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?