Issue 4B - I created myself the problem

[unhuzpt]

Hello everyone!

So here is my call for help and kindly ask if people can help me find and solve whatever problem i created.
For starters i have a very basic understanding of electronics but still trying to figure things out; then i bought the basic tools to try to help finding the problem: digital multimeter, more or less decent microscope and solder stuff.

Here is a description of how i got to this situation:

- it was working fine all stock i could load games with mobile phone and play, sometimes some would crash but seemed ok
- then i found there is a replacement for the power input converter - TSR1-2450 - that we could swap in to remove the heating and the big heatsink so i replaced it and it was working just like was before
-then i found the composite mod and i did it and all worked as before no issues
- then i found the advise to recap that seemed logic to me due to very old caps so i ordered a recap kit; here i made a mistake as i replaced all without 1 at the time and test (i think i tested after the first 2 that were the ones near the video out circuit and was ok)
- after replacing all it does not work anymore correctly.... i had very few some good boots, with color in the image and dandanator board giving ok in the tests, them random failed boots with not even sinclair text and quite random results. Then i did my best to find any solder issue or short, then i resoldered many many pins - all lower memory, ula, caps, and many more.
- then i put an lower memory ic backwards and things got worse so i ordered the lower ram board that replaces the ics

So after learning the hard way the "if ain't broke don't fix it" i now struggle with this. I have no idea why and how but every time i retouch the solder of pins i get different behavior after.
Until this morning it would boot again to the normal sinclair text, then after like 10s make some beeps and change to a white border and black screen in middle.

Then I re-soldered some more pins (i think around the high memory ics) and now i have boot to the sinclair text, then it beeps a few times and nothing else - no colors shown also.
Ah and if i boot with dandanator board it boots run those checks and then "ram fail 01234567" and no colors

Now this morning i recaped again (new caps), did my best to check for shorts or bad solders, re heated all pins in the lower/upper ram, ula, whatever else.

Now i try to start to look into power delivery to the board and i think i have issue there. But as now i have the lower ram board from retroleum i am not sure how to check the voltage on lower ram.

I have uploaded to my google drive a couple of videos and picture of my machine:
https://drive.google.com/drive/folders/ ... sp=sharing

[1024MAK]

Okay, best not to do anything else until I've had a chance to think about this...

You say you have a digital multimeter, do you know how to measure resistance, continuity and voltages?

Mark

[unhuzpt]

Okay, best not to do anything else until I've had a chance to think about this...

You say you have a digital multimeter, do you know how to measure resistance, continuity and voltages?

Mark

Hello Mark

Far from me to disrespect anyone or anything. Yes i have posted also in the forum i did not know existed until today, under the same username and email.
I started here not even realizing this was mostly aimed to a different kind of computer.
So if in anyway i have made wrong i apologize for that.

Now for me the plan is to try to get the best of both and update both as things develop. With nothing more than solve my zx problem and then explore +30 years of people doing amazing things that i am not aware/informed/educated about.

Let me know if i should remove from this forum so i do not disrespect anyone in any way.

[1024MAK]

No need to apologise, or edit or remove any posts.

There are actually three English language forums that cover 8 bit Sinclair machines, this being one of them (yes, I'm on all three). All three cover both the ZX81 and the ZX Spectrum. This site is primarily for ZX80, ZX81 and clones/compatables, but also for ZX Spectrum and Z88 computers.

I'll happily reply either here or on Spectrum Computing. All I ask is that you keep the members of both forums up to date on where you are with your repairs.

Mark

[unhuzpt]

No need to apologise, or edit or remove any posts.

There are actually three English language forums that cover 8 bit Sinclair machines, this being one of them (yes, I'm on all three). All three cover both the ZX81 and the ZX Spectrum. This site is primarily for ZX80, ZX81 and clones/compatables, but also for ZX Spectrum and Z88 computers.

I'll happily reply either here or on Spectrum Computing. All I ask is that you keep the members of both forums up to date on where you are with your repairs.

Mark

Agreed!
Thank you for taking time to help me!

[1024MAK]

So, before jumping around randomly, I would like you to start with two things.

First, can you take a photo from at least two angles of each capacitor that you replaced, Plus a photo of the underside of the board for each capacitor. Please label the photos, as I don't have an issue 4B board to hand to compare with.

Second, measure the power rail voltages as per this list:
Across C50 (next to C34)
Across C34 (near the coil, right hand side of board)
Across C44 (near front of board near the Sinclair logo)
Across C45 (next to C44)
Across C79 (between the 'upper' DRAM chips and the speaker)

By 'across' I mean touch the black (negative) meter lead to the negative (-) lead of the capacitor, and touch the red (positive) meter lead to the positive lead of the capacitor.

A layout of the similar issue 4A board is here but note it does not show C79 as this 4A board has different circuitry for that part.

Mark

[unhuzpt]

Good morning!

I am jumping right into this, just wanted to share a couple of things after so many hours searching for information.

For reference to me and trying to know what is where i reference to:
https://zxs4b.com/wp-content/uploads/20 ... ue-4B.html

Updated the google drive with some more pics on the microscope i bought and the ram and diag cards.

Also i ordered the following:
- vLA82 - Spectrum 48K ULA replacement (should take about 4 weeks to arrive)
- 2 x ZTX651
- 5 x ZTX653

Now i go to do what you told in last post!

[unhuzpt]

Measurements (without ram board plugged in):

Across C50 -> 11.83v
Across C34 -> 5.01v
Across C44 -> 1.92v
Across C45 -> 1.89v
Across C79 -> 0v

Went further if would be useful:
Across C80 -> 9.90v
Across C46 -> 11.56v
Across C74 -> -0.17v
Across C27 -> 4.90v
Across C25 -> 4.88v
Across C65 -> -1.59v

Am now working on naming the photos in google drive.... that will show some solders i am particularity not proud off
But to the best of my capabilities i have tested for shorts and continuity from soldered leg to the next point.

[1024MAK]

So, the +12V and -5V are not working. Without a +12V supply, the video will only be monochrome.

It's likely that TR4 and maybe TR5 are damaged. But do not remove them yet.

Before hunting for other non-power supply problems, the power rails need sorting out.

In the meantime, can you also take a photo of the underside of the board where the 'lower' RAM is.

I'm in an all day meeting today. So will post again this evening (after 20:00 GMT).

Mark

[unhuzpt]

Thank you for the feedback!

Do i suck at taking pictures....
https://drive.google.com/file/d/128KqVc ... sp=sharing