HP 8903A-Calling all experts, CPU card isn't right - Page 1
Forum section: electronics / repair

author: gdnichols31
author_rank: Regular Contributor
timestamp: June 27, 2020, 05:15:46 pm
content: I purchased a couple of these devices from EBay. I have gotten one of them working, but the second is being a bigger pain than the first one.First, all the power supply voltages are good.Second, I can swap the cpu cards into the other device and the problem follows the bad CPU card. So that eliminates something being defective in the balance of the device.Third, the cpu does run and it appears to be getting interrupts from the keyboard. It does NOT appear to be running the POST (Power On Self Test) routine. The display shows random segments on and no response from key presses other than on the 4 LEDs on the cpu board. The 4 LEDs on the cpu board blink. The 2 MHz clock is good.I forced a reset and that didn't help.I have reseated all the socketed chips on the board.I have an HP 5005A signature analyzer. But what I don't have is the special HP 08903-60018 cpu extender board that is required to use the HP 5005A. At least that's the way I read the service manual.Has anyone seen one of the extender boards? I've been looking for 3 or 4 months and I haven't seen one.Is there a way to use the sig anal without the extender card?I'm stuck on this one and need some help.Any input is greatly appreciated.Gary

author: George Edmonds
author_rank: Frequent Contributor
timestamp: June 27, 2020, 11:37:27 pm
content: Hi GaryYou stand more chance of finding hens teeth than a HP08903-60018 card extender, but you probably know that.It is efectivley a double 44 way (22 way one side) connector card extender with provision to open some traces. Sometimes HP released full details of these special card extenders, but I cannot find the information for a 8903AIt may help you if you obtained two standard 44 way card extenders, at least you could obtain better access to the A8 board that way.Either search on eBay for Sparko6079 of google WA1ESO for HP card extenders.George G6HIG Dover UK

author: gdnichols31
author_rank: Regular Contributor
timestamp: June 28, 2020, 12:09:50 am
content: Thanks George,I have the 30 & 44 pin extenders.I just can't find the fancy one for the CPU card.....Like you said hens teeth

author: George Edmonds
author_rank: Frequent Contributor
timestamp: June 28, 2020, 12:20:06 am
content: Hi GaryAs you have two 8903A's why not just compare them to fault find.There is a school of thought that is of the opinion that signature analysis does not work, after one bad experience I have some doubts myself.If you want to see what the 08903-60018 board does look at the service manual for a HP5342A, it must be similar.Have you checked the EROM's and the RAM, most likely cause of problem.George G6HIG in UK

author: Tony_G
author_rank: Super Contributor
timestamp: June 28, 2020, 12:24:19 am
content: You might want to reach out to sparko6079 on eBay (https://www.ebay.com/usr/sparko6079) to see if he can create a custom extender for you.I had him build an extender for my 8340B and he did a great job at very reasonable prices. I did a video on his cards here:TonyG

author: WA1ESO
author_rank: Newbie
timestamp: June 28, 2020, 08:17:45 am
content: Hello Gary and the group,At this time I produce and stock the 15X2 and the 22X2 extenders for the HP-8903.I would like to be able to offer an extender board for the CPU board.Can I be provided with more information on the CPU board? Perhaps send me a CPU board photo and other information?If you prefer to contact me directly my e-mail address is:JPNery@Yahoo.com JPNery(at)Yahoo(dot)comThank you very much.John WA1ESO

author: George Edmonds
author_rank: Frequent Contributor
timestamp: June 28, 2020, 03:43:13 pm
content: HiNow before the Youtube video promoters get busy on this subject I think that it would be beneficial to point out a few facts and inject a touch of hard reality.HP CPU board extenders are a massive PITA, they are as far as can be readily ascertained they are almost specific to each series of instruments. Some HP service manuals give a reasonable amount of detail regarding them and some give no detail at all, for example the HP8903A service manual gives no information. The HP8903A CPU extender card looks to be a particularly difficult one as it also involves a ribbon cable extension, or so the service manual sketches shows.The complexity varies considerably from just enabling the opening of data lines to having additional logic on board. I have attached a photograph of a HP very simple double sided counter CPU card extender that was home manufactured using the toner transfer method, this one just opens some address lines. Also attached is the schematic for the CPU extender for a HP5342A, this not only has provision to open the address and data lines it also has onboard switching and some logic.I cannot see how the two example CPU extender cards can be made using the milling process, but by far more importantly I cannot see how it would be financially viable to do so given the multiple types of them in use and the very low potential demand for them.I certainly do not wish to detract from the excellent and much needed card extenders manufactured by WA1ESO, they fill a large void and provide a cost effective way of at least being able to make measurements on most HP instruments PCB’s. Just wish that there was a way that HP CPU board extenders could be made in a viable way.George G6HIG Dover UK

