Very quiet ZX81 MIC.. What's... Uh the deal?

[siggi]

Sorry for the late reply. But meanwhile most questions seem to be answered

Yes, the passive components R and C do NOT ACTIVELY increase the signal, but avoid, that it is too much decreased by the original components. Thus the voltage at the output connector is higher than before.

The R/C combination is a low-pass filter to filter the SYNC-pulses of the video signal (16 kHz), which are also output to that ULA pin even during LOAD/SAVE.

Here a pic of my ISSUE 1 board (used in my Laptop-Zeddy), where this modification has been done:

Issue-1.JPG (103.49 KiB) Viewed 5825 times

The new components are located just above the 9 DC connector (visible at the lower border of the picture.

Siggi

[user@lab1]

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...

[kokkiklhs]

Oh, this is fine additional info, thank you both siggi and user@lab1!!
Well, of course a little more sensitivity does not hurt, but since I have never tried to load from any device other that my laptop's direct headphones out, I hadn't realized that it might be needed!

So it's R33 for the EAR, eh? Next time I open up my Zeddy, I will try it, as well!!

[user@lab1]

If it can help, in the instructions of Z-XLR8 (program similar to Q-Save; FasLoadSave; etc.) it is written:

"For more reliable loads with both Z-XLR8 and Sinclair LOAD routine, replace the 0.01 ufd (10nF) capacitor at location C10 (near the EAR jack) with a 0.1 ufd (100nF) ceramic disk capacitor."

Errata Corrige: the R33 must be replaced with a 3.3Kohm resistor, 4.7Kohm is the original value. Sorry...

Greetings.

[Tezz]

Hi all,

Following the info in this thread I bought some parts from Farnell last week to do both the EAR and MIC modification to all of the Issue 1 and Issue 3 boards that I have. For the MIC mod I ordered 100kOhm resistors and 470pF ceramic capacitors. For the EAR mod I ordered 3.3 kohm Resistors and 0.1UF ceramic capacitors. Both the ceramic caps are the correct type that I was expecting but I've made a mistake ordering the 100kOhm resistor as they are very large. I guess I bought ones with too high wattage? These are the ones I ordered: 2w 100k resistor, 2w 5% 100k. The 3.3 kohm Resistors look a little larger than I was expecting too but perhaps acceptable? These are the ones I ordered: 3k3 resistor, 0.5w 1% 3k3. Can anyone tell me please which ones I should I have ordered?

Many thanks

[1024MAK]

Resistors are rated by resistance, tolerance, material, voltage, size, and power handling capacity.

In small signal applications like the ZX81 ear and mic socket modifications, the tolerance, material, voltage, size, and power handling capacity are not critical, so were not specified.

Higher power resistors have to be physically larger in order to cope with the amount of heat they have to dissipate into the air.

You can use resistors with a higher power rating, they just take up more space and the spacing of the leads may be awkward.

What you want, is resistors with a power rating of
250mW or 1/4W carbon film or metal film
or
500mW or 0.5W or 1/2W metal film
or
600mW or 0.6W metal film.

All of the above can be 1% or 2% or 5% types.

Mark

[Tezz]

thanks for that Mark, I'll make a new order tonight.

[Tezz]

I eventually bought the correct components for the two modifications shortly after my last post and I'm now getting around to look at installing them. On my UK issue 1 boards R29 is clear to see but I can't seem to spot C12? The info I'm following in this thread shows images of a USA issue 3. Could somebody possibly please point out C12 in the attached image? Thanks

issue_1_board.png

(1.7 MiB) Downloaded 246 times

[Tezz]

I had some time to look at the board more closely this evening, it looks like the C12 screen print is partially obscured by the MIC socket so it's the square type cap inbetween the disc capacitors C10 and C11?

[PokeMon]

Indeed - I think it is between R33 and the second socket (between the two other caps).