Storage rad PCB

Hi All,
We have had a couple of storage rads in here since I installed them yonks ago and they actually do a pretty good job. A while ago both started 'clonking' as the power relay dropped in and out (maybe 10 times at 1-2 second intervals) till now one won't fire up at all.
I've taken the PCB out and it looks like there is a dry joint on the main relay leg:
https://dl.dropbox.com/u/5772409/Dry%20joint.jpg
https://dl.dropbox.com/u/5772409/Relay.JPG
Now, when you operate the relay manually there is a very slight 'loading' of the spring as the relay pole closes so I believe there is some contact pressure but there was a bit of blackening around the contact and inside the cover by that brushed off fairly easily.
So, the question is, was there a dry joint that has become worse with the action of the relay or has the joint opened because of an h/r relay contact heated it up?
It looks like I can get new relays but could the relay not pulling in cause the clonking? There doesn't appear to be any feedback between the relay output and the PCB (so it wouldn't know if it had actually pulled in or not).
I understand the system to work like this:
The E7 switches in at say midnight and the board determines from an external probe, what the ambient temperature of the room is and therefore if the heater needs to power up or not. If not then it holds off until such time as it determines it needs to be able to take in enough heat to be able to last for the next day (that's the idea anyway, not sure if it's that scientific in practice. In practice however it does seem to work, ranging from not coming on at all in the summer to getting very hot in the winter).
So, if all else was to fail I could simply bypass the PCB but then it would come on all year round and to whatever I set the input (thermostat) and output (bi-metallic flap) controls to.
I've yet to contact the suppliers of the heater (Unidare) or the PCB (Diamond H Controls Ltd) as the components on the board suggest 1988- so they are all probably well obsolete by now?
https://dl.dropbox.com/u/5772409/PCB.jpg
Cheers, T i m
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On Tue, 02 Apr 2013 20:45:15 +0100, T i m wrote:

Combination possibly, though it looks like the has been some arcing at the joint, hence the "crater". I'd fuly reflow the joint and drag some paper through the closed contacts a few times to clean 'em up, unless you happen to have a proper contact file.
If it's taken 20 odd years to fail with only minor soot in the cover a simple repair and clean as above will probably get another 20 years out of 'em.
--
Cheers
Dave.
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On Tue, 02 Apr 2013 21:35:38 +0000 (GMT), "Dave Liquorice"

Ok.

I do but of course can't find it right now. So, I've gently run the back of a scalpel over the contacts and finished with the paper to clean.

Let's hope. ;-)
I can see how this particular fault would stop the fire working but I don't think it explains the relay pulling in an out as it starts up. I think the IC MC14541 might control the relay with the others (LM339, LM329, comparators?) handling the external sensor (and possibly one on the PCB to give the 'internal' temp?).
https://dl.dropbox.com/u/5772409/PCB.jpg
I was wondering if the electrolytics caps might have 'dried out' in the heat, and not providing sufficient current to allow the relay to pull in first time? <shrug>
The caps look ok and don't seem to be of a make that suffered the electrolyte issue so do they lose some capacity over time (especially ~25 years in a fairly warm environment)?
Luckily, the whole board is covered in discrete components so with wouldn't take long or much money to sawp out the main suspects (electrolytic caps) first?
Cheers, T i m
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On Wednesday, April 3, 2013 1:17:04 AM UTC+1, T i m wrote:

If the relay clunks, its closing. I'd definitely clean the contacts.
Lytics do lose capacity sometimes. No need to replace them to test, just clip some on to give added capacity and see if it changes anything.
NT
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On Tue, 2 Apr 2013 18:45:40 -0700 (PDT), snipped-for-privacy@care2.com wrote:

Sure, it's closing (and opening) and closing ... It's like a slightly irregular (asymmetric) clock ticking.

Done.

Ok.

Not quite so easy (/safe) given it's a mains powered board and in a difficult to access location. I could run it on the bench I guess but would then possibly need to attach a real load (electric kettle / bar heater?) but at least I might easier be able to see what is going on *if* it does start clonking. I haven't been near the cleaned up system to know if it is still doing it and I'm not sure (because of how it works) it would even start to do it if you just interrupted the power during the E7 period.
Still, it's a good thought though and something I might rig up should I find out it's still doing the clonking etc (it was doing it on both the heaters and wasn't for a good few years).
Cheers, T i m
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On Tue, 02 Apr 2013 21:35:38 +0000, Dave Liquorice wrote:

The good news is that re-soldering the bad joint seems to have fixed the 'fault' and the heater was nice and hot this morning. ;-)
However, I wasn't there when it first kicked in so I don't know if it did the clunk-donk ... clunk-donk <repeat what seems like loads of times when you are trying to get to sleep> thing.
I mentioned previously that there was no positive feedback on the relay state but I was wondering if there was any way the board (compatriots?) could be used to sense a voltage drop / pulse if / when the heater pulled in?
eg, If the board told the relay to activate but didn't sense such a pulse, to release the relay and re-energise a second later. I'm thinking that if the relay contact was bad (or one of the pins were intermittent) that it might explain the clunking thing?
Just thinking out loud etc. ;-)
Cheers, T i m
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