the synth doesn’t power up. Lucky me, Dan Newcome has been kind enough to upload the service manual online. This helps a lot with analysing the fault, as there’s no need go about it the hard way and spend (a lot of) time reverse-engineering the power circuit. With the help of the manual, I was actually able to analyze possible problem areas before I even got my hands on the synth!

Schematic for the power supply circuit of DG-20

If you look at the picture above, there are two main subsections in the power supply circuit. When divided vertically in half, there’s the “switch on” section formed by transistors T1-T3 on the left, and the “output” section formed by transistors T4-T6 on the right. Quoting the service manual, the “power on” sequence goes like this:

  1. When the power switch is turned on, battery voltage is provided to the base of transistor T3 causing the transistor to turn on. Current from T3 emitter charges the 10µF capacitor.
  2. The electric charge from the capacitor turns T2 on compulsorily.
  3. Transistor T1 turns on providing the battery voltage to voltage regulating transistors T4, T5 and T6.
  4. After being charged, the capacitor does not provide voltage to T2 base however, receiving VDD (+5V), the CPU provides +5V to T2 base by signal APO.
  5. If the guitar is not played for six minutes, CPU lowers signal APO causing the transistor T1, T2, T4, T5, and T6 off.

..so in other words, the circuit also has a simple “battery saving function”, controlled by the signal ‘APO’ . In case you’re wondering “APO wtf?!”, this is probably derived from something like ‘Auto Power Off’ and not eg. Asian Productivity Organization..

..

Briefly checking for internal operating voltage, the +5V bus measured clean zero. Luckily no short-circuit to GND and no input voltage on the “output” section either, so the fault was now pretty much narrowed down to “switch on”. Checking for the functionality of this subsection step by step, it immediately turned out that T3 isn’t doing much. Here, the voltage “visible” to the base of T2 measured about 0.2V. Just to verify that there isn’t a connection issue between transistors T2 and T3 (they’re on separate boards), I tried to bend T3 back slightly to get a better contact for the multimeter lead. When doing this, a piece from the T3 shell fell off! Take a wild guess which is more likely cause of problem, a damaged T3 or a connection issue between the two transistors ;)

Pretty obvious damage on transistor T3..

The service manual lists T3 part type as 2sc1310F-T or 2SC1740LNSR NPN. Both seemed a bit obscure to be sourced from anywhere (read, very little plausible search results) so I didn’t bother looking deeper into compatible replacements. 2N3904 I have a plenty of, so I figured I’ll just give that a spin. It’s not like the part is doing more than off/on switching, so anything remotely similar should work just fine. Come to think of it, you could also bypass the whole “switch on” subsection, if the APO function is not needed.. Comparing spec-wise, the 2N3904 isn’t exactly a value-for-value match with the 2Sc1730. However it does have higher maximum values on all common ratings (Vceo, Vcbo etc.).

Other than this, 2N3904 isn’t pin compatible either. The base is in the middle pin, whereas on the 2SC1730 this is on the right pin.. A bit of plier work needed, so to say :)

2N3904 viewed from both sides, the bent base terminal is more visible on the flipside.

And that’s about it, synth powers up once again \o/. Gotta admit, the internal sounds are pretty (horrible) bog-standard “Casio 80s keyboard style”, but definitely have to test running them through guitar pedals and such later on ;)

To keep this post short, I’ll save the mech stuff for later. Dan has a bit of DG-20 disassembly pics in his blog post here, in case you want to check out a bit of the mech stuff beforehand. To boot with the situation is this for the headpiece screw:

Side-by-side, a complete and a damaged headpiece screw

I’m thinking a screw and a lathed board riser should work here just fine, but let’s see what I come up with.. Luckily these parts are all metric, so I can get away with M3 hardware I have plenty of. No need for silly imperial body part dimensions ;)

Advertisements

Tags: , ,

6 responses to “Casio DG-20, power supply”

  1. Steve says :

    Thanks for your post. I am having a power supply issue at the moment. Not sure what the problem is yet. This very helpful, thanks Steve (Australia)

  2. Preston Dow says :

    My Casio Dg-10 stopped working a few weeks (or months, I really can’t be sure) after I lost the Casio 9v adapter that came along with it and started using a different 9v adapter. I’m not sure if this is related to it’s failure. It no longer powers on. I’m pretty tech illiterate. I was wondering if you could help me figure out the problem or translate some of this to layman’s terms? Thanks!!

    • Arto says :

      My Casio Dg-10 stopped working a few weeks (or months, I really can’t be sure) after I lost the Casio 9v adapter that came along with it and started using a different 9v adapter. I’m not sure if this is related to it’s failure. It no longer powers on.

      Check that your new 9V adapter outputs DC and that the polarity is correct (negative on center tap). If not, get one that does and test again. If that doesn’t work either, try using batteries. If your DG-10 still doesn’t power up, then it’s broken somehow.

      You’d also want to ensure that the center tap diameter of DG-10 matches the recess on the 9V adapter connector (measure both with a caliper). If the latter is wider than the former, the two won’t make proper contact.

  3. Simon says :

    Hey,
    I just used my DG-20 again and found out that the Drum machine is not working properly anymore. So I tried to google and stumbled on your blog and thought you might have an idea what the problem is.

    So everything works fine but the drum sounds. I can just hear cracks and noise. I also tried it with headphones.
    Do you have any ideas what I could check?

    Oh and I think I once accidentally used an AC adaptor because it says “AC adaptor AD-5” on the back (which was dumb of course but even worked).

    • Arto says :

      So everything works fine but the drum sounds. I can just hear cracks and noise. I also tried it with headphones.
      Do you have any ideas what I could check?

      If you look at page 3 on the DG-20 service manual (thanks Dan!), there’s a custom Percussion Generator chip (PG) around section M-6. It’s output is fed to a filter circuit and then on to the summing op-amp mixer which combines the Percussion and Melody signals (page 4, sections F-6 & G-6).

      If the synth sounds are ok on your DG-20, then the mixer section is likely just fine and you should look for faults in the remaining circuitry. What you should (and can) check are:

      1. All PG circuitry solder joints are ok (no fractures etc.)
      2. VDD on pin 28 of PG reads +5VDC and doesn’t fluctuate
      3. Reset on pin 1 of PG is not active (+5V) or fluctuating above hysteresis level (usually around +3.5V for 5V logic levels)
      4. 256kHz clock signal on pin 2 of PG is steady and it has good logic states (clean 0’s and 1’s)
      5. _WR & _CS signals (active low) on pins 4 & 5 of PG are working and have good logic states
      6. Data bus signals D0-D7 on pins 6-13 of PG are working and have good logic states
      7. Transistor T2 in the filter circuit is working

      You will need a magnifier (or microscope), a multimeter and an oscilloscope to perform these checks. Most of the steps above revolve around verifying that the PG chip has all the “basics” it needs in order to operate properly. _WR & _CS signals will likely only be active when you press a button to play a drum sound or select a rhythm. If everything on the list is good then it’s safe to assume that the PG chip is busted.

      Unfortunately I don’t have my DG-20 anymore so I’m unable to help you further with the problem.

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s

%d bloggers like this: