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HP 1740A Oscilloscope no HV

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Has anybody have a HP 1740A scope that could take the time and make some resistance measurements on the HV transformer secondary. My unit is in excellent condition for being 35 years old. I was using it last week and the HV quit. It appears to be the transformer that is no longer available from HP. The secondary is center tapped to ground. The readings across the full winding is 159 ohms, the reading from the center tap to one side is 27 ohms and center tap to the other side is 185 ohms which makes no sense if the full winding is 159 ohms. The HP part number is 01740-61101. The scope is about to become a wheel chock. Any help would be appreciated.
 

JimB

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Most Helpful Member
159 + 27 = 186 ie very nearly 185

I think the (centre) tap is not arranged quite how you think it is.
It is a bit off centre and the way it looks on the schematic is not the way it is physically wired.

JimB
 
I works out mathematically as you stated, but makes no sense the way it is shown in the schematic. If the schematic is drawn wrong, then the center tap would have to be start of the winding, then next leg the 27 ohm and the 159 ohm. Thanks for the input.
 
JimB....I walked away from it for a while and grabbed a coffee and the IB. This time I assumed the transformer was OK and the problem elsewhere. It turned out the HV 4X multiplier block is defective, shorted, etc. It was loading down the secondary of the transformer to the point it would not allow it to oscillate. As soon as I removed it from the transformer connection the HV test point went to -3000 vdc. Now the problem is to find the diode multiplier block. HP 0960-0429.
 

JimB

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Most Helpful Member
I had been thinking that, the problem is more likely with a capacitor or a diode.

Now the problem is to find the diode multiplier block. HP 0960-0429.
If a quick trawl of the internet shows that they are made of unobtainium, try making one!
I had a similar problem some years ago with a Telequipment scope.
The EHT multiplier had a problem, it was "leaking", the eht was tracking out by the mounting screw.
The schematic showed what was inside the block, but not the component values, so I took a guess and bought some diodes, capacitors, a potting box and some potting compound and built one of my own.
It is still working 20years later.
Success!

JimB
 
Thanks! Still looking.........I thought I found a site last night that had a used one but can't find it again. It is a popular item. I like your idea and will probably come down to that vs. trashing it. I did find the company that made it, VARO, and part number is MH924A07. However they went bankrupt and bought up by I think MicroSemi. Have feelers out asking if they can make it.
 

KeepItSimpleStupid

Well-Known Member
Most Helpful Member
I'd also suggest you join the HP/Agilent Yahoo group index at groups.yahoo.com. I don't know the name of the group.

There has been a lot of success with Tek stuff rebuilding multipliers or using a TV HV multiplier. The Tek group is Tekscopes.
 

rumpfy

Active Member
OK so you're making good progress.
Remember there is no such thing as an oscilloscope that is not worth repairing; so soldier on.
the tube in the 1740 is a PDA (post deflection acceleration) type of tube and these CRO's had a good bandwidth and plenty of brightness so you can see very fast waveforms quite easily.
The good news is that its only the multiplier block, and you have several choices. In order of probability of success:
1. rebuild it/ repair it.
2. make a new one yourself.
3. see if you can get a multiplier from a CRT type TV set. These things are often triplers.

The final voltage is 13 kV according to the manual. Changing this to a lower value will probably only reduce the display brightness.
I will watch this thread to catch yr progress. I have no doubt the you will succeed and for a small cost.
Incidentally, which country are you in/ Might help in suggesting options for you to consider.
Hope this helps.
 
OK so you're making good progress.
Remember there is no such thing as an oscilloscope that is not worth repairing; so soldier on.
the tube in the 1740 is a PDA (post deflection acceleration) type of tube and these CRO's had a good bandwidth and plenty of brightness so you can see very fast waveforms quite easily.
The good news is that its only the multiplier block, and you have several choices. In order of probability of success:
1. rebuild it/ repair it.
2. make a new one yourself.
3. see if you can get a multiplier from a CRT type TV set. These things are often triplers.

