ARP Pro/DGX (Pro Soloist) No Output Repair - Synthchaser

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We kick off the New Year on the Synthchaser channel with troubleshooting and repair of this ARP Pro/DGX Mk I, which is the same as the ARP Pro Soloist with the exception of having pushbutton switches for voice selection instead of toggle switches. This Pro/DGX has no output at all, so together we troubleshoot and repair it, and in the process I explain how one of the least understood circuit boards in the synthesizer works.
My oscilloscope: amzn.to/3rm6b8d
My desoldering tool: amzn.to/3PikaEf
My soldering station: amzn.to/3rudVF4
Parts, service and synthesizers available on my website. I buy broken synthesizers!

[@perry3928]

Hello. My 1st synth was the eml500. Wish I still had it I'm 64. My father owned 2 TV stores. I managed 1 and did component level repairs on tvs, vcrs, and stereos. Just wanted to say thanks. This brings me back to reading schematics and with scopes, meters, bench power supplies, and fixing them. Thanks for a blast from the past.

[@fetaldreamcore]

Thanks for the detailed walk thru of function, schematic, and trouble shooting.

[@danieleden1856]

Mum.......! New synthchaser video is out!

[@jimgiantomasi2121]

Thanks brother. Your videos are great and so much appreciated. I bought a dead DGX many years ago and haven't looked at it in a long time. Same problem, no output, PS good, the buttons all work. Great tip to look beyond the sound generation. I'll recap this thing when I eventually get it working. Renewed interest in these things thanks to the Cherry Audio VSTi. Rock on!

[@StKnightis64]

Excellent work here. I used to have one just like this. Looks like you’re hot on the trail of getting it playing back like new. 👍

[@Pablo668]

Nice work. I'm no technician, but a couple minutes before you said it, I wondered why no IC sockets. All works out in the end though I guess.

[@pepemogt]

Happy New Year!

[@blenderbuch]

Your videos always inspired me. I always wanted to repair a vintage keyboard. Three weeks ago I found a logan string melody mk1 on the flea market. 😮 water damage. But now it plays again. Still plenty to do, a handfull keys and voices are missing so i am on my way tracing it down. All the vintage chips seem to be fine!

[@daffyduxxx]

Thank you for sharing you extensive knowledge, that was a really interesting fault with a great explanation of your fault finding journey.

[@stormstereo]

Just discovered your channel. I'm no electrician or service engineer but this is so darn interesting for some reason. I dabble a bit but only with super easy stuff. Boom shakalaka, you've got youserlf another subscriber.

[@Sjobeck7]

Excellent information and breakdown. Thank you!

[@sawsquaresinetube]

Happy New Year Synthchaser 🎉 Great video, love seeing that clean keybed! Thermal imaging was “cool” 😎

[@edda673]

Happy New Year Synthchaser!!! Great video again! It ended way too soon :-)

[@shahruzmoshtael]

Great work

[@justinmijnbuis]

Great video! Although you had good reason to remove the board, if the only goal was to replace the chip you could simply cut off the +5V pin close to the chip, piggyback a new chip (or even a socket if there's headroom) and reconnect +5V separately.

[@PWMaarten]

You could. But seems the better solution to me.

[@justinahrens1868]

Great video!

[@SCOTTYD2031]

Happy New Year! I still ❤❤❤ your videos. Keep 'em coming. I already watched all of them. I even repaired a Clavinova CVP-7. Thanks!

[@JerryZard]

Very interesting video Synthchaser, thanks for all your great work... my question is, why did Arp make such a complicated keyboard scanning circuit for a 3 octave monophonic synth? What benefit did it offer compared to the typical kind used on something like a Minimoog?

[@Synthchaser]

It seems unnecessarily complex to me as well and I don't see any upside. Besides being more costly to make and more failure prone, it precludes the synth from being easily modified to take a CV in, and the high frequency VCO creates bleedthrough into the output (you can always faintly hear the last note played).

[@JerryZard]

@@Synthchaser Ah interesting... the only thing I can think of is maybe they were using this synth as a testbed for some new tech that could be applied to a later product (ie micro-controlled polysynths).

[@DestroyER82]

All the best in 2024, have a fantastic year! A little question regarding this repair. Wasnt it worthy to replace all those old logic chips from the painful board or it is generally rare for them to fail or they are not available (obsolete) anymore?

[@Synthchaser]

The TTL chips in these synths have a much lower failure rate than the CMOS and op amps they used. But yes, some of the chips are obsolete (like that big 24 pin 74150 multiplexer chip and the DM9601 one-shot used to generate the trigger).

[@DestroyER82]

@@Synthchaser Oh ok, thank you very much for reply. Makes perfect sense then. And yes I was suspecting that the "specialised" chips might be obsolete and problematic to source eventually.

[@Danbalambalam]

I’m working on a Pro/DGX V2 myself right now and I’m stumped. The ADSR is not working. I’m getting a gate signal to board b but i’m not sure if i’m getting a proper trigger. When I probe the trigger input to board b on pin 7 of J4 i see a very tiny spike when I hit a key. So small and fast that you barely even see it if you’re not looking for it. I’m just not sure if it’s a proper trigger signal. Does that sound like what I should be seeing or should the spike be a more prominant?

[@Synthchaser]

It's a quick pulse at 5V TTL logic levels. I usually will need to change triggering mode on the scope when looking at it.

[@Danbalambalam]

@@Synthchaser thanks, it looks like it’s a good trigger then, at the same voltage as the gate. 🤔