Help with HP 1707A oscilloscope repair

[picofarad]

Hi guys,
just after a month from the purchase, my vintage HP 1707A oscilloscope doesn't work anymore. The CRT doesn't show anything at all. Besides the CRT problem, just changing the settings it seems to work (trigger, sweep, timebase and channels). It's not a settings problem: even pressing the find beam button the display doesn't work.

I assume that the problem is related to the tube power supply, so I searched the service manual, but I only found the one for the 1707B (not much a problem, they are pratically identical), and I found the CRT "driver" board schematic:

http://imageshack.com/a/img199/5972/5ffo.jpg

I tested all BJTs and almost all resistors and capacitors and I also did some measurements: on the emitter of Q3 there are 14V (pretty much as the schematic says) but on the base there are -0.1V instead of +12.1V as reported on the manual.

Can you help me to repair it?
Here there are the manual and schematics http://www.hparchive.com/Manuals/HP-1707B-Manual.pdf

P.S. Sorry for the bad english and the tricky explanation.

[lowimpedance]

Are you sure Q3 is not dead??, can you find a substitute to try.

[tautech]

Have repaired a good few CRT scopes over the years and you never quite know where or when they will fail next.
However I usually look at circuit areas that are under stress(voltage, heat, current etc).
Mostly the ageing componentry just fails and finding replacements can be troublesome.
In some areas component values are critical but many are not, just think about how a substitute could affect accuracy.
Be aware that supply voltages are often co-related or linked so a low voltage can be a symptom of a problem on another rail.
I acquired an AVO with 3Kv range especially for scope HV work.
Watch out for the cathode and grid voltages at negative potentials.
 Based on your symptoms you are in the right area...don't take anything for granted.
Check everything, even transformer windings.
Good luck
Rob

[picofarad]

I've checked all transformer windings and they're ok.
I thought that could be a transistor failure, but I checked all BJT (simply with the diode tester on my multimeter) and they seems to be ok. But some time ago I read that older transistors can raise the polarization voltage need in the years. How can I check if it's this case?

[sync]

I tested all BJTs and almost all resistors and capacitors and I also did some measurements: on the emitter of Q3 there are 14V (pretty much as the schematic says) but on the base there are -0.1V instead of +12.1V as reported on the manual.

Q3 is dead or you have an open circuit/ cold joint. 14V between emitter and base is impossible for a working Q3.

[picofarad]

Solder joints are ok. Next days I'll make more measures, so the diagnosis can be more precise.

[lowimpedance]

To eliminate Q3 as the possible suspect you really should try a new part of either the same or equivalent to be sure.
If Q3 is ok then I would look at Q1 or 2 or both more closely. I assume the transformer pins 3-4  and 1-2 have continuity ?
As has been stated be careful that all high tension charges have been discharged before poking around with soldering irons (Not sure of your prior experience with this sort of servicing, so still worth the warning!).

[picofarad]

After three months I finally got the time to get more accurate measures (the first values were taken without connect a wire to the board).
Here are the values for Q3:
 - On the base there are -1.1 volts instead of -0.1 volts;
 - On the collector there are -1.8 volts instead of +12.1 volts;
 - On the emitter, that is connected with Q2 base, there are +1.8 volts. No reference values from the manual here.

Q1 and Q2 transistor are 2N3904 and 2N3906, I have tons of them and I can replace them very easily, but I'd prefer to find the issue first.
I can't identify Q3. It's in a TO-5 package and it's labeled F4-023 141, but I can't find any information on the Internet about this component. The only thing I know is that is a NPN BJT.
Anyone can help me to identify this component?

Yes, I've checked the transformer and it's all ok.

[sync]

- On the emitter, that is connected with Q2 base, there are +1.8 volts. No reference values from the manual here.

That doesn't match the schematic.

Q1 and Q2 transistor are 2N3904 and 2N3906, I have tons of them and I can replace them very easily, but I'd prefer to find the issue first.
I can't identify Q3. It's in a TO-5 package and it's labeled F4-023 141, but I can't find any information on the Internet about this component. The only thing I know is that is a NPN BJT.

In the service manual is a list of components (table 6-2 "Replaceable parts"). It's the A3A4 board (A3 = power supply assembly, A4 = high voltage oscillator) and the transistor is A3A4Q3.



