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Commodore pet 4032 cassette port dead

Traindriver69

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Nov 2, 2015
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Hi all
I recently brought a commodore pet 4032 computer with no picture which I managed to repair following a post on here about a 8032 computer.
Now I have it booting I brought a c64 tape player so that I could try and load some software but I am getting nothing.
I type load enter the computer says press play on tape but the tape does nothing there is no power.
I have tried the cassette player on my commodore 128 computer and works fine.
Anyone any ideas where to start I have cleaned the socket on the pet but made no difference.
Many Thanks in advance.
 

Hutch

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Duplicate post.
My first guess would be a bad 6520. Cassette #1 is controlled by the 6520 at B12.
Try swapping the two 6520s at B12 and B15.
 

Traindriver69

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I tried my meter on pin 2 of cassette port 2 looking for 5v nothing I then tried for a reading on C60 looking for 9v and still nothing with the idea of swapping B12 and B15 around neither are socketed so would have to get a replacement so less chance of causing any problems messing around with the two.
 

Gary C

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Just a left field though, try the second cassette port ?

plug in and try LOAD "filename",2 (also just try LOAD, in case you have it plugged into the wrong one ;) )

Port 2 I think on 4032 is on the side
 

powerlot

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What exactly doesn't work? The motor doesn't spin when you press play or it spins and doesn't read any data?

If it doesn't spin check the voltages, if it does,
apart from a case where I had to replace the electrolytic capacitors in my datasette, my issues where mostly with corroded connectors and plugs.
It's a bit unlikely that both interface ICs are dead so if you swapped them and the issues persists try to give both the edge connectors and plug a good cleaning (scrape off the black gunk if necessary) and verify that there is continuity.
 

daver2

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The usual culprits are the drive transistors that switch the unregulated 9V to the motors.

If memory serves me correctly, these can be damaged by plugging the cassette connector in the wrong way round. Of course, the previous owner could have done this and not you...

Check the output bits of the PIA/VIA changes state correctly when you enter the LOAD command. If this is the case, follow the signal through the Darlington transistor switch to find the faulty component(s).

Dave
 

Hugo Holden

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The output stage that sends the approx +6v out to the cassette units are wired as Darlington transistors in the emitter follower configuration. These are two transistors, one as the driver in UE16 and the other as the output TIP29.

Because there are two base emitter junctions in series this drops around 1.2 volts, so when 7.5V is applied at the base of UE16 pin 6 and pin 9, that voltage appears at the emitter of the TIP29's as about (7.5-1.2 ) volts, or about 6.3V. And it can source a lot of current, because the bulk of the current comes via the TIP29's collector. This is what the Darlington configuration does, in the emitter follower configuration, it is a current amplifier (not a voltage amplifier) and the base currents into UE16 pin 6 and pin 9 are only tiny and about 1/2500th of the the output current (assuming say each transistor of the Darlington pair had a current gain of 50). On the average you can assume a Darlington pair will have a current gain of about 1000.

As you can see, to switch the 6.3V output voltage on and off, they simply use a driver transistor UE16 collector-emitters, pins 1&3, and 14 &12 to short out the 7.5V zener reference, and that kills the 6.3V output. That happens when base current is injected into those two transistor's base-emitter junctions, pin2 and pin 13, which happens when pin 14 of IC UB15 goes logic high. Or pin 19 of UB12 for the other cassette. So generally with the cassettes not running these lines from the UB12 and UB15 (VIA & PIA) will be logic high. They go logic low to power the cassette units.

PS: You can check the transistors (the base-emitter and base-collector junctions, which act like diodes) fairly easily in this sort of circuit with the power off and the DVM set on diode mode. If that doesn't reveal a problem it requires tests with the circuit powered.
 
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Hutch

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When you press play, does the computer recognize that you have pressed it? Does it proceed and say "Searching"? or does it stay stuck on the press play prompt?
If it doesn't sense that you've pressed a key, it won't turn the motor on.