The final voltage is 13 kV according to the manual. Changing this to a lower value will probably only reduce the display brightness.
I will watch this thread to catch yr progress. I have no doubt the you will succeed and for a small cost.
Incidentally, which country are you in/ Might help in suggesting options for you to consider.
Hope this helps.
3/12/14
Progress if I can get a used one......currently awaiting a quote. Others are asking $350.00 for a new one. Also I going to request a quote from a firm that builds multipliers. I need someone to confirm if the output voltage of the x4 multiplier is positive with respect to ground that connects to the post accelerator. The manual does not have any mention of the polarity, only 13KV approx. The input is around 3KV @ 30Khz. I will need to send the spec's with my quote request to build. The last resort will build my own.
 

rumpfy

Active Member
OK.
The output voltage is positive wrt ground.
The output current is likely to be around 200 to 400 micro-amp, AT THE MOST. A check of the CRT specs will tell you more. For a small screen black and white TV, the cathode current is around this value, but not more, so I would expect the same kinds of values.
The multipliers used in colour TV (for the EHT) are mostly triplers I think, but the input frequency is around 45 KHz with a repetition rate of 15.6 KHz. This is important for the dimensioning of the capacitors in the tripler (4rupler), but in any case I'm sure they won't be too far from about 200 to 1000 pF. Early Philips receivers used a separate tripler block but much asian stuff seems to incorporate the tripler into the line output transformer moulding and encapsulation.
Frankly, your problem is quite trivial, so a simple solution is called for.
If you decide to gut your dodgy unit and repair it, the main problem you face is getting a suitable equivalent potting compound. You need to be aware of the different dielectric constant of various potting compounds and to select something that is compatible with the original. You should apply a vacuum to the potting compound to extract air voids if you decide to repair.
Hope this helps.
 
:) Thanks for all the information. I got a couple of replies to RFQ's and the price the vendors were asking were around $400.00 US for a new assembly. One company needed more details in order to build one and one came along with a $200 price. After putting the 4x multiplier back in the scope and powering it up without connected to the tube, it still loaded the oscillator confirming it was bad. I had to do that because originally I disconnected only the input side assuming it was the multiplier and it could have been the tube. After confirming it was multiplier for sure, I went ahead and bought the new one. I figured by the time I would order all the parts and potting material it would close to the same amount including my time. Will only repair this scope one time, then buy something new and about 1/4 the foot print. I can't believe what some of the parts houses that have bought up old inventory elsewhere are asking for parts.
Now if I can get somebody with admin rights on this site to fix my first name to William it would be nice. I apparently had fat thumbs when creating the account.
 

rc3po

Banned
:) Thanks for all the information. I got a couple of replies to RFQ's and the price the vendors were asking were around $400.00 US for a new assembly. One company needed more details in order to build one and one came along with a $200 price. After putting the 4x multiplier back in the scope and powering it up without connected to the tube, it still loaded the oscillator confirming it was bad. I had to do that because originally I disconnected only the input side assuming it was the multiplier and it could have been the tube. After confirming it was multiplier for sure, I went ahead and bought the new one. I figured by the time I would order all the parts and potting material it would close to the same amount including my time. Will only repair this scope one time, then buy something new and about 1/4 the foot print. I can't believe what some of the parts houses that have bought up old inventory elsewhere are asking for parts.
Now if I can get somebody with admin rights on this site to fix my first name to William it would be nice. I apparently had fat thumbs when creating the account.
I guess I got real lucky. I bought a Tek 2465A DV on ebay for $400 and it works like a champ and doesn't have a scratch on it.
This is my first scope and it's a lot of fun. A very useful tool.
 
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Excellent.....I guess it comes to those who wait! Now all I need is the 4x multiplier for the scope and I am set. Thanks!

Well....got the 4X multiplier diode block in the mail today, installed in the oscilloscope, powered up and it works fine. 3/26/2014 :)
 
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