Re: HP 9821 - display and tape transport issues #VintageHPComputers


A lot of transport problems are caused by a bad working EOT sensor, the LDR’s used in the system to detect the end of tape deteriorate and the resistance changes permanent to a low value.

The system then won’t detect the end of tape or thinks end of tape has arrived when it hasn’t, with all kind of nice tape behavior as result.

The LDR and Lamp are place left from the tape head and accessible when you remove the tape door, you can check the working of the LDR with a multimeter the resistance should vary between 100-1K and >1M roughly (light – no light condition).

Other problems with the tape boards are the 7403 (1820-0269) made by NI they die without notice 😉

They’re used in several boards but the most important is the control board.

Another problem early multi-layer boards suffer from via contact problems causing intermittent or permanent errors, boards I know off are mostly memory and memory control boards (M-register, T-register, display) dual-layer boards typically don’t suffer from this only multi-layer boards.


Hope it helps a little,



Van: <> Namens mariuszmleczko@...
Verzonden: maandag 3 februari 2020 1:30
Onderwerp: Re: [VintHPcom] HP 9821 - display and tape transport issues #VintageHPComputers


9821A finally opened

Here you can see photos of all the boards.
Some of them are not the best quality (sorry).

So far I've repaired the lever for paper feed.

Next thing to do is a tape transport.
O-rings looks OK without any dents, cassette pulling strength and speed seems to be OK as well.

But it not behaves properly....

1) Rewinding.:
- when I execute "<>REW" command, it is rewindig but only for about one second and then stop immediately.
Here you can see video (few times executed <>REW command on execute keypress).

2) Program load "<> LDF 0".:
- after few tape turns when clear leader is finished it starts to behave strangely.
electromagnets on the both sides flip back and forward, then tape finally is rewinded back to begginnig.
This test was made on origin HP cassete which is OK in 99%.

3) Mark command:
- when I run "<>MARK 1, 5"; command it's running and running....don't stop.

Do you have any idea what shall I check first ?

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