Commodore pet 4032 cassette port dead

[WimWalther]

I keep hearing over and over again that the contact 'fingers' on the datasette connections become really heavily oxidised over time ..

Sure. But oxidation won't prevent a modern hi-Z DMM from detecting the presence of low-Z voltage on the pins, particularly if your probes are sharp.

 

[WimWalther]

Now I don't know what to make of this..

Removed the cover off of cassette port #2. The machine acts dead if I connect the datasette to the port. No startup tweedle, just black screen, dead.

Quick question - Is the 9V un-reg'd supply used for anything other than cassette motors? I know that on C64 the raw ~9V AC is used as the tinebase for the TI jiffy clock.

 

[Hutch]

Removed the cover off of cassette port #2. The machine acts dead if I connect the datasette to the port. No startup tweedle, just black screen, dead.

Port #1 and #2 depend on the machine. One is internal and one in the rear next to the user port.
Connecting a datasette should not cause any change. The cassette drive draws +5v from pin 2, on or off.
The computer should not enable the motor voltage unless a key is pressed on the drive.
If the datsette is creating a problem it could only be a faulty datasette drawing too much current on the +5V line.
or perhaps, a faulty 6520 or 6522 if the cassette switch or cassette read inputs are being pulled high or low.

Quick question - Is the 9V un-reg'd supply used for anything other than cassette motors? I know that on C64 the raw ~9V AC is used as the tinebase for the TI jiffy clock.

No. The PET uses the vertical sync from the video to generate a 60Hz interrupt for the jiffy clock. It will run completely on DC.

 

[Hugo Holden]

Its four 2N2222s in an IC case. There may now be SMD versions of it, but the datasheets I'm seeing are for DIP-12 packages.

i'd be surprised to find many SMD parts in a machine of this (1982) vintage.

At one point transistor arrays were quite popular and not just with single transistors. Some had the transistors in various configurations like long tailed pairs, to make differential amplifiers, and some in a very elaborate 4 quadrant application with crossed collectors, like the MC1496 IC used for multipliers, mixers, modulators, phase detectors, DSB generators etc etc. The configuration in these IC's came to be known as the Gilbert Cell, after Gilbert patented it, sometime in the 70's I think. But the architecture had existed in tube circuits before and was invented by a fellow named Jones, but due to the obscurity of his work, he didn't get the credit. Also the Nagra company in Switzerland had used the configuration with discrete transistors in the Nagra tape recorders prior to the patents. These beautiful tape decks were used especially by the BBC for film audio. Nagra used the arrangement as a phase detector to lock the capstan motor to a crystal reference. The idea was it prevented the lip sync drifting out time. I still have an NOS Nagra MK2 deck. Those small pocket sized tape recorders that you see in James Bond movies with the tiny metal spools are also made by Nagra.

 

[SiriusHardware]

Its four 2N2222s in an IC case. There may now be SMD versions of it, but the datasheets I'm seeing are for DIP-12 packages.

It was just a guess. I am familiar with ICs which are an array of transistors in a DIP 'box', like the CA3046. I assume the quad 'Package' is still available, if not you could remove the whole IC and replace it with four actual 2N2222 transistors with their leads formed accordingly.

 

[WimWalther]

It was just a guess. I am familiar with ICs which are an array of transistors in a DIP 'box', like the CA3046. I assume the quad 'Package' is still available, if not you could remove the whole IC and replace it with four actual 2N2222 transistors with their leads formed accordingly.

Great minds..

Right. I looked at that possibility yesterday and came to the conclusion that there's no reason it cannot be replaced with 4 individual 2N2222 or similar. In fact, it's probably possible to replace it with a socket and just plug the transistors into that, until the correct IC shows up. (One correction, it's a DIP-14 pkg, not -12).

Of course, I still haven't made an attempt to actually locate the trouble.. so should probably get after that. But what I *really* want yo know is why connecting Datasette to port #2 is preventing the thing from booting. Excess current draw was my first thought, too, but that doesn't seem to be it.

I do have 3 Datasettes, so no shortage of test subjects.

 

[WimWalther]

. These beautiful tape decks were used especially by the BBC for film audio.

They were also used heavily by National Geographic for both straight audio and film sound. And you're right, they're really handsome machines.. that work as good as they look.

 

[Hugo Holden]

Great minds..

In fact, it's probably possible to replace it with a socket and just plug the transistors into that, until the correct IC shows up.

Yes, and if that is done, the correct IC socket to use for the task is the round machined pin type (not dual wipe) , because the round lead diameter of the 2N2222 transistor is 0.45mm, absolutely perfect for the unused machined pin socket.

I collected a stack of vintage JAN 2N2222 mil spec transistors with gold plated leads, because they have great specs and have multiple uses and are an excellent transistor.

 

[WimWalther]

"Well now THERE'S your problem!"

 

[Hutch]

That'll do it. No power to the motor.

 

[WimWalther]

That'll do it. No power to the motor.

Crazy part is that it looks intentional. Both TIP-29s (Q1 & Q2) are damaged in the same way - it looks like the leads were struck with a sharp chisel. Q1 had lead 3 totally cut through, with leads 1 & 2 barely hanging by a thread. Q2 has lighter damage to all three leads, so it should still work.

 

[WimWalther]

I was able to borrow a TIP29 from a dead C64 board. Nice, used vintage part.. and almost 100% guaranteed NOT fake.

 

[SiriusHardware]

So it seems there has been a historic tendency to 'disable' the cassette port perhaps on machines originally used in educational environments (someone else mentioned cassette edge connectors being painted with actual varnish in order to make them non-functional). Interesting, if a little annoying.

 

[daver2]

Like you say, that 'damage' looks deliberate doesn't it?!

It is a possibility that this was a 'non technical' attempt to disable the cassette ports. But why, that is another question of course...

EDIT: Good point about educational environments. From an audio perspective, schools use the opposite gender XLR connectors to the rest of the audio world. Mainly because it is much easier to get chewing gum out of one gender of XLR connector than the other! You normally have FEMALE XLRs on the stage and MALE XLRs on the microphone cable that plugs into the stage. A school generally has them the other way round...

Dave

 

[WimWalther]

The cassette motor drive now behaves as it should. Read & write are dodgy, though to be expected.. the C2N Datasette hasn't been touched in decades, and the only tape I had handy is an old C-90 high-bias with a stereo music recording.

So for now it's not being taken seriously. Like most cassette decks, I'm sure basic servicing will bring it around.

 

[WimWalther]

Demagnetizing the heads and cleaning them as well as the pinch roller & capstan (91% IPA) made a great difference. Surprisingly the belts seem to be OK, based on the operation.

And BTW - It seems that the only times the cassette motor will NOT run, is when a SAVE, LOAD or VERIFY process ends. Otherwise FF, RW, PLAY etc. can be used any time. This is pretty much how I remember it.