HP 1345A display too dim  - Page 1
Forum section: electronics / repair

author: hjf83
author_rank: Contributor
timestamp: October 11, 2025, 11:18:32 pm
content: I'm trying to figure out what's the problem with a 1345A display. it's extremely dim (completely unusable). I thought it may be a worn tube but it's not burned in at allI've checked all voltages, even the -2500V and the anode at 9kV. 105V is fine too.intensity controls work to a point and then it just starts going out of focus.is it really a worn tube?it belongs to a 4145B semiconductor parameter analyzer. I know these are from the 80s, so a worn tube isn't out of the question of course. but i would expect burn in after enough power on hours, especially on static texts and grids like these instruments display.it's a vector display and it has some control for adjusting the stroke end. when I change this, I get a little bright but very focused dot at every stroke end which is strange. I'm not sure if the beam is "dwelling" there for a bit and that's why it does this or if there really is an issue with stroke intensityI've also checked the Z amplifier in the HV board and it looks ok as well

author: floobydust
author_rank: Super Contributor
timestamp: October 12, 2025, 02:05:47 am
content: It's not easy to troubleshoot this.I would say losing focus at higher intensity points to the HV power supply sagging under the load.But which rail? It looks like the regulation feedback is from the -2,500V control grid so I expect that to be rock solid.The heater and cathode are around 50V more positive at -2,450V so that can indicate if the tube bias is keeping it dim. Is is around 50V?I hate thinking and measuring with high common mode voltages present, that crackle lol.The 9kV is the first thing I would suspect, the multiplier had a engineering change for it but no idea what was wrong with the first part. Is it sagging or not, with intensity?The beam bright dot could point to the blanking or "intensity amplifier" not working right. Is the "-Z Level" goal of +100V positive peak around TP2/R31 there?What happens is HV capacitors go low value or leaky, HV rectifier diodes get lazy with age. C15, C16, C17 0.01uF I would check but only after carefully discharging the CRT and the caps!I use a 10k-20k resistor HV rated say 1W for that. Shorting them out is hard on the other parts nearby.

author: hjf83
author_rank: Contributor
timestamp: October 12, 2025, 03:50:49 am
content: well, i was able to get some more brightness from it by touching the pot nearest to the screen in the HV board. this pot reduces the picture size in X and Y (looks smaller but gets a lot brighter)then i compensated with X/Y size in the A1 board. it's still dim but usableanother thing that helped was to adjust R70, drawing time. the problem with doing this is that the screen flickers when there are too many things in it.as for the voltages. i dont remember exactly, but they're a little lower than 2500. there is a tiny handwritten label that said 2350 or so, and the voltage is close to that. the tiny label seems to be as old as the monitor as the glue has failed.9KV at the tube's anode input measures 8.9KVI adjusted TP2 to an amplitude of 40Vpp as the manual stated, I don't remember anything about 100V around there?

author: wn1fju
author_rank: Frequent Contributor
timestamp: October 12, 2025, 05:17:40 pm
content: Just a thought, but I suggest you go through the entire adjustment procedure, in sequence, per the manual. At least if that doesn't improve things, it may point to an area of the circuitry that is misbehaving if a specific adjustment cannot be made. Turning pots at random until the screen looks better, if that is what you have been doing, is generally not the best approach.I had a 1345A that was also dark (but not as bad as yours probably) and the full adjustment procedure rectified it.Also, I have fixed a few of these 1345As over the years. Most all of them had problems with the cylindrical 2.2 uF tantalum caps which shorted and took out the positive and/or negative 15V rails. Keep an eye on them.

author: hjf83
author_rank: Contributor
timestamp: October 12, 2025, 05:33:48 pm
content: I may have missed something in the manual so I'd have to double check. But so far, following the manual just led me to the original dark image. That's why I've started touching things at random too.I guess the silver lining here is that it SEEMS the tube isn't worn because it can display bright images without becoming blurry, so there's that.I goth this 4145A for the price of shipping because it was sold as "no picture". I found that the brightness was just too low. Which does seem like a problem of "something went bad". Still after that adjustment the screen was still so dim that "i have to turn all the lights off in the lab to be able to see anything".

author: TERRA Operative
author_rank: Super Contributor
timestamp: October 13, 2025, 04:48:18 am
content: I have found those small Bourns 3329 trimpots gone bad on a couple display modules so far. I also had good results in replacing the electrolytic capacitors too.

author: hjf83
author_rank: Contributor
timestamp: October 13, 2025, 11:50:22 am
content: Quote from: TERRA Operative on October 13, 2025, 04:48:18 amany particular ones going bad? there is a small, round trimpot in the HV board (different from the square blue ones) that does nothing when I adjust it.

author: TERRA Operative
author_rank: Super Contributor
timestamp: October 13, 2025, 02:36:31 pm
content: The small round metal ones with the white turny bit are the dodgy type. Any of them can be bad.If the original trimpot does nothing, swap it, if it still does nothing, trace out the circuit.I had a few in both my 4145A and 4145B that were bad, I ended up replacing all those type of trimpots with new ones just to be safe.

author: hjf83
author_rank: Contributor
timestamp: October 14, 2025, 03:10:23 pm
content: In the HV board there is only one round, vertical one. 5K, that seems to do nothing. i think it's the "focus gain" one. it measures OK with a multimeterThe only adjustment that had any effect was one called H.V. adjust. it did increase brightness. But when I checked the HV lines, they were at around 2900V instead of the specified 2450VI adjusted it back to 2450 and I read 2450 in the one marked 2450, and 2536 in the one marked 2500VSo I'm not entirely sure what's going on. It COULD be bad caps on the HV side, i suppose, and my multimeter is reading 2450 but it's actually less and noisy. I suppose I'd have to connect the scope and see.It could also be, i guess, a worn tube and the higher voltage is helping it a little.

author: hjf83
author_rank: Contributor
timestamp: October 14, 2025, 10:13:06 pm
content: UPDATE: I've tried rejuvenating the CRT. I applied 50% more filament voltage, and applied 50V to the grid. not much happened. then 80V, then at 100V the 40mA compliance limit in my SMU triggered.I gave it a couple "zaps" and after that, the screen is much brighter. Not eye-searing bright but significantly more usable.I'll try giving it a couple more zaps at a little higher current and try again. All seems to indicate it's a worn tube after all.NOS tubes are available on eBay anyways so I have nothing to lose.

author: hjf83
author_rank: Contributor
timestamp: October 15, 2025, 02:50:55 am
content: More zaps didn't do much more. But the display is now usable in a lit room.I guess CRT rejuvenation does work.We'll have to wait to figure out how long the fix will last.

author: HoangLong
author_rank: Newbie
timestamp: October 15, 2025, 04:17:41 am
content: Can you try to cut the intensity modulation signal from the Z amp from going into the tube? That will keep the tube at maximum intensity. Then you can see if it's a tube problem or a control problem.

author: squadchannel
author_rank: Frequent Contributor
timestamp: October 15, 2025, 04:54:16 am
content: 1345A makes maximum use of the Z axis signal. what would happen without it?