ZX Spectrum 48K Issue 4B No sound

Hi, I have a ZX Spectrum 48K Issue 4B without sound.
I tested the ULA on another Spectrum and works fine, tested the speaker and it has the correct resistance, tested D9 no issues, tested continuity from pin 28 ULA to emitter of TR7 no faults. TR7 tested ok. The only unusual thing is that the collector of TR7 has +10V and it's connected to Input of 7805.
I don't know if this is ok for an Issue 4B. I checked the diagram from WoS and it show +5V.
Any ideias where to look next?
Kind regards

Comments

  • bganhao wrote: »
    The only unusual thing is that the collector of TR7 has +10V and it's connected to Input of 7805.
    I don't know if this is ok for an Issue 4B. I checked the diagram from WoS and it show +5V.
    No, that’s an error in the schematic. The collector of TR7 should be connected to the +9V rail.

    Try disconnecting the speaker and probing the emitter of the TR7 with an oscilloscope (if you have one) or at least a multimeter in AC mode, which having the Speccy play a continuous beep.
    Every man should plant a tree, build a house, and write a ZX Spectrum game.

    Author of A Yankee in Iraq, a 50 fps shoot-’em-up—the first game to utilize the floating bus on the +2A/+3,
    and zasm Z80 Assembler syntax highlighter.
    Member of the team that discovered, analyzed, and detailed the floating bus behavior on the ZX Spectrum +2A/+3.

    A few Spectrum game fixes.
  • Hi, thanks for your reply.
    What am I looking for by testing the emiter of TR7. It tested ok using diode function
    Kind regards
  • The schematic is wrong. In all issue boards where a transistor is used, TR7 collector is always connected to the +9V rail (which can be anything from +8V to +12V).

    Some questions:
    If you connect an amplifier to the ear socket, then get the ZX Spectrum to make some sound, can you hear it through the amplifier (and attached speakers)?

    If you try loading a program from your cassette recorder to the ZX Spectrum, can the ZX Spectrum load the program? Can you hear any sound from the speaker in the ZX Spectrum?

    How did you test D9?
    Did you test D13?
    How did you test TR7?

    There is no direct connection between ULA pin 28 and any terminal/leg of TR7.

    Mark
  • Hi, You are right pin28 of ULA connects to D9.
    And yes I did test D9, TR7, just tested D13 as recommended by you, no faults. I will try to load a game and see what happens.
    Thanks
  • Hi, you are right! The spectrum is deaf!
    I tried to load a game and it won't load bars or anything!
    Do you think it might be D13 like you said? If so do you know the ref? It doesn't show on WoS schematics
    Many thanks! Now I feel I'm going somewhere.
  • Just found out its the 1N4148. I'll replace it and let you know.
    kind regards
  • D13 is directly under D9 on the issue 4A / 4B schematic.
    On the board, it should be near the Ear socket.

    It would help if you could describe how you are testing the diodes and transistor.

    Both D9 and D13 are 1N4148 signal diodes, but a similar specification signal diode can be used.

    Diodes most often fail short circuit, so when this happens, any audio signal would be severely clipped.

    If there is any doubt about D9 and / or D13, replace with a new diode.

    Mark
  • Ok. So D13 was faulty. I tested it out of circuit and it was shorted. Replaced with a new one. But still no sound when I try to load a game. Now when I press "play" on datacorder it shows a red bar around the screen (before replacing D13 it wasn't) but it just stays there, it won't load the game. Tested again D9, TR7 and they tested ok
  • edited May 22
    I see you've tried the ULA in another working machine, did it confirm the sound was working? Also have you tried the ULA from the working machine in the board with no sound and test for sound again?
    Post edited by Your Spec-chum on
  • Hi, thanks for your reply.
    Just tried another ULA and still no sound...
  • edited May 23
    So, having found one faulty diode, we start again from step one again with the fault finding process...

    If you connect an amplifier to the ear socket, then get the ZX Spectrum to make some sound, can you hear it through the amplifier (and attached speakers)? If you don’t have an amplifier, some cassette decks can be used to do the same thing (sometimes called ‘monitor’ in the instructions).

    If you try loading a program from your cassette recorder to the ZX Spectrum, can the ZX Spectrum load the program? Can you hear any sound from the speaker in the ZX Spectrum? Does turning up the volume control on the cassette deck make any difference?

    What voltage do you have at the collector of TR7?
    What voltage do you have at the base of TR7?
    What voltage do you have at the emitter of TR7?

    Also, apart from a multimeter, what test gear do you have?

    Mark

    Post edited by 1024MAK on
  • Hi, It loads games from a tape using a datacorder.
    I don't have an amplifier to test the MIC out.
    TR7 E- 0.9V
    TR7 B- 0.7V
    TR7 C-10.5V
    I have a multimeter and a logic probe.
    I'm not a pro (obviously!), I'm just trying to fix my late uncles speccy that's why i'm reaching for you guys.
  • 0.9V on the emitter with 0.7V on the base, that’s a bit odd.

    If it loads games from tape, that means a lot of the circuitry is okay.

    Does this ZX Spectrum have a working keyword attached/connected?

    Mark
  • Hi, thanks for your reply. It has a brand new keyboard membrane and it was connnected
  • Only a couple of things I could think of. I see you've tested the little speaker for resistance, what happens if say you connect the terminals of a 1.5v AA battery to the speaker terminals? It should hear a bit of a crackle if working. Could possibly have a stuck voice coil. Also non of the connections on the ear and mic sockets are touching together are they? That would put a load on the audio/amplifier circuit. You might have tried those but as you've checked all the obvious possibilities, it a can be simple faults that seem like major one's with electronics sometimes. Good luck and hope you get it sorted.
  • Thanks I'll have a go on that one
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