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Success Stories If you have successfully repaired or restored a piece of equipment, why not write up what you did and post details here. Particularly if it was interesting, unusual or challenging. PLEASE DO NOT POST REQUESTS FOR HELP HERE!

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Old 15th Nov 2021, 6:19 pm   #1
ajgriff
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Default Commodore PET C2N Datasette Restoration

INTRODUCTION

The subject this time is a ‘parts only’ PET datasette which I bought for a reasonable price to replace the missing unit in a Commodore PET 2001-8. The datasette really had been quite seriously abused in its previous life. I suspect it would have ended up in a skip if it weren’t for the fact that the black PET style units are becoming harder to find and so attract a corresponding price premium.

The datasette was referred to sporadically in this long thread about the PET so I thought it might be useful to specifically summarise the refurbishment of this C2N. A more digestible summary of the PET’s restoration can be found here.

ATTACHMENTS AND LINKS

I mention the attached files as they form useful reference points when reading the rest of this post. The first attachment is a copy of Commodore’s 1984 Datasette Service Manual and the second is a portfolio of photographs taken during the work on the case plus an image showing Commodore’s recommended approach to head alignment. The third file is a PDF copy of this post for retention if required.

As ever Bo Zimmerman’s site is an invaluable resource for all things Commodore. This page provided much of the information used during this mini-project.

This nicely produced little booklet tells the story of the various C2N models and may be of a general interest. To download a PDF copy at no cost you do have to provide your name and a valid email address but this didn’t cause me any problems. Personal choice of course.

INITIAL ASSESSMENT

Here’s a summary of the datasette’s condition when it first arrived:

1. The two halves of the case were held together by tape because the fixings were badly damaged.

2. There was a small chunk missing from the front upper half of the case.

3. There were ugly heat marks to the top above the keys.

4. The datasette was originally supplied as an external drive as confirmed by the length of cable. Internal drives had much shorter cables.

5. The silver trim (paint) around the cassette window had largely worn away.

6. The case was scuffed in places particularly on the base.

7. The drive belt was slack and perished.

8. The azimuth adjustment screw had obviously been played with.

9. At this point I had no idea whether or not the machine would work with a new belt although the mechanism seemed to operate freely.

ELECTRONIC FAULT FINDING

Testing

After fitting a new drive belt I was keen to test the datasette before giving the case the attention it deserved. New belts are still easy to find and are inexpensive as the belt used in this version was also used in all the variants that followed. There’s nothing too difficult in terms of fitting the new belt. It’s just necessary to undo a couple of screws to free the bracket positioned over the large flywheel/pulley.

After experimenting with a blank tape and a couple of simple programmes (typed into the PET) it soon became apparent that the unit wouldn’t successfully load or write back any of them. However when the tape was played back on an ordinary cassette player it was evident that the datasette had recorded something but it wasn’t possible to tell if the recording was satisfactory or not.

Spare C2N

One of the challenges I faced was that I didn’t have to hand any kind of pre-recorded Commodore programme tape. However I did have a similarly battered C2N of the type (lozenge shape) often associated with the VIC20 or C64. This had remained untouched since being bought as a source of spares for 50p at a boot sale many years ago. Fortunately after a bit of fiddling I managed to get the lozenge to load and write, despite the broken key, using the PET’s external port. It was also reassuring to find that the alignment adjustment screw looked undisturbed despite the screwdriver marks on the outside of the case around the aperture. Anyway I went ahead and recorded a couple of simple programmes for the purposes of testing the PET’s newly acquired machine but it still didn’t read or write.

Azimuth Tape

There didn’t seem to be much point in proceeding further until the previously twiddled azimuth alignment had been reset. There are different ways to do this of course but I opted for the method advocated by Commodore (I think) in image 11 of the photo portfolio. The only challenge being that I didn’t know what sort of signal was recorded on the “azimuth tape”. For inspiration I turned to the service manual. Pages numbered 1, 12 & 17 are of particular relevance. The schematic for my PET C2N (P12) doesn’t show any waveforms but the schematic for a later version (P17) does. The only real difference between the two circuits is that the former uses two dual op-amps in read mode whereas the latter uses a single quad op-amp but the waveforms should be identical. Note also that the C2N attempts to record a kind of symmetrical sawtooth although in read mode the head reproduces something much more like a sine wave due to the limited bandwidth of magnetic tape.

