help needed with old detector

[dbanner]

Keith 777 thank you very much for your drawings. It's exactly what I needed. Tomorrow I will examine in more detail. I will show you how to test each transistor without having to remove them from circuit board. It actually very simple using the multimeter. And also the diodes. What value resistor did you use to replace the damaged one?
I am sure you will be able to fix this detector with a little help but we will have to take some measurements with the multimeter to see exactly which component
Has failed. It's late over here now.

[keith777]

Hi again dbanner.
i replaced the resistor with the one in the pic bellow. i sill have the toasted resistor. so i had a good look at it with a magnifier eye peace. plus looking at the photos i had taken of it before it fell apart. i think i got it right.
also thank you again very much for the help. what a great bunch of guys on here.

[dbanner]

looks like correct value resistor but just to be absolutely sure i would like you to measure the resistance of old resistor. Set up your multimeter as shown in picture then clean ends of old resistor with some abrasive thing and place multimeter probes on each end as shown in picture( any probe to any end, it doesnt matter). Read resistance on meter. What does it read?

[keith777]

Hi dbanner.
i have done what you said and i can not get any kind of reading what so ever.

[kt315]

KT is this any good for you.....

thank you. now you can draw a schematic

[dbanner]

O.K. No problem. We'll go with 30K as replacement for now. Next I want you to check this transistor for me. Set up meter as shown and place probes as shown on picture. what does meter read?

[keith777]

Hi again dabanner.
sorry for the late reply. i was keep refreshing page 2 for a new post and not realizing there was a new page 3 added with your reply on. sorry about that.
anyways i tested that part you asked me to and i got a reading of. depending on which way i put the meter pins on which way. 467 and 540
should it matter which way round i put them?

[dbanner]

Yes, but the reading you got looks O.K. Ideally you should desolder the base leg of transistor(the leg in the centre) in order to isolate it from rest of circuit so it can be checked. I'll be back a little later, have to do something right now. Meanwhile, do same test but on the other transistor as shown in pic. Then test each diode as shown in pic. Remember the color indicates which probe goes where.

[keith777]

Hi again dbanner.

for the tests i got.

first test 482
second 437
third 451

[dbanner]

These results are what i would expect. So far everything appears to be ok on small board. Now one more test, please set up multimeter as shown and turn on detector, measure voltage between points as shown on picture.

[dbanner]

Something seems odd about the way the power supply(batteries) are hooked up on this circuit. I think i will attempt to draw(reverse engineer) schematic of circuit using the pictures and diagram posted. This way I'll know for sure what's going on. It will take a little time though.

[keith777]

Hi again dbanner.

i get no voltage at all doing that test.

[keith777]

i have checked every wire on the buzzer sound test and all sounded off.

[dbanner]

Check voltage between these points with detector on. By the way, where is the on switch located?

[keith777]

Hi again dbanner.

i am getting a voltage reading now on testing. the cheap replacement 1.5v holder that i bought is cr*p. the minus spring doesn't connect properly to the solder part. i have sorted it for now but i will have to get a better one.
here is the last two voltage test reading that i get........ see pics.