It might be helpful to explain how the computer controls the cassette port.
*There should always be +5v on pin 2. That's is not switched.
*The 6v on pin 3 should normally be off. You won't have any voltage there unless the CPU has enabled the 6520 output to turn the motor on.
The computer looks for the sense line on pin 5. When that is asserted (grounded) it generates an interrupt that tells the computer to turn the motor power on.
This allows the computer to control the motor power and turn the motor off when the load is complete.
The switch inside the cassette drive only grounds that sense line. There's no direct motor switch in the transport, it's only controlled by the CPU and the 6520.
That switch is independent of the logic PCB in the datasette and will work even without the +5v

The sense line also goes through the 6520. If it's not sensing that you have pressed a key, you definitely have a bad 6520.
On the PET, the default state for the cassette motor is ON. As part of the startup process, the CPU initializes the 6520 and turns the motor off.
For example, if you remove the CPU and turn the power on, the motor should spin continuously.
The "cassette sense" or "cassette switch" input goes directly to the 6520 for both ports, there's nothing else in between.
Are you getting to the "Searching" indicator?

Also note, when checking the signals and voltages on the port, check the top and bottom contacts on the edge connector.
If there's no voltage on pin 2, there might be on pin B.
The datasette connector has contacts only on one side, it doesn't connect to the top and bottom of the PCB edge connector. Usually they only make contact on the top side, but it depends on the datasette.
Some very early PETs only have signals on the bottom, making newer datasettes incompatible, but that shouldn't be the case on a 4032.
 
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Traindriver69

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Hi all
Switched it on now to have a look it it and mysteriously the cassette is now powering up and doing what it should do apart from the fact I have not got anything to load yet.
I have been trying to copy wav files via my pc through to an old school cassette recorder.
It says searching but it's not finding anything.
 

Hutch

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Recording WAV files to tape is tricky. Personally, I've never gotten it to work for me.
A mono recorder is recommended. Stereo recorders never seem to work well.
Set the record level HIGH. Very High.

The signal polarity is important for Commodore tapes. Most audio recorders ignore it since audio is AC, but if the signal polarity is reversed, the pulse widths will be wrong and you won't read anything.
You could try swapping the + and - wires going to the audio recorder but it may not work anyway.

Try writing a quick simple basic program and saving it, then see if it will load.
 

Traindriver69

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I have noticed that if I use cassete port 1 I have to fiddle about with the cassete connector to get power but if I use port two it is fine for power but I lose the sound from the pets speaker.
 

powerlot

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Sorry for repeating myself, but really clean both parts of the interface until there is no black gunk on either connector and plug. I also sprayed some contact lubricant into the connector.
Try writing a quick simple basic program and saving it, then see if it will load.
Have you tried this method? Keep in mind to advance the tape to a writable part when it's rewound.

You can also run a tape alignment program on your c128 with a preferrably original cassette to check if data is read with a good enough signal strength.
 

Gary C

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Paper, folded until its a similar thickness as the PCB, pushed in and out of the cassette plug can often clean connections.
 

daver2

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I have noticed that if I use cassete port 1 I have to fiddle about with the cassete connector to get power but if I use port two it is fine for power but I lose the sound from the pets speaker.

I can't see what the two have in common though - unless misalignment is upsetting the PIA or VIA.

Dave
 

WimWalther

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Hey, what a coincidence! The cassette won't work on my 4016/32 either.. =)

First I thought it might be the datasette itself, but I tried another unit and got the same results. Only the rear-facing port is available on this machine, which I'm guessing is port #1? The mainboard does have a second port that faces right, but there's a cover panel on the case that's blocking access to it.

ETA: Found 5V on pins 2 & 6, a few mV on pin 3 with screen displaying "SEARCHING". Time to check transistors I guess?

Did they really use one of those silly Q2T2222 chips for this?!

Remind me, should I be able to FF/RW the tape at any time, so long as the deck is connected to the port? Which POKEs switch the cassette motors on & off on the Universal board 4016/4032 etc?
 
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SiriusHardware

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I keep hearing over and over again that the contact 'fingers' on the datasette connections become really heavily oxidised over time, so maybe just cleaning those up with a PCB cleaning rubber might do the trick - I wouldn't use sandpaper of any grade - too harsh, it will leave fine scores in the plating / tinning on the contact fingers.
 

SiriusHardware

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Did they really use one of those silly Q2T2222 chips for this

That number makes me suspect they are an SMD clone of the common 2N2222 transistor - if you are good at soldering you could solder a conventional 2N2222 to the PCB pads vacated by a SMD version. I have occasionally repaired surface-mount circuits this way, if I have only had a conventional part available.
 

WimWalther

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That number makes me suspect they are an SMD clone of the common 2N2222 transistor
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.
 
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