Bearing the above in mind I reasoned that a continuous sine wave of moderately high frequency would enable me to align the head and trace the signal through the read circuit using an oscilloscope. Blank tape in hand I recorded a series of continuous tones at varying frequencies using an audio signal generator and a decent quality cassette tape deck with correctly aligned heads. After some experimentation with the lozenge I settled on a 15 min long 4 kHz sine wave which the ‘scope tracked nicely (in accordance with the schematic) through the read circuit.

Head Alignment and Signal Tracing

To begin with I didn’t have much luck with the azimuth tape in the PET’s datasette. I then noticed that one of the fine wires to the read/write head had become detached, probably caused by my careless prodding around. Also the sliding read/write switch needed some more doses of switch cleaner and further exercise even though I’d given it similar treatment previously. With these issues corrected I was able to adjust the azimuth to obtain a happy looking peak at the output of the first op-amp (LM358 IC1-B Pin 7). Next I tracked the signal as far at the output of the third op-amp (LM358 IC2-A Pin 1) where it disappeared. A few in circuit checks with a DMM suggested that nothing dramatic was amiss with the associated components so it looked like the dual op-amp IC2 had failed so I placed an order for a small pack of LM358s. In the meantime I proved that the write circuit was working satisfactorily by writing to tape from the PET and then reloading the programme using the lozenge.

End Game

The LM358s duly arrived so I removed the suspect chip, soldered in a socket and fitted a new IC. When tested with the azimuth tape there was now a healthy signal appearing at the output of IC2-B Pin 7. Sorted I thought but wrong again as the C2N still wouldn’t load a programme so I checked the output signal just after the two inverters and got nothing. Either the hex inverter was faulty or the PET was dragging down the signal. I knew that the datasette ports were linked directly to the PIA1 & VIA and I had spares for both of these socketed chips so as a quick check I fitted them to the PET. Eureka! I now had a fully operational datasette (actually two counting the lozenge).

If I’d paused and studied the PET’s main board schematic a bit more closely I’d have realised that PIA1 looks after the internal port (J6) and the VIA deals with the external port (J3). A bit more chip swapping proved that PIA1 was indeed the cause of the problem. This explains why the lozenge worked perfectly well as I had it connected to the external port throughout.

COSMETICS AND CASE STRUCTURE

This part of the refurbishment ran parallel to the fault finding exercise. This is a summary of the work carried out (see also photo portfolio):

Missing Piece

The only way I could see to undertake this repair was with two part epoxy (JB Weld in this instance). After positioning some masking tape to provide support I was able to mould the slow setting epoxy into position. Once hardened the JB Weld was easy to shape with a small file and some fine grade carbide paper. I was quite happy with the shaping but had to find a way to colour match the grey epoxy to the black plastic of the case. There was also the problem of how to recreate the appearance of the original textured finish. To begin with the area surrounding the repair was rubbed down to create an evenly matt surface to the front right corner of the case (upper half). Next I masked off the entire raised area around the keys and applied gloss black acrylic paint with a rattle can. By spraying at an excessive distance I was able to create a reasonable approximation to the textured finish. Please see photos.

Broken Case Fixings

Three self-tapping screws hold the case together by means of three opposing pairs of pillars. Five of these had suffered damage in the past probably when someone attempted to close the case without making sure that the cable grommet wasn’t trapped out of position. The various cracks/breaks had been repaired with super glue but some of the joints had failed to one degree or another. A combination of super glue, epoxy (JB Weld again) plus short sections of plastic tubing were used to reconstruct and reinforce the pillars. Hopefully everything has been returned to full strength. I tidied up the repairs with black paint even though none of it can be seen with the datasette fully assembled. See photos for examples.

Cassette Door Paint Loss

Painting fine lines can be difficult unless you’re a time served sign writer so my approach was to mask and spray. The key to success is to use the right type of masking tape. Ordinary decorators tape allows paint to bleed and it pulls at the fresh paint when peeled away. Modellers’ masking tape overcomes such problems and is easy to obtain in varying widths from modelling suppliers. Positioning the tape in this instance was fiddly and took some patience but I’m reasonably happy with the result as illustrated by the photos (the modellers’ tape is yellow).

