HP 9825A Tape driv



#3

I'm wondering if Tony Duell in his tracing of the 9825 calculator circuitry has noticed whether the tape drive has the ability to control the start up acceleration of the tape and associated components. I have just tried two Athana refurbished tapes (40-100A) that should be compatible with the HP9825B. Both tapes self destructed apparently during rewind. In both cases it involved the drive belt. It broke on the second tape directly after rewind (writing seemed normal). The first tape refused to work after initial rewind. The drive wheel would not move. Taking the cartidge apart showed the drive belt jammed between the drive wheel and the baseplate. I'm wondering if the start up acceleration can be reduced to limit drive belt stretching.


#4

I am afraid not. According to Dyke Shaffer,

> [...] the drive technology used in the 9825 has two issues, 1) no
> acceleration control and 2) read sensitivity [...]

however, I have never had a problem with cartridges being damaged
by the 9825 unless they were already marginal. If the belt is old, it stretches much more easily and hence under speed it fails to contain the tape.

Also tapes need to be retentioned from time to time. Retensioning involves fast winding to EOT followed by fast rewinding to BOT.
This is what the HP-85 CTAPE command does. If your tape cannot
handle this, then the tape is faulty.

**vp


#5

That what I was thinking. These were refurbished and checked out on another HP transport but not an HP9825 calculator. Being refurbished ones, I have no idea was was replaced besides the tape (if anything). Other transports might be easier on the tape than the 9825.

Edited:
It seems the statement by Vassilis Prevelakis may not be correct.

In the 9825A/B Service Manual available from the Computer Museum (Australia) under Theory of Operation 4-13: "An analog reference voltage is generated from the three digital signal GO, FST, and REV. This voltage drives a ramp generator to control the slew rate; this limits the acceleration of the transport motor. The steady state output of this circuit is a forcing function, or reference voltage, of either 0 volts, +/- 2 volts, or +/- 7 volts."

Now if I can just find out where the components for the ramp generator are.........


Edited: 14 July 2006, 4:45 p.m.

#6

If you grab 'my' schematics from the Australian site, the motor
control circuity is pages 8 and 9 of the tape controller section, that's pages 46 and 47 of the pdf file

The first page gives the circuit for getting the speed signal from the tacho sensor, and for the motor drive amplifier. The second page gives the circuit to generate the reference signal -- this is what the speed signal is compared against. It's the latter that you want.

It would appear that IC25b is an integrator, presumably that's the ramp generator.


#7

Thanks, That is the conclusion I came to with the 0.47 microfarad capacitor controlling the ramp rate. Larger capacitor - slower ramp. Now to find it on the board and experiment.

Looks like Tony was nice enough in his schematics to put black dots on componets that are easily replaceable (plug in). The resistor associated with the capacitor is one of those. I call them S.A.T. components. (Select At Test) The 74K5 resistor shown in his schematic is a 88.7 kOhm 1 percent in mine with the 0.47 microfarad capacitor being what looks like 20% tolerance. (Easier to adjust resistors than capacitors.) I think I'll try doubling it and see if I notice any difference.

Edited: 19 July 2006, 10:18 a.m.


#8

Where a component plugs in, I put little socket symbols (look like parentheses ;-)) on the ends of the circuit board trace lines, and plug symbols (black dots) on the ends of the component. The exception is that I don't do this for socketed ICs (normally).

And yes, these are almost certainly select-on-test components. There's a text file on the HPCC CD-ROM (not on the Australian site, though) where I comment that there may well be SOT components in these machines, and the values given on the schematics are the ones found in the machine I took apart.


#9

socket symbols? circuit board trace lines? Were these only on the HPCC CD-ROM? I have not seen anything with circuit board trace lines.


#10

Sorry, I wasn't clear.

None of 'my' diagrams include PCB layouts, trace patterns or anything like that.

The 'trace lines' I refered to are simply the lines on the schematics indicating connections (wires, PCB traces, etc). The socket symbols are the schematic symbols for sockets.

AFAIK the Australian site's pdf files include all the information I provide (and indeed have produced) for a particular machine.


#11

Now I get it, the ( symbol or ) is where a connector to another board would be.

24 July 2006
It is beginning to look like this unit has a rewind problem. But I have not gotten a storage scope on the signals yet. With a 3M tape it does not always stop in the correct place for end of tape sensor. On another 9825 the 3M tape does stop correctly and the tape seems to move more slowly.

Edited: 24 July 2006, 9:59 a.m.


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