Pintek 60MHz Oscilloscope PS-605 Issues

[wattsjake]

Purchased a Pintek 60MHz Oscilloscope off of ebay around 3 years ago.

The unit works and powers up. I'm able to see a trace on the screen. The issue comes when I try to focus the trace on the screen to be more focused. It seems it can only focus to a point but the trace is still not in focus. I hooked up a probe to the calibration hook and I'm able to see a wave form on the screen and adjust it using the volt/div and time/div. There is a knob labeled delay time/div which doesn't seem to do anything when I turn it to different positions.

I've made a video and posted it on Youtube. Here's a link to it for more information -  https://youtu.be/E281JIRxb_w

Thanks for any help!
 

[tautech]

Welcome to the forum.

I suggest you email Pintek in Taiwan and ask them nicely for the user and service manual and/or schematics.
Get the focus sorted before going further.
FYI, all older scopes were without settings readout and to this day 1KHz is the standard probe adjust frequency.

[wattsjake]

Thanks, I'll see what I can do to get schematics and a user manual. That would probably help a lot to have more information on the scope. I'm not sure what I can do to sort out the focus issue. Should I try to change out the potentiometer. I have doubts that it's the pot but I guess that's the first place to look...

[tautech]

Thanks, I'll see what I can do to get schematics and a user manual. That would probably help a lot to have more information on the scope. I'm not sure what I can do to sort out the focus issue. Should I try to change out the potentiometer. I have doubts that it's the pot but I guess that's the first place to look...

No the EHT circuitry or EHT voltage is most likely at fault.
Sometimes it's the mega ohm value divider resistors have drifted from their marked values and that's what I would check first, in fact and 1M+ ones in the EHT area. Sometimes they're used in series to make values like 20-30M and you need check them all.
Careful in there, voltages can be (minus) -1 to -3 KV.

If everything measure good then suspect the EHT DC restorer caps but let's get to see a schematic before getting ahead of ourselves. 

[wattsjake]

I was able to get a schematic for the Oscilloscope. I'm not certain where the EHT circuit is located in the schematic. I'm able to see the focus and intensity pots on the CRT circuity but I'm not certain if that's where the EHT circuit is located.

[tautech]

I was able to get a schematic for the Oscilloscope. I'm not certain where the EHT circuit is located in the schematic. I'm able to see the focus and intensity pots on the CRT circuity but I'm not certain if that's where the EHT circuit is located.

Pages of concern are 6 5 and 10. EHT (Extra High Tension) is called HV in these schematics and is -1.9KV.

Note, where there are two pots for an adjustment, one is the trimmer to set the minimum behavior for the front panel pot.

Really you need more than this to correctly set/adjust the scope at service level however if a faulty part can be found and replaced everything else might fall into line and negate the need for any adjustments or the guidance of how to perform them.
So for now don't adjust anything instead go seeking components that have failed or drifted beyond spec.

[wattsjake]

Alright I was able to find some resistor values here they are...

Identification Code - Specified Value from Schematic - Actual Value - Color Code

VR813 - 1M - 0.766M 

R812 -  390K - 1.359M - (Brown, Green, Green, Gold/Yellow)

R814 - 2.2M - 1.878M - (Red, Red, Green, Gold/Yellow)

R815 - 1.5M - 1.36M - (Brown, Green, Green, Gold/Yellow)

R816 - 1.5M - 1.89M - (Red, Red, Green, Gold/Yellow)

VR818 - 500K - 230.7K

VR819 - 500K - 333.8K

VR817 - 500K - 296.7K

My confusion comes when I look at the color code for some of the resistors. They don't match that of the schematic. I'm not sure if I have the correct schematic for this board. The board does have an identification code which is PWB-045H-F. The schematic calls out for PK-045H4 on page 10, and PK-044V4 on page 6. But it could be the same board but just a different version. I'm going to email Pintek and find out if I can have to exact schematic for this PK-045H4 board. Also when I looked to find the resistors on page 6 I wasn't able to locate them on the board. I'm assuming it's the same board for page 10 as for page 6 but I'm not certain. I've included some photos of the board.

[wattsjake]

Here is the schematic for the Pintek 60MHz Oscilloscope

[wattsjake]

I was able to figure out the board confusion. I have found the following values for the schematic on page 6.

Identification Code - Specified Value from Schematic - Actual Value - Color Code

VR519 - 1K - 0.671 ohms

VR516 - 100 ohm - 26.6 ohms

VR523 - 1K - 0.742 ohms

These values seems to be far off from the specified values. Should I go ahead and replace all of the variable resistors on the board? I feel like if most of them are off then all of them are off and will need to be replaced. I have checked with two different multimeters and have gotten the same results...

