Dead Ultrasonic cleaner.

[MotorBikeLes]

I have a 'bike carb that needs cleaning, so I dug out my ancient ultrasonic cleaner. Plugged in, switched on, and the internal fuse, a 5A glass type, blew. I replaced it with a proper porcelain one, slow blow microwave type, but that blew instantly is well. I dug out the circuit, to see if there was any "other" possible fault, since the obvious "disaster" was a faulty mains transformer. As you can see from the attached circuit, there is nothing else.
This machine is rated at 600VA, so obviously a big transformer with a very low resistance primary. It measures about 0.7 ohms.
I assume the transformer is shorted, but my previous experience transformers (older 'scopes mainly) is that it either blows the fuse after a few seconds, or heats up the transformer over a longer period before either cooking or (probably) catching fire. No sign of any other faulty item (e.g,, the switch).
Quite obviously it is not worth getting the transformer rewound, and i don't image anybody would have one, so it looks like a skip job. Comments welcome.
Les.

[HamishBoxer]

Is the fan short circuit?

[TonyDuell]

What about a short on the secondary side of the transformer, such as C1 being shorted or breaking down. That would blow the mains fuse I think.

[G6Tanuki]

Disconnect the 'jumper' in the secondary circuit and see what happens when you power it up.

[Ed_Dinning]

Hi Les, these designs were taken over by Kerry Ultrasonics many years ago and were pretty robust. The mains transformers were normally good, but the output trans and caps were weak in the presence of the "RF".

Ed

[MotorBikeLes]

Fan S/C? - Probably unlikely, but I will disconnect it.
Secondary side short is where I expect more time before fuse blows.
Yes, I will lift the jumper.
I have spares for EVERYTHING except transformer. There were two of these plus three tanks. Two tanks "dead" when connected to power unit. The second unit had a different "build" mains transformer, but that WAS definitely S/C.
Ed, I contacted Kerry (I think) when I was after that capacitor you sent me. Nothing. I decided NOT to ask you about a rewind, it would probably cost £25 just to post one way, and it would not be a five minute and free copper wire job.
If all tests prove negative, I MAY buy something new, but then again, cost.
Thanks all, Les.

[Herald1360]

Sounds like a microwave oven transformer might do for the EHT with maybe a separate heater one?

[MotorBikeLes]

Herald1360, I have at least one MW transformer here, but maybe almost one quarter of the weight of this one. Not sure it would have the power. This one is noted at 2KV and 200mA, which in round numbers is 400W, am I right in thinking the MWs are only around 100W. Not sure what voltage the heater of the TY-125 sits at, but does not look too high. Perhaps others would comment. Is not the construction of MW transformers a bit "odd"?Not that I know much about transformer design or construction. Ed?
Les.

Your average microwave oven is in the 600 to 900W range.

[Silicon]

Some Microwave transformers have one end of the HV winding riveted to the laminations.

[Ed_Dinning]

Hi Les, check C3 for shorts and values (it acts as a tuning cap) and check the transducers are not S/C. Do not dismantle without trying to find out what the bolts are torqued to. It could be a damaged insulator on these bolts that is giving the problem.
Run the transformer unloaded and see if it blows fuses. They were pretty rugged and seldom fail, I might have one here but not certain.

This kit is pretty old for Kerry's now as they have been bought over a few times since the Mullard days; probably why you had no luck.

Ed

[MotorBikeLes]

Not touched it today, suffering a very rare cold and don't feel like going out to the shed.
MW transformers. I have one upstairs, so I just "measured it up". Whilst nothing like a heavy as the Mullard unit, it is pretty hefty.Three windings. Mains (I think) with resistance of 1.8 ohms, not too dissimilar.
A well insulated small winding, only a few turns, could be removed I suspect. Resistance measures same as probes.so very low.
HV winding, Yes, one end earthed. Resistance 101 ohms, BUT SO IS the Mullard!
E and I laminations, with a weld run at the junction of them (All the same, EEEE and IIII, not EIEIEI) Two more runs where the junctions would be if EIEIEI construction.
I don't know what the output voltage is, and obviously would need care measuring (the only death I ever heard of in white and brown goods servicing was a very competent engineer who managed to zap himself working on a MW).

