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

[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.

​Is 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  .
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 

[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 

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.

[Ian.M]

It is very unlikely to be biased at exactly 1/2 Vcc.    I would expect it to be somewhere in the 2V-3V range, as the datasheet specifies the input thresholds (beyond which the unloaded output will rail) are 1V and 4V.  The exact bias voltage will be determined by how well matched the N and P MOS transistors in the process are matched, and how that matching tracks with Vcc and temperature.

N.B. the bias is set by negative feedback, via the 1 Meg feedback resistor.  As the input is AC coupled, and CMOS input impedance is very high, provided the muting transistor is fully off, the input must settle at the same voltage as the output.

[iMo]

Sure not exactly..
When he looks at the output of the inverter with his o'scope, and he touches the input with his finger he should see full swing digi signal, my bet (provided the transistor there does not interfere much)..

[vvmm]

So after more investigation, I can confirm that:
- The input bias is almost at V/2, and the amplification works as expected when inputting a dummy generated sine wave.
- The sound output from the computer is very low, under 200mVpp but it records fine on tape/digital recorder.
- The input sound level must be at least 2.5Vpp to properly trigger the inverter and the signal to pass to the next gate.
- My portable recorder headphone jack level is way lower than this to work properly.

What simple circuit do you recommend to build internally to amplify my digital recorder's headphone output in order for the signal to be detected correctly?
The computer circuit works from 4xAA NiMH batteries, the positive terminal being considered the ground, so it's negative logic.

[Ian.M]

Is there a spare gate on the inverter?  If so, another stage the same could work well, or maybe even let the existing linear inverter bias the extra one (i.e. DC coupled from the linear inverter's output).

[vvmm]

I just refurbished a Philips D6600 portable cassette deck (from the same era) and connected to the Sharp PC. After testing some save-load cycles, it seems that both saving and loading works properly.
I think that both the level and the input/output impedance must match to make loading work properly. The output impedance on the cassette player says it must be 8 ohms or less (like a normal internal speaker).

Looking at the schematic, the internal speaker is replaced directly by the load on the headphone jack output, which is not what modern devices do (see snipped attached).

I think I don't want to mess with modifying the circuit if the original cassette deck works fine with it after all. What do you think?

[Ian.M]

Its not a matter of impedance, its just a tape recorder that dates from the era of speaker level on headphone jacks.  You were assumed to be sensible so wouldn't turn it up loud enough to hurt.  A generation of teenagers with hearing loss as young adults proved that wrong so now headphone jacks are power limited.

There's no way I'd mod a working or reasonably reparable original piece of computing history like that, but each to their own, so I didn't dump that on you.  As I suggested, an external amplifier is the way to go for compatibility with modern devices, powered by USB if available or 3x AA alkaline if not.  Probably just a low voltage RRIO OPAMP, AC coupled, input biassed mid-rail with variable gain of x1 to x10.

[iMo]

2.5Vpp input for a proper working of the first inverter looks pretty much, frankly.
Your first deck perhaps keeps its output wired at the 4-8ohm speaker, therefore the output amplitude is lower.
You may try to do the same with it as the second deck's schematics shows - the 100ohm load of the output instead of the 8ohm speaker.

Or, do make yourself a single transistor stage and put it into the first deck as the special output for the Sharp (see below an example, set the R4 such you get aprox half of the Vcc at the collector, any npn transistor will do).

[vvmm]

Thank you both for your thoughts.
I decided to leave the PC working as it is with the tape recorder from the same era. The system is way to nice to hack and it works fine now.
Will definitely use an external amplifier or record to tape to copy data to and from the modern PC sound card.