25K5515 black level pot

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Can a black level pot make the screen look very dim ?

I am working on two 25K5515s form two HD's at the moment.

One had a HVT that went off in fireworks. Replaced it and it works again. HV is at 25 kV.
I tried increasing the Screen pot but pretty soon I get the usual greyish screen with visible retrace lines.
Since there are no other controls for brightness I figured Id check the Black Level pot which is supposed to be 2k but it measures up to 2 Megaohms between the runner and both legs, no matter what position I turn it to.
The pot also feels grindy when turning ite so it's 100% dead but I wondered if it could give this effect.

The other chassis also works but has vertical deflection problems. Its not just only a horizontal line, it incidently shows lines up and down from the horizontal line, like indeed a bad contact or a bad pot so I'll check the pots on that one too.

Checked all caps with an esr meter and replaced the bad ones.
 
I just desoldered and tested all the main PCB pots of both monitor chassis. The result was pretty shocking.

Only 1 tested OK ! ALL the others showed either a resistance value somewhere in the mega-ohms, others "only" showed crackling when moving/jittering the dial. (Remember those old radios when you heared crackling when turning the volume ? That's what is happening on these.

I must say that the pots on 1 chassis were in worse condition than on the other, but still they all showed problems except 1.

The pots for the colors on the neck board all tested out fine, so did the one's on the over-voltage PCB and the extra little PCB for Vertical Position.

I have very good hopes that "my" chassis's vertical deflection issue will be solved by replacing the pot, as the vertical size one showed mega-ohms values....

Pots ordered at RS !
 
By the way, for the record,
VR 351 is listed as 2k in the manual but it is 3k. There is a fixed resistor soldered in parallel which measures (in circuit) as 8k1.
 
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I replaced all pots but I also had to rejuvenate the tube (first time) but it works pretty well now.

For everyone with this monitor, check your pots, they are POS cheap crap....or this cab must have spent it's entire life at the sea-side...(which I think it did ;))

I only have some jitter in the picture but this seems to be "activity" related that is, when it's displaying a steady picture it looks completely solid but when the PCB needs to do some processing-work (f.i. zooming in the Hard Drivin' logo, I can see the jitters. It is repetitive and also f.i. when a demo race is showing you see it.

I suspect (and hope) it is caused by my test set-up with a switching PSU that may barely keep-up with the 3 PCB set (and only 2 wires for the +5V).

Pics, video etc. as always here:
http://www.dragonslairfans.com/smfor/index.php?topic=3095.msg45416#msg45416
 
I'm dealing with the exact thing right now with my 720 chassis. I've replaced my pots, but I'm still getting that little jittering. Even though I saw that VR351 pot was 3k, I swapped it with a 2k and really doesn't seem to make any difference. I'm also getting some bowing in on the sides. I've done a full cap kit and replaced the flyback. Still can't figure it out.
 
How does "your" jittering look ? with me it's like horizontal lines interfering with the picture. It's almost as if it was a vector game that needs to draw lots of texts: when it gets busy it's starts jittering...

Now, would that be caused by the PCB set interfering somehow, or is the monitor simply handling changing images badly ?

The test screens are 100% rock-solid....
 
I couldn't find a 3k pot either so I used a 5k (figuring I could set it anywhere between 0 and 3k that way). Doesn't seem to matter much. The pots are a big improvement though, the old one's were so bad you could already feel by moving them they were "grindy".
 
I think my theory of the PSU not keeping up with the demand of the board-set being the cause of my jittering is correct.

I removed the inter-connect power cable (from the main PCB to the sound board) and the flat-cable and powered up again and now the jittering was reduced to about maybe 10% of what it originally was. I can also hear some 'jittery" noise from the PSU when it jitters on screen, I'm sure there are some bad caps in it or maybe it's plain not powerful enough to deliver the required current.

So, maybe try another (new?) PSU on your 720 to see if that helps. I also thought about putting a Big Blue in parallel over the +5V output of the PSU but figured that if the required amount of current was the cause that that wouldn't really help.

Hopefully I have time to put the monitor and PCB set back in the cab tomorrow and we'll see how it looks.

