Goblin Time Spot Clock Radio S25

[ViragoRider]

First power up of the clock radio was disappointing. Not much happening on the radio side, but that can be fixed. More of a worry is the clock which doesn't work either. I've stripped it down and it appears the stator coil is open. I hope the break is near the start and not deep inside. I'll know more once I've got into it. Worst case scenario I'll be looking for a replacement or a rewind.

[Ed_Dinning]

Common fault I'm afraid, but easily rewound (if tedious)

Ed

[Viscount]

Look out for a D25 Goblin Teasmade, it's the same coil.

[Mike Phelan]

Rewind it - D25 Teasmade has considerable value.

[ViragoRider]

Looks like a rewind. The binding tape was so brittle, getting it off proved a real challenge and took the wire with it, so I now have lots of frayed ends. Rewinding the coil is beyond my capabilities, so I'll look for someone in the services offered section that would do it for a fee.

[ViragoRider]

The clock coil has been rewound thanks to Ed Dinning and is on its way back to me. I'm looking forward to reassembling the clock and seeing it working again.

I've moved onto the radio and am stuck. It looks like I have a short somewhere and I think the transformer might be where it is but it's confusing me. I've measured the AC voltages coming out of the transformer and they seem fine. The 5v rectifier heater winding is measuring around 4.9v, the 6.3v heater winding is measuring ~6.1v and the centre tapped secondary is measuring 278v per side of the centre tap where the transformer states it is 280v.

All good I'm thinking, but with no valves plugged in, my dim bulb is glowing brighter than I would like as shown in the attached photo - it's a 42w bulb for reference.

I purchased a NOS 5Z4 rectifier as I wasn't happy with the rectifier that came with the radio and wanted to have the correct rectifier according to the schematic. If I plug that in, with no other valves present, when the rectifier comes into emission, my dim bulb glows very bright and the B+ is at around 40v. In my troubleshooting efforts I decided to disconnect all of the circuitry and just have the power supply in an attempt to eliminate any shorts down the line, but the problem persists. I've included the schematic of the power section for reference. C4 and C3 have been replaced and they are across the field coil speaker which is being used as a choke I believe. Everything after C3 has been disconnected For reference, I have replaced C4 with a 22uF 350v and C3 with a 10uf 400v cap.

I've never dealt with a field coil speaker so I'm not sure what to expect in terms of coil resistances, but the coil measures 1150 ohms when measuring across the solder connections and there appears to be a connection to the speaker basket - one of the terminals to the basket measures 1150 ohms and the other terminal to basket resistance is around 500 ohms. I'm suspicious about this but as I have no experience of field coil speakers I would welcome comments on whether this is normal or not.

That having been said, if I disconnect the choke I still get no B+ and a bright glow from my dim bulb, so I think the fault lies elsewhere.

As an aside, I believe the schematic is wrong. According to the data sheet on the 5Z4, the cathode is taken from pin 8, but the schematic has the B+ line coming from pin 2. The radio has the B+ tap from pin 8 as you can see in photo 3. You'll also notice that the rectifier base is broken as someone at one time has forced a valve into the socket in the wrong orientation. I have a replacement for that on order and will change it.

My conclusion at the moment is a fried transformer. Photo 4 shows the state of it when I pulled it out of the cabinet. It looks like all the wax has melted out if it when it got very hot and there is obvious signs of charring in the windings.

I'd welcome any comments on what could be the issue here as I'm still a novice but trying to improve my knowledge.

Pat

[kalee20]

The snippet of circuit diagram shows a 0.01uF capacitor across the mains.

Keep your fingers crossed, snip it out, and again try powering-up via the lamp (all valves unplugged). Hopefully the lamp will stay out this time.

It's not a good idea to have just the rectifier valve in - the HT rail will be unloaded and the voltage could rise to a rather high figure. Better to get 100V or so from somewhere else, and put it across the HT reservoir capacitor, to see if it is comfortable. Certainly, the lamp coming on pretty bright as the 5Z4 warms up isn't a good sign - at a lower voltage like 100V things will happen less dramatically and you can see if anything is getting warm, rather than watching where bits fly out from!

