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8050 drive no LED flashes on DRIVE 0 during power on

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anti77

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Oct 9, 2023
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I have a 8050 drive in my collection for about 25 years, now that I have a working PET 8032-SK, wanted to see if the two will work together.

After a bit of a cleaning, powered it up, middle led stays green, Drive 1 led lights up long, goes off, lights up short, then stays off.

Drive 0 led never lights up (the led is good, as on power off I can see a faint light on all three leds). Never operated such drives so have no idea how it should behave.

According to the manual on page 12, all three LED's should flash, so I wonder what to check next. Initially I thought it's the 6532's according to the flash codes, but as it's not repeating the flashes, I assume there are no errors on power up diagnostics.

Keeping the drive powered for a minute, something blew inside, probably a capacitor by the smell, I still not found which one (there was smoke between the transformer and the 15mF big green cap when opening it up). The LED's behavior has not changed after a power cycle. AC voltage with no load on the transformer checks out, maybe a bit exceeding specs on pin 5 & 6 with the 11.2V measured. DC +12v, +5v checks out on pins of P3 even if CR5 and CR6 are hot to the touch (can't keep hand on them - LE: as they were working but had a crack, replaced both with BY255, now it's cool). Switching UE1 with UC1 no change. Service manual is not really helpful other then providing the schematics.

I'm afraid hooking it up to a working PET like this, so any suggestion is welcomed. Board assy 8050002 and 8050006.
 
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As I've edited above, I'm fairly sure the one of the rectifier diodes CR5/6 blew/crack, while still remaining operational. Both measure as diodes with a multimeter...

Here are the chips fitted in the board:
UH1 901482-06 2364-091 ROM DOS 2.5 rev. C000-DFFF
UL1 901482-07 2364-092 ROM DOS 2.5 rev. E000-FFFF
UK6 901467-01 6316-017 ROM GCR
UK3 901483-03 6530-038 RIOT DOS 2.5 Micropolis

Replacing UN1, no change. UN4 (CD4066) was replaced in the past, as it has a socket.
 
If the voltages are ok, check the 6502 in UN1 (with a scope) for a proper clock signal, reset sequence and if there's activity in general
 
Reformed the 23000uF capacitor overnight through a 5w 3k3 resistor and 15V. This morning connected it directly to a PSU and slowly raised the voltage to 15V and left it until it draw less than 5uA. I believe for now this is all it was needed after 25 years of rest. CR5/6 are warm, but not hot after 10 minutes. I can keep my hands on them. So PSU checks out.

I've started from the LED's, removed UE1 6532 as it was in a socket. Without it DRIVE1 LED stayed green, ERR LED stayed green, DRIVE1 LED was off. That is strange as according to the schematic ACT LED 0 and 1 at least should behave the same. So I took out UN2 7406 and it proved to be bad with a TL866II logic test.

Put a socket and a known good 7406, now I have:
ERR LED starts RED, both DRIVE0 and DRIVE1 LED light up long green, followed by a short green, then only the ERR LED remains GREEN. Expected behavior I would say, but I'm not sure.

For me this raises the question why this happened, and there is P2 pin 8 and 5 on the schematic linked to the two LEDs. I think one possible cause may have been that the analog board flat cable was at least once connected wrong, so a whole lot more of logic IC's like UJ6, UH6, UL2, UM5, UA5, UK6 6316, UK3 6530 may be affected. And I don't know yet the original cause why would someone tried to "repair" it. But this is just an assumption.

So I'll follow now your suggestion @powerlot and start probing around UN1. I'm not that certain onboard diagnostics runs. I will keep posting here the updates.
 
If you already get a pattern of lights and a steady green in the middle, you could be good to go. Try to hook up a PET and see if it repeats after turning it on, if it does it should be possible to issue commands to the drive.

7406 ICs can go bad for many reasons...
 
IMG_20240319_185318_5.jpg
Surprisingly easy fix so far just with two diodes, a bad led driver logic and a capacitor needing reforming.
 
I'm trying to format a brand new HD disk (I still have like 200 masterdata ones):

HEADER "test",D0,I12

are you sure? yes
ready

The ready reply in instant. Same for D1. Even if I put a piece of paper instead of the disk should do more with it I believe, so it's not the type of the disk.

There is no motor spinup, no noise, no LED, nothing. When I close the door of any of the disks, it spins up briefly. I don't believe I saw the heads ever move.
 
My 8050's logic worked out of the box. I only reformed the big capacitors before turning it on.

The drives are another cup of coffee, I had to completely degrease the worm gear, reapply it and also clean and lube the head carriage sled. Later on I had to replace the felt pad because the worn ones were scratching my disks due to uneven pressure.

If you want to see some quick results you could drench the hardened grease with contact cleaner and manually spin the worm gear back and forth few times to get it going

The ready prompt is normal, the drive executes the commands independently from the computer. The heads spinning briefly is also a good sign (this is required to center the disk correctly)
 
Do you know if the IEEE port is good? It would be handy to eliminate that if possible by using another device on that port successfully.

I have a working 8050 here (but not an 8032-SK) which I had trouble with and managed to fix OK.

Also the 8050 should really use Quad density diskettes. Double density diskettes sometimes work in my experience, but not always.

Colin.
 
Unfortunately I don't have any double or quad density disks. In 1541-II drives the masterdata HD ones worked most of the time, if not, I "helped" them with a speaker magnet. The CP/M I wrote on such a HD disk 15 years ago still works today on a C128.

Cleaned the head in the 8050 with IPA, cleaned the rest of the mechanics with brake cleaner, added new silicon grease, gear oil to the ball bearings which all made the drive very silent. Used contact cleaner on all plugs and sockets. Decommissioned the EMI filter from the 8032-sk that went silently open circuit. Now the behavior changed a bit, it waits after confirming the "are you sure?" for about two minutes before it fails with "? bad disk". ?DS$ replies "21, READ ERROR,01,00".

For one disk, the head progressed slowly to the middle of the disk before failing. Repeating it on the same disks failed on track 0. For me this tells me the drive is possibly good, I just don't have the correct disks.
 
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