Yet another 48K restoration (no tape loading sound on speaker)
Posted: Thu Mar 12, 2020 2:02 pm
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.
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.
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
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.
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.
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