author: gdnichols31
author_rank: Regular Contributor
timestamp: June 28, 2020, 04:03:16 pm
content: Thanks to all that have replied!George,I have thought about using the working CPU card as a 'go by' to trouble shoot the bad. I'm a little nervous about damaging the good one. I may not have a choice and do that.I will look at the service manual for the HP 5342A and see if it will help the cause.I agree the most likely problem is the ROM and/or the RAM. Might be the easiest thing to do is swap out the ROM1 and RAM chips and see what happens.....Tony_G and WA1ESOI have the 30 pin and 44 pin extenders from Sparko6079.If I can find the wiring diagram for the CPU extender I would love to have you make one for me. Hopefully the SM for theHP 5342A will shed some light on the wiring and function of the card.I know there is a ribbon cable that jumps the address bus down to the extender and a DIP switch. Not sure the function of that. There are also 2 or 3 additional test points.George,Just saw your latest post. I agree it will be tough to make the CPU extender given the lack of documentation on them in general. But I wanted to at least try to find one or the info to make one.Thanks again everyone, I'm off to study the HP 5342A service manual.Gary

author: Tony_G
author_rank: Super Contributor
timestamp: June 28, 2020, 04:07:08 pm
content: Quote from: George Edmonds on June 28, 2020, 03:43:13 pmThank you George, I appreciate you impugning my motives. This is always an important addition to any conversation.Yes, CPU board extenders are often very complex, including dip switches, attachment points, etc that can't be duplicated without schematics for the board and more complex manufacturing processes.That said, utilizing a simple extender board enabling the CPU board to be lifted above the case for easier access is still very valuable. It might not enable the use of the signature analyzer but it does provide Gary with access to be able to check things like clocks, line levels, and whether things are changing. Given that he has a working system then being able to easily make comparison measurements is something valuable in and of itself.Artek Manuals has good quality scans of HP manuals - I have many of their other manuals and heartily recommend them - If you're using the scans from the Keysight website then these are a good upgrade.Good luck with the repair Gary.TonyG

author: WA1ESO
author_rank: Newbie
timestamp: June 30, 2020, 07:44:21 am
content: Hi Gary,I was wondering if you could send me a photo of the CPU board to be extended. Perhaps some dimensions as well like board height and finger area width.With this information I might be able to say I think I can make a simple extender (riser) or I cannot.Over 8 years of making extenders there have only been two I could not make. Both of these boards to be extended require an edge connector that was no longer available either new or used.ThanksJohnWA1ESO@JUNO.COM

author: gdnichols31
author_rank: Regular Contributor
timestamp: July 01, 2020, 03:13:03 pm
content: The connectors are the standard 22x2. I have a couple of your extenders and they work great.The problem is that HP made a custom extender card for the CPU card. So far I haven't been able to find any details on it. If I do you'll be the first to know.ThanksGary

