Thandar Function Gen Cutout Mains transformer

[llama]

Actually it's an RS 610-635 with the symptom of no power.

After taking much longer than I should have I've deduced that the cutout that seems to be thermally bonded to the mains transformer is open circuit.

Do these devices just fail of their own accord? Shouldn't it have reset itself when it cooled down? What should I replace it with, please?

Does anyone know what the DC load current should be so I can meter it before calling it "fixed"?

Thanks
Graham

[McMurdo]

they're usually a one-way trip for those thermal fuses. Yes they can fail with age and yes they can fail with a fault downstream.

Once you're in a position to try a new fuse it might me possible to fit another in there. They're often not buried too far inside the winding and often on the surface under some poly tape or similar.

RS and farnell do a range of transformers with built in thermal fuses nowadays if you'd rather go that route. You can try shorting it out for test purposes to make sure the rest of the instrument is ok. Something as subtle as an open circuit reservoir capacitor can cause these to run too warm, you'll have to use your intuition!

[llama]

Thanks Kevin, that's great. Yes I can see the device at the surface.

I should have mentioned that the split secondary winding measures 3.8 Ohms one half and 4.4 Ohms the other half. That sounds a bit too much difference to be accounted for by one being wound over the other so I wonder is there are shorted turns in the 3.8 Ohm winding.

Graham

[Nickthedentist]

You could try temporarily shorting-out the thermal fuse, and running it under close supervision via a lamp limiter, to see what happens.

[McMurdo]

it would be unusual for a secondary to burn out on a low voltage transformer, I'd be more suspicious of the primary if it boils down to faulty windings. But let's not put the cart before the horse, see if you can find the ends of the old fuse without tearing into the winding first!

[llama]

I have taken the secondary windings off their respective rectifiers and run the input off load like this. The windings both measure 17.5 Volts so I guess the resistance difference is just the different winding diameters.

All 4 rectifiers are 1N4002 and they measure OK.

Having reconnected the secondaries to their rectifiers and powered the whole unit via the lamp limiter (thanks Nick) I note the transformer now gets hot. After allowing it to cool down a quick measure of the input to the rectifiers gives both a reading of 12 Volts.

The fact that both rails show 12V input suggests to me that the fault is something loading both rather than, for instance, a leaky reservoir or smoother since they'd be unlikely to fail equally.

It looks a right pain to extract the PCB from its case but it's getting close to inevitable...
Any tips to avoid that happily received!
Graham

[llama]

I've just run the transformer alone, via the lamp limiter, into a 110 Ohm resistor on each of the secondaries in turn. 16.5 Volts was developed across the resistor which of course got rather hot. The transformer, however, remained cool.

So I guess the problem is downstream.

Kevin, could you describe the mechanism whereby an open circuit (or low capacitance?) reservoir capacitor can cause overheating, please? I could possibly tack another in parallel, topside, as a cure.
Graham

[MrBungle]

This is a Thandar TG102 from what I can see. I've got one - very nice when they work. Complete pains when they don't.

Firstly Google "Thandar TG102 service manual" if you haven't already. Check schematic on the last page - the PSU is at the bottom. Check the following things in order:

* Check all electrolytics for short circuit.
* Check all transistors for shorts and open circuits (standard diode test)
* Check all resistors aren't open.
* Check the two reference zeners on the front end for shorts.

If you dare, power it up and stick your fingers on the BD135/BD136 that are the pass regulators for the rails. That should indicate which rail is being pulled hard because it will be hotter than the other one. Follow this around and check everything that touches it carefully.

Usually when diagnosing this stuff, manufacturers like to stick 0.1 ohm resistors in the lines of each circuit block so you can measure current via voltage drop and isolate this to a particular circuit block but Thandar never bothered annoyingly.

I reckon it's a blown up electrolytic or transistor somewhere. Possibly on the output amp as that's the most fragile thing closest to the end user.

[llama]

Well that's fantastic inside information Mr Bungle - thanks very much. There seems to be some variation in build version of these but I can work my way round that. The first thing I pick up from your post is that I need to extract the PCB so now starting to do that.
Cheers
Graham

[MrBungle]

You will indeed have to extract it - they aren't terribly difficult to get out fortunately.

Mine matches the service manual exactly so I'm interested to know if you do find any differences as I was going to grab another one as a spare and like to be prepared.

[llama]

Yes, the PCB came out easily once I realised the front panel came out with it. Haven't spotted any differences in the few areas I've checked.

Out of interest, a previous owner has disconnected the front panel mains switch and put a little 6mm (fixing hole) toggle switch on the back. Don't know if this is a typical failure for the TG102 or maybe a squirt of switch cleaner would've done the job. I shalln't change it back.

As reported in my thread about tants, I've found one of the output amp supply decouplers to be completely short circuit. I plan to pop a modern normal electrolytic in its place and hope this completes the repair.
Graham

[llama]

That's it working now. Powered via the lamp limiter it still worked fully. No heating of the mains transformer and no real difference when the series lamp was shorted out.

However - the thermal fuse is still duff and the service manual states it must be replaced with the correct component. The manual I found on line doesn't permit searching; the only reference in there is the first one that mentions it.

So now to do a Storm search for a suitable replacement.
Graham

[llama]

I see Maplins do a range of thermal fuses with different temperature settings. They say to not solder it in so I could use a tiny choc-block.
Can anyone suggest what temperature I should select, please? I'll probably get one in Colchester or Norwich Maplins next week.

Alternatively, are Thandar and I being over-cautious here and perhaps a 100mA fuse would be just as effective?

Graham

[McMurdo]

The ones I've seen range from 100-120 deg C.
If you're quick with your iron you'll be ok, maybe use a croc clip as a little heatsink barrier. After all, the ones fitted from new are usually twisted and soldered.