 

[wattsjake]

Also found the following user manual from a BK Precision 2160A Oscilloscope. They are the same as far as I can see. Also I got a copy of the values shown on the PWB-045H-F board from Pintek. It looks like they changed the R812 to a 1.5M ohm resistor and R816 to a 2.2M ohm resistor without changing the schematic. They probably changed these values after release and found issues after. Not sure though

[tautech]

I was able to figure out the board confusion. I have found the following values for the schematic on page 6.

P6 is for one of the CRT output stages so sorry you need look at P5 for the EHT generation from +12V
900 # component vales.

[wattsjake]

BK Precision 2160A Schematic and board layout

[wattsjake]

I was able to figure out the board confusion. I have found the following values for the schematic on page 6.

P6 is for one of the CRT output stages so sorry you need look at P5 for the EHT generation from +12V
900 # component vales.

I'll check the values of the resistors and compare them to the schematic.
What should I do about the large different between the specified values of the variable resistors and the actual values. It seem they are much out of tolerance. 

[tautech]

I was able to figure out the board confusion. I have found the following values for the schematic on page 6.

P6 is for one of the CRT output stages so sorry you need look at P5 for the EHT generation from +12V
900 # component vales.

I'll check the values of the resistors and compare them to the schematic.
What should I do about the large different between the specified values of the variable resistors and the actual values. It seem they are much out of tolerance.

Nothing for now.
2 look like they're fully adjusted which implies someone has been in there before.

Focus (excuse pun) on the Focus and EHT as it's one part of a CRO that's under considerable stress.
Fine adjustments can come later if required.

Like any gear that's not behaving the PSU circuitry is always where you look first and most often when corrected everything else falls into line.
Check all the LV rails for value and ripple then check the EHT is the normal procedure for fixing CRO's.

In the sticky thread at top of this Repair board is a link to the Tek troubleshooting CRO's guide, download and study it.

[wattsjake]

I have received another board layout from Pintek. It's for the main board that contains the power supply, and HV supply etc. The board I.D. is as follows PWB-044V-D.

[wattsjake]

I have come across something that might be causing the issues that I'm having with the Oscilloscope.

As I was checking resistors I saw that there were some in a corner that have been burnt. They are the following and can be found on page 4 of the schematic.

1) R417
2) R418
3) R424
4) R425
5) R427

I'm going to go head and order replacement resistors but I'm worried that when I replace them they'll end up just like the others. I'm thinking there might be something else that I can't see wrong with the board, but I guess replacing them is a good start.

[wattsjake]

Replaced all of the burnt resistors and double check them to make sure they were the correct values. All went well, I put the CRT back in and hooked everything back up as usual. The moment I turned power on, R427 started to smoke. I quickly turned off the oscilloscope. I'm lost as to what could be causing the issue. It looks like R427 is connected from +140V to a transistor. Not sure what to do. The schematic I'm looking over is on page 4. R427 is close to the top right of the schematic layout, just below the +140v label.

Edit-  I did take voltage measurement before replacing the resistors. On page 5 of the schematic, W801 connector.(Upper right hand corner of the page is where W801 is located) Here is the following...

Pin # - Specified Voltage/Name - Actual voltage measurement

1 - CHOP BLK - Not applicable
2 - GND - 0v
3 - +5v - 4.7v
4 - +12v - 11.83v
5 - -12v - -11.79v
6 - ALT PLUSE - Not applicable
7 - +140v - 175v

I'm thinking something is wrong with the power supply for the +140v line. I see the R427 is connected to +140v. If the power supply is producing +175v then I'm pretty sure that would burn out the resistor. Still confused though and not sure what to do.


 

[tautech]

Q901 must be turned full ON or gone short. Fix this first.

The LV rails are a little low but get the scope operating correctly first then adjust them if necessary.

[wattsjake]

Q901 must be turned full ON or gone short. Fix this first.

The LV rails are a little low but get the scope operating correctly first then adjust them if necessary.

I have taken out the B861 transistor that's labeled Q901 on the schematic and it's looks like it's not shorted. That means that it's probably full ON. What could be causing it to be full ON?

I have also checked the voltages across the following on P-901 connector from the transformer. Refer to page 5.

