Sinclair ZX80 / ZX81 / Z88 Forums

patters wrote: ↑Sun Apr 04, 2021 10:14 amIs it worth replacing C10? Or bypassing it even as a test?

Further to my advice above, if you have a digital source, that does not output any negative signal and which does not have a constant DC level. But is a ‘5V’ TTL type digital logic level. Then this can be fed in at the junction of C10 / R33.

The output from a 386 audio amplifier does not count as suitable.

Mark

No, I don't have a digital source. Rather than modifying the LM386 circuit (which I'll order the capacitor to try) would it not be easier to simply swap one or both of the two resistor values on the ZX81 so it's not dropping the input voltage as much? Would that have the same effect as making the signal louder? My logic is that since the WESPI isn't working in this ZX81 either that could solve both problems. Ultimately I do plan to use a WESPI in this machine too.

However, I don’t believe that failure of the Ear/tape input part of the ULA is common, as it’s a low power system.

Great, this is the kind of insight I was after.

One thing I noticed when I removed one of the modulators was that a resistor sheared in half at the slightest touch. Is this kind of degradation likely? Perhaps I should order replacement resistors for the two on the audio circuit.

Various types of electronic components are fairly fragile if subjected to too much force (especially bending forces) or vibration. This includes various types of capacitors, resistors and signal diodes (1N4148 for example, in glass packages). Resistors are however very easy to test, use a multimeter. If it tests okay, and looks okay, then they are normally fine.

To reduce the low sensitivity, you could use a higher value for resistor R34. Try a 470Ω. You could also reduce the value of R33 to say 3.3kΩ.

Mark

I,m sure someone on the German forum actually did something similar but posted it on here,I will have a look to see if I can find it.

The thread concerned was about increasing the output of the MIC socket but someone did ask about making the ear socket more sensitive and this answer was posted...

"An old issue of "EchosSinclair" suggests 100pF for C12, 470Kohm for R29 and 4,7Kohm for R33 (to increase the EAR jack sensitivity)"

I don't know whether this is for issue 1 or 3 though.

Brilliant, thanks for digging that out Moggy. I need to order a few bits to fix that defective composite mod from eBay so I'll add this stuff to the order.

One thing though, I followed the components back from the EAR jack to the ULA and got labels R34, C10, R33 - so R33 seems to be the only common one. Is that sentence out of context I wonder, so perhaps the first two listed components are for the MIC circuit.

The existing R33 should already be 4.7kΩ...

C12 and R29 are part of the Mic circuit.

Mark

1024MAK wrote: ↑Sun Apr 04, 2021 6:12 pm The existing R33 should already be 4.7kΩ...

C12 and R29 are part of the Mic circuit.

Mark

Hmmm, the magazine was incorrect then?

Full text of post which is all I can go by so make of it what you will.

Quote.

"A bigger capacitor and smaller resistor would simply increase the output voltage even more! Since a magazine suggests it, seems that someone has already tried it and it *should* be safe for the Zeddy! But as the 50mV seems to be enough for a good quality playback or recording of the sound produced, I'd say *not* to push this ancient machine's tolerances to their limits...

Many have tried:
On an old Italian electronics magazine, there is a modification that uses a JFET for amplifying the output signal of the MIC jack.
An old issue of "EchosSinclair" suggests 100pF for C12, 470Kohm for R29 and 4,7Kohm for R33 (to increase the EAR jack sensitivity)
There's something for every taste.

As for higher EAR input sensitivity, may I ask "what for?"
You can already load anything flawlessly, I even use OTLA-converted turbo loading stuff, which does not allow input mistakes at all, and it works like a dream!!

I also use O.T.L.A. (in the past: QSAVE, FastLoadSave, etc.) to load programs in the ZX81. But to do that, I have to keep the volume high enough (compared to the ZX Spectrum). If you use an MP3 player (like I do), it must be preceded by an audio amplifier.
I simply say that a little more sensitivity in the EAR jack does not hurt..."

Moggy wrote: ↑Sun Apr 04, 2021 6:37 pm

1024MAK wrote: ↑Sun Apr 04, 2021 6:12 pm The existing R33 should already be 4.7kΩ...

C12 and R29 are part of the Mic circuit.

Mark

Hmmm, the magazine was incorrect then?

Not sure what happened, but I suspect if they are mainly concerned with the SAVE output level on the Mic socket, then changing R27 (rather than R33) to 4.7kΩ makes more sense.

It’s strange, because R27 and R33 are not next to one another on either the issue one or issue three boards

I posted my suggestion to increase the sensitivity of the Ear (LOADing) above here.

Mark