HP 5890 Series II TCD board diodes

[NSChemist]

Good day folks,

I have an HP 5890 Series II GC into which I have installed a TCD. When powering it up, however, the solenoid valve wouldn't switch. Some testing on the board that I bought off eBay revealed two shorted diodes. One of them appears to be a rectifier diode (no problem to get that), but the other one is either a Zener or a Shotkey (I don't know if that's how that's spelled).

Does anyone have any further information about that diode? What's the power rating and breakdown voltage (assuming it's a zener/shotkey)? Looking at pictures of other boards it appears that the rectifier and zener diodes are wired in parallel, and that various other brands of diode have been used there in the past. I've looked at the writing on the diode, but it didn't reveal anything.

If someone has a wiring schematic of the board, that would also be immensely appreciated.

P.S. Alfonso -- no thanks, I don't need your services.

[KM-USA]

See viewtopic.php?f=7&t=27106&start=15

I have some 5890 GC boards available. Post your E-mail address if interested.

OK, here's what I found in the attic. Make offer if interested. Christmas sale, I need the space.
82335 HPIB board
82341 HPIB board
82350 HPIB board
19232-60020 TCD board
19231-60010 FID board (two)
19257-80040 Board (two)
18594-B0170 Controller box board for ALS
5061-3302 Board CIB board (five)
K9260EU-01 Board
05890-60945 Oven flapper motor 5890 (new)

18597-60255 ALS tower mount (rare type that fits both “A” and “B” 7673 towers)
03380-60070 Print head for 1084, 5830, 5840 (new)
Oven fan blade 5890
Oven fan motor 5890
FID housing 5890
0-30 psi pressure gauge
FID/TCD gas valves/ignitor switch

19251-60560 Capillary split/splitless injection solenoid (new)
19257-60020
19246-60530 Controller
Oven heater and support 5890
05890-80300 Main board 5890
Oven flapper with motor 5890
19300-80710 Oven heater wire (new, two)
05890-60050 Board
Pressure controller 5890
Front keyboard and attached circuit board 5890
19362-60570 Gas controller (new)

[uzman]

Hello ,

Those diodes are connected parallel to solenoid valves or relays , to prevent back emf voltages which may be harmfull for the rest of the electronic circuits.
They simply short the back emf.

You may use a 1N4007 diode for testing your circuit , simply cut the leads of shorted diodes , and solder it in place . Before removing the old diodes locate the cathode lead and solder the cathode of 1N4007 to that point.

Schottky diodes have lower forward voltages than normal diodes and safer , but short term testing is not a problem.

After testing , if everthing is o.k. try to find the original coded diodes if possible .

[NSChemist]

Ahh, OK. The problem I've been specifically having is that I'm not getting a pulsing 12V DC square wave out through junction J24. I've tried all the pin combinations and all I get is a steady 27V DC with the detector on or off.

Would shorted diodes cause that or do you believe that there are other bad components on this board?

[uzman]

Hi NSChemist ,

Actually, I made an assumption depending on your explanation of non-functioning solenoid valve and shorted diodes.
If your solenoid valve is not working after changing the diodes , of course there may be other faulty components.

You need the circuit diagram in order to locate the problem , and some electronic repair experience .

Finding a cheap instrument for parts ,on e-bay ,and using its electronic board may be a better solution.