ZX Spectrum 48k Issue 2, No loading sound (was working previously)

[shazeal55]

HI,

Hoping someone might have some insight into this problem.

I have had this 48k Issue2 speccy for a few years now, bought off an older couple who had not used it since the 80s clearing out "junk". I recapped it when I got it and before using, changed the 7805 for a retroleum switch mode regulator and swapped out original PSU with a switching one.

I use it with an old CRT TV that only has RF in, a 1987 National Cassette Recorder to load stuff, and an original Turbo joystick interface.

When I first had it running I got the full speccy experience with the loading noises when loading from tape. It was always much quieter than I remembered from the old 16K speccy we had in the 80s

But at some point the loading noises just stopped, but the beeper still worked fine in games/basic. Nothing had changed since recapping it.

I have swapped out the speaker with a retroleum issue 2 speaker module, and swapped out the ULA for a vLA82. This made the beeper sound better in games, but made no difference to the missing loading noise. I have tested it with the bytedelight diag cart and it passes fine.

So then I swapped out the D9/D10/D13 diodes and noticed that R34 was too high so replaced that too. Still no effect on the loading noise.

I am not really sure what to do now, I keep reading that the original ceramic caps are fine, and with my limited multimeter skills I can't see anything abnormal around the audio circuit but I am not really sure as to what I should be looking for. From the issue 2 schematics it seems like the audio from the mic should just go MIC -> C31 -> R35 -> D9 -> D10 -> Speaker?

-- As a side note, not sure if this could be related to the loading noise issue? --
If I plug in some powered speakers to the line out jack while loading games, there is a massive amount of humm but the loading noise is there in the background. Also if I turn up the volume on the TV it humms a lot and the loading noise is there in the background too.

Plugging in speakers with speccy off has a "normal" poorly grounded humm, turning the speccy on with TV unplugged changes the pitch/volume of the humm slightly. Playing a tape there is still nothing from beeper, but with speakers the audio is much more distinct than with TV plugged in. Plugging in the TV to RF port the humm is significantly louder and loading noise becomes muffled.
--

Any help would be appreciated thank you.

[1024MAK]

Hello and welcome

It was always much quieter than I remembered from the old 16K speccy we had in the 80s

The issue one and issue two boards were known for their speaker not being very loud. The issue 3 board (and later boards) had a change in circuitry, a transistor was added, this improved the speaker volume somewhat.

So then I swapped out the D9/D10/D13 diodes and noticed that R34 was too high so replaced that too.

R34 (15Ω) is in a power feed line to the ULA. That is, an originial Ferranti type. The replacement modules like the vLA82 don't use this as far as I am aware.

From the issue 2 schematics it seems like the audio from the mic should just go MIC -> C31 -> R35 -> D9 -> D10 -> Speaker?

Um, you are connecting the "Ear" output from the Cassette Recorder to the "Ear" input socket on the ZX Spectrum aren’t you?

The "Mic" socket on a ZX Spectrum is suppose to be a LOW level output for a sensitive "Mic" (microphone) input on a Cassette Recorder.

If you feed the audio output from the Cassette Recorder to the "Mic" socket on the ZX Spectrum, there will not be enough power for the internal speaker to work.

If I plug in some powered speakers to the line out jack while loading games, there is a massive amount of humm but the loading noise is there in the background. Also if I turn up the volume on the TV it humms a lot and the loading noise is there in the background too.

The 16K/48K/+/128K (toastrack) models don't have a "line out" socket. And the 16K/48K/+ models don't (officially) have audio on the RF/UHF output to the TV. That's why Sinclair fitted an internal speaker.

For use with an external amplifier or powered speakers, you can either connect to the "Mic" or the "Ear" sockets. The "Ear" socket will produce a louder output compared to the "Mic" socket.

You may be confused by this. But both the "Mic" and the "Ear" sockets connect to the same ULA pin. The difference being the values of the resistors in the two circuits.

The reason that you are getting a humm with a weak loading noise in the background is due to your powered speakers being sensitive enough to pick up both electrical noise from the mains, electrical noise from the computers digital circuitry as well as the loading noise via a "roundabout" route.

Mark

[shazeal55]

Um, you are connecting the "Ear" output from the Cassette Recorder to the "Ear" input socket on the ZX Spectrum aren’t you?

No sir I was not!

It all makes so much sense now, I swapped out the original case for a replica some time ago so I could restore the original and the replica is not labelled, at some point I have switched the jacks around and been completely mixed up of which port did what. I mean it loaded games and even saved things to tape

I Just swapped the jack from mic to ear and it sounds amazing, so much better than before changing the speaker so it wasn't a total waste of time. I checked the old speaker again and it reads 240ohm (supposed to be 200 on issue 2).

Anyway thank you so much for unwinding that loop in my brain!

And thank you very much for the explanation of the Humm noise, I do remember most things used to humm in the 80s especially with the volume up so the sensitivity of modern hardware makes sense.

R34 (15Ω) is in a power feed line to the ULA. That is, an originial Ferranti type. The replacement modules like the vLA82 don't use this as far as I am aware.

Also good to know thank you

[1024MAK]

Good to know that you are now sorted. Now time to have some fun

Mark