Sinclair ZX80 / ZX81 / Z88 Forums

I think I may have broken the TR5 transistor in a +3 while doing the sound distortion fix mod (viewtopic.php?f=21&t=4666).

Is it possible to remove it and test it?

The only test equipment I have is this:
If so how would I go about testing it?

Many thanks!

That meter will only test its hfe or gain value as explained at the link below but at least it will give some indication of its condition.

https://www.learningaboutelectronics.co ... transistor.


So find data sheet for given transistor then follow instructions attached below for that meter.

Thanks Moggy.

Either the transistor is completely dead - it reads 0 - or the legs are too short to engage with the testing mechanism.
I tried an unused BC549 and that gave a reading fine so the tester is doing its stuff.

You can use the diode test range.


With a digital multimeter, the red probe is + and the back probe is -

A pass means a voltage of between 0.5V and 0.75V, a low resistance indication (but not less than 100 ohms) depending on the actual multimeter being used.
An open circuit means the meter showing OL, OPEn or other high resistance/open circuit indication.
A fail is anything else.
A resistance of less than 100 ohms means it’s fried…

For an NPN transistor:-

1. Test between base (+) and the collector, should pass the test. Swap the leads around, it should indicate open circuit.
2. Test between base (+) and the emitter, should pass the test. Swap the leads around, it should indicate open circuit.
3. Test between collector (+) and the emitter, it should indicate open circuit. Swap the leads around, it should indicate open circuit.

BTW, bipolar transistors are actually fairly hardy and hard to accidentally kill. Unless you connect them up to short out a supply rail, allow more than their current rating to flow (100mA collector current or more than 10mA base current for small signal transistors like the BC549) or let them overheat (500mW for small signal transistors).

Mark

Thanks Mark.

I ran the tests on a new 549C which obviously passed. The readings were around 810 on my multimeter (or 1 when open circuit).

I then ran the tests in my suspect TR5 and it gave readings of around 850 and then 1 when open circuit and therefore looked like it might be ok? I refitted it and still no sound.

I have sourced a 549B in case that makes a difference.

Much to my surprise, I don’t have a BC549 or near relative in the bit box that I have to hand.
But I do have a couple of other ‘European’ NPN transistors to hand. Here are the results on two inexpensive multimeters.
Results will vary slightly between different transistor types, their gain, their current temperature and the meter used to test them.

Mark

Lardo Boffin wrote: ↑Tue May 24, 2022 6:06 pm I ran the tests on a new 549C which obviously passed. The readings were around 810 on my multimeter (or 1 when open circuit).

I then ran the tests in my suspect TR5 and it gave readings of around 850 and then 1 when open circuit and therefore looked like it might be ok? I refitted it and still no sound.

I have sourced a 549B in case that makes a difference.

BC549, BC549A, BC549B, BC549C are all the same specification transistor, except that the suffix (A, B or C) is indicating that they were tested by the manufacturer and sorted into gain (hFE in data sheets and data books) groups. The BC549 (no letter suffix) has not been sorted, or are actually BC549A. BC549C have the best (highest) gain figure.

Hence a BC549B can replace a BC549A in most circuits (*) and a BC549C can replace a BC549B in most circuits (*).

I very much doubt that it will make any difference in a ZX Spectrum +3.

(*) I’ve yet to find a circuit where this is not the case, but there is likely an exception somewhere!

I’ve not read your other topic yet. When I have, I will reply there.

Mark