Here you find the HP part number (1853-0036). According to the HP part number cross reference (www.sphere.bc.ca/download/hp_xref-free.pdf?) it's a 2N3906.

BUT I think you mixed up Q1 and Q3.
Q1 is a 1854-0023  (selected 2N2484)  TO-18 metal case
Q2 is a 1854-0215 = 2N3904  TO-92 plastic case
Q3 is a 1853-0036 = 2N3906  TO-92 plastic case

And the the label F4-023 141 matches 1854-0023.

[picofarad]

Yes, sorry, I misread the schematic when I copied the values.
Here are the CORRECT values:
For Q1:
 - On the base there are -1.1 volts instead of -0.1 volts;
 - On the collector there are -1.8 volts instead of +12.1 volts;
 - On the emitter there are -1.8 volts.

For Q2:
 - On the base: -1.8 volts;
 - On the collector: -1.8 volts;
 - On the emitter: -1.34.

For Q3:
 - On the base: -2.6 volts instead of +12.1 volts;
 - On the collector: -2.7 volts. The reference on the schematic is -2.3 volts. I guess that this value is ok;
 - On the emitter: +15.2 volts instead of +12.8 volts.

I guess that the issue could be one of those three BJTs, probably Q1.

Thanks for the help to identify Q1. I didn't know this cross reference table. 
Sorry again for the mistakes.

[sync]

For Q3:
 - On the base: -2.6 volts instead of +12.1 volts;
 - On the emitter: +15.2 volts instead of +12.8 volts.

Q3 is dead. A BJT has a diode between emitter and base. The forward voltage between them should be about 0.7V. You measured 17.8V. Not possible for a working transistor.

Replace Q3 and see want happens. If you are lucky it was the only fault and the scope works again. Or maybe the real fault will blow Q3 again. But you have tons of them.

[picofarad]

So, I replaced Q3 but now the scope doesn't power on at all! But I hear a buzz coming from the inside, I assume from one of the transformers in the high voltage section. As I wrote before, I checked all transformers and they seem to be ok.
Although the scope doesn't power on, the screen shows something (very weak, seems to be noise).

[picofarad]

Nobody can help me?

[tautech]

Nobody can help me?

Are all voltages to Spec ?
What front panel lights are on ?

[sync]

So, I replaced Q3 but now the scope doesn't power on at all! But I hear a buzz coming from the inside, I assume from one of the transformers in the high voltage section. As I wrote before, I checked all transformers and they seem to be ok.
Although the scope doesn't power on, the screen shows something (very weak, seems to be noise).

How do you checked the transformer?
Remove Q3. Does the scope power up again?

[picofarad]

Front panel lights are off.
I checked the transformer simply using the continuity test on my multimeter and verifying if the windings are not interrupted. I know that it's not the best way to do it, but it's the only thing I can do.

I didn't have the time to take the new measures (just replaced Q3 and tried to power the scope on). I will take them in the next days.

[picofarad]

Ok, now the values are:
Q1:
-base= -0.09v
-emitter= -0.60v
-collector= the value changes from 0.15v to 0.50v
Q2:
-base= -0.60
-emitter= -1.20
-collector= is wired to Q1 collector, so value changes from 0.15v to 0.50v
Q3:
-base= 0.50v
-emitter= 1.10v
-collector= 0.43v

I removed Q3 and the scope power up. All front panel lights are on.
I also taken measures without Q3 mounted:
Q1:
-base= -0.03v
-emitter= -0.67v
-collector= -0.67v
Q2:
-base= -0.67v
-emitter= -1.32v
-collector= -0.67v
Q3(or where Q3 is supposed to be):
-base= -0.67v
-emitter= 15.14v
-collector= -6.74v

I've also noticed that with Q3 mounted, the two lamps (DS1 and DS2 on the schematic) sometimes blink. The two lamps are not on the front panel, but they're mounted on the PCB (I can't figure what is their function).

[oldway]

I don't know if this could help.
On the HP1740A, there is a safety feature : HV is shut down if 120V power supply voltage is too high.
Perhaps it is the same on this scope.
Check the signal on R8 coming from A1 - 21 (61).
Did you check the power supply?
Voltages are ok?

