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Help with Elston DM30-09BO VDU board

JonB said:
I bought a wire wound part that I think should do the trick. It's a bit high rated (7A) which I may come to regret, but it'll do for a test. Not expensive, but I will seek a metal film resistor anyway.



I'm pretty sure I did that but found no correlation (I posted the traces). Of course, I can't profess to being an expert with the Rigol as it took me so long to switch the signal inversion off.. :rolleyes: But I do love it. One of the most useful tools I have apart from the Hakko.
Click to expand...
But the post #32 showing the power supply rail pin 2, I think, was set to see the H pulse rate around 15.6 kHz with a number of cycles shown, showing small glitches, not Vertical rate around 60 Hz, so the recording would have completely missed the much broader disturbance at vertical rate. That is why better to lock the scope on the vertical rate scan on one channel, to see at least one or two cycles and view the supply rail on the other, at the same time, so there are no mix-ups. Also when you scope the supply rail for this sort of test, ideally that channel is set for AC coupling, so you can windup the scope gain without it going off screen because of the large DC offset.
 
Yes, I see your point. I was thinking of the traces I posted showing horizontal / vertical signals (idea being to see if the H section was interfering with the V section).

Meanwhile a bunch of caps arrived for the PSU apart from one of them, and the 1.2R resistor hasn't turned up yet, either. I swapped out what I could. Hopefully another day and it'll all be ready for testing.
 
ldkraemer said:
Jon,
In the past we used the OHMITE Series 93 - 3.25 Watt Resistors that were Vitreous Enamel Molded.
I forget the specific application but they worked really well. We likely purchased them from Newark
Electronics or another larger Electronics supplier. If you could locate a few of these, I'm sure they
would do the job.

Larry
Click to expand...

Hi Larry

Looks very similar to what I ordered. They are ex UK military radio parts, I think and likely rather BIG. New old stock though.
 
Righty ho, last cap for the PSU arrived. Popped it in and the PSU is humming along nicely. But.. the VDU is still drawing too much, making the PSU click. I still have the burned resistor to replace, but for now it’s pulling nearly 1.5A which seems a bit heavy.
 
New resistors arrived, they are 1.6R which is very annoying. Plugged one into the board and the VDU is running, but the resistor still gets very hot (enough to boil water on the end of my finger) and the image stays steady for longer than before, but ultimately ends up glitching but not as badly as before. I tend to doubt that a 1.2R will solve it. I also note some static like clicking sounds coming from the thing. I would expect this if I'd disturbed the HT lead but I haven't. Anyway, I've asked the seller of the resistors what's going on and maybe will have a replacement 1.2R to try at some point next week.

After shutdown I felt around the board thinking to find some component that's drawing too much and getting hot. In particular the BU406D (the HOT), but it seems to be OK. It's only just warm. Any ideas?
 
I swapped the BU406 out for another one, which promptly went up in smoke... Grrrr.... have ordered another, along with a 1.2 Ohm 2W metal film resistor. The 7W wire wounds that are the wrong value are going back.

I feel like I'm chasing my own tail here...
 
Thanks Larry.

I've replaced the HOT with a BU407 (same as the originbal part) and fitted a 2W metal film resistor at R478. I have the picture back and it is stable, but as before R478 is heating up and it is pulling over 1.4A. I've not attempted to soak test it as I know the resistor will go again, or it will damage the HOT - or even the PSU.

I'm hoping someone can offer some more suggestions. @Hugo Holden ..?

Meanwhile I will review the thread as there were some comments by @gertk that I need to check up on.
 
Hi Jon, thought while I had some time I would pop back onto the forum and see whats going on, the answer to your problem would be apparent if you looked at it with a thermal camera, I have one and a kettle, give me a shout if you fancy a drive to kent.

Merry Christmas mate.
 
JonB said:
Hi

I'm trying to repair a monochrome video board from a Kaypro 16, made by Elston. It's a DM30-09BO. It was working for a short while, then yesterday a resistor started smoking and it died.

@ldkraemer was kind enough to send a schematic which I reproduce below. The smoked resistor is the 12R 2W R478 which sits between +12v and the board (the tube filament heater is also connected but direct to the PSU, not going through the resistor). What is the purpose of this resistor? Is it current limiting to protect the circuit in case of failure such as I have? The fuse, also inline, has not failed.

At the moment, it will power up the tube filament but there is nothing on the screen. However, a reset of the Kaypro causes a faint horizontal line to be displayed which fades out to a dot. All the while, the resistor is heating up... but I see no evidence of other components getting hot and I don't really want to touch anything to check for heat.

Seems like something is pulling too much current...

Is there anyone here who can advise on how to diagnose and repair this board?
Click to expand...
Hi, how did you get a schematic for this board? I have dm30-09bd(09BO?)-404-c42 out of a kaypro 1,1986..
it has no high voltage..so far.. just filament..and I'm not analog troubleshooter.

that looks like a real scehmatic..so maybe you can tell me a source?

Do these boards turn on without video signal?
 
Last edited:
drfrzog,
I ordered the Schematic direct from Elston back in the Mid to Late 1980's.
I'm sure they are out of Business now, but I wanted the Schematic that
applied to my Kaypro II. I didn't purchase any other Schematic from them.
Sorry.

Larry
 
ldkraemer said:
drfrzog,
I ordered the Schematic direct from Elston back in the Mid to Late 1980's.
I'm sure they are out of Business now, but I wanted the Schematic that
applied to my Kaypro II. I didn't purchase any other Schematic from them.
Sorry.

Larry
Click to expand...
does your board need a video signal/ or h/v sync signals before it produces high voltage?
 
From looking at the Schematic for the DM30 you need the Horizontal Drive to get
High Voltage. Q414 has to be working properly to switch T402 primary winding
which feeds the secondary for High Voltage Output.

Larry
 
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