Sencore Mighty Mite VII / TC162 Valve Tester runs hot.

[tri-comp]

I recently won an eBay auction for a Sencore TC162 valve tester to complement my B&K 667.
The are actually very similar, partly transistorized, but the B&K lacks sockets for testing Loctals and D-series battery valves (DAF92, DL91 etc. filament pin-out different from 'regular' 7-pin valves).
The tester arrived in a fair to good condition.
Mostly needing a good scrub down and replacement of out-of-spec's resistors.
Nothing to it but when I finally applied power (117VAC/50Hz) the power transformer made a seriously loud hum.
Output voltages are fine but obviously the transformer hasn't been wound to perform on 50Hz.
So, after a while the core and windings turns hot, too hot for comfort, and I'll have to switch off to cool down.
I suppose I could live with that, as I seldom have more than a handfull of valves to test in one go.
However, if someone knows a way around this problem, short of replacing the transformer, I'd be happy to know what to do.

rgds

[Ed_Dinning]

Hi, it sounds like a poorly designed transformer with not enough lams for operation at 50Hz.
Simplest solution would be to rewind on a lam stack with a 20% greater core area but the same turns (you could also increase primary turns for 230 aft the same time, reducing pri wire dia for the now lower current).

Ed

[M0SOE_Bruce]

The manual states that it should run from 50Hz or 60Hz. Ed may be correct in that the actual xfmr is too light. Unlike the older sets with valves inside, therefore needing more current, the TC162 only has one transistor. Check primary voltage and current as power draw is only 32 watts.

Here's a website that gives info on repair of this unit.
http://tubesound.com/2008/08/12/repa...e-mighty-mite/

[Boater Sam]

What is " after a while runs hot" How long? most valve testers are for short cycle use 'cept the monsters. Is this when actually powering a valve or just sitting turned on?
Check just how much current is being drawn on the secondaries, that will give us a clue.
The loud hum from the transformer could simply be loose lams. I am assuming that it is an I & E laminations, and not a toroid?

[tri-comp]

After a while' is running idle for about 5 min.'s w/o mounting any valve for testing.
The AC filament voltage supplied to the valve-sockets seems to be in range.
I'll have to do closer inspection but as far as I remember 12,8VAC idle is supplied when set for 12V.
The DC-voltages for the FET-circuit is fine and the tester IS working.
It needs to be calibrated and shows a little on the low side for emission but that's a minor problem.
I do see that the spec's says 50/60 Hz but I think it's not happening with this transformer.
Hum caused by loose lams can usually be influenced by 'squeezing' the transformer but not so here.
Loose lams will also not cause excessive heating of the core/windings.

Ed, is it at all within reason to have a new transformer wound ?
BTW, the one you did for a CNC-controller circuit is still running very fine

[Boater Sam]

If you have conviction that the transformer is defective in some way, then please go ahead and get Ed to do a new one, it will be excellent of course.

I was just trying to establish if there was a reason for the apparent over heating before you had to take this step, terribly sorry if my interest was not required.

[tri-comp]

Originally Posted by Boater Sam

I was just trying to establish if there was a reason for the apparent over heating before you had to take this step, terribly sorry if my interest was not required.

You input is ALWAYS welcomed

[tri-comp]

Here's a thought.
I need to find a supply of around 117VAC/60Hz for the tester.
How will I find that living in Europe?
Obviously not from the wall outlets.
I have a prefab. mono audioamp on a small PCB with TDA7293 and on-board power-supply rectifier and 2200uF/50V cap's.
All it needs is a dual AC supply voltage from 2x10VAC to 2x40VAC AND a 60Hz signal input to supply lots of output Watts at 60Hz.
A small 240VAC 30~50Watt supply toroid should fit inside the case.
Another small 30~50Watt toroid in reverse with a split 230V/115V primary and suitable secondary voltage should work as output transformer for the amplifier.

Did anyone do this in the past ?
It won't cost a lot to try it out so unless someone says no-go I suppose I'll give it a try.

[Ed_Dinning]

Hi, the audio amp and transformer is not a bad idea, but you will need to monitor the waveform with a scope as the frequency response does not always go down to 60Hzon all amps.
Drop me an email with a pic of the transformer and I'll see what is involved.
Glad to hear the other transformer is going well

Ed

[tri-comp]

Pictures for Ed and all.
Measurements are obviously outher and not exact core-size.

[tri-comp]

I did some more checking.
A forum-member send me a PM suggesting the buzz is caused by bad quality sealant and may be cured by generously applying super-glue to the hot windings after removing the top-sealant.
In his own experience it should do the trick with exactly this transformer.