[Ed_Dinning]

Hi Pat, as Kalee says, the suppression cap is a prime suspect.
You could try (after removing the cap) powering the set with all valves and pilot lamps removed. With the lamp limiter still in circuit is it still very bright? It will depend on the bulb wattage of course.
Leave running like this for an hour or so, but monitor. After this time the transformer should be only mildly warm. Repeat without the limiter and it should be a little warmer.
If getting hot you may have shorted turns on one of the windings, probably the primary
Looking at the wax that has exuded this is a possibility, but not a huge amount has come out, so not definite

Ed

[kalee20]

I'm certainly concerned about the wax that's come out - and the limiting lamp glowing very bright as the 5Z4 warms up.

But it could be there's something dodgy on the HT line, a duff smoothing capacitor maybe - that's overloaded the transformer in the past and melted its wax. Hopefully the transformer is OK.

You need to be able to leave it powered with the valves all out (so no load on it) for an hour or so (supervised!) and with your 40W lamp having at most a very dim glow, so cut out the capacitor and see if it makes a difference. Hopefully it's the culprit! Then, once satisfied, it's move on to the HT-line fault. It certainly seems as though there's two faults.

If as Ed says, the transformer gets hot enough to drip wax, then it's curtains, and another rewind job!

(Incidentally Ed, what's the turns count and the wire gauge for the clock coil? I have a Time Spot which may need its clock rewound!)

[ViragoRider]

Here is a better snippet of the schematic. There are two caps across the line and I removed both of them before testing. The bulb still glows like in the photo. I removed the restriction and ran the transformer for over an hour. It felt only mildly warm even after an hour or more.

I retested the voltages with the transformer unrestricted and they were higher than I would like. The 5v heater line came out at 5.5v the 6.3v heater line was 6.8v and the 280v line was 305v. Could this be indicative if a short in the primary winding as all the voltages were up by a similar factor?

The voltage selector has 3 positions but they aren't marked. I tested the resistance of the primary in all three positions and went with the highest resistance position assuming that would be the 240v winding. When I got the radio. the selector was in the middle position but connecting that winding leads to a higher voltage at the output.

Pat

[PJL]

Higher voltages offload is to be expected and is good news. Some current flow in the transformer when offload that generates heat from the primary resistance is also to be expected, not sure what your halogen lamp behaves like so unable to comment on brightness. I would suggest it is probably OK but just keep an eye on it until you are confident it is not overheating.

[kalee20]

If it's run for an hour without frying, it's probably OK. Maybe not very conservatively designed, and with a high magnetising current, but OK.

Swap the bulb for an AC ammeter and tell us what current it pulls, off-load.

The voltages being a bit high - when the transformer is loaded, they'll drop, and hopefully by the right amount!

But there's still the low HT, and the excessive current when the 5Z4 has warmed-up, to investigate next, before worrying about a 10% voltage error.

Looking forward to the next instalment!

[Ed_Dinning]

Hi Peter, most clock coils come out at about 10 to 12K of .063 wire and work quite well at that level. Even the ones wound with 0.043 wire and have higher turns counts work well with above figures and still only run warm.

Ed

[ViragoRider]

It's the weekend so time to do a bit more to the Time Spot. I replaced the dodgy valve base and it looks a lot better now, with no change of plugging the valve in the wrong orientation now.

I hooked an ammeter in and the unit is pulling 91mA with no valves plugged in. No wonder my 42w bulb glows as just the transformer is pulling 20 odd watts if my maths is correct.

As a test, I upped the bulb to 100w just to see what would happen with all the valves in place. I'm still getting a bright glow when the rectifier comes into emission so there's definitely a short somewhere. I've been tracing the circuit and am at a loss to find anything. One thing a have done today is to put a bit of tag strip in. The previous repairer left a lot of interconnected wires floating in mid air which I didn't much care for both from a safety aspect and aesthetically, so I've tied them all to the tag strip and the wiring looks a lot neater now - although the fault still exists.