Pin # - Specified Voltage - Actual reading

1,2 - 140v - 158.8v *

3,4 - 16v - 17.45v

3,5 - 16v - 17.47v

6,7 - 12v - 13.48v

*The data sheet for B861 states the following - It looks like 158.8v is too high for this transistor if I'm looking at the Collector-Emitter voltage: -150V unless I'm supposed to be looking at the Collector-Base Voltage: -200V

Characteristics of the 2SB861 bipolar transistor
Type - PNP
Collector-Emitter Voltage: -150 V
Collector-Base Voltage: -200 V
Emitter-Base Voltage: -6 V
Collector Current: -2 A
Collector Dissipation - 30 W
DC Current Gain (hfe) - 60 to 200
Operating and Storage Junction Temperature Range -45 to +150 °C
Package - TO-220

[tautech]

I have taken out the B861 transistor that's labeled Q901 on the schematic and it's looks like it's not shorted. That means that it's probably full ON. What could be causing it to be full ON?

Too much base drive.

Simply it's a negative side Vreg controlled by the 741 op amp.
As I see it the 741 will add additional DC smoothing to the 140V rail over what the bridge and 100uF main cap offers by sensing variations on pin 3.
If we remove/neglect the active components, the negative side has 82R, 2k7 and 4R7 all in a default series path to 140V ground.
However Q901 is the active component that will pass current at some lower resistance value than the 2k7 shunted across it and regulate the 140V rail to approximately 140V. That there is no trimmer indicates the exact value is not crucial but needs to be stable. Typically there would be a ripple spec for this rail and it needs to be down in the 100's of mV.
Still, check all component values associated with the 140V regulator plus the Q901 base drive should be ~0.6V higher than the its collector if I have that right.
I also suspect without the regulator circuit working correctly some ripple is getting to the CRT plate output amps which could well be related to the wide blurred trace seen on the display.

I'd check main caps C901 and 909 first then C904, D903 and then associated resistors.

[tautech]

Rethink.................
Actually the base drive for Q901 is OFF so without Q901 being partially ON it's not shunting out the two 2k7 paralleled (1k35) across it in the resistor chain to ground.
Sorry I put you crook. 

Cropped PSU

[wattsjake]

Rethink.................
Actually the base drive for Q901 is OFF so without Q901 being partially ON it's not shunting out the two 2k7 paralleled (1k35) across it in the resistor chain to ground.
Sorry I put you crook. 

Cropped PSU

Looking over the board there actually are no two 2.7k ohm resistors, R902 and R903. It does look like R9 which is 1.2k ohm 5W is connected to the same place as R902 and R903. I'm unable to find R9 on the schematic though.

Edit - That would make sense though putting a 1.2K ohm 5W resistor there instead of two 2.7K 0.5W resistors in parallel. I'm sure this is the reason for the change. They probably need a higher rated resistor to be there.

I'm going to go ahead and check the components in that area and see if there's anything I find. Is there anyway the U-904 could be causing any issues?

[tautech]

Is there anyway the U-904 could be causing any issues?

Yes of course but it's easier to replace 2 wire components than an IC so work through checking them all first.
Drifted resistors, dying caps and open/short diodes are were I look first.
I'd probably replace the 1N4148 just because I have a bunch of them.
Op amps are not something I've spent time with so you might have to hunt out some theory and have a swat up.

[wattsjake]

I've taken some time and checked all of the following components...

Identification Number - Specified Value - Actual Values Measured

R901 - 82 ohm - 85.7 ohm

R9 (Replacement for R902 and R903) - Around 1.35k ohm - 1.2k ohm

R904 - 4.7 ohm - 5 ohm

R905 - 100 ohm - 99.8 ohm

R906 - 220 ohm - 219.5 ohm

R907 - 560 ohm - 555 ohm

R908 - 10k ohm - 9.26k ohm

R909 - 121k ohm - 16.57k ohm 121.0k ohm

R917 - 6.8 ohm - 7.1 ohm

C901 - 100uF 200v - 95.8 uF 200v

C904 - 1uF 200v - 1.052uF *It's a 350v cap

C909 - 2.2uF 200v - 2.325uF *It's a 250v cap

D903 - 1N4148 - Tested it and got 0.52v one way and O.L. when I switched the leads *I'm assuming it's okay.

I didn't check the BD902 1A/300v I'm thinking it's okay if I was getting a DC current reading when I previously tested the 140VDC and got 175VDC.

I've ordered some UA741 and R909 resistor which is 121KOhm, and D903 which is 1N4148 just fun. Do you think the drifted resistor R909 could be the cause of the issue?

[tautech]

Do you think the drifted resistor R909 could be the cause of the issue?

Most certainly but as a double check what color is the banding ?