Author Topic: Pintek 60MHz Oscilloscope PS-605 Issues  (Read 1830 times)

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Offline wattsjake

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Re: Pintek 60MHz Oscilloscope PS-605 Issues
« Reply #25 on: June 24, 2019, 03:33:58 am »
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 ?

Just double checked and it looks like it's (Brown, Orange, Brown, Red, Brown), which is 13.1k ohm +/- 1%. It looks like the first band could be Black, but that wouldn't make sense, that would be 3.1k ohm. Anyway 16.57k ohms is still not within +/- 1% of 13.1k ohms, if we assume it's (Brown, Orange, Brown, Red, Brown) also on the board layout I got from Pintek it shows R909 is 121kF(I think the F next to the number means a +/-1%, not certain though), on the schematic it's 121k as well.
« Last Edit: June 24, 2019, 03:36:45 am by wattsjake »
 

Online tautech

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Re: Pintek 60MHz Oscilloscope PS-605 Issues
« Reply #26 on: June 24, 2019, 07:57:47 am »
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 ?

Just double checked and it looks like it's (Brown, Orange, Brown, Red, Brown), which is 13.1k ohm +/- 1%. It looks like the first band could be Black, but that wouldn't make sense, that would be 3.1k ohm. Anyway 16.57k ohms is still not within +/- 1% of 13.1k ohms, if we assume it's (Brown, Orange, Brown, Red, Brown) also on the board layout I got from Pintek it shows R909 is 121kF(I think the F next to the number means a +/-1%, not certain though), on the schematic it's 121k as well.
Hmmm, does the PCB show signs of rework like R909 has been replaced ?
13k1 would be the value convention used by whoever labelled the PCB component layout however it's clearly marked as 121K in the pdf you supplied. 

If we look back at the PSU:



We see R909 is part of a divider between +140V and -12V and 121K makes more sense there with the diode protecting the op amp input to ~0.7V which after all is only powered with +12V.

Now the worrying bit, with only 13k1 in circuit for R909 the input into the 741 has likely destroyed it and maybe stress R908 and D903.
So you see where I'm going on this ?

I'd replace the 741, both resistors and the diode.
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Offline wattsjake

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Re: Pintek 60MHz Oscilloscope PS-605 Issues
« Reply #27 on: June 25, 2019, 12:48:51 am »
Okay, I've made a mistake. I measured R909 without taking it out of the circuit. I just took it out and measured it and got 121.0k ohms exactly. I guess I read the color code wrong. It's actually (Brown, Red, Brown, Orange, Brown) which is 121.0kohm at +/- 1%. I read it backwards.

I'm going to replace the diode, and 741. Just because I ordered replacements. I'm not sure now where to look for more problems. It looks like everything passes, at least the components I have check so far...
 

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Re: Pintek 60MHz Oscilloscope PS-605 Issues
« Reply #28 on: June 25, 2019, 01:32:51 am »
Okay, I've made a mistake.
Does that make us even ?  :-DD
No matter, that's how we learn.  :)

For now I'd swat up on op amp theory so to understand how this regulator works and if you have a transistor tester double check Q901.
You've listed the gain as 60-200 and if it tests close to either end of the range it might be suspicious.
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Offline wattsjake

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Re: Pintek 60MHz Oscilloscope PS-605 Issues
« Reply #29 on: June 25, 2019, 01:59:18 am »
I've made another video going over what I've done. During the video I had a revelation you could say. So I have measured the voltage across pins 1 and 2 on the connector P-901. It's labeled ~140VAC so I'm assuming the voltage should be within a couple of volts. Well I measured it and got 158.8VAC. I don't think it should 18.8V higher then what it says. The other voltages on the P-901 connector are within 1-2VAC of their specified voltage. I have a felling that it's something with the step down transformer. I'm going to take a look at it and see if I can find anything with it. I've never tried to test a transformer and tips?

https://youtu.be/J47J3YgVMHo - link to Pintek PS-605 Oscilloscope Repair Update
 

Online tautech

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Re: Pintek 60MHz Oscilloscope PS-605 Issues
« Reply #30 on: June 25, 2019, 02:34:24 am »
Two comments, check the primary side transformer tap is set correctly to match you mains voltage.
Transformer secondary AC out is fine.
Measure the DC LV just prior to the regulators (or across caps)......look up full wave rectification and capacitor smoothing and the voltage gain from using it. AC * 1.41V IIRC.
The regulators each need some voltage headroom so whatever they're labelled at, 5, 12, -12V you want a couple of additional volts to supply them with.

