Korg DW8000, Mono/Poly, MS-20...

Based on the voltages on the regulator IC’s  1-4 it looks like they are creating a virtual ground so the –ve of the bridge is not ground, the centre pin on one of the 7805’s would be a better ref, in which case it looks like a problem around IC3 as this is probably shutting down due to something further on in the circuit drawing to much current.



I would disconnect the power and check the resistance between the output and ground on IC3 which I suspect will be very low if not a short.



Obviously this means that you only need to concentrate on the parts of the circuit fed by IC3 for the moment.



The voltage on IC4 also looks wrong but I would just check this again with the correct ground ref as this may actually be OK if it reads -5V



Many thanks



John Rose



From: DW8000@yahoogroups.com [mailto:DW8000@yahoogroups.com]
Sent: 26 March 2019 16:35
To: DW8000@yahoogroups.com
Subject: [DW8000] Korg EX-8000 display flashes once, then nothing! Please help!





I recently picked up a Korg EX-8000 which unfortunately I have not been able to see in working condition.



I'll try to explain where I'm at:



Upon startup the display flashes once (very quickly) and then flashes off (never to return).

I haven't tested for audio output because I figured it's not possible without a display.



First thing I tried was to check all cable connections and to replace the bridge rectifier.

Next was the capacitors - first the larger ones (6800u, 4700u, 1000u). No change.



I replaced D10 and D11 with IN4148's and tried replacing IC1 and IC4 (7805 and 7905 voltage regulator).

Still no change.



I noticed that LED1 is not working (the service manual says it's a LT-8201p - I've no idea how to replace this or of it's importance/relevance to the circuit).



Checked all resistors for o/c, changed out all the 16v electrolytics (they're all 25v and up now), diode tested the SR1K-2's in circuit (some were reading both ways which i think is normal in circuit - I still may replace these with IN4007's out of desperation).



These are some DMM voltage measurements which I took after I'd replaced the components as above:



10v AC out of each transformer secondary.

25v DC between the + and - pins of the new bridge rectifier (W04).



Using the DC -'ve pole of the bridge rectifier as reference):



IC1 (7805): 25v- 13v -18v

IC2 (7805): 25 - 13 - 18

IC3 (7805): 25 - 13 - 13

IC4 (7905): 13 - 0 - 8



To KLM771: 14.6 - 9 - 16 - 7 - 19.6 - 29.1

To KLM662: 14.6 - 9 - 19.6

To KLM766: 14.6 - 19.6 - 14.1 - 29.1



I have not yet looked at: 2SB744 (PNP) or either of the 2SA1175K (PNP's also - where's the NPN's ??)

as these could be difficult to replace.

---



I am concerned that perhaps some of these high voltages have damaged chips on adjacent boards but any thoughts/suggestions/assistance will be very gratefully received.



Thank-you!