author: gdnichols31
author_rank: Regular Contributor
timestamp: November 10, 2020, 01:54:43 am
content: It’s been awhile so I thought I should update this thread.I wasn’t real excited about using my working A8 CPU card as spares for the bad one for trouble shooting.So I waited to see what showed up on EBay. I wound up purchasing 2 CPU cards. They aren’t the same part number but the boards are identical except for the ROMS.The new cards are part number 08903-60185, see photosThe card that came out of my 8903A’s is 08903-60003, see photoOne of the new cards didn’t have a ROM installed.The first thing I did was to install the card in my defective 8903A. It WORKED! Well kind of. The display came alive and it responded to keyboard input. The only thing that didn’t seem to work was the oscillator. I know with my good CPU the oscillator worked(more on this at the end).I THINK the new CPU cards may be from an 8903E. My understanding is this instrument doesn’t have an oscillator, just the metering. If anyone knows for sure please respond.Next I swapped out all the socketed chips (except the ROMs of course) onto the defective card and still nothing.At this point I didn’t think I had any choice but to use the ROMs in my good A8 to swap into the defective board. When I swapped out U9 (ROM 1) it came to life! Oscillator and everything else.So I ordered some 2716 EPROMs and I’m searching for a burner. Does anyone have any recommendations?Now the oscillator. It has about a 5% distortionSee the waveform shot,See the spectrum,Pretty ugly, I’ve swapped boards and it is the oscillator board. I guess that’s my next opportunity on the device!Gary

author: gdnichols31
author_rank: Regular Contributor
timestamp: November 14, 2020, 03:09:56 am
content: Bought a few 2716 EPROMs and a burner, they came today.Burned a new EPROM for it and it now runs!See the Pic below, the first shows the 5% THD,The output level isn't correct, so the attenuator board needs work also.

author: gdnichols31
author_rank: Regular Contributor
timestamp: November 16, 2020, 02:14:02 am
content: Since the CPU card has been repaired I thought I would start a new topic just for the oscillator problem. The link is below if your interested.https://www.eevblog.com/forum/repair/hp-8903a-a5-oscillator-board-distortion/Gary

author: 2016kat211
author_rank: Newbie
timestamp: October 01, 2022, 04:03:25 am
content: Hello Gary,I am facing same issue on A8 CPU board, where I can find the Roms data for HP8903A?Would you consider to share the Roms image?Last week, got a non-working HP8903A from local seller,before plug in, checked the Main capacitors, C02 (for -15V) was short.replace C02/03 with new same type,feed-in power through VARIAC, the unit start to respond around 80~90VAC,once over 90VAC, it shut down.seems the U2 -15V regulator does not work, the output increase when main AC increased.then triggered the over voltage protection.since difficult to find a TO3 replacement locally in short time, i tried to mod a TO220 L7915 into TO3 socket. ugly but works,now the unit can output correct voltage (+5V/+12V/+15V/-15V) within VAC 85~120VNow I can run the unit happily,the oscillator seems within spec. low distortion and correct output voltage checked by Panasonic VP7722afeeding OSC to input BNC, gave THD 0.003~4% (1kHz,2V) , seems not so bad.then I tried to setup the GPIB connection to PC by Pete Millett's software, (NI USB GPIB)through hours of trial, still can't get connections.then suddenly, HP8903A was frozen, no response to any of keypad.after reboot, only strange pattern shown on left LED section (similar to Gary's descriptions)tried to reset, not work,checked the voltage at A8 (+5V/+12V are good), Tantalum caps are checked OK by ESR meter.what I can guessed is one of those ROMs might damaged after long time no powered.......any comments and showing the location of HP8903A Roms would be super appreciated.Thanks again.Katdesu2016kat211@gmail.com

author: 2016kat211
author_rank: Newbie
timestamp: October 01, 2022, 05:58:50 pm
content: Dear Gary,here is the update to HP8903A,The problem was found, not caused by ROMs, CPU should be blamed.I did remove the Roms and re-seated, but left the two 40 pin ICs no touch,since no help, only step is removing CPU U5 and U6,when I tried to remove CPU with care, quite a few pins fall off from both sides,I compare CPU pin with other ICs, CPU Pin shows dark and non metallic surface,but all the other A8 ICs shows shining surface.when the CPU took out, try to clean the pins, more pins fall off. some pins are very very soft.I guess, no Boot up issue, might cause by CPU, some pin could be weak or loose contact.try to rebuild the CPU pins!Choose a Pin type 40 pic IC socket, try to rub the pin and remove the dark stuff,Install CPU into IC socket, then solder all the pins to socket . after continuity for each pin was confirmed,install to A8 board then recheck the continuity.It works!Seems no reason to request the Rom images,thanks again,Gary.Next step will is calibration works.Katdesu

author: gdnichols31
author_rank: Regular Contributor
timestamp: October 02, 2022, 11:47:02 pm
content: Congrats on the fix!Gary