Typically for a scope we'd have Service manual defined rail values and a tolerance for each plus a ripple spec too.
As we don't have these we can only go on experience of other scope repairs where 5V would be say +-50mV and the 12V rails +-100mV or better. Ripple needs be probably 20mV max for those rails.
140V will be more forgiving, say +-2V and maybe something like +-200mV ripple.

Edit to add
If the input voltages are sufficient to allow for Vreg dropout then we have two possibilities for low rails.
You DMM is inaccurate.
Something is loading the rails down, eg leaky devices......normally caps.
« Last Edit: June 25, 2019, 02:59:48 am by tautech »
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Offline wattsjake

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Re: Pintek 60MHz Oscilloscope PS-605 Issues
« Reply #31 on: June 25, 2019, 04:10:10 am »
So you think 158.8VAC out is alright?

Also I'm unable to test the voltages when I hook up power, I'll blow out the resistors again. I'm only able to test the P-901 connector because I can disconnect it from the board. Any suggestions on what I can do about that?

Also I emailed Pintek and asked about a replacement transformer, they replied and I was told by their engineer that it's probably Q901. I just ordered a Q901 (B861 PNP transistor). I'm going to replace it as well.
 

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Re: Pintek 60MHz Oscilloscope PS-605 Issues
« Reply #32 on: June 25, 2019, 04:19:06 am »
So you think 158.8VAC out is alright?
I do as the regulator should adjust for the excess voltage providing its componentry is correct and operating properly.

Quote
Also I'm unable to test the voltages when I hook up power, I'll blow out the resistors again. I'm only able to test the P-901 connector because I can disconnect it from the board. Any suggestions on what I can do about that?
Disconnect the PSU from the rest of the scope by breaking connector W801.

Quote
Also I emailed Pintek and asked about a replacement transformer, they replied and I was told by their engineer that it's probably Q901. I just ordered a Q901 (B861 PNP transistor). I'm going to replace it as well.
Good.
See if you can get all the voltage rail specs from them, especially for tolerances and ripple.

I buy a little from Pintek and they're good people do deal with.
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Offline wattsjake

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Re: Pintek 60MHz Oscilloscope PS-605 Issues
« Reply #33 on: July 06, 2019, 04:14:37 am »
All of the parts have arrived from ebay and other places and I have replaced the following components.

Q407, Q408, Q409, Q410, Q411, and Q412 they can be found on page 4 of the schematic outline on the vert final ampl circuit.

I have also replaced U-904 as well as Q901 which was the B861 transistor.

I powered up the oscilloscope, and it blew smoke. Again resistor R427 started to smoke and is visibly burnt, I turned the oscilloscope on for about 3 seconds before it started to smoke and quickly turned it off.

I spoke to Pintek about the issue and the engineer told me it was most likely a bad Q901, and also told me to check Q407, Q408, Q409, Q410, Q411, and Q412.

I'm lost for what could be causing the issue of the burning resistor. I'm unable to see anything on the scope because I have to turn it off so that nothing else gets damaged.

*Looks like I have blown the fuse which is a 0.8A fuse for 115VAC. I tried turning on the oscilloscope this morning and test some voltages but nothing. So I check the fuse and it's blown. So I looks like replacing the components has changed something. There must be a short somewhere in the circuit. I looked over my solder joints and saw nothing, and I have checked my components and their direction and everything seems to be in order.
 
« Last Edit: July 06, 2019, 04:58:18 pm by wattsjake »
 

Online tautech

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Re: Pintek 60MHz Oscilloscope PS-605 Issues
« Reply #34 on: July 08, 2019, 09:25:27 am »
That part of the scope that uses the 140V rail (CRT plate output stage) has been subjected to higher voltages than designed for so all/any components there need be suspect.
The resistor has smoked as it's there for current limiting and excess current is being drawn. This may be related to the excess voltage or more likely a damaged component.

You have 2 areas to pursue, the 140V supply itself to get it regulating properly and the output stage to find the reason for excess current draw.

I certainly would disconnect the 140V supply by whatever simple means you can, be it a connector, removing a linking component or even cutting a PCB trace however if you do you want to make it a widish cut some 20+ thou.
In the unlikely occurrence there's a short in the CRT you can also remove the connections to the plates to eliminate them as a problem.
Any part between the 140V rail and ground or another rail should be carefully checked.
C424 and 425 are the first suspects to check as shorted.

« Last Edit: July 08, 2019, 09:34:38 am by tautech »
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Offline wattsjake

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Re: Pintek 60MHz Oscilloscope PS-605 Issues
« Reply #