However, before getting overly hot I had a chance to check AC voltages around the transformer.
It has it's supply from a 240:120VAC toroid of sufficient wattage. I don't exactly remember but I think around 60-70Watt.
Input voltage is then 118VAC/50Hz.
Output voltages, both for test-elements and for filaments are very close to spec's.
As an example the 12,6V filament output rides at 13V, idle.
Then I hooked the set up to a vario-transformer and all became clear !
My vario has a built-in amp-meter and it clearly shows the disaster.
At 118VAC input the small transformer draws 3/4 amp., idle !!
I have to lover the input to around 90VAC for the idle consumption to be around 0,1Amp which is still too much.
Raising the input to a mere 100VAC forces the idle consumption to almost 300mA or 30Watt !
Just to be 100% sure I disconnected the 'low' end of both secondary windings and it doesn't make any difference to the idle consumption.
Imho no doubt this shows a saturated core and the transformer was never build for 50Hz despite advertizements.
Solutions:
1) Ed does a new transformer
2) The audio-amp solution described above
3) You think of something else

[Ed_Dinning]

Hi, it could be shorted turns on the transformer, if so it will get worse.
It could also be a transformer that is not laminated correctly and has air gaps in the core, difficult to see from the pics and difficult to correct if transformer is varnished

Ed

[Boater Sam]

I have this problem in reverse, 50Hz record player on 60Hz mains, the voltage is the same.
Best I could come up with was a 50Hz inverter running of a car battery.

I would not of thought running on 50 rather than 60Hz would make a massive difference, I have run loads of US gear in the UK without any such problem, seems there is a fault in the transformer as well.

[tri-comp]

I think just for the fun of it and to see if it's doable, I'll whip together the Audio-amp solution over the week-end.
If it runs it'll be possible to decide for sure if the transformer is defective (shorted turns, air-gap) or not.
By far the most elegant solution will of course be to ask Ed to calculate and wind a new 50Hz transformer.
...I'll be back.

And to Boater Sam:
I do think it makes a lot of difference if you feed a transformer 50 or 60Hz.
I've encountered this problem before with US equipment.
Some runs others don't.

[ms660]

I also have encountered problems with transformers designed for 60Hz running on 50Hz and getting a bit too hot, one failed and fried, the other survived but I never let it run for more than half an hour or so.

Same problem with some motors apparently, they have to be de-rated voltage wise.

Lawrence.

[Boater Sam]

I'm surprised that it makes a big difference. I used to import American ovens and waste disposers, all 60Hz of course. The disposers were 1/2 HP motors, 1700 RPM on 60 Hz but 1440 on 50Hz, they worked fine but of course they are short duty cycle.
The ovens had clocks that we had to re-motor obviously but the rotisserie motors worked OK on 50Hz.
Perhaps the situation is worse now we have Hertz, it was OK on cycles per second.

[tri-comp]

Originally Posted by Boater Sam

I'm surprised that it makes a big difference.
Perhaps the situation is worse now we have Hertz, it was OK on cycles per second.

I'll have to say Yes.
The situation is much worse with Hertz !
Anyway, I whipped together the Audio-amp solution with a TDA7293 chip.
I already have a pre-assembled mono-amp that arrived in the mail from China a couple of months ago.
It was ordered to be an electronic thermostat regulator for keeping my wifes outdoor tree-furn safe from January-frosts.
So cheap I didn't even have the heart to complain thinking it may come in handy some day.
That day was today !
It performs admirable as a 60Hz amplifier into a 30Watt toroid with a 18V primary and a 110V secondary.
DC Supply voltage about +/- 26V, 52V in all
I put the chip on a small Alu heat-sink, not nearly enough to sustain prolonged use but for a few minutes it was fine.
And now the conclusion....
The Sencore transformer is running absolutely fine on feeding it 117VAC/60Hz.
It was NEVER dimensioned for 50Hz and will never work with that low frequency input.
Ed, what's a new transformer comming to ?

Btw,
The tree-furn's doing well.
Conrad Electronic supplied me with a good digital read-out thermostat ....at a price !!!

[Ed_Dinning]

Hi, still away I'm afraid, I'll try and get some ideas to you shortly. Pm me a reminder in about a week

Ed

[Boater Sam]

I've learnt something today, again!

[tri-comp]

Originally Posted by Ed_Dinning

Pm me a reminder in about a week

Certainly, no rush.

Originally Posted by Boater Sam

I've learnt something today, again!

Isn't that what life's about ?