Troubleshooting - constant tone, no beep

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Troubleshooting - constant tone, no beep

Postby DRAM19 » Tue Oct 20, 2020 4:01 pm

Hello All,

I recently acquired a PCJr that had been in storage for a few decades. It does not currently boot, here are the things I see:

1. The floppy fan spins
2. I get nothing on the screen
3. I get a constant tone out of the monitor speaker
4. I get zero diagnostic beeps

I read that many of these older computers initialized sound in an 'on' state, and then during boot the CPU would send an 'off' signal to the sound chip. And so, I think the issue is somewhere in the early boot steps, but I am not sure (and not fully sure what steps the PCJr goes through to get to that point).

I pulled the extra 64K mem card, and the floppy controller card from the board so I could do a minimum configuration diagnosis. During this test, I suspected 1504037 was bad, because it got very HOT if I left it on, while 1504036 did not (and 1504037 did not get hot on another PCJr main board I could compare with). I programmed a new BASIC ROM to replace 1504037 (and verified the new ROM is good), but that changed nothing on my not-working board. I still get power, I still get tone from the monitor, I still get nothing on the screen, and no beeps.

Where can I go next?

(if it helps, I can take pin voltage measurements, but I do not have an oscilloscope)

Thank you in advance for the help!
-DRAM
DRAM19
 
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Re: Troubleshooting - constant tone, no beep

Postby DoctorOctal » Wed Oct 21, 2020 3:26 am

DRAM19 wrote:1. The floppy fan spins


This and the tone from the speaker suggest that your power rails are OK, but you should still check them if you haven’t already. I’d check voltages directly at the power supply card but also spot check several chips across the motherboard, including the CPU and ROMs.

3. I get a constant tone out of the monitor speaker
4. I get zero diagnostic beeps


These suggest that your CPU isn’t executing the BIOS code, either because the CPU isn’t running at all or because it can’t read the BIOS code.

I read that many of these older computers initialized sound in an ‘on’ state, and then during boot the CPU would send an ‘off’ signal to the sound chip. And so, I think the issue is somewhere in the early boot steps, but I am not sure (and not fully sure what steps the PCJr goes through to get to that point).


It’s true that the PCjr has a TI sound chip that needs to be quieted at the outset, which is one of very first things the BIOS code does. It appears your system isn’t getting to those early instructions, never mind the POST.

If the power is OK, I’d first try re-seating your socketed chips. Pry them up a little with a flat head screwdriver, then press them back in. You probably don’t need to pull them out completely, and doing so would add the risk of bending pins. Examine the pins of each socketed chip carefully to make sure they’re all present, reasonably clean, and inserted properly into the socket holes.

If re-seating the chips doesn’t get the POST started, you can try swapping in a different 8088 if you have a spare. Since you appear to have an EPROM burner and images of the ROMs, you can use those to verify both ROMs (not the character ROM behind the IR receiver).

Beyond that you’re in for some more advanced troubleshooting. A logic probe might suffice for the task. A simple multimeter might not.

Check the clock pin on the CPU for clock pulses. With a multimeter on DC volts, a working clock will probably read as about 1.8 V. Also make sure the reset pin is low.

Beyond that, check the address/data lines for activity, both at the CPU and the ROMs.

Anything amiss in these checks will point to other signals and chips to check.
DoctorOctal
 
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Re: Troubleshooting - constant tone, no beep

Postby DRAM19 » Wed Oct 21, 2020 7:40 am

Thank you DoctorOctal! I will give it a try and report back what I find.

Some relevant info:

This board is a later revision, with an AMD 8088, and everything is soldered (no sockets). I will still carefully check and re-clean pins (I used air, but not ispropanol, so I will do that).

Is the BIOS in the 8088? Because the board is not socketed (except 1504037 now because of me), I cannot do a quick swap try of the 8088. If the other tests point there, I will have to commit to removing, socketing, and replacing the 8088 (lets see what your other advice reveals first).

Part of me was really thinking about the clock last night, but I haven't checked it yet. I will be sure to check that.

I did check the voltages on 1504037 before replacing it and found that on pins 2 through 6 it was 0.4 and 7-10 it was 0. On the good board, these are in the 2V range, so I suppose the signal there is wrong (but do not know how those pins are driven). I have not measured the pins since replacing 1504037.

Thanks again!
DRAM19
 
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Re: Troubleshooting - constant tone, no beep

Postby DRAM19 » Wed Oct 21, 2020 6:27 pm

Update with pin readings:

I've got 24.5, 17.5, 12.5, 12.2, 7, and 5 on various (pin to pin)s of the power supply card into its socket.

I measured the ROMS 1504036 and 1504037 and found both had almost the same voltages (expected). Here is the readout from 1->14 then 28->15 for 1504037:
3.8, 3.8, 3.8, 3.8, 3.8, 3.8, 160mv, 160mv, 160mv, 3.8, 3.6, 3.6, 0, 0
5, 3.8, 3.8, 3.8, 3.8, 3.8, 100mv, 3.8, 100mv, 0, 3.8, 0, 0, 0

I noticed that 1504037 was not getting hot (since I replaced it). So maybe it was bad, but not the only issue.

