DuoDock Power Supply issue (Un-solved agin)

[Elemenoh]

I have a Duo Dock that works okay with a known good PSU, but it's own PSU isn't working correctly.

It has been recapped, but still just gets a faint tick when the power button is pushed. The eject mechanism works okay.

5V for the eject button and ~20V for the eject motor work okay.

The power button does enable a steady on/off state, but the +12V/-12V rails shut down in less than a second and the +5V rail stays at a steady 0.3V. The eject motor +20V and +5V rails work okay and are unaffected when the power button is pressed.

I already double checked the caps and spot checked the D5 shottkey and main transistors as well as a few easy to get to diodes. But I'm hoping someone can help me think through this strategically.

Primary must be fine since the eject motor rails are working. So something is likely wrong on the secondary that handles 12V, but what would be best to check? Maybe something is wrong with the optocoupler? A zener somewhere?

I'd greatly appreciate any help!

 

[Elemenoh]

I'm going to journal here the things I've tested so far

Capacitors (ESR OK)
C04
C09
C10
C11
C12
C13
C15
C17
C19
C24
C30

Diodes
D7 (Diode ahead of the -12v voltage regulator)

Transistors
Q4 TYN682 silicon controlled rectifier (seems OK in circuit, but haven't tested outside yet)
Q5 Q6 BUZ80 MOSFETs OK

Voltage Regulators
U1 L7912CV negative voltage regulator (Not sure how to test this out of circuit. Not getting proper input voltage in circuit)
U3 7805CT 12 to 5V Voltage Regulator OK

I'm going to continuing along the 12V path to see what's broken there. From an old 68KMLA post it sounds like there was a Bomarc schematic for this PSU at one point. Anyone here happen to have a copy?

 

[Elemenoh]

My local parts place didn't have a TCDT1100G optocoupler in stock, but did have IL74 which I thought was basically the same (but is not). I dropped it in anyway and the voltages were all higher, but not high enough and the supply would switch on as soon as it was plugged in rather than waiting for the button to be pressed. I'm going to try swapping over the optocoupler from the known-good PSU to see if that does the trick next.

 

[Elemenoh]

I removed the optoisolator and tested it out of circuit. It seemed fine. Diode check on the LED pins was OK. I attached a 300 Ohm resistor to its anode and bench power supply positive lead. Attached the cathode to the PSU negative lead. measured resistance across the collector and emitter from 0V to ~4.5V which would decrease as voltage increased. I figured that meant it was good.

I swap tested the optoisolator from the known good PSU anyway and bam, it fired up. It powered itself off after 5-10 seconds the first couple of times, but now seems to be fine. It ran for 10 min without issue. So mystery solved I guess

Fortunately these parts are still available. Mouser sells small quantities, but DigiKey only sells in bulk.

 

[Mac84]

Yay! Glad you got it working again!

 

[Dana Does Stuff]

Yay! Glad you got it working again!

I like ya new glasses...

 

[Elemenoh]

I got some new optoisolators. Before installing I tried the PSU again and it had its same symptom of low voltage. I replaced the opto with a new one and it had the same issue. The new opto works okay in the already known-good PSU. Bummer!

Since then I've:
Applied hot air to warm up the PSU. No change.
Tested every resistor. All were fine.
Tested all of the diodes on the secondary side. All were fine, but I just tested with a diode test on my multimeter. Perhaps I should

I'm kind of at a loss now. Perhaps there's some marginal component?

Any suggestions for what to try next?

 

[Trash80toG4]

I'm going to continuing along the 12V path to see what's broken there. From an old 68KMLA post it sounds like there was a Bomarc schematic for this PSU at one point. Anyone here happen to have a copy?

That's a new one on me? I've collected everything I could about the Duo System.

IIRC, the tick of death involved replacing the guilty component with one of higher capacity, so testing what you have in there at present might not do the trick? Is it still ticking and not booting up?

 

[Trash80toG4]

The Duo Dock Tick of Death | Low End Mac

 

[Mac84]

The Duo Dock Tick of Death | Low End Mac

This is what fixed my Duo Dock. Those two tiny little capacitors wedged in the power supply (which I had to drill open) were the culprit!

 

[Trash80toG4]

Yep, mine too. Pop rivets knitting it back together look good in white.

Posted that fix link for reference. Unfortunately such doesn't appear to be a solution here unless higher capacity caps might do the trick? Inadequate capacitance doesn't bode well for longevity in a PSU.

 

[j-man]

Hello everybody, this is my first post here and I'm writing it because just yesterday I managed to revive my Duo Dock PSU
After I replaced all the caps it didn't start switching, although there were 400 volts on the primary caps.
In the end it was my fault... removing those big caps for testing I managed to cut off the through-hole connections between the PCB sides, so there was a missing contact there (and maybe around other caps as well). I checked and restored all the caps PCB connections and... bingo, the PSU was working again!
Hope my experience could be useful to others. Good luck!

 

[AndrewG]

hi
Perennial problems with the Duodocks is power supply failure. The deadly tick as the motor tries to pull the the computer to the sci connnector. I checked the bulk literature and its seem capacitor c28 100 MF consistently fails. So first replace it. I have 7 DD's . 280cs and 2300 PPC versions 4 still functional with installed 2gig HD . Always same problems the deadly clicking . Some one here reported a diode was hot. I checked the diode I think is the same one ( his pic is bit strange ) D15 on the magenteck 3359 -09-001 . The diode is marked UF4/g19 . Hope that helps . Probably better replace all caps if possible when it fails .