What are my options? Transformer fault.

[PETERg0rsq]

I have a nice Hameg 204-2 oscilliscope I am hoping to repair.

Unfortunatly it looks like the HT winding on the transformer has failed.

I get an open circuit voltage of 150VAC and an in-circuit voltage of only 90VAC. It should be giving 491VAC in circuit.
The following doublers are working , as I get approx 350VAC HT to the tube, not the 1962VAC required.

What are the options to get the 500VAC needed to restore the HT?

I cant find any small step up transformers that I could use.

The transformer has 8 secondaries on it, so finding a replacement transformer is unlikely.

The scope doesnt owe me much, but i hate giving up without a fight.

Thanks

Peter

[MotorBikeLes]

Get a little (NOT big) mains supply 230v:230v (or other suitable secondary) transformer, and put an appropriate multiplier to get your desired final voltage.
Use the smallest capacitors which will drive the multiplier sections at mains frequency and you will get the required final voltage without it being too dangerous. Maybe somebody else will do the calculations for you, or just experiment.
I once recovered a TQ D43 with a multiplier of about 8 stages. It worked OK.
Les.

[Richard]

How much room have you got?
What current do you need?

Richard

[PETERg0rsq]

I was looking for a small 240-240 transformer, but all are quite large (100VA ish).

I was thinking also of using two 240-240 transformers, and putting the secondaries in series to give 500V, but cant find any physically small 240-240 transformers.

In answer to richard, not a lot, and not a lot There is space for very small transformers, but not much.

The voltage is for a CRT tube in a scope, so only a few mA, possibly 10 or 20 would be my guess. Dont know how I could determin exactly what the load would be from the schematic. Only resistors to ground are about 2Meg. No other DC path to ground via transistors that I can see.

I guess a 10VA transformer would be OK, but cant find anything anywhere near.

If I could find a very small 240-240 transformer I could add an extra stage to the voltage multipliers I guess.

I will put the two relevent parts of the schematic here in case someone can help identify the current requirement of the circuit.

Peter

G0RSQ

[WME_bill]

Replacement transformer for Hameg 204. The current for the EHT will be 1ma or less.
I had to replace the transformer for a Comark Insulation tester. The original used a RadioSpares transistor audio output transformer, driven by a simple 5khz oscillator. My replacement was actually identical to that originally fitted. The problem of course is the quality of the insulation. Input 12v to 215v, quadrupled to 860v pk. Circuit attached. Will need a bit of experimenting to settle the diode multipier stages.
Otherwise, have you asked Ed Dinning. He rewound the transformer for a UHF Frequency counter with Scope display for me. wme_bill

[radiotechnician]

How did the HT winding fail? If is open circuited, well and good
but if it has shorted, the primary will over heat. Adding another
transformer (s) may additionally cause magnetic beam hum
especially if located close to base of the CRT.

You might consider sourcing the beam transformer from an
outboard unit connected with plugs.

If the transformer failed, there might not also be much life
left in the CRT cathode.

[dave cox]

What about a DC-DC convertor ? I've attached a link to a 1000V design from Jim Williams using AD parts.

http://www.youtube.com/watch?v=e9RX_UPownc

dc

[PETERg0rsq]

Thanks for the information.

The winding has failed open circuit (how i got any measurement from it is odd!)

It is wound on the top of the transformer, so a re-wind might be possible, but it looks like thousands of turns on there, so might not be very practical (this is just an old cheap scope). Also, as mentioned, I dont know if the tube is OK. I need the HT to test! All other voltages on the tube are within spec, and the deflections seem to be good)

I like the idea of a DC to DC converter, as it gives me something new to look into and hopefully learn. Will do a bit of research and see what i can dig up. I like a challenge!

That link was interesting, but didnt really tell me much, other than this is possible.

Thanks again

Peter

[dave cox]

The app note gives the detail :
(it would be easier to find if I gave the RIGHT manufacturer)

http://cds.linear.com/docs/en/applic...te/an118fa.pdf

[glowinganode]

Hi Peter, I have a small transformer that may be of use to you.

Pri; 17.5 V @ 30 mA

Sec; 220 - 0 - 220 V

Not sure what it's from, approx. 1 1/4" cube. Should give you a mA or so to play with, sure you'll have a suitable winding on the main tx to power it.

Cheers, PM me if it's of use.
Rob.

[dave cox]

Or, maybe a ready made option like the CCFL driver TDK CXA-0256 for 750V ac (plus tripler) ?

[Silicon]

Many valve radio mains transformers have 250v-0-250v windings for the HT.
If you ignore the centre tap you have a 500v AC winding.
You will also have windings for the CRT heater.

[PETERg0rsq]

Thanks Dave, and Silicon

Nice find about the CFL PSU-s. I have been replacing some industrial screens at work to LED, and just scrapped some inverters, so well worth investigating. Hope they are not on the skip already!

I have been looking around for some 250-250 secondary transformers, but they are quite large and expensive new.

However I will now have a careful look around rallies to see what I can find from old sets.

Thanks, some great ideas, and plenty of leads to follow.

Peter

[newlite4]

Peter, I have some spare Hameg transformers here. I have a BV031/0061 (6.3v/31v/486v/12.4v/145v/15v/15v) from a HM203-7 and a BV004/0006-2 (outputs unknown) from a HM1005. The former has flying lead-outs, the latter has an 11 way connector. Either of these of any use to you?
Neil

[PETERg0rsq]

Neil

Thanks for the offer, but it looks like there are several voltages missing from the transformer you have, including 140V and 80V, so I probably cant use it.

I do have now a transformer with a 250-0-250 winding on it, so hoping this will generate my missing 491V.

Unfortunatly been tied up with some long outstanding radio projects (including putting up a new HF antenna) to return to this, but hoping to look at it again shortly.

Thanks again

Peter

[Chris55000]

Hi!

Might be worth checking if there's a HM103 or HM203 going cheap on ebay or the transformer on it's own - the HM203 and HM204 have the same power supply circuit that I can see!

I do have some Gould transformers, but they seem to be a bit on the large side for Hamegs!

Chris Williams