82161A casette drive



#2

Augh! I'm having a problem with my casette drive.
I cannot read any tapes anymore. Looks like
read errors all over.

The setup is a 41CX, HP-IL, Casette driver and video
unit.

The IL is working fine. The video unit is working fine.
The cassette driver is responding fine to commands.
I've tried NEWM a casette. Doing a DIR shows that it
is empty, but trying to save something, or creating
a file gives me MEDM FULL. Trying to DIR a cassette
with known contents give me lots of junk entries.
These entries have junk characters, and shows file
sizes in excess of 3000.

I've tried cleading the heads with isopropanol with
no success. I've disassembled the drive, cleaned
things and reassembled it again. No change.

I had the thing lying without power for a couple of
years, and a tape in place. I wonder if that could
have damaged something.

Does anyone know a way of fixing this? Anyone seen
something like it?


#3

Hi Johnny,

Since you left it with a tape inside for years and considering all things you've already checked, I would say the Head itself is damaged (magnetized) due to the tape being next to it. This is only a possibility, as I'm not a expert to that cassette drive and I'm sure there will be more help from someone. Hold there and you'll see.
Cheers,

Luis

#4

i did the same thing to mine; i left it unused from 12/96 to 5/99 unused with a tape in it. i could only see part of the directory and could access nothing. i demagnetized the head using a wand type demagnetizer and it would do a newm on a new tape and works fine now, accessing and reading everything. but the old tape was usless.

one warning; someone who used to post here a lot offered to try to transfer my old programs onto a new tape using some 71b function. i sent him the old tape, a new tape, a disk, and money to ship the tape back to me. i never heard from him again and he did not answer my email. so d.i.y.

good luck.

#5

Others have suggested demagnetising the head. which would probably be what I'd do next. Actually, I'd clean the ehad again as well, just to be sure.
However, one thing seems odd to me. Althoguh the HP41 is getting garbled data, the drive is not reporting any errors. This suggests to me that the drive is reading the tape correctly, the
checksum is matching, but the data is somehow getting mangled. Now, there are 3 main chips in the digital part of the 82161. A microcontroller (containing an HP custom program), an HPIL chip, and a RAM. Most likely the microcontroller is fine -- the drive will respond to commands so it's doing something.
Ditto for the HPIL chip. But maybe the RAM is faulty. IIRC it's a 1K*4 device, like a 2114, and those are notoriously unreliable. Try replacing it -- it's the 18 pin chip on the board _not_ on the drive mechanism itself.


#6

Hi, Tony. Seems like we have several shared interests. :-)

Is that ram pin-compatible with a 2114, or should I
check out what it really is before I find a replacement.

Also, I hope the checksum calculation is done
within the control logic without involving the RAM
in that case, since otherwise replacing it won't help.

How do I de-magnetize heads by the way? I say someone
mentioning a wand. What exactly is that?


#7

Hi;

a demagnetizer (also a Degausser) can be made by building a coil with some turns of insulated cooper wire. I once built one to demagnetize audio heads on tape recorders. I just build the coil with a few turns (about 20-30) of wire over a carton paper base in the shape of a small box, where the head would be inserted in. To energize it, I used the secondary of a small 6+6 transformer with a 47ohms series resistor.

The trick is switching the Degausser ON while the coil is far from the head; then you get the coil closer to it until it is around the head; move the coil slowly far away from the head (as far as possible) ad switch the apparatus OFF.

When dealing with audio heads, I disconnected them from the circuit and shorted their terminals previous to demagnetize them. THIS IS MANDATORY for audio equipment, cause current flow through the internal head coils when degaussing is A LOT HIGHER then the microamps generated by the magnetic tape.

I would expect some other suggestions before trying, cause I do not know and even have (how I wish...) an HP82161A. Not even an HP82160... Hey, do any of you trade one HP82160 for a working HP31 plus case plus charger plus Owner's Manual (Portuguese)? Oops, sorry... Classified Adds, Yes, I know...

Success.


#8

FWIW, I've demagnetised many heads in audio, video and computer data drives. Not once have I disconnected them and shorted the terminals, and not once have I damaged any components in the read
amplifier. I've also never seen a service manual that states you have to disconnect the head before demagnetising it.
Amybe the 82161 is particularly fragile, and the R/W chip is HP custom, but I doubt it.
Most chips will stand the full supply voltage (+5V in this case) or slightly more on any pin without damage (see the 'absolute maximum ratings' in a data sheet). I'd be very supprised if you could iuduce 5V in a head winding using a normal demagnetiser.


