Sugden A48 Mk 2 Amended version Relay issue

[myredbook]

Hi there! I just picked up one of these and as much as the internals all look in amazing condition the relay is not engaging on power up and I'm trying to ascertain whether its the relay or the circuitry powering the relay thats the issue.

Theres about 9v DC being sent to the coil via C51, through RLY J1, and VT51 (please see attached schematic).
The only markings on the Relay are IMO 48=VDC, which suggests that maybe it needs a good bit more voltage to energise the relay and would put the blame on the earlier circuitry? On the other hand all the electrolytics are rated @40v which makes me wonder if the required coil voltage is lower and the 48vdc is what its capable of switching......

any suggestions would be gratefully received.

Thanks for your time

Ben

[Radio Wrangler]

Timing capacitor gone leaky?

David

[myredbook]

Hi David, thanks for your reply. Theres no visible signs of it, was trying to avoid having to take the board out unless totally necessary but maybe I'll have to do it....
It charges up to about 9v through the transistor before going to the relay.
Would you reckon its meant to charge a good bit higher than that?

Thanks

Ben

[Radio Wrangler]

I reckon it needs to go high enough to pull that 48v relay coil in.

VT51 is simply an emitter follower driving the relay coil.

So C51 could be leaky, R53 high, X51 (zener) almost open circuit, or VT51 low on gain to create your problem.

I note that there is no transient catching diode on the relay coil, so VT51 will get a zap every time it turns off. Putting a modest diode across the relay coil, cathode to the top ought to improve reliability. Generally considered good housekeeping.

David

[myredbook]

Brilliant, thanks very much for your help. I'll pull the board out and see where I get to.

Cheers

Ben

[trsomian]

Providing the base is never open circuited completely then a diode is not required across a relay coil when it is driven from the emitter. If the transistor base is suddenly taken to the negative rail (with an NPN) the back emf will drive the emitter lower that the rail, which simply keeps the transistor on until the current has decayed. In that circuit the base is always biased, so no diode is required

[Radio Wrangler]

It relies on C51 to hold the base.

If C51 fails open, R53 will be a large value to make the time constant easy with a moderate value for C51, so now the transient can give a surge in collector-emitter voltage on the transistor.

So, good housekeeping, a clamp stops a failed capacitor taking out a transistor.

David

[myredbook]

Hey guys, thats it working! Thanks so much for your help. So, the zener X51 had gone open circuit and the cap was acting pretty weird. Transistor seems to have been unharmed in the process but I may do as you suggested regarding the diode on the coil. What would you consider a modest value? And it would go cathode between the transistor and the coil and anode to GND?

Btw, the relay seemed to kick in around 40vDC and take around 5 seconds or so for anyone else's future reference.

Cheers again

Ben

[Radio Wrangler]

A one amp diode and 200v would be comfortable BUT don't assume 1N400x - that family is very slow. The higher voltage ones being the slowest. They aren't fast enough to catch spikes and things that go twang.

The diode goes across the relay coil, oriented so that the diode does not conduct when the relay coil is powered.

David