Grundig Concert Boy 1100

[poppydog]

Thanks - tested all 3 with the - on B and + on E and it reads: AC121 is 191, AC188K is 163 and AC187K is 1. Any ideas on the costings and if I need to unsolder the RC650 to test? Thank you

[Restoration73]

Looks like the AC187 is dud. You don't need to unsolder the preset, just measure
across the two outer legs, ignore the middle leg. 40-60 ohms is ok.
AC187K and others are around on the web.

[poppydog]

Apologies R650 (post from Restoration73) - sorry been a long day!!

[Herald1360]

AC187K is NPN, so B-E will conduct with + on B and - on E. It should read 1 the way you described. It could still be dead though..... check with + on B to both C and E, should get around 150-200. Also check + on C to - on E, should be 1 or at least go to 1 if B shorted to E.

Worth doing same with polarity reversed to AC188K. It's possible for transistors to fail C-E short whilst still checking out OK B-C and B-E.

[poppydog]

I measured the resistor and it says 85. However when I click to the 200 ohms scale it measures 58.7?

[Restoration73]

Thats OK for the resistor, I may have misinterpreted your readings on the transistor, I usually use an Avo on the ohms x 1 range.

[poppydog]

I just as well order all 3 of those transistors so it is all done. Is there anything else I need to check? Would the transistor that is no good be what is giving me the problems? Sorry for all the questions!

[Restoration73]

My final tip is after replacing the transistors (ensure the wires of the new ones go to
the same holes as the ones removed !) you can set up the no signal current as follows;
1.Do not solder the C wire of AC188K into the hole, instead temporarily solder a piece
of wire to the hole. 2. Connect your meter on mA range, one lead to C wire, one lead
to the piece of wire in the hole. 3. Connect 9V battery, turn radio on, set volume to
minimum. 4. Adjust R650 so meter displays 7.5mA. 5. Remove meter, solder C wire back.

[poppydog]

fantastic thanks. I will post when all is complete. thankyou

[poppydog]

My new transistors have arrived, however the pins on the AC121 are miles too short for some reason so instead of sorting it out this evening it is looking like it may now be after Christmas. I do hope this thread will be kept open.

[Herald1360]

Pins can be extended using offcuts of tinned wire of suitable gauge - the old transistor might come in handy for this if it's not been binned. Form a small hook in the transistor leg and the extension wire, crimp the two hooks round each other and solder quickly.

Threads aren't normally shut down quickly unless requested or a definite resolution has been reached.

[poppydog]

On our AC187 we are making sure which one is the Collector as there is a C printed on the board so we are checking to make sure everything is right. We are using a digital multi-meter to do this. We are putting our + on B and - on one pin and getting a reading of 160 and then putting the + on B and - on the other pin and we are getting a reading of 158, which means that the pin with the highest reading should be our Collector. We also tested our old AC187 using the same method however according to our readings on the test meter the Collector should be where the Emitter is and vice versa. It is getting very confusing and I need to get it right. Is it to do with my multi-meter?

[Station X]

You don't need to go through all that palaver with a meter.

Just look at the datasheet for the AC187 (google it) and it'll tell you exactly which lead is which.

Isn't it the K suffix version?

[poppydog]

I had to check this to make sure because of the circus we had with the rectifier. What is the reason for the reading on my old transistor being higher for the emitter than it was for the collector?

Thanks.

[Station X]

As long as you see a volt drop one way and resistance the other it doesn't matter which reading is highest.

A transistor isn't like a rectifier. With very few exceptions the lead out configuration is always the same for a given type.

[poppydog]

hello.so far so good ive soldered in the new transistors,spent ages trying to adjust the resistor [its so sensitive] but managed to get it down to 7.47ma I hope that's ok.AND when I turn the volume up I have a nice loud hiss.i will clean and reassemble and then see what its like

[poppydog]

We have come home and switched the radio on to start putting it back together, it was still connected to the multi meter and is now showing 7.10ma. I readjusted the R650 but if you turn it off and leave it for a while when you turn it on again the reading is a lot lower than it was previously. Is there is a reason for this? Is the resistor faulty? It tested ok the last time I tested it.

[poppydog]

ive tested the r650 and its reading 56ohms however its still not staying at 750ma. I adjust it till its right then switch off then switch on again and its always a lot lower,ive even set it so that its a little higher than 750 but when switched on then off its always much lower.can anybody please help. im so near yet so far

[TrevorG3VLF]

I have not seen the circuit but

This is a temperature effect. Vbe drops by 2mV/degC. You will not get stabilisation without a temperature compensating device.
I would just use a low current and keep the volume down.

[poppydog]

thank you for the reply i was worried something was wrong but if its safe to reassemble then i wil do so later. i haven't yet tried it through the mains yet, thanks.