Help with HP 1707A oscilloscope repair

[HackedFridgeMagnet]

Sorry I haven't read the whole thread, disregard if I am off target.

Sounds like a protective thermal shutdown. With the 10-15 seconds being time to cool.
It could also be some other type of protective measure.
I realise it is hard but you may have to disconnect the different subsystems from the power supply and see if one of them is dragging the whole power supply down.

[picofarad]

I just noticed that when I power on the scope, the high voltage board fuse filament becomes incandescent for 1 second or less.
The fuse is a 0.75A one. So there's something that is consuming too much current.
But it's strange that before I replace the capacitor this behavior was inexistent.

However, it's a good idea disconnect all subsystems, but it's an hard work to do!

[oldway]

Check R2 (90.9K), ajust R1 (20K) for max resistance.
The high voltage oscillator seems not to oscillate.
The most likely reasons:
Faulty transformer.
Faulty HV multiplier
Also possible:
C8, C9, C11 of C13 shorten.

[picofarad]

The most likely reasons:
Faulty transformer.
Faulty HV multiplier

They are also the worst reasons, since neither the transformer nor the high voltage multiplier can be replaced. 

EDIT:
Just saw that C8, C9, C11 and C13 are 5000pF 3000 volts! I'm not sure to find them easily. Even Farnell seem to don't have a matching capacitor.

[oldway]

.......
EDIT:
Just saw that C8, C9, C11 and C13 are 5000pF 3000 volts! I'm not sure to find them easily. Even Farnell seem to don't have a matching capacitor.

You don't need to replace them.
Check with ohmeter.
Unfortunately, it's often an internal short in HV transformer.
I have a HP1740A with such a problem.

[picofarad]

I guess it's a risk to rebuild the HV multiplier. It's built in a resin/silicone block. Maybe if I can remove all the resin, I should been able to see components values.

[sync]

Look at this thread: https://www.eevblog.com/forum/testgear/philips-pm-3232-repair/15/

Have you checked that the HV multiplier is the problem? Does the scope power up without it?

[oldway]

Unfortunately, it's often an internal short in HV transformer.

[sync]

Unfortunately, it's often an internal short in HV transformer.

Probably. Do you know a way to test it?

[oldway]

Yes, but to be sure, we need to compare the result with a transformer in good condition.
That's the problem.

The test principle is to supply one of the winding with implusions through a 1K resistor and verify the damped spurious oscillations (in case of internal short circuit, there will be no oscillation or they will be quickly damped), or to form a parallel tuned LC circuit by connecting a 0.1?F capacitor in parallel with one of the windings and to supply of this coil windings via a resistor of 1 Kohms and to search the resonant frequency of the circuit.

Overvoltage on the winding must be as high as the transformer in good condition.
But as we do not have another transformer to compare, it will be difficult to be sure of the results of these tests.

With HV multiplier desconnected and if Q3 has a high enough Hfe, the HV oscillator MUST oscillate. (auto-oscillator).
If it don't oscillate, that means that the transformer is dead.

[picofarad]

So there isn't a simpler way to test the trasformer? It's a crazy idea to use a DMM and check if there are any shorts in the windings? I'm a beginner and until now this is my more difficult repair.
Anyway, if the HV trasformer is dead, there isn't a way to replace it.
Mmm, if I can't repair this scope, at least I have a ton of salvaged components 

[oldway]

HV coil of the transformer has hundreds of turns.
If you have a short circuit between two layers (NB: that's what happen !), it only short-circuits some tens of turns.
Measuring the resistance, you can not see the difference.
But this short circuit is enough to dampen the oscillator by induced current in the short-circuited turns.

[GiskardReventlov]

I learn a lot from these repair threads. And I admit that I always like the "happy ending" on these.  But it's sounding like that ain't gonna happen on this one.  But I don't understand why the HV transformer cannot be fixed or replaced. Is it simply that it would have to be hand wound? Or is it something else?

[picofarad]

The HV transformer has a strange construction. It isn't simple to re-do all windings. It's more simpler to replace the whole transformer, but since it's a custom part, and spare parts for my model are not longer built, the only way to do it is buying another 1707 and salvage the part.
However, these are the worst hypotheses. I have not yet checked those capacitors in the HV oscillator board. Maybe the issue it's a shorted cap.

[oldway]

the only way to do it is buying another 1707 and salvage the part.

Yes, that's right.
With two faulty HP1740A (100Mhz), i made a good working one and with one faulty HP1715A (200Mhz) + one faulty HP1725A , i made a good working HP1725A (275Mhz).
I like very much de HP1740A, it's a very good analog scope.
Only drawback: it is a huge scope, take a lot of space on your bench.
For this reason, i am using more often an HM605 Hameg. (it has also a component tester very useful for repair)

[GiskardReventlov]

The HV transformer has a strange construction. It isn't simple to re-do all windings. It's more simpler to replace the whole transformer, but since it's a custom part, and spare parts for my model are not longer built, the only way to do it is buying another 1707 and salvage the part.
However, these are the worst hypotheses. I have not yet checked those capacitors in the HV oscillator board. Maybe the issue it's a shorted cap.

It's considered custom because it's made specifically for this scope but are the inputs and outputs also custom? In case you haven't figured it out I know very little about this topic but I'm still learning and curious to know why an alternative part couldn't be found or made that's not from another scope.

