Help in resurrecting my ZX81

[mrtinb]

Measuring pin 6 on CPU is still 5V.

With multimeter on 200mV setting I get the values on TR2: Base 3,3 Emitter 2,5 Collector 2,2. That's a puzzle to me.

You mention I should try a higher value for R6. I have the following values: 2K7 3K 3K3 4K7 5K1 6K8.

[mrtinb]

On Andys ULA pin 35 OSC, I get 3,4 when multimeter is set for 200mV.

[Andy Rea]

pin 35 on the replacement ULA is not used for oscillator at all.... it shouldn't even have a pin in it.

do you get oscillation out at pin 14 ?

regards Andy

[Andy Rea]

my ula should provide a fairly clean square wave near to 3.2v p - p

regards Andy

[mrtinb]

my ula should provide a fairly clean square wave near to 3.2v p - p

Measuring with a multimeter I get 2,27V

[dessony]

TR2 is a ZTX313 in both the schematic and on the pcb. I need a replacement. I don't have a ZTX313 but I have listed below the transistors I have. Could anyone of them be used

2N3904
2N3906
2N5401
2N5551
A42 A331
A733
A1015
C945
C1815
KSP92-706
S8050
S8550
S9012
S9013
S9014
S9015
S9018


mrtinb, I quoted the listing of the transistors of yours. I had an experience similar to yours. I searched for the ZTX313 all over the websites. Then, I just observed close enough to read the existing transistors' labels on my working ZX81/ TS1000 motherboard and ordered a few new MPS2369s. These parts are equivalent to the ZTX313. With the two transistors replaced, the motherboard is working fine now.

Cheers,
Dessony

[McKlaud]

Martin: That is an expected value. Your multimeter is a True RMS or RMS type and the clock signal is a square with 50% duty cycle. So, 2.27Vrms = 3.2Vp-p/sqrt(2).

You can try with any NPN type transistor from your list but bear in mind that the ZTX313 is a switching transistor but most units on your list are general purpose silicon transistors (mostly low frequency amplifying type). For a try I would go with 2N3904, S9016 or S9018.

[mrtinb]

So I seem to have a choise.

Try replacing R6 with a higher value.
or
Try replacing the resistor with a resistor that might work.

[1024MAK]

Nearly all transistors will have too slow a turn off time compared to the fast ZTX313.
The nearest equivalent is a MPS2369.

Martin, can you test the continuity of the PCB tracks between the components surrounding TR2 first, just in case a broken track is leading us astray.

Mark

[mrtinb]

That's almost impossible, as the Z80 is soldered without socket, and most traces goes beneath it.

Edit:
I'll get a desolder gun in a few weeks. That might help.

I've ordered MPS2369s but they only show up a month from now.