A Grand Day Out

[rubber_jonnie]

So should be back together and fingers crossed with a working tape deck soon. What's interesting is that it seems like the external cassette connections appear to rely on the internal cassette deck for the amp circuit, which surprised me a bit.

Why duplicate the circuitry? Sounds like a smart thing to do as then the computer only needs a single input and doesn't need extra circuitry itself.

I'd agree to some extent, but if you remove the cassette unit, which is a modular part, the on board cassette ports will cease to work and won't have any options for program loading/saving.

And both inputs fail if the cassette unit fails. I suppose it was probably always cheaper to just replace the cassette module than start digging into the mainboard.

[stephen_usher]

The alternative would be to move the amp onto the motherboard, which gives similar problems if that circuit fails.

Of course, the designers never thought that someone would want to run the machine without the tape drive fitted as that's how it would always be supplied.

[rubber_jonnie]

The alternative would be to move the amp onto the motherboard, which gives similar problems if that circuit fails.

Indeed, it is problematic, so I do prefer it this way, just 2 screws and a plug to get the unit out for disassembly

Of course, the designers never thought that someone would want to run the machine without the tape drive fitted as that's how it would always be supplied.

Indeed.

[PhilC]

Any luck then @rubber_jonnie

[rubber_jonnie]

Any luck then @rubber_jonnie

Not today I'm afraid, work was uber busy and then I had training, and I'm up in the morning at 05:30 to go into the office, so early night tonight.

May be Thursday before I can take another look, but still waiting on the uPC358C's anyway. I did manage to give the whole machine a good clean though and it looks great.

Very good build quality compared to other contemporary machines.

Just a bit weird from the OS situation, even BASIC needs loading from tape.

[rubber_jonnie]

I have put the demon OSI600 to one side, so I will be looking at the MZ-700 next, see if I can figure out at which point the cassette input fails.

More on this later.

[rubber_jonnie]

Well thats a bit weird.

The MZ-700 has a great service manual, puts others to shame and gives these waveforms and tells you the test points to look for them:

Screenshot 2024-02-09 at 00.00.32.png (1.06 MiB) Viewed 1141 times

Said test points can be found here on the schematic:

Screenshot 2024-02-09 at 00.02.02.png (1.47 MiB) Viewed 1141 times

And the PCB has a decent silkscreen making part location and therefore test points, easy to find.

The trouble is, I get zero signal at TP 1. Just nothing. I did move on to look at TP 2 & TP4 , but given there was nothing at TP1, I wasn't surprised I found nothing, since TP1 was flat.

I will compare with another cassette deck to see what the resistance should look like for the head, as even connecting the scope directly to the head read nothing, so that feels suspect.

It doesn't quite answer the question of why the external cassette doesn't work, though if it is using the same circuit on the cassette and that is bad, then all bets are off.

There is 5v at the op amp, before anybody asks. Bit of a head scratcher.

[HigashiJun]

I've been away for a 5 day business trip abroad only to discover that our fellow member @rubber_jonnie got two retro machines meanwhile...

[stephen_usher]

Don't go away again, he'll get more!

[rubber_jonnie]

Don't go away again, he'll get more!

You know me so well