Sinclair ZX80 / ZX81 / Z88 Forums

Hi,

The issue 4b I've repaired a couple of weeks ago has a very very low sound, being loading from tape or just the usual beeps. It's really barely audible, special the load tape sounds. The sounds are good if I connect some columns to the mic output.

I have identified TR7 or D9, as probable culprits? Am I right in this assessment?
The speaker measures 40Omhs between the 2 pins, so I'm leaving this one to the end.

And just for my info, what are the replacements?

Speaker model?
D9 is a 1N4148
TR7 can be replaced by what model?

Thank you all.

Sounds like TR7 (ZTX450) is the culprit, specially if the 5v line has gone high at some stage, this can happen if a lower ram chip has badly failed and spiked the 5v line, Ive not seen D9 fail before, but might be worth replacing if TR7 doesn't remedy it. And the standard speaker should read 40 Ohms.

Spectrum65535 wrote: ↑Sun Feb 28, 2021 2:32 am Sounds like TR7 (ZTX450) is the culprit, specially if the 5v line has gone high at some stage, this can happen if a lower ram chip has badly failed and spiked the 5v line, Ive not seen D9 fail before, but might be worth replacing if TR7 doesn't remedy it. And the standard speaker should read 40 Ohms.

TR7 is fed from the +9V (nominal) supply. Yes, I know what the schematic shows, but most of the schematic diagrams have errors, and this is one of them.

A failure of diode D9 or D13 (or both) is just as likely as a failure of TR7.

Anyway, first off, test the collector of TR7 to confirm that you have between +7.5V and +11.9V (obviously the actual voltage depends on the type of PSU you are using).

Next with the power off, assuming the ULA is in a socket, remove it. Now try testing D9 and D13 on the diode test range on your multimeter. Do the same with the two junctions of TR7.

Each forward junction should conduct (about 0.5V to 0.6V if your meter shows voltage). When tested in reverse, each junction should be open circuit. If you get a short circuit indication, or open circuit both ways, replace the component.

D9 and D13 are 1N4148
TR7 is a ZTX450

All these are still in current production.

Mark

Hi all,

Thank you for your help.

I have tested both D9 and TR7 and they appear to be fine, changed the speaker and the volume is now good, so in the end the problem was from the speaker.

Thank you both for the help