Analog Oscilloscope - Horizontal Problem

[rack]

Hello.

I've a "horizontal" problem with my HungChang analog oscilloscope mod. 5502, 20MHz dual tracks.

The instrument shows the tracks only in the left part of screen.

If I turn the "X-POS" control knob the track moving only on the left part of screen, on the right direction the track "collapse" or overwrite the the one that exist.

I supposed the problem was in "horzontal stage" so I've checked all the transistors and diodes: they seems all ok.
There aren't burned resistors or swollen capacitors.
The solders seems good and I redoing some ones with a bad look (poor).

Vertical controls works fine.

here the schematics about "horizontal stage":

**broken link removed**

**broken link removed**


Measuring the out voltage for the "H+" and "H-" plates (see red marks in schematic), with "TIME/DIV" sets in "XY" :

"X-POS" knob at min (left) -> "H-"=160V, "H+"= about 4V.

"X-POS" knob centered -> "H-"=about 3V, "H+"= about 3V.

"X-POS" knob at max (right) -> "H-"=about 0V, "H+"= about 5V.

So the problem should be in "H+" output voltage.

If i turn the "TIME/DIV" control, sometimes the track fill all the screen and in this case the "H+" voltage reaches the 160V (157V).

The calibration signal (square, 1Vpp, 1KHz) is shows fine but only in the short left part of screen but, when the "X-POS" knob is turned, the square wave stands over the one that exist.

I have replaced the transistors for the output stage (Q809,810,811 and 812) and some of the polarized capacitors in this schematic part.

Any helps about identify the trouble are welcome.
Thanks in advance for answers.

 

[karesz]

Hi rack,

What about the inputsignals/voltage of Q801 & Q802 are there on the bases OK?
Or the Signals of IC602(TC4053)?

May be your problem is to find at IC602, or the H-Position potentiometer and the +/- 12 Volts on that.... And so on.

K.

 

[rack]

Hi karesz, thank you for the fast reply.

I forgot to mention the first check I done about the voltage on the power supply: all are ok and in the ranges indicated on the service manual.

The +12/-12 at "X-POS" potentiometer are present and I've made adjust at the "X-CENT" trimmer (VR413, see FET Q403) as explained in calibrating procedures.

I haven't another oscilloscope for test the saw-tooth and one of my suspected are around the TC4053 chip (CD4053, triple 2 Channel analog multiplexer).
My local electronic shop haven't this spare chip, so I'll try another vendor.

I've tested the voltage at the bases of Q805 and Q806, the values are strange as the measures on the final stage: on Q805 the volts changes when turn the XPOS potentiometer but not at Q802 base (around 0V or a bit negative).


Note:
The schematic contains many errors about the cross references (arrows), who drawn the layout was a bit confused.
In the schematic, "H+" signal are from Q810/Q812 and Q809/Q811 drive the "H-".

Clarification:
This "horizontal" problem not happened immediately but in the past weeks it starts nearly the end of the screen (right) and moved so on to the left till now where is about 3 divisions of graticule.
I thought a capacitor, but which one and in which stage ?
It been a long time and my knowledge of transistor circuits are a bit "rusty".

 

[E-design]

Don't overlook resistors going high in value or develop intermittent connections to the pcb due to heat. Have a look at R825 & 826

 

[karesz]

Hi rack,

Nothing to thank!

Or don`t overlook C825/826 too.
It`s possible, that there are time to time going to short...

I think you need to take a better (2- 5x) magnifier glass and check your PCB for cold solders!

Do you have some sockets/connectors on this pcb for ICs or the driver transistors?

Situations is practically some failure of contacts, but its very possible, that if you check the components than they are OK (in static modus"), but under stresses as Voltages/Currents or even heat as wrote E-design too_ they will be wrong...

What about failure effect if you pull the output signals(P803/804) from your Scope-tube (load less output stage)?

You must be carefull with cross types!

TC4053 has a delta of Vcc-Vee up to 20V and you need jet +5 to -12=17V...
The typical 4053 orHC4053 versions are usually specified only for max. +/_7V!
May be you can find at "hxxp://octopart.com" the right part that has your distributor too...

May be it exist some other type for example from Maxim with a little bit other coding, but with the needed min. 17V Vcc-Vee delta and same pin out?
Sorry-I don`t have time to check it jet....


K.

 

[rack]

@E-design & karesz

Many Thanks for yours useful suggestions.

The guilty was R822 (100K, at the base of Q812) open.

It's hidden under some big ceramic capacitors so escape the visual check.

Replaced this resistor and the old iron runs again!

 

[karesz]

May be you can give us "a copy" of the rest of your Scope-Manual please?

Tnx!
K.

 

[rack]

Of course, karesz.

For now I've scanned only the schematic sheets posted above, but I'll do the rest for the other pages soon.

May I upload it on sharing server (rapid, megaupload) or there is a way to post it directly in forum ?

 

[FvM]

I think, manual and schematic uploads are a wanted contribution in the "service manuals" forum, as you can see from previous posts.

 

[karesz]

Tnx FvM,

I finded just some web-sites over (other) scope (leader, hung chang ...) repair-manuals:

hxxp://www.dxportal.com/stats/content/128465-oscilloscope-service-manual-Hung-chang.html

K.

 

[rack]

I completly forgot to send the manual, I'm sorry.

The attachment contains two pdf, the full manual and the schematic pages in a best resolution.

Please, follow the schematic carefully because it contains some errors about the reference arrows.

In some pages of the Instructions Manual there are references about a "Components Test", I think this is implemented in a similar instrument model with the same name.
The mine one haven't this test function, so the schematic not shows this part, maybe the PCB layout was outfitted for that (see the marked areas).

 

[karesz]

Thanks rack & happy New Year!
K.