[Fixed] and "need repair again" - Signal Burst after repair oscilloscope

[chihaxinh]

Hi All

I have a problem when after repair osc . When power on in 5 min so everything ok . Signal clean and i can change focus ok. But after 5 min , signal go to burst like picture.

I open case and turning trimmer in HV PSU ( focus trimmer ) not work.

Pls help me repair and take equipment back to work.

Many thank you !
Best Regards
Nam Nguyen Hai

[siggi]

Hi Nam Nguyen Hai,

It would help if you told us the make and model of the oscilloscope, and what you've done to repair it so far.

That being said, it looks like you may have a classic case of degraded or bad mains filter capacitors, or possibly output filter capacitors if your oscilloscope has a switch-mode power supply.
Try set your oscilloscope to "line" trigger and a time base of 1ms or so, see whether there's a period of 50/60Hz to the "noise"?

Siggi

[singapol]

I think he's refering to his TDS520 scope.

https://www.eevblog.com/forum/repair/need-help-repair-tektronix-tds520b/

First I think he needs to check the voltages of the crt circuit and then adjust the focus eg. adjust until a small
dot (trace) appears and then the front panel pot (focus) as they are interactive. Hope this helps.Please refer to the service manual for instructions.

[chihaxinh]

Hi All

My osc it's Iwatsu SS-7810 ( 100Mhz ) . I Have 2 scope for measurement tube amplifier . Digital OSC i using for solidstate amp.

So one scope i repair LV PSU , and when i swap LV PSU from work scope to fail scope . Problem not gone .

I thing HV PSU have problem . But i want make sure about big burst trace ( Noise come from gain or att or only problem on HV PSU - focus or etc...)

Many thank you !
Best Regards
Nam Nguyen Hai

[singapol]

First thing first, get SS7810 crt voltage (focus) working then can you determine if or what noise is present.
Can you find service manual?

[chihaxinh]

First thing first, get SS7810 crt voltage (focus) working then can you determine if or what noise is present.
Can you find service manual?

Hi Singapol

This it's link to service manual
www.iti.iwatsu.co.jp/manual/pdf/SS-7810-05-04_ServManl.pdf

[singapol]

From Page:2-23 SS7810 service manual:

2.3.3 Focus

Description
The trace should be focused when the􀊲 FOCUS􀊳 control is positioned between plus minus 45
from the midrange.
 
Procedure /Set the instrument as follows:
INTEN : To such an extent that the trace can be slightly seen
READ OUT : To such an extent that the characters can be slightly seen
FOCUS : midrange
 
Check  P3 #1 with the digitl multimeter (high-voltage probe used) and adjust to -625V with R51.

 Input a sine wave (1KHz) and adjust the amplitude to 4 div.
 
Adjust the following variable resistors to optimize the focus.
 R23 (200Kohm) -325V  [+/-50V],  R41 (5Kohm) +295 [+/- 30V] ,  R54 (50Kohm) +75V [~ -12V]
 
Locatio: P3,  R51,  R23,  R41,  R54 : See "Figure 2.1.4 HV BOARD "


--------------------------------------------------------------------------
2.3.4 Intensity

Description
 The trace should disappear when the􀊲 INTEN􀊳􀃧control is turned fully counterclockwise.
 The characters should disappear when the􀊲READOUT􀊳control is turned fully counterclockwise.

Procedure:  Set the scope as follows so that a spot is displayed.
INTEN : Fully counterclockwise
READOUT : OFF (not displayed)
HORIZ DISPLAY : X-Y
 Adjust  R46 (50Kohm) CUT OFF so that the spot disappear thoroughly.
 Set HORIZ DISPLAY to A.
 Set TIME/DIV to 100 􀐖s/DIV.
 Set the INTEN to 10 O'clock position.
 Adjust the brightness of Intensity by using adjustment menu so that trace line is slighly
appear.
 Set READOUT INTEN to the 12:00 position and make sure that the characters have not
disappeared.

 Refer to "2.1.3 Manual Adjustment Menu".
Location of R46 : See "Figure 2.1.4 HV BOARD "

SS7810 can make adjustments from menu with Jig IE-1066 but you don't have this jig so everything is done manually.

For HV schematic see page : 5-27

Also see  V output AMP schematic page 5-10 for crt power connections.

