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HP 8568A Spectrum Analyser not booting up. Both INSTR CHECK Leds on
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Hi everyone. I bought a broken HP 8568A for dirt cheap.
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The display part works (test pattern can be displayed properly) but i can't get it to boot.
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Both INSTR CHECK leds are on and never shut off.
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The only sign of life i get outside the test patterns is a line appearing whenever i push the INSTR PRESET,
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Has anyone here experienced something similar ? Any ideas?
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Thanks |
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? Hi, If both LED remain on then you probably have a problem in the A15 processor board in the RF unit. If you have not already done so you'll need to download (purchase) the service manual set from Regards Lou VK3ALB On Sun, Sep 22, 2024 at 8:01?AM azpo1928r via <azpo1928r=[email protected]> wrote:
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The first thing I would check is the bus cable connecting the two boxes. Then, I would verify that all the power supplies are working.
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If that doesn't help, I would re-seat the boards in the processor cage. It might help to find out if yours really is a 8568A. It may have been converted to a "B". The "A" has a separate memory board, while the "B" has memory on the processor board. If it is an "A", check also the NiCd battery pack which usually leaks and does some damage. The service manual has useful information regarding the two Check LEDs.
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Vladan |
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Thanks for the info. I actually forgot to mention the stuff i tried :
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- Cleaning all the board connectors and removing them + visually checking them (no rust, no leaking capacitor, no oxidation)
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- Checking voltages (all seem good)
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- tried to give it 10MHz from a signal generator and switching the 10MHz reference to external, absolutely no difference
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I also have the original service manual.
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The memory is separate, it's on A14 board btw.
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I'm gonna check continuity for all connections on the bus cable. I tried wiggling it around etc but i got nothing. Maybe there's a broken wire in there.
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Btw the seller i bought the SA from told me he tested the display part on his hp8568B and it worked just fine. But who knows if he used the same bus cable or not... the fault may be here
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If it's not the bus cable, what else should i try? I'm gonna buy a cheap usb logic analyser and use it as a signature analyser, i may be able to determine where is the fault located...
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Does anyone here have some "tricks" to find out where is the fault, without having a signature analyser? I have a scope.
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Thx |
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What happens if you disconnect the bus cable and then turn on the analyzer? LED 1 should stay on, while LED 2 should extinguish. I suspect both will stay on as they have so far, but if things change it may be a clue.
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As far as any other easy tests, I would also check all the front panel keys. They have leaf springs in them so that they return to their normally-off position. These springs can dislodge and leave a key in a state where it's always acting as if it's pressed. During preset, the processor checks for that condition. If a key is in such a position, you would get just one of the LEDs remaining on, but I would check anyway because of the age of this analyzer.
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The easiest way to check for errand springs is to press each key and note if there is a click. The key should firmly return to its resting position. Keys with missing or dislodged springs will feel ¡°mushy¡±.
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Other than that, check that the processor clock is working. Everything else becomes more difficult to troubleshoot because of the need for a signature analyzer. Even with one of those, chasing a problem can get circular until you have replaced almost every part on a board.
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Pragmatically speaking, if you can find a processor and memory board for sale, it would be easiest to just swap them. Most 8568A (and 8566A) have been updated to ¡°B¡±, which left many unwanted ¡°A¡± boards around. I am not sure if they still show up for sale, but the cost should be minimal due to lack of demand.
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Vladan
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Thanks for your answer,
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If i unplug the bus cable i still get both LEDs.
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The processor clock is also running, i've checked it yesterday. 10MHz signal is present, it gets divided into two 5MHz out of phase signals which are also there.
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There's only ONE weird thing i've found when doing tests.
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Manual says "UMC Pin (A15U13 pin 25) should be high when INSTR PRESET button is pressed"
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The thing is that i've got the exact opposite. The "idle" state is HIGH instead of being low and when i push on INSTR PRESET, it oscillates back and forth from high to low and then stops and goes back HIGH instead of low.
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I can't really know for sure if it's supposed to do that, maybe someone here has an hp8568a and could try probing the UMC pin? That would help me a lot
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Other than that, everything else i've tested seems fine. I also checked every single wire inside the bus cable, all good.
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Checked all voltages for ripple (in case of a dead capacitor) , all good too.
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I just realised something really important :
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The service manual says "if the ADRS'D LED flashes without INSTR PRESET pressed it indicates A13 is functionning but cannot communicate with the controller processor"
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The ADRS'D Led is indeed flashing on my unit... never realised it wasn't normal...
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Btw, INSTR PRESET button is working just fine, i've tested it.
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So that probably narrows it down to A15 right?
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If i remove A13 (hpib board) it doesn't blink anymore but still got the same two instr check leds.
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Also, i completly forgot to mention, if i flip the power off then on pretty fast i get random stuff on the screen, like lines, garbeled text-looking stuff and things like that. It always did that. They last for multiple seconds.
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I just ordered a cheap usb logic analyser, i will do signature analysis checks.. in the meantime i'm searching for A15 and A14 boards online... people are selling them for more than double the price i paid for the whole unit (the 2 parts + the two cables + 4 volumes of manuals about it).
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Seems cheaper to just buy a non-working RF part and try swapping the boards like you said... |
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On 9/23/24 16:35, azpo1928r@... wrote:
Also, i completly forgot to mention, if i flip the power off then on pretty fast i get random stuff on the screen, like lines, garbeled text-looking stuff and things like that. It always did that. They last for multiple seconds.That's a great way to blow up the power supply in just about anything electronic. -Dave -- Dave McGuire, AK4HZ New Kensington, PA |
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I 100% agree with you, that's not something i'd like to do, i personally wouldn't have thought of doing that. That's what the seller told me so i tried it too.
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Pushing on the INSTR PRESET button can also sometimes cause the same thing to happen, it's just less likely to do it.
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Thanks everyone for the help, i'll probably be back in a few days after i'll receive the logic analyser and do signature checks. If i can't find any issues then i'm gonna wait till i find another broken 8568a and try swapping the A15 board. |
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Yes, having a spare parts box may not be a bad idea. Here in the U.S., the 8568A/B is almost unwanted. It seems from your posts that you are in the UK, AU, or NZ. It may be harder to find a cheap parts box there.
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The 8568 is a really great analyzer. In fact, if you don't need to see above 1.5 GHz, it's more attractive than the 8566 because it barely has any microcircuits. Not to mention the extremely tedious front end alignment in the 8566.
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One more thing you can do is to check the traces on the motherboard that connect the processor and memory boards. Maybe one of them is open or shorted.?
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Vladan
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Thanks i'm gonna try that, i did not check that. ?
I'm actually from France, where those are even rarer. I've been looking at spectrum analysers online for more than a year and i never came across a 8568 (A or B). ?
Thx for the help again. |
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Alright, i've got some updates?
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It looks like the A15 board is fine, but the issue seems to be with the A14 board. The MAR/CS check isn't returning the expected signature.?
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The RAM check is showing the correct signature (PF59), and both the digital storage check (211A) and partial interface check (5669) are passing as well.?
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However, the ROM/Processor check is failing, returning the signature CPU1, which indicates a possible failure in the ROM (A14U8).?
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At that point things start to be a bit too complex for me. So i'm planning on just finding new boards...?
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Does anyone have an A14 board for sale? Ideally, I'd also like to buy an A15 board if available.?
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Thanks everyone for the help again |
Dave McGuire
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