RE: [DW8000] Korg EX-8000 display flashes once, then nothing! Please help!
2019-03-30 by John Rose
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.
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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!