Yet another 48K restoration (no tape loading sound on speaker)

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Bora66
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Joined: Thu Mar 12, 2020 9:35 am

Yet another 48K restoration (no tape loading sound on speaker)

Post by Bora66 »

Hello all,

I found this forum while searching for a fix to my problem. My 48K issue 2 was sitting in a box since late 80's. I decided to bring it back to life. A brief "what I did" is like this:

1) Did a complete recap.
2) Changed the broken keyboard membrane with a new one.
3) All the IC's removed, brand new sockets fitted for each of them.
4) Did the composite mod. (just with a capacitor)
5) Removed the insides of the original power brick, replaced the smoothing cap and added an adjustable DC-DC stepdown, adjusted to 7.5V output to keep the original 7805 a lot cooler.
6) First power up, worked but only intermittently, sometimes it did not boot, just stayed on a white border, black screen.
7) Checked voltages, all rails OK.
8) Changed all the 16K RAM chips, no joy, same problem remained.
9) Changed all the upper 32K chips, nope...
10) Changed all the 74LS logic chips, nothing...
11) Focused on the ULA, noticed that if I put my finger on certain pins of it, it seemed to work better, thinking there is a capacitive issue on the board I tried a 100n bypass cap to ground on those pins one by one, nothing... Forced my luck and tried the by pass cap with +5V rail and managed to kill the ULA completely.
12) The whole project halted and started looking for a new (or at least verified working) ULA. A whole year passed and finally I found an issue 3 board that was otherwise unusable due to a butcher's work on trying to remove RAM IC's and transistors but luckily, the ULA turned out to be working. Installing that ULA on my board cured everything and now I have a fully working 48K issue 2 that passed any test I threw at it. :D
13) Now with a working Spectrum, I did the color adjustments (allignments?) detailed in the service manual and managed to get a decent enough picture on my TV via the composite video input.
13) Time for games... No tape player, no tapes, so I Built myself a TZXDuino, (Yes, an Arduino-head here) which worked perfectly, everything loads at first try. So, I downloaded every piece of software ever written for the 48K in .TZX format (a little over 1GB!) and put them on a SD-Card. Started playing with them and I noticed my problem:

THERE IS NO LOADING SOUND FROM THE SPEAKER!

The speaker works otherwise, ie all the game sounds are there, but I surely remember the loading sound coming from the Spectrum's little speaker in the days that I used it with a cassette player. So there must still be something wrong. Looking at the board schematics and reading from here and there, I believe there must be a faulty diode or a transistor in the audio section which prevents the loading sound being fed into the speaker.

I would appreciate anyone helping me on what to check. This seems to be the last fault remaining, though it is not a big problem, after working on this project this much, I really want everything to be as good as 1983. :D

Oh, by the way I can gladly help anyone with a similar restoration project. I learned a lot, mostly with trial and error, but I can prevent someone repeating my mistakes. Also I can help with building your own TZXDuino, it is an open-source project and quite easy to make if you are somewhat familiar with Arduino's and basic level electronics.

Best regards to all, sorry for the long post.

Bora Yurtören
Istanbul / Turkey
Lardo Boffin
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Joined: Sat Nov 26, 2016 2:42 am

Re: Yet another 48K restoration (no tape loading sound on speaker)

Post by Lardo Boffin »

Welcome! Just seen this post.

I can’t offer any help on fixing your Spectrum but have just tried one of mine and yes you can hear the tape loading from the speaker when you load from tape.

Is a ULA from an issue 3 fully compatible with an issue 2 pcb? I know they changed the ULAs over the years.

Good luck finding a solution.
ZX80
ZX81 iss 1 (bugged ROM, kludge fix, normal, rebuilt)
TS 1000 iss 3, ZXPand AY and +, ZX8-CCB, ZX-KDLX & ChromaSCART
Tatung 81 + Wespi
TS 1500 & 2000
Spectrum 16k (iss 1 s/n 862)
Spectrum 48ks plus a DIVMMC future and SPECTRA
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1024MAK
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Re: Yet another 48K restoration (no tape loading sound on speaker)

Post by 1024MAK »

Hello Bora and welcome :D

Does this issue TWO board have diodes in positions D9 and D10? Do they test okay (diode test on a multimeter)?
Test the values of resistors R36 (680Ω) and R37 (1kΩ).
Test diode D13.