CPU pin reading summarized:
GND=0, VCC=5
A17, A15-A8 = 4v
A0, A1, A6 = 3.6v
All other addresses at 120mv or less (floating i guess, some 120mv, some 25mv)
NMI = 180mv (was floating?)
INTR = 5
CLK = 1.8
RST = 120mv, RDY = 180mv (maybe both of these were floating)
TST was progressively increasing, in the mv range.
SSO = 25mv
MN/MX = 5v
RD=40mv
HOLD=150mv
HLDA=50mv
WR=5
IO=50mv
DT/DEN/ALE = 30mv
INTA=5V

So the really operative question for me: where is the BIOS code that the CPU uses to initialize, tell the sound chip to quiet, and the POST to occur? I'll look to follow that around once I know where to go (if its in the CPU, or a ROM, or elsewhere).

I'll get to cleaning the entire board tonight and tomorrow.
DRAM19
 
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Re: Troubleshooting - constant tone, no beep

Postby DoctorOctal » Thu Oct 22, 2020 4:42 am

DRAM19 wrote:This board is a later revision, with an AMD 8088, and everything is soldered (no sockets).


Ah, sorry. I figured you had a board with sockets because they’re much more common and you had already replaced a ROM.

I will still carefully check and re-clean pins (I used air, but not ispropanol, so I will do that).


I suggested checking the cleanliness of the pins just to make sure that no oxidation or other stuff could prevent good contact with the sockets. Since the chips are soldered in, the surface cleanliness shouldn’t make any difference. Cleaning shouldn’t hurt, though.

Update with pin readings


Glancing over the readings, nothing stands out as wrong. It’s hard to conclude much from just voltage readings, but I haven’t gone through it all in detail. It looks as if the board is getting good power, the CPU is getting a clock, the CPU is enabled, and the ROMs are enabled.

So the really operative question for me: where is the BIOS code that the CPU uses to initialize, tell the sound chip to quiet, and the POST to occur?


The BIOS spans the two ROMs, 1504036 (XX1) and 1504037 (ZM63). Cassette BASIC and the built-in diagnostics are also in those ROMs.

The code that quiets the sound chip and runs the POST is in 1504036. However, the CPU starts executing code at FFFF:0000 in 1504037, where there’s a jump instruction pointing back to F000:0043 in 1504036. In other words, you need both ROMs working just to pass Go.

Where did you get the image for 1504037…Minuszerodegrees? What type of chip did you write it to, and using what burner? Can you read the original 1504037?

You were probably right to replace 1504037. I’m just wondering if there was a hitch somewhere in the replacement process.

Other things to look for: broken traces, bad solder joints, and any signs that someone else monkeyed with the motherboard in the past.
DoctorOctal
 
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Re: Troubleshooting - constant tone, no beep

Postby DRAM19 » Thu Oct 22, 2020 9:39 am

The BIOS spans the two ROMs, 1504036 (XX1) and 1504037 (ZM63). Cassette BASIC and the built-in diagnostics are also in those ROMs.

The code that quiets the sound chip and runs the POST is in 1504036. However, the CPU starts executing code at FFFF:0000 in 1504037, where there’s a jump instruction pointing back to F000:0043 in 1504036. In other words, you need both ROMs working just to pass Go.


That is a big piece of information for me, thank you!

Where did you get the image for 1504037…Minuszerodegrees? What type of chip did you write it to, and using what burner? Can you read the original 1504037?


I did get the image off Minuszerodegrees. It was a BIN. I wrote it to an Intel D27256-25 using a TL866ii+ with the xgpro software. I told it to start writing on that chip at 0000 (IIRC).

I bought a spare, socketed PCJr main board on eBay (with an Intel 8088 of course), so I could test/compare, and then hopefully re-sell. I plugged in the 1504037 I burned into that known working board, and that new board worked with the ROM I burned (passed POST, got to BASIC, ran a few commands). So I feel good about the replacement 1504037. I can read the 1504037 from the working board, to compare to the one I burned. But unfortunately because I lack a desolder gun I had to cut pins on the suspect 1504037 (which I was very hesitant to do) to remove it.

I don't believe anyone did anything odd to the board, since I bought it from the original owner and it sat in storage for so long. I'll get to cleaning it and carefully inspecting chips/pins/traces while I do. I will also double check all of the soldering I did (make sure the socket is in there right, continuity to somewhere else down the trace, etc).

Again, thank you so much for your advice!! It's not working yet, but your help has been invaluable!
DRAM19
 
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Re: Troubleshooting - constant tone, no beep

Postby DoctorOctal » Fri Oct 23, 2020 2:50 am

The Minuszerodegrees images match what I’ve read myself with a TL866II+. The PCjr ROM sockets are wired the same way as those on early XT motherboards and thus may run into problems with certain brands of 27(C)256 chips, but the fact that your new EPROM worked in the second motherboard clears up that concern.

I had one more shot-in-the-dark idea: since you have another working motherboard with socketed chips, you could try piggy-backing the 1504036 and 8088 from that board onto the ones on your non-working board. There’s an off chance that one of those chips has gone bad on your first board and that piggy-backing a good chip will overcome the problem. It’s something relatively easy and quick to try.

Anyway, you seem to have it well in hand. Good luck, and let me know how your troubleshooting goes!
DoctorOctal
 
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Re: Troubleshooting - constant tone, no beep

Postby DRAM19 » Fri Oct 23, 2020 8:12 am

Piggy-backing would be just placing the good chip right on top of the bad chip, with the pins in contact; correct? Yeah I can try that. Did a bit of cleaning on the board last night, but not done yet. I'll post an update with what I find tonight.

Thanks!!
DRAM19
 
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