PS: Make sure your DMM can read up to 1000V and have adequate DMM protection quality.
Avoid checking R51 (1 Mohm) -625V first.Just check those below 600V and see what happens.
If possible wear eye protection goggles.

DANGER HIGH VOLTAGE!

[chihaxinh]

Many thank singapol !

I Fixed device, i replace resistor in focus function and equipment back to work very well . 

Nam Nguyen Hai

[chihaxinh]

Hi All

After finish repair, i shipping equipment to my friends, and shipping service make broken HV PCB  . So i fix by epoxy bond and rewire broken track.

After repair , when power on i missing all HV voltage , B+ , G123 , Vk , and Heater .

I check Q1 2SC3570 ok , Q2 2sc1815 ok . L1 same specs not burn or short .

IC1 i repalce new ( 4558 JRC) .
I check in Secondary of HV Trans, CP3003 haven't osiclator . and haven't Voltage.

I Checking CP3003 not short or burn. 

Thank so much !
Best regards
Nam Nguyen Hai

[singapol]

How can pcb break when it's inside scope unless you only shipped pcb only and did not put enough packing and
protecting filler in package? It's amazing you are willing to go as far as gluing both pieces together but common sense will tell you that you must add connecting wire links to EVERY BROKEN copper trace which first you scrape off the protecting varnish layer to see bare copper ( use solder flux to help with solder wetting). Have you done this? Very sad to hear this has happened.

[chihaxinh]

How can pcb break when it's inside scope unless you only shipped pcb only and did not put enough packing and
protecting filler in package? It's amazing you are willing to go as far as gluing both pieces together but common sense will tell you that you must add connecting wire links to EVERY BROKEN copper trace which first you scrape off the protecting varnish layer to see bare copper ( use solder flux to help with solder wetting). Have you done this? Very sad to hear this has happened.

Yes , i rewire all broken track . You can see picture , HV PCB Below MCU PCB and isolation with MAIN PCB in other side . Gap within 2 PCB about 3cm . OSC keep in sponge box, but HV PCB not make by FR4 material all weight of HT Trans press to PCB when it's have vibration ==> Bingo, pcb crack to 2 part.

[singapol]

Hi, actually it's easy to troubleshoot by working backwards. From HV schematic L1 needs +50V from main power supply schematic 39. Also -295V and - 325V needs +150V to bias Q7 and Q8.

If there is +50V at L1 then Q1 and Q2 will start switching and AC is produced at primary coil 9 & 10 and 11 & 12.
Then there will be step up voltage at secondary side of T1 - CP 3003. Do not measure pin 5 & 6 with respect to pin 7 ground as they are at 2Kv.  Pin 1 & 2 = heater 6.3V. Pin 7 & 8 is AC voltage multiplied to higher voltage for CRT. If there is AC voltage at pin 7 & 8 it means HV transformer is working.

Because of the accident use magifier glass to inspect coil windings of T1 at the pins ( wire break)  if +50V is present at L1 and no HV.

Good luck and be careful, personally I try not to work at night as one may be tired after day's work.

[chihaxinh]

Hi singapol

I checked 50V unreg. It's OK , but i check pin 12 - 6.3V heater and 7-8. Don't have any voltage.

Do you thing circut don't have oscillator ? 

[singapol]

Hi singapol

I checked 50V unreg. It's OK , but i check pin 12 - 6.3V heater and 7-8. Don't have any voltage.

Do you thing circut don't have oscillator ? 

Check Q1 & Q2 if there is switching waveform output with another scope? If not check the caps and other components.

[rf+tech]

Hi chihaxinh,

I agree that the circuit does not oscillate.

Is -12 Volts present as shown in the partial schematic below?
What are the voltages measured on the emitter, base and collector of Q1 and Q2?

RF+ Tech

[chihaxinh]

Hi chihaxinh,

I agree that the circuit does not oscillate.

Is -12 Volts present as shown in the partial schematic below?
What are the voltages measured on the emitter, base and collector of Q1 and Q2?

RF+ Tech

Many thank by your support, after checking i find out one track broken, see by eyes verygood , no crack but check open/short by DMM it's broken.
It's track from pin 12 to 1SS120 switching diode ( like 4148 ) 
Now we have oscillator and everthing run very well . 

I think my friend will happiness in tomorrow 
Thank so much again !