exxos's Atari, Amiga & retro forum

More family members nice addition

Tom

Tomswork wrote: 05 Feb 2024 16:30 More family members nice addition

Tom

I'm definitely happy with the additions :)

The Sharp needs some work on the cassette side, it isn't loading anything, either using the internal or an external deck, so going through the service manual ATM.

Given basic needs to be loaded from cassette, it's a pretty important thing to get working.

It suggests the amp is a problem, but I'm struggling to find it and wondering if it's on the PCB for the internal cassette deck. More digging required.

Check your -5V line. That's usually the problem on the BBC Micro if there are loading problems as the amp uses it.

Nice cheese Gromit

stephen_usher wrote: 05 Feb 2024 21:13 Check your -5V line. That's usually the problem on the BBC Micro if there are loading problems as the amp uses it.

I'm trying to figure out if the amp is on the mainboard, or on the PCB inside the cassette deck.

The cassette deck does play OK, so it's clearly getting +5v there, but the mainboard seems to be just a sea of 74LS chips with nothing that looks like an amp. Need to read the service manual some more.

alexh wrote: 05 Feb 2024 21:23 Nice cheese Gromit

😁

And yes, finally found the op amp :)

And yes, it was in the cassette player module:


There is a uPC358C, and then the motor controller is an LA5522. I don't think there is an issue with the motor controller at the moment, so I'll focus on the tape input first.

There are also two transistors, Q3003 & Q3004 which seem to be the input/output stages either side of the uPC358C, and I have all the waveforms and test points along the signals journey, so I can get to testing.

I've ordered replacement op amps just in case.

Yeah, Q3003 does pre-amplification and Q3004 squares the waveform and makes it TTL level.

So it seems Sharp were like their name in that the construction of the MZ-700 and built it very sharply, with a minimal number of plastic screws and the majority going into brass inserts.

They were also good enough to mark up their PCBs with component locations:


Which means finding the test points and checking agains the waveforms in the excellent service manual should be easy :)

The PSU is a dinky little fella with no RIFAs thank god and was actually very clean inside:


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.

rubber_jonnie wrote: 06 Feb 2024 10:30 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.