[picofarad]

Ok, after few months I decided to get back on this scope. I want to replace all capacitors from C8 to C21, because some of them get pretty warm when the scope is on.
They are all ceramics caps, .005uF +/- 20%, 3000 volts.
I'm getting crazy finding caps with this value! .0047uF are more easy to find. Can I use the .0047 ones instead of the original caps value? .0047 is a value that it's included in the 20% tolerance of the original caps, so I guess that it is possible.

[tautech]

.047 will be fine. Voltage rating must be the same or better.

[picofarad]

Just a quick update. I disconnected the high voltage multiplier block from the board and tried to power on the scope. Same problem: a buzz and a high frequency tone coming from some component. At least is not the H/V multiplier. The buzz disappeared replacing a ceramic capacitor, but the high frequency tone still remains, but I can't define the component that produces it. I also noticed that the previous owner repaired the A3A1 board (motherboard for other two high voltage power supply boards) replacing the CR5 component and I guess that the tone is coming from that board. The manual reports CR5 as "thyristor, diac" and the component code is 1884-0094. I don't understand if CR5 is a thyristor or a diac, beacuse the 1884-0094 on the internet is classified as thyristor, but the real component on the board has only two pins instead of the classic thyristor three legs pinout.
Moreover, I can't find precise infos on the 1884-0094 on the internet. I can't even find the datasheet. How can I replace it?
Thanks

[mij59]

Hi,

Its probably,  as noted, a short in the transformer T1, before you replace it, check if there are any shorts on secondary side of T1.

Assuming that you checked all of the components on the primary side of the transformer T1.

Check the diodes CR7..CR9
As noted if the capacitors C8 etc are not shorted you don't need to replace them.
Check the resistors, high value resistors could be open ( disconnect one lead to be sure you measure the resistor and not the rest of the circuit).
DS1 and DS2 are probably some kind of gas filled tubes acting as an over voltage protection, so they shouldn't be shorted.
Check the resistors in the feedback path from R16 to R3.
 

[picofarad]

Ok, a whole year passed since last reply. It's been a long year, and the scope is not working at all. I decided to give the last try before I give up and scrap the whole thing. As said before, the issue is probably a short in the HV transformer, and this seems to be confirmed by the high pitch tone coming from the high voltage assembly due to the magnetostriction on the trasformer.
I searched all over the internet for a spare part, and eventually found a complete hv board, but it was already sold.
Other 1700 series scopes are rare on eBay and when they come up, even if are for parts, the prices are very high.

So, now my question, it is possible to rewind the transformer? And if it's not possible to rewind, could I get a similar part and adapt it?

Thanks for the patience. 

[picofarad]

Ok, I fiddled a bit with the trimmers on the HV board. These two trimmers sets high voltage and intensity limit. Now the high pitch tone is gone and the scope turned on, but still no trace.
The HV oscillator is not oscillating, no output on A3Q1.
I've done some measures:
Q1:
    - base: 7.06 V
    - emitter: 4.46 V
    - collector: 4.46 V
Q2:
    - base: 4.46 V
    - emitter: 5.78 V
    - collector: 4.46 V
Q3:
    - base: 10.7 V
    - emitter: 11.44 V
    - collector: 11.44 V

After 10 minutes, something probably absorbed too much current and an inductor has burnt on the main power supply board, so now the scope doesn't power on again. 

[mij59]

Ok, I fiddled a bit with the trimmers on the HV board. These two trimmers sets high voltage and intensity limit. Now the high pitch tone is gone and the scope turned on, but still no trace.
The HV oscillator is not oscillating, no output on A3Q1.
I've done some measures:
Q1:
    - base: 7.06 V
    - emitter: 4.46 V
    - collector: 4.46 V
Q2:
    - base: 4.46 V
    - emitter: 5.78 V
    - collector: 4.46 V
Q3:
    - base: 10.7 V
    - emitter: 11.44 V
    - collector: 11.44 V

After 10 minutes, something probably absorbed too much current and an inductor has burnt on the main power supply board, so now the scope doesn't power on again. 

And the burnt inductor is ?
So the problem could also be on the power supply board, (page 21) case off bad caps e.g.
Maybe a problem with the -15V rail.

[picofarad]

And that's the problem: the schematic I'm following is for the 1707B, while my scope is a 1707A. They are pretty much the same, except some details, and the board with the burnt inductor is one of these  . On the schematic I have, the board A3A3, which has the burnt inductor, has only two big wire-wound inductors, while mine has more resistor shaped inductors, if I remember correctly, 4 inductors.

EDIT: I attached a photo of my board and a photo of the same board from the manual. My board is actually more different, having less capacitors and more inductors.
I've marked the burnt inductor.


I've checked multiple times all voltage rails, before the inductor blew, and they were OK.

[mij59]

And that's the problem: the schematic I'm following is for the 1707B, while my scope is a 1707A. They are pretty much the same, except some details, and the board with the burnt inductor is one of these  . On the schematic I have, the board A3A3, which has the burnt inductor, has only two big wire-wound inductors, while mine has more resistor shaped inductors, if I remember correctly, 4 inductors.

EDIT: I attached a photo of my board and a photo of the same board from the manual. My board is actually more different, having less capacitors and more inductors.
I've marked the burnt inductor.


I've checked multiple times all voltage rails, before the inductor blew, and they were OK.

Looks like a 18 uH inductor, is it open ?
Could be connected to tap 10 of the transformer, which is +15V in the 1707B schematic.