transistor and relay problem

[pwdiya12]

Hi

I have an APT Pre amp with a relay which I am upgrading.

There are 3 transistors providing the drive and control to the relay(the circuit diagram does not match the actual circuit)- and the only way I can get the circuit to operate is by putting a DMM (either free standing battery version or rack mount version) across the base and collector, and hey presto it works- otherwise nothing no reassuring click.

The relays for unit this are unobtanium- I have replaced all the transistors in the drive circuit and all the diodes and resistors as well-

I feel I am missing something basic here, so any help appreciated

 

[Kirchhoff]

Try a high hfe transistor. ZTX690B or similar.

 

[pwdiya12]

thanks for the fast response, I do not have any ztx690 to hand, any other thoughts (I have KSC1845)

original was a 2N3904 Hfe 266, replacement 2N3904 hfe 180

 

[rayma]

I do not have any ztx690 to hand, any other thoughts (I have KSC1845) original
was a 2N3904 Hfe 266, replacement 2N3904 hfe 180

Can you post the actual circuit that you have?

 

[pwdiya12]

I have a badly hand drawn partial of that part of the circuit, which I have attached:

Q17 is a 2N3906
Q18 was a 2SC1345 now a KSC1845
Q19 is a 2N3904

thanks

 

[pwdiya12]

here is the relevant section form the manual that describes the circuit operation, however some of the described components dont exist

 

[rayma]

I have a badly hand drawn partial of that part of the circuit, which I have attached:

Have you also replaced all the electrolytic capacitors in that circuit?
Which transistor do you touch with the DVM to trigger it?

 

[pwdiya12]

yes assiduously..

and the rectifiers (7918/7818) and a bunch of film caps and (as I have done on a previous unit that I use) all the IC's

I do like these APT's- but this relay thing has me flummoxed

 

[pwdiya12]

the Q19 transistor across base and collector

 

[rayma]

yes assiduously..

and the rectifiers (7918/7818) and a bunch of film caps and (as I have done on a previous unit that I use) all the IC's
I do like these APT's- but this relay thing has me flummoxed

If worst comes to worst, you can remove that circuit and install a different, new one.
There are several variations available, or you can design your own. Not hard

 

[Enzo]

Sounds to me like the base lead is open. Your drawing shows a 10k resistor from the base, but it is not clear what it connects to. if that path is open, the resistance of your mater to the voltage on the collector is apparently enough to turn on the circuit. Instead of a meter, try touching a 1 meg resistor from base to collector, does that also trigger the relay?

 

[pwdiya12]

unfortunately 1meg resistor does not trigger relay whereas meter does

 

[rayma]

unfortunately 1meg resistor does not trigger relay whereas meter does

Try a smaller resistor, 10k?

 

[pwdiya12]

ok that was on Q19, I think you may have meant q17- I will try and report back

 

[pwdiya12]

thanks rayma and enzo!

10k across the base and collector of Q17 and relay works!
10k across the base and collector of Q19 and relay works!

so now that there is a solution, any thoughts as to why?

greatly appreciated

 

[rayma]

now that there is a solution, any thoughts as to why?

That's the hard part, so try to draw a good schematic first.
Don't despair, this is a good preamp with a minor problem.
Are the power supply voltages all ok?

 

[Kirchhoff]

yes check the voltages, as the manual says, the circuit is voltage sensitive

 

[pwdiya12]

unfortunately all there are a number of versions of the pre amp- which a number of relay types, a narrow type (earlier I think), a wide type (the one I am working on- this is Revision 11) and a wide type on a daughter board (later than narrow, but before the one I am working on- I think)- and there is only one manual out there, which refers to the earlier versions. APT originally printed a circuit and attached this to the bottom of the units, however they are all the same and dont match the specific unit

thanks again for the help, I will figure out where to put the 10k (Q17 or Q19) and try and divine "why"

 

[pwdiya12]

and thanks Kirchfoff- I have measured them to try and diagnose the problem and will now compare with the 10k inserted and see if I can figure it out

 

[rayma]

and thanks Kirchfoff- I have measured them to try and diagnose the problem and will now compare
with the 10k inserted and see if I can figure it out

See if the relay seems to function correctly now, with a delay upon turn-on, and an immediate mute upon turn-off.