[kalee20]

That sounds very wrong.

It might be worth pulling out the other valves, leave the rectifier in, and repeat.

Don't leave it on very long, whether the bulb brightens up or not. If it does, the short is probably in the HT reservoir or smoothing capacitor, and your rectifier valve is taking a hammering. If it doesn't, then one of the valves (almost certainly the output valve) is pulling far too much current when fitted, and a simple process of elimination will prove which. While power is on and your limiting bulb isn't dimming, there's almost no load on the HT rail so the voltage will be higher than normal, giving the capacitors a hammering. So leave on for long enough to know one way or the other, then switch off!

Originally Posted by Ed_Dinning

Hi Peter, most clock coils come out at about 10 to 12K of .063 wire and work quite well at that level. Even the ones wound with 0.043 wire and have higher turns counts work well with above figures and still only run warm.

Thanks Ed!

[PJL]

The mains transformer is an inductive load so power dissipated is not as simple as multiplying the applied volts by the measured AC current.

The set may not work with your 42W halogen lamp limiter. With only the rectifier valve plugged in, what DC voltage are you seeing on C4? Do you still have L18 (field coil) disconnected?

[ViragoRider]

I've tested the radio with just the rectifier valve in and the bulb glows very brightly was soon as the valve comes into emission. B+ starts to rise before the bulb lights up (as the rectifier is warming up) gets to about 90v then falls rapidly as the bulb brightness increases. I didn't leave the radio on for long so the last voltage reading I saw was 40v.

I replaced the filter caps with new ones and tested them, albeit with a low voltage tester before I fitted them. ESR on both of them was ~0.5ohm. The wiring is back together and the field coil speaker is connected.

I tested the radio with the full compliment of valves and get the same result with the bulb glowing brightly - I'm now using a 100w bulb. I removed each valve in turn to see if it was one of them that was causing the issue but got exactly the same result as with just the rectifier valve and no other valves fitted.

I can't see any obvious shorts in the wiring and I know it is not a fault in the rectifier as I get the same result when I plug in the original rectifier that came with the radio.

I'm stumped. The only things in the mix are:

Transformer
Rectifier valve
Smoothing caps
Choke (field coil of speaker)

My schematic has the B+ line coming off pin 2 of the rectifier, but on my radio it is actually coming off pin 8. Is the schematic wrong?

I've included some shots of the speaker for reference although there are no details marked on the label, someone might recognise the model.

[ms660]

Originally Posted by ViragoRider

My schematic has the B+ line coming off pin 2 of the rectifier, but on my radio it is actually coming off pin 8. Is the schematic wrong?

The info in the ERT service sheet that I'm looking at shows pin 8 of the rectifier as the B+ which is correct so far as I know.

Lawrence.

[ViragoRider]

I Checked and saw the Cathode was connected to pin 8 according to the data sheet. So it looks like the schematic I have is in error.

I've been checking things over and noticed that C4 has 360v across it and I only have a 350v rated electrolytic in there. I'm wondering if it is breaking down under load? I'm going to source a better rated cap to put in there to see if that helps. C3 has 260v across it and I've got a 400v rated cap in that position, so I'm good with that one. Just got to replace C4 with a higher voltage rating.

[kalee20]

I wouldn't worry about 360V on C4, rated at 350V... it's not good practice, but it should survive.

Electrolytic capacitors behave as a sort of soggy Zener diode, passing current above a certain voltage - which is not usually fatal unless there is significant self-heating.

But the fault: I can't see it being the mains transformer, because nothing really changes when the rectifier gets plugged in. It just gets loaded.

My money would be on the loudspeaker coil, unfortunately. You could try disconnecting it at the C3 end and see if the bulb brightens-up, hopefully it won't. But if it does, disconnect it then at the C4 end so that the coil is isolated, and see then.