At least I'm quite sure it's not in the chassis, because I had exactly the same issue with the 2nd chassis I have been working on, so this too indicates to the PCB (PSU) as the cause...
 
My whole screen shakes slightly side to side...I don't have the lines you're talking about. I know mine is a monitor issue because I have 3 720 video boards and 2 game boards. It's the same with all of them. I left it running for 5-6 hours and it seemed to get better occasionally, but I'm still on the look out for a brand new monitor any way because I have a decent amount of burn in mine.
 
Polderarcade and I (we both have a HD cockpit now) discussed the possibility of hooking up a more modern tri-sync monitor which should of course work in theory. I think I know. where to get some NOS but they are 19" and I sure want to keep. this 25"

Ithink 720 is 25" too?

Did you do a cap-kit and replace all pots ?
 
Yeah, 720 is the same 25" k5515. Chad rebuilt it about 2 years ago with new caps and transistors (before I was brave enough to pick up a soldering iron). I had someone with an ESR meter check the caps and they're all fine. I checked all the resistors and swapped out a few that seemed suspect (even though like VR351, it seems like some were different from what they have listed on the schematic, namely r207, r209, r211, and r359 isn't even listed on the parts list and mine was missing on the chassis but it was listed on the schematic). I spoke with Chad and he thought maybe the fly might be bad, but a new one didn't make much of a difference on mine. I'm thinking of sending the chassis back to Chad to see if he can test a few other things, but I'm not in a real hurry to send it off and not see it for a month and a half, as it's at least playable now...especially if I might be investing in a new monitor anyway. I've got a few calls out and a few people looking checking some warehouses right now.
 
Oh...and yes, I swapped out a few of the pots, but not all. I swapped out the horizontal hold, position, vertical hold, and vertical damp(?). The other pots, like my black level, work fine, so I figured I'd leave them be. How do you test them anyway? Do you just put 1 lead on one of the double side leads and the other on the single side, turn it and see if you get the 2k or 3k reading or whatever?
 
Exactly. I was getting nuts values like 2 Mega ohms sometimes (those were the really bad one's) but also you have to turn it steadily while measuring. The ohms value should increase steadily as well and not be jumpy. You have to measure two times (center and both sides) if one side is good. Usually both will be bad. In doubt, replace anyway.

I tested the neck pots this way and they looked pretty ok but once I powered it up and started adjusting them the made the colors jump instead of steadily increase/decrease so I had to replace them too. It's worth replacing all of them, they're dead cheap and will rule out problems...even if it's only in the future.

I put the monitor back in the cockpit today and as expected the jittering was gone, so indeed it was the PSU I was testing with.

Of course, in this hobby nothing goes smoothly do now I suddenly get a Sound board time-out error and no sound......sigh....but that will have to wait a bit...

Anyway the monitor really went from dramatic with a very dark screen to a very workable bright picture. It was a lot of work but it payed off....
 
Of course, in this hobby nothing goes smoothly do now I suddenly get a Sound board time-out error and no sound......sigh....but that will have to wait a bit...

You did rmember to replace the cable to the sound card, right? ;)

I removed the inter-connect power cable (from the main PCB to the sound board)
 
Yeah checked that and the interconnecting cable between the main PCB and the soundboard.
Also reconnected the header connector that goes to the APU board and AFAIK thats all there is to the soundboard and of course that last header cant produce the time-out error....
 
Resurrecting this thread as I'm having trouble with my K5515 too in my 720. Black level pot seems to just blank the screen with the smallest movement and I'm just not getting the brightness and contrast out of this monitor that I feel I should.

Recapped, new flyback, rejuved the tube but I just can't make this screen POP the way it should. If I drive my guns even slightly high then I start to lose focus and can't pull it tight with the flyback focus pot.

Maybe that black level pot is killing my signal?
 
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I've long sold my Hard Drivins but I remember that these monitors are not up to par to any others I've worked on. I heard the tubes are shit. (ANd indeed so far they were the only one's I actually needed to rejuve).

But, as mentioned a couple of times in this thread: replace all pots. Really.
 
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