Darlington-transistor problem

[Poul Erik]

I have a darlington-transistor problem and need some help

In my newly bought Hammond Aurora 231000 there is a defect B1 key. I have found out, that it probably is the couplet that has a 1/3 defect. the 2 other thirds is OK.

I have tried to find the couplets on the internet, but with no succes.

The couplet is covered in some plastic-stuff, so i cant measure or repair.
I have measured all the connections to and from the couplet and they are fine. If i use one of the other keys to activate the IC on the respective pin (24), that generetes the tone, so the IC works.

I am sure, that there is a short or disconnection in the couplet for the B1 note.

I have inclosed the pcb and diagram. The B1 key is connected to pin 1 and the signal is lead from pin 18 to the IC pin 24 with a 10K resistor to ground.

Now to the problem. I will try to make a new couplet and think i can get a suitable darlington using MPSA63 or 2SA1048. The one used is named 001-022103 and that is translated to NTE232 (https://vetco.net/products/pnp-si-da...v-300ma-nte232)

The capacitors are 220 nF and i guess i will be able to use 1N4148 for the signaldiodes

But the diagram only tells the value for the 3 resistors R13-R15. The other is not mentioned.

How would i find out the values for resistors R1-R12 to make a similar couplet.

I would be glad, if any one could help.

Best regards

Poul Erik

[gtc]

Uploading for you:




Redrawn showing 1/3 of it (hopefully without errors):



Because of blurring, it's not clear to me what the label on pin 5 is. Also, I'm not sure of the value of R15 -- looks like 3K3 to me.

Using a multimeter on one third of the couplet that is known to be OK, and adjusting positive and negative leads to accommodate the series diodes:

The value of R3 should be obtained between pin 1 and pin 4
The value of R6 should be obtained between pin 1 and pin 5
The value of R9 should be obtained between pin 1 and pin 8

The value of R22 will need to be calculated so that Q1 is fully turned on by whatever is available at pin 1.

As for the Darlington, your choice is probably OK. The two main features of Darlingtons are high gain and high input impedance. In this instance, I believe high impedance is the key characteristic.

Any small signal diode should be fine.

I'd construct a 1/3 circuit 'in the rough' and use a resistance wheel to experiment with values for R22.

BTW: a shorted C3 would cause the circuit to fail.

[gtc]

Need to see the schematic but your image upload hasn't worked.

[gtc]

Uploading for you:




Redrawn showing 1/3 of it (hopefully without errors):



Because of blurring, it's not clear to me what the label on pin 5 is. Also, I'm not sure of the value of R15 -- looks like 3K3 to me.

Using a multimeter on one third of the couplet that is known to be OK, and adjusting positive and negative leads to accommodate the series diodes:

The value of R3 should be obtained between pin 1 and pin 4
The value of R6 should be obtained between pin 1 and pin 5
The value of R9 should be obtained between pin 1 and pin 8

The value of R22 will need to be calculated so that Q1 is fully turned on by whatever is available at pin 1.

As for the Darlington, your choice is probably OK. The two main features of Darlingtons are high gain and high input impedance. In this instance, I believe high impedance is the key characteristic.

Any small signal diode should be fine.

I'd construct a 1/3 circuit 'in the rough' and use a resistance wheel to experiment with values for R22.

BTW: a shorted C3 would cause the circuit to fail.

[Poul Erik]

Dear GTC

Thanks a lot for your effort. Much appreciated.

Pin 5 is according to the generator diagram "Cutoff bus". C/O for short. I dont know what it means.
The R15 is definitely 3,3K
I have made a similar drawing of the 1/3 couplet, but I haven't thought about the C3. Of course it vil disable the output if shortened.
I think it will be a problem to measure the resistance between the pins while the couplet is mounted in the motherboard, as they all are parallel connected. So I will have to take it out, but that is doable.

Again, thanks for your effort.

Best regards
Poul Erik

[gtc]

Yes, definitely remove it from the circuit. Let us know what you find.

[Poul Erik]

I have made som measurements this afternoon.

The output of pin 18 is zero, so the C3 must definitely be shortened to ground.
I have compared the output with some of the other couplets and they are not zero at the output.
Then I took the output from C1 key from IC 9 (it was reachable) and took it to the B1 input on IC6.
Then the C1 tone disappeared until I released the connection. Looks like the circuit for the B1 key is pulling the -3,5 volt to ground.

It looks like a winter-project taking the motherbord/generator out and making a new couplet. Looking foreward to do that.

Thanks again for your help

Best regards.

Poul Erik