Has the speaker been changed? Issue 1 and issue 2 boards use a different speaker compared to later issue boards.

Replacement diodes should be 1N4148.

However before replacing anything, note that the issue 2 boards did NOT use a transistor to amplify the signal to the speaker, so when loading, the tape sound from these boards is significantly quieter compared to issue 3 and later boards. When using a cassette tape deck, the volume has to be turned up fairly high.

The ULA DOES NOT drive the speaker when loading from tape, so it can’t be a faulty ULA if games load okay.

Mark
ZX81 Variations
ZX81 Chip Pin-outs
ZX81 Video Transistor Buffer Amp

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1024MAK
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Re: Yet another 48K restoration (no tape loading sound on speaker)

Post by 1024MAK »

Lardo Boffin wrote: Mon Mar 23, 2020 12:21 am Is a ULA from an issue 3 fully compatible with an issue 2 pcb? I know they changed the ULAs over the years.
No such thing as ‘fully compatible‘, but yes, a ULA from an issue 3 board should work fine in an issue 2 board. Generally speaking you can fit a later (newer) ULA in an earlier issue board, but not always the other way round.

Mark
ZX81 Variations
ZX81 Chip Pin-outs
ZX81 Video Transistor Buffer Amp

:!: Standby alert :!:
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Moggy
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Re: Yet another 48K restoration (no tape loading sound on speaker)

Post by Moggy »

Just my two cents worth.

I have one of those lovely Samsung made spectrums and when loading it's silent as the grave yet squeaks normally when asked to do the old BEEP dance and does everything a spectrum should so I wouldn't worry about it at all.

I have made my own power amp for this spectrum with a very sensitive input which, although requires no pre-amplification, has an op-amp set at unity gain which acts as a buffer for the ULA so as not to overload its output.
This then Is driven via the MIC socket and is very loud.
zx-heinz
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Re: Yet another 48K restoration (no tape loading sound on speaker)

Post by zx-heinz »

I had the same problem: No tape loading sound and no BEEP. There was even no signal at pin 28 of the ULA. The reason was a faulty ULA.
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Bora66
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Re: Yet another 48K restoration (no tape loading sound on speaker)

Post by Bora66 »

Guys,
Sorry for the very late update on this. Somehow I did not get notified of the replies, and then I totally forgot I posted here. I sincerely apologize to the ones that replied.
Here is the update:
1) No loading sound problem fixed. Looking thru the issue 2 and issue 3 schematics, I noticed just how logical and better was the issue 3 audio section compared to issue 2, so I modified the issue 2 accordingly adding a transistor to drive the speaker and of course I replaced the speaker with a suitable sized 32 ohms one. Works perfectly now.
2) I also fixed the butchered issue 3 board while at it, now that one works too. My issue 2 (bought new in 1983) came with the same ULA (6C001E-6) that the issue 3 had on it. Apparently, the very last issue 2 boards came out with the same ULA with the issue 3's so, there is no compatibility problems.
3) With 2 working rubberkeys, and only one working ULA, I bought a vLA82 from Charlie Ingley. It is a clone of the latest 48K ULA, 6C001E-7, and works perfectly both in issue 2 and issue 3.
Take care, and thanks!
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1024MAK
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Re: Yet another 48K restoration (no tape loading sound on speaker)

Post by 1024MAK »

Thanks for (eventually) replying :lol:

Good to know that you have fixed and sorted out the issues.

You can select notifications in the user control panel.

Yes, despite Sinclair being known for using whatever components that were available at the right price, for some reason people think that the type of keyboard (be it the rubber mat, the membrane, or the method of fixing the metal plate to the plastic) and the ULA always tie up to the board issue number/version. But they don’t.

Some issue 2 boards had the first ULA version chip fitted along with the cockroach. Some issue 2 boards did not have the spider fitted. While all later issue 2 boards did. So it’s not surprising that some issue 2 boards had the later 6C001E ULA.

Mark
ZX81 Variations
ZX81 Chip Pin-outs
ZX81 Video Transistor Buffer Amp

:!: Standby alert :!:
There are four lights!
Step up to red alert. Sir, are you absolutely sure? It does mean changing the bulb :!:
Looking forward to summer later in the year.
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