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Sharp PC-1211 tape loading problems (TRS-80 PC-1)

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vvmm:
Hi,
I just finished refurbishing a Printer/Cassette interface CE-122 for the Sharp PC-1211. I am still waiting for my replacement LCD to arrive for the computer itself, but the old LCD is still partly usable.

The PCB of the CE-122 interface had some corrosion because of the NiCad batteries inside, but I managed to clean it up and repair the broken traces. The printer works fine now, also does the save to tape, the signal records good on tape/PC.

But I have an issue with loading from tape. It seems that the earphone signal from my recorder (or PC headphone jack) has a very low amplitude (<1Vpk-pk), and it does not get detected by the CE-122 circuit.
I tried outputting directly from PC and it is barely detected (=can be heard in the pc speaker) when setting the volume to 100% and amplification in VLC media player to max 125%. I got one program loaded once, but after numerous tries and it still doesn't get loaded properly every time.

I investigated the schematic and checked out the signals with and oscilloscope. Nothing seems wrong, except that the earphone signal is fed (almost) directly to the input of a logic gate inverter and then used as it is further down the line (see partial schematic attached).
I generated a 4kHz sine wave with varying amplitude until I saw on the oscilloscope that it is picked up by the next logic gate and found out that the normal audio signal has a 1 Vpk-pk amplitude, but in order for it to trigger the logic gate, it must be more than 2,5-3Vpk-pk to properly trigger the next stages. When the amplitude is right, the whole next stages are working fine.

Was the original tape recorder that was compatible with this that higher of an amplitude to directly trigger the logic gate? I expected the signal to be amplified within the CE-122, but it isn't. Never saw the audio signal being used directly to drive a logic gate.

‚ÄčIs there anything bad that I missed that should trigger the normal operation of the logic gate?
How can I fix the issue?

Thanks,
Mihai

Ian.M:
The unbuffered inverter is biased for use as a linear amplifier with a gain of less than 10, but almost certainly greater than 3. (see: https://www.eevblog.com/forum/beginners/how-to-design-with-cmos-inverter-amplifiers/ ) Check the DC voltage at its input (pin 1) - if its operating correctly it should be fairly near 1/2 Vcc.

However, 'back in the day' portable cassette players typically didn't have line out,  (and if they did it would have been on a DIN connector together with line in, not a separate 3.5 mm jack), so it would have been a headphone output, capable of maybe 4V pk-pk at max volume.  Typically (for all 8 bit computers with a cassette interface but without a dedicated data cassette player),  one would turn the cassette player volume up midway, fiddle with it to find the sweet spot where program loading was reliable, then mark the volume control as some adjustment was often required between your own tapes and pre-recorded or 3rd party program tapes.

If you read the inverter amplifier topic I linked, you will know that the odds are there isn't much more gain that could be squeezed out of the TC40H004 inverter by changing its input and possibly feedback resistors, so the best option would be an AC coupled external amplifier with an adjustable gain of up to x10, powered from a spare USB port on your PC.

iMo:
I spent my youth messing with cassette interfaces and various gadgets I owned  :D.
Almost all worked "reliably" when used with decks of that era, as described by Ian.M.
The input of your biased inverter is muted by the transistor - that could be a source of potential issue, for example.
Closest to yours is my PC-1251 I still own, there is a hw interface plugin for a cassette deck, it is just some R and C inside with two cables for mic and speaker (I added an external battery wire).
There are PC apps today working as the external memory for those computers, not sure what is the amplitude of the PC soundcard output, however.
You may try a single transistor amplifier, AC coupled, with a pot for level adjustment, powered from a 3V battery for example.

PS: I opened the box after perhaps 15y (I got the PC1251 in '83 or '84) and the interface is CE-124, inside is one transistor and couple of R and C. In my schematics the signal from the 11pin connector goes straight into the MCU.
Moreover, the half of my display got pretty dark, so the same destiny as yours, it needs a new LCD.
Otherwise it works  :D

vvmm:
@iMo, yes, checked the transistor, it "mutes" the input except when the CLOAD command is given, then it unmuted and allows signal to pass.
I tested an 80s cassette tape recorder that I use reliable with my Spectrum and it only partially work, most of the signal is too "low" for this to be interpreted correctly, but some parts are passed through correctly, although the tape recording volume is maxed out. Will try a couple of times more, but what I really want to use is a portable recorder with SD card to transfer my programs back and forth and not mess with tapes.
I am tempted to try amplifying the signal as you said.

@Ian.M, If it is used as a biased amplifier, which I missed (thinking the transistor is only used to mute the input when not needed), I will take a second look to see the exact DC bias. It must be set precisely to 1/2 Vcc, correct? If it is not, then that must be my issue. I used both a portable recorder connected to the headphone out (not line out) and an old tape recorder that works fine with a Spectrum, but only partially works here.
As @iMo suggested, I might be tempted to add a simple transistor amplifier, but I also would like to keep it as original as I can. 1Vpk-pk amplitude sounds should be enough to trigger state change in my opinion if the bias is set correctly.

iMo:
Perhaps there is an app for emulating the cassette recorder for smarthpones as well, thus no need for an sdcard interface.
Yep, the tape storage was a big pain with those gadgets in 80ties, I can remember that time well  ;D

Sometimes the problem we had with cassette stuff was wobbling (unstable speed).
The amplifier in your schematics should be sensitive enough to amplify from a cassette deck, imho.
It could be the inverter is damaged, so it does not work as the amplifier.
Also doublecheck the input capacitor, it could be it is a tantalum (Edit: 47nF - it will be ceramics, imho, but check it out as well), and shorted or what.
Then you may get DC at the inverter's input and the DC shifts its bias off.
Without signal you should see aprox Vcc/2 at the input of the inverter, if not - there is something going wrong.

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