Issue 4a and 3b burning speaker

[poiex]

Hello im new to here and need some help fixing my speccys.
on my issue 4a speaker heats so warm it melts and then burns same problem with my issue 3b.
I have changed TR7 and D9 no luck i even tested new ULA same result, what could the problem be?
i saw one post where someone had same problem but there was no fix for this burning fault.
on TR7 the voltages are 10v 0v 10v that can't be correct.
both speccys are working fine just the no sound fault.

[Ast A. Moore]

Do the MIC and EAR ports work okay? Can you load games or hear sounds through them?

on TR7 the voltages are 10v 0v 10v that can't be correct.

I believe the collector is fed 9V directly from the input power jack, so the voltage could vary, but that alone shouldn’t be killing the speaker. What could be killing it is if it just stays on all the time. Check D9, D13, and—just to be sure—C35. Also, make sure there are no shorts or solder splashes on the traces around. I’d also desolder the base from the PCB and measure the voltage on the emitter while pulling the base to ground/+5V (via a resistor). If it doesn’t change, the transistor is dead.

It’s worth testing the speaker, too. It’s a 40-Ohm one; not the more common 4–16-Ohm variety.

[1024MAK]

TR7 is controlled (or should be) by the ULA via diode D9. As TR7 is configured as an emitter follower, when switched on, the voltage at the emitter should be around 1.2V to 1.4V less than pin 28 of the ULA (assuming there is no audio signal from any cassette deck).

Despite what the schematics may say, TR7 collector is connected to the unregulated +9V supply, which is normally around 10V to 11V.

If you have 10V on TR7 emitter, then TR7 is faulty or bypassed (shorted out).

Mark