ZX Spectrum + : no boot

[dizzy33]

Sorry Mark, finally I could find out the answer about the jumpers "N" or "H".
But I still have this question for you regarding my no sound problem :

I replaced TR7, same issue : no sound.
I tried to connect the existing speaker to one on a working Spectrum : still no sound.
I know that D9 could be the culprit, but I need to find one to buy, it may take time before I receive it.

I have read on the service manual here https://spectrumforeveryone.com/wp-cont ... Manual.pdf that I need to check load input waveforms. But I do not know how to do it as I am not able to understand the diagram on this documentation (I'm a noob).
On which pin of which component should I put the probe of my oscilloscope ?

Thanks !

[1024MAK]

Issue 6A board showing D9 and the ROM H/N selection links

Issue 6A sound/ear/mic/speaker circuit

I think the section you are talking about is section 6.10 on page 35 of service manual
(https://spectrumforeveryone.com/wp-cont ... Manual.pdf).

I think this is where a square wave test signal is input on the ear socket (Point A). The applied test input signal is shown at point A. The diagram shows what you should see on an oscilloscope if the input test signal is a 5V peak to peak square wave.

The diagrams for points B and C then show how the resistors and capacitors reshape the waveform.

If you attach a cassette deck or other suitable audio source to the ear socket, when you play a tape (or otherwise send sound signals), does the speaker make any sound at all? If you connect your oscilloscope to the anode of D9 (Point C)(or any other point where R90 / D9 / R35 / D13 / C32 / C35 join) do you see any AC signal showing on your oscilloscope?

When the ZX Spectrum is supposed to be making a sound, if you connect the mic socket to a cassette deck or other suitable audio system, do you hear any sound produced by the ZX Spectrum?

If no, if you connect your oscilloscope to the anode of D9 (Point C)(or any other point where R90 / D9 / R35 / D13 / C32 / C35 join), when the ZX Spectrum is supposed to be producing sound, do you get any AC signal showing on your oscilloscope?

If yes, D9 may be open circuit. Now move your oscilloscope probe to the cathode of D9 or the base terminal of TR7. If you get a signal here, D9 is okay.

If no, D13 may be short circuit. Cut one leg and try again. If this restores sound, renew D13.

D9 and D13 are both 1N4148, but any silicon signal diode will work in these positions.

Mark

[dizzy33]

Fantastic answer, thanks a lot, this exactly what I needed !
I will do all these tests tomorrow and let you know.

[dizzy33]

Here is the result :
- if I connect a headset to the "mic" connector, then I can ear the Spectrum sound.
- if I connect a sound source to the "ear" connector, then I don't ear nothing from the speaker.
So I connected my oscilloscope to the anode then cathode of D9 (while playing music to the ear connector), but there is no signal.
So I disconnected one leg of D13, but no change, still no sound from the speaker.
I finally wonder if the problem may come from the speaker (maybe my test of connecting the speaker of another Spectrum was wrong), but I guess I should have seen some signals on D9, right ?

[1024MAK]

If you heard the sound from the Spectrum via a headset plugged into the mic socket, then there must be an audio waveform making it to point C. However, that does not mean that it is getting to D9 anode. There could be a broken PCB track or dry joint.

Just to test for this, with the power off, use your multimeter on resistance (200Ω range) or continuity range and test between ULA pin 28 and D9 anode.

Mark

[dizzy33]

I tested the continuity between pin 28 of ULA and D9, the continuity is OK.
But that's very difficult for the moment to do these tests as my only way to let the Spectrum produces sounds is via the Smartcard, which only beep once every minute ... I have no working ROM on my motherboard so I cannot try games.
I will try to find a solution before receiving my new ROM and socket, I will let you know when I will be really ready to re-do the tests you asked.
Thanks again Mark, you are very helpfull.

[1024MAK]

Do you have a NE555 timer chip, or any other source of a AC signal or pulsating DC signal?

In fact any astable multivibrator circuit that can work at 1kHz or thereabouts will do.

Mark

[dizzy33]

Thanks a lot Mark, but I should receive my 28 pins socket very soon (Monday or Tuesday), then I will be able to use the ROM of another Spectrum.
I will then re-do all the tests you suggested. I will let you know of the result.

[dizzy33]

Hello Mark,

here is me real result (my sound tests worked better with a working ROM ...).
The sound is working perfectly on my headset connected to the "EAR" connector of the Spectrum, but no sound on the speaker.
I connected my oscilloscope to the anode of D9, I see a signal. But on the cathode, no signal.
No signal on the base of TR7 (TR7 has already been replaced).
I also tried to remove one leg of D13, but it did not restore sound on the speaker.

So does this mean that my D9 diode is dead ?

[1024MAK]

So does this mean that my D9 diode is dead ?

It sure looks like it’s dead.

Mark