Is there a problem with MW transformer construction? Ed?
Ed, if I confirm the transformer is faulty, it would obviously be uneconomic to consider rewinding it, but if you do find one, I have pal who visits his family just up the road from NoT, who would surely collect it for me.
Perhaps tomorrow, but I have agreed to go and look at an old, big compressor which won't start, star / delta 3 phase job. Perhaps I should cry off from that.
Les.

[Maarten]

The MW transformer is magnetically shunted to limit the current through the tube so it would be less efficient than a normally configured tansformer but it should work allright. Most are rated around 2.1kV 0.5A and can operate continuously for at least 30 minutes with a bit of air flow around them. One secundary leg is usually grounded but so is the original.

The well insulated winding is for the heater. Since a magnetron tube is directly heated, the heater supply is sitting at full cathode voltage (4kV since a doubler is usually used).

[Techman]

I may be proved to be completely wrong this time, but I think it's highly unlikely that the transformer is at fault. I can tell you for a fact that with a big transformer like that one, that a short in the circuitry on the secondary side of the transformer can blow the primary fuse into oblivion instantaneously. If you remember my little story from a few months ago where I stupidly forgot to remove a shorting link from the HT line on a big RF linear amplifier. When I switched it on, the fuse blew with such violence that the arc produced, carbonised the fuse and set the fuse holder on fire. So as always, just disconnect everything from the transformer and power up just the primary and see if it bangs the fuse again. If you don't want to bang anymore fuses, then just power it up through a Variac, with an AVO on its highest AC Amps range in series and start from nothing and slowly increase while watching what's going on with the meter - I expect you know all this already anyway.

[Refugee]

I tried that on my Variac with a transformer that I later discovered had windings that were burned to charcoal.
The moving iron meter on the variac waved goodby to the transformer and no fuse holder got set on fire.

[Herald1360]

If the filament winding on the MW TX can be removed easily, it could also be rewound to give the filament voltage for the TY-125-..... 6.3V?

[MotorBikeLes]

My cold seems to get worse, and I need a bit of cheering up, which you seem to be managing.
Techman, I hope you are right. I will check everything once I feel like getting out there to play.
Herald, I did wonder about that. I can see why it is thickly insulated if heater at xKV. I just need to be careful near to that "Lucar" spade at HV. I am sure I could "un-thread it" and replace with a few turns of enamelled copper, or maybe some 20awg PTFE insulated wire I use for ignition pickups on the bike.
Maarten, I knew there was "something" about that transformer construction. The welds "calm it down" but generate extra heat?
Looking at my photo, would it make sense to grind away the welds that simple run across the laminations, and maybe partially grind away where they fix the E-I lam junctions?
Les.

[Ed_Dinning]

Hi Les, TIG welds on MW transformers are a mass production technique. They are also sometimes used on "yellow pot" transformers.
They do help with lamination buzz and also slightly increase the iron loss, but make the transformer cheaper to produce.
Most MW transformers have pri and sec side by side. This is good for creepage and clearance but makes the transformer regulation much worse.

The welds can easily be ground off to re-use lams and bobbin.

Ed

[MotorBikeLes]

Ed, the question is, would you grind off as much weld as practicable, without it falling to bits, or leave "as is"?
Stayed out of shed again today, but I did go upstairs and connected up the MW transformer. Measured the HV at 2050v using my Selectest Super 50. I also checked the heater winding which showed 3.0v off load. The windings are all side by side, mains bottom, small gap, then heater and finally the HV. The earthy end of the HV winding is accessible, but tied to the earthed lams.
If I establish the Mullard one is shorted, this looks as if it will replace it quite easily.
Les.

[trsomian]

The only microwave oven repair I have done was to my sister's when it blew its fuse instantly on switch on. It turned out to be the HV capacitor gone low resistance. changed that and it worked and I believe it is still working. So, yes a short circuit on the output of the transformer will blow the fuse quickly