[picofarad]

What do you mean with 120V power supply? Do you mean the mains voltage? Here in Italy the mains are 230V.
I didn't checked the power supply completely. It will be the next thing to do.

[oldway]

What do you mean with 120V power supply? Do you mean the mains voltage? Here in Italy the mains are 230V.

In the HP1740A, there are several power supplies, -15V, +5V, +15V, +43V and +120V. Nothing to do with mains voltage, it is dc voltage.
 

I didn't checked the power supply completely. It will be the next thing to do.

It is the first thing to do before looking after other problems.

[picofarad]

In the 1707 the voltages are +5v, +15v, -15v, +50v, -50v and +80v. There isn't the 120v voltage.
I checked the power supply, and the voltages are all ok. The +80v voltage actually is +86v, but I don't think it's a problem.

On R8 there are 15.4 volts.

[oldway]

The +80v voltage actually is +86v, but I don't think it's a problem.

When you repair, don't think, be sure of it.
Check why this voltage is too high.
Is this a regulated power supply?
EDIT: I found the service manual:
http://cp.literature.agilent.com/litweb/pdf/01707-90916.pdf
80V comes from a CC/CC converter and is not regulated.
Then, 86V is not a concern.
(61) -15V
(62) +15V
(63) +15V (from another +15V power supply)
Q1 and Q2 : this is a darlington.
Initially, Q3 was dead.
After you changed it, Q3 was conducting too much and Q1 had a high base current and thus also a high collector current, what overloaded the +15V power supply and the dc/dc converter.
Why was Q3 conducting too much ?
First, replace C4 1µF , it can be shorted or have leakage

[picofarad]

Ehm... my scope has been hacked by the previous owner. The 1707 is designed to work with 110/220V AC or using a DC power line. The previous owner took  out the old transformer for AC powering and mounted inside the scope a regulated power supply from Farnell, wired to the DC power circuit.
Basically, the Farnell power supply bypass the transformer, the rectifier and the filtering, giving directly an already stable tension (24v).

It's a very intricate explanation... I hope you'll understand.

For the +86v, I'm using the 1707B service manual instead of the 1707A one. They should be pratically identical, apart for the buttons design and other small things, but I'm not taking the schematics and the values as an absolute truth.
Anyway, I'll check why there are +86v instead of +80v.

[oldway]

Read the EDIT of my previous post

[picofarad]

Replaced C4 and put back a new 2N3906 for Q3. Same result. The screen doesn't show anything and a buzz is present. But now when I turn on the scope, for about 1 second front panel lights light up, but then they turn off. But if I turn off the scope and turn it on again just 1 second later the lights doesn't light up. I have to wait at least 10-15 seconds  between the shutdown and the power on to see the panel lights on.
This strange behavior makes me think that the problem can be a electrolytic capacitor issue, right?

[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.
 

[picofarad]

Yes, it's connected to transformer pin 10, through a diode.
It doesn't seems to be open (I measure about 1.2 ohms through the inductor) but it's not working (it's burnt and with a magnify lens I can see a crack in the middle of the component).

[mij59]

Yes, it's connected to transformer pin 10, through a diode.
It doesn't seems to be open (I measure about 1.2 ohms through the inductor) but it's not working (it's burnt and with a magnify lens I can see a crack in the middle of the component).

Don't think there's a problem with the inductor, you can compare it  the resistance of the inductor just below.
Check the diode, capacitor and the power supply rail for shorts.

[picofarad]

The resistance is the same as the other inductor. The diode is ok and the cap isn't shorted. But when I've turned on, smoke coming from the inductor and voltage rails all messed up: +80V rail is now 24 V and +15 V is now 2 V.

[mij59]

The resistance is the same as the other inductor. The diode is ok and the cap isn't shorted. But when I've turned on, smoke coming from the inductor and voltage rails all messed up: +80V rail is now 24 V and +15 V is now 2 V.

The +15V powers the low voltage regulator (page 20), check Q3, Q4 , and the -15V rail.

[guitarmike000]

WOW PICO! this is crazy stuff! very glad i have 2-1707b's don't envy you at all but i truly hop you can fix it.   mike