#9

Hi, Tony;

thanks for the info.

The first degausser I saw was built as a little black holder (coil inside it) with a thin metal tube built in. The tube's end was covered with a plastic layer, so it would not scratch the head. It's owner showed me how to use it: wave it in front of the head and allow the plastic layer to gently touch the head's gaps.

After building my own degausser, I tested it in a low-end tape player and the first-stage preamplifier burned out. The B-E junction of the BC550C became oppened.

I know this could happen for many reasons (maybe the transistor was already burned out, I did not test it before); also, magnetic heads are just transformers. Coincidence or not, I decided never to use my degausser that way, and all other heads I demagnetized from then on were successfull. If I am using a conventional degausser, I'm sure that would not happen, but I felt as if careless procedures resulted in the damaged transistor. And when dealing with HP products, this is a one-way street.

I would like to thank you for your (as always!) valuable information. Based on them, I've been learning a lot more.

Best regards.

#10

To take your points in order : Yes, we do seem to share several interests -- like PDP11s, HP calculators, etc :-).
The RAM is an 1818-0643, which my equivalents list gives as P2114A-6. So it's not just pinned out like a 2114. It _is_ a 2114 :-). Do be warned that if you attempt to trace out the circuitry of the 82161, it's a bit odd. The microcontroller has no external memory bus, so the HPIL chip and the RAM are linked up usign a couple of the controller's I/O ports. And since the RAM has 10 address lines and 4 data lines but the HPIL chip has 8 data lines and 3 address lines, one port is used for the HPIL data _and_ (most of) the RAM address lines while another set of port lines are used for the HPIL chip address lines and the RAM data lines. It sounds crazy, but as everything is software driven, it can work.
I have no idea how the checksum is done. It might be done on the data in the RAM (in which case you might have a microocontroller failure), or it might be done 'on the fly' as I suspect. However, as the RAM is a standard part, and as I've had a lot of trouble with deffective 2114s, it would make sense to check this first before attempting to find another dead drive to raid a microcontroller from.
To demagnetise the heads you use a demagnetiser (aka 'degausser' or 'defluxer'). This is basically an electromagnet that's conencted to an AC supply (normally the mains). You turn it on, bring the pole piece up to the head face, move it around a bit, draw it away slowly (to at least arm's length) and turn it off.
At one time every shop that sold tape recorders sold demgnetisers (along with other useful things like splicing tape and jigs). I have one from that era, so I have no idea where you'd get one now. But surely they're still being made.


#11

Well, I got to hand it to you. You did pinpoint
the problem.

It is defective ram. Once you got me starte thinking
about it I jkust figured I'd write a small diagnostic
program to check it, and sure enough, two out of
four bits are stuck.

Writing all zeros to the buffers reads back as 0xCC.

Having the Extended I/O module helped. :-)

Now I'll just have to get a memory chip, and do some
soldering. Thanks a lot!


#12

Now that's what I call a lucky guess :-). Actually, you'd still better hope that the problem is the RAM and not (say) a port pin on the microcontroller. But I think the latter fault would cause HPIL problems too as the the microcontroller wouldn't be able to communicate with the 1LB3 properly.
Anyway, given the unreliability of 2114s, I think it's likely that the RAM is the problem and that I ahve another example of a dead 2114 to add to my list...


#13

Considering the fact that I could communicate
perfectly fine with the drive, and just saw that
two bits out of four are stuck (the other two
responds just fine), makes it very unlikely that
there is any kind of HPIL problem.

However, I'll not get the 2114 until monday,
so I can't replace it and verify before the middle
of next week.


#14

Okay. I can now confirm that it was the
2114 that had fried.
I've changed it, and the drive works like a charm
again. Thanks to all of you!

#15

<The setup is a 41CX, HP-IL, Casette driver and video unit.>

Maybe the cassette drive is too close to your monitor. I have found this to be a problem. Move the drive as far away from the monitor as possible and try the drive again.

Christopher


#16

Nope. That is definitely not the problem.

I also ran without the video unit, and not even
close to any other electrical equipment. :-/

#17

I searched long and hard for a tape head demagnetizer. I finally found one made by TEAC at the Mars Music store in Dallas on Walnut Hill. They special ordered one for a guy, and got in a case of them. When I asked about one, I got strange and baffled looks. Then a passing clerk said "Oh, yeah... we have some stashed in the back. Nobody knows what they are for. You are the first person to want want in a couple of years"


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