Heat Scars

I used a really sharp craft knife blade to gently peel away the raised edges of the scars. This was followed by some careful abrasion with fine grit carbide paper to produce a matt finish. I took great care not to scuff the silver strip close to some of the damage. The marks are now only visible on close inspection and barely noticeable at a distance. I’m at a loss to know how they were acquired in the first place. Something hot seems most likely but it could have been a solvent of some description I suppose.

Scuff Marks

There were quite a few surface marks all over the case as well as plenty of tape residue of course. Everything cleaned up well with white spirit, meths and silicone/wax free furniture spray. The exception was the base where there were some deeper scuffs so I rubbed them down gently and sprayed the base only, taking care to mask the label with modellers’ tape

STING IN THE TAIL

It was only when I’d used the C2N a few times that it dawned on me that that the auto stop function wasn’t working. In other words the power wasn’t switching off at the end of a tape with the datasette in play or record mode. The mechanism relies on a quite complex system of levers and sliding actuators to perform this simple function. All was well again once the old grease had been carefully washed away with meths and fresh lubricant applied.

I also gave the datasette to PET connectors a good clean as the external port in particular didn’t always work without a good wiggle.

SUMMARY

I guess there won’t be many people around who might need to restore such a dilapidated black cased C2N especially as the later models are cheap and easy to find. However I’m hoping this summary will provide some guidance on fault finding and possibly act as a source of inspiration to anyone faced with awkward plastic case repairs in general. This wasn’t a promising candidate for refurbishment but I enjoy a challenge and am quite pleased with the outcome. As always if there are any questions please ask.

Alan
Attached Files
File Type: pdf C2N 1530 1531 Service Manual.pdf (1.66 MB, 121 views)
File Type: pdf Datasette Photos.pdf (3.26 MB, 104 views)
File Type: pdf PET C2N Success Story.pdf (55.5 KB, 64 views)
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Old 16th Nov 2021, 8:34 am   #2
electronicskip
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Default Re: Commodore PET C2N Datasette Restoration

Nice to see it restored, i have the cream version of the original oblong C2N like yours , its been with me for ever and still works fine.

The later 1530 is much more common as you say.
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Old 16th Nov 2021, 8:41 am   #3
Nickthedentist
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Default Re: Commodore PET C2N Datasette Restoration

Amazing work, nice to see something that most would not have given house room, restored to its former glory.

BTW, I suspect the supposed heat scars may have been caused by a chemical reaction with the plasticiser in a cable which was in contact with the unit while in storage. It's common to see this on the acrylic covers of turntables.
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Old 16th Nov 2021, 12:37 pm   #4
ajgriff
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Default Re: Commodore PET C2N Datasette Restoration

Quote:
Originally Posted by electronicskip View Post
....... I have the cream version of the original oblong C2N like yours , its been with me for ever and still works fine.
Commodore thoughtfully added a tape counter to the cream version and the lozenges that followed. Generally I think C2Ns are pretty reliable. I only bought mine out of frustration as a result of 'losing' the original and I didn't want to pay too much for it.

The booklet referred to in the refurbishment story gives a good overview of the different models. Intriguingly I noticed one on the internet recently which doesn't appear in the booklet. It was a black oblong C2N identical to mine but without the word commodore or the logo embossed in the area above the cassette door. Quite rare I'd imagine.

Quote:
Originally Posted by Nickthedentist View Post
...... nice to see something that most would not have given house room, restored to its former glory.

BTW, I suspect the supposed heat scars may have been caused by a chemical reaction with the plasticiser in a cable which was in contact with the unit while in storage. It's common to see this on the acrylic covers of turntables.
I was surprised how rarely the early black PET style machines come up for sale and when they do the prices asked can be ridiculous. For me it was worth buying a reasonably priced project C2N especially as we were in the middle of lockdown at the time.

I think you've hit the nail on the head with the plasticiser theory. Just hadn't occurred to me but now you've mentioned it I reckon it's the most likely explanation.

Alan
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