Diagnosing tape loading issues

Discussions about Sinclair ZX80 and ZX81 Hardware
Moggy
Posts: 2347
Joined: Wed Jun 18, 2008 2:00 pm

Re: Diagnosing tape loading issues

Post by Moggy »

That makes sense,Mark.

Bit by bit I think we'll get there. :D
patters
Posts: 86
Joined: Thu Mar 11, 2021 12:34 pm

Re: Diagnosing tape loading issues

Post by patters »

1024MAK wrote: Sun Apr 04, 2021 5:04 pm 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.
Thanks Mark, will give this a whirl. In the meantime I measured both resistors and they are as per specification.
patters
Posts: 86
Joined: Thu Mar 11, 2021 12:34 pm

Re: Diagnosing tape loading issues

Post by patters »

I'm ready to order a bunch of components to attempt various remediations. Is it ok to replace C10 on the ZX81 with something this physically small?
https://www.bitsbox.co.uk/index.php?mai ... cts_id=207

I kind of need a wider leg pitch ideally but I can't seem to find that in a ceramic.
User avatar
1024MAK
Posts: 3554
Joined: Mon Sep 26, 2011 10:56 am
Location: Looking forward to summer in Somerset, UK...

Re: Diagnosing tape loading issues

Post by 1024MAK »

patters wrote: Tue Apr 06, 2021 5:40 pm I'm ready to order a bunch of components to attempt various remediations. Is it ok to replace C10 on the ZX81 with something this physically small?
https://www.bitsbox.co.uk/index.php?mai ... cts_id=207

I kind of need a wider leg pitch ideally but I can't seem to find that in a ceramic.
It does not have to be a ceramic disc, any 10nF capacitor should work. Sinclair used a ceramic disc because this type (for this value) is the cheapest.

So take your pick from these (well, until the search tool starts finding things other than capacitors :lol:).

Also, you can go up to about a 47nF value here if you want. A higher value will make the Ear input a bit less deaf. The only downside being the frequency response will be a bit wider. But I don’t think that will be a problem here.

In this application, leaving the leads slightly long and bending them so a smaller component fits in the PCB holes is not a problem. Just don’t bend the leads too close to the capacitor body, and ideally use small round nose pliers or the shaft of a small screwdriver so that no stress is applied to the capacitor body.

Mark
User avatar
1024MAK
Posts: 3554
Joined: Mon Sep 26, 2011 10:56 am
Location: Looking forward to summer in Somerset, UK...

Re: Diagnosing tape loading issues

Post by 1024MAK »

Note: post above edited...

Also buy a 1kΩ resistor.

Mark
patters
Posts: 86
Joined: Thu Mar 11, 2021 12:34 pm

Re: Diagnosing tape loading issues

Post by patters »

Good, I just wanted to rule out that the larger existing ceramic wasn't a different voltage rating for instance.

What do I need the 1K resistor for?
User avatar
1024MAK
Posts: 3554
Joined: Mon Sep 26, 2011 10:56 am
Location: Looking forward to summer in Somerset, UK...

Re: Diagnosing tape loading issues

Post by 1024MAK »

patters wrote: Tue Apr 06, 2021 11:47 pm Good, I just wanted to rule out that the larger existing ceramic wasn't a different voltage rating for instance.
Well, it may be, but in this circuit, the voltage will be well below the 50V rating of the 10nF capacitor that you linked to.
patters wrote: Tue Apr 06, 2021 11:47 pm What do I need the 1K resistor for?
When I get some time, I’m going to try a 1kΩ resistor in parallel with R3 or in parallel with R2 on the composite video board to see if I can get the black in the picture to be a bit darker (more black).

Mark
patters
Posts: 86
Joined: Thu Mar 11, 2021 12:34 pm

Re: Diagnosing tape loading issues

Post by patters »

Great. I'll add some to the order. Bitsbox has a min order of 10 resistors of each type which is pushing cost up a bit, but they're the only cheap supplier that has the 7555IPA which I also need for fixing the bad composite mod from eBay.
patters
Posts: 86
Joined: Thu Mar 11, 2021 12:34 pm

Re: Diagnosing tape loading issues

Post by patters »

Moggy wrote: Sun Apr 04, 2021 2:34 pm There is something you can do to increase the output of the 386,if it hasn't been done already by the makers and will take a bit of confidence on your part to try.

Like most op-amps used for audio they rely on a bias circuit to control the strength of the out-put.
The 386 is a bit unusual in that it already has a small bias built in which gives it an amplification of X20.
The addition of a 10uf electrolytic cap' can boost this to X200 (the chips max out-put) and would certainly put to bed any thoughts of the out-put being too low.
LM386 boost.jpg

I've just had a closer look at my Maxduino and I can see a 10uF cap between pins 1 & 8 on the LM386 - it's very clear on my unit's PCB layout, and it's even labelled. So by your reckoning this should be plenty loud enough for the ZX81. Things are not looking hopeful for my ULA then. I'm currently waiting for a component order to see if I can tweak the resistor/cap on the EAR input to make it less deaf. I'm hoping C10 has developed a crack or something, because the resistors are fine. I guess I should have checked tape loading first before I went to all the effort of replacing the modulator and doing the 16K mod...
User avatar
1024MAK
Posts: 3554
Joined: Mon Sep 26, 2011 10:56 am
Location: Looking forward to summer in Somerset, UK...

Re: Diagnosing tape loading issues

Post by 1024MAK »

As you are still having problems, try the test program that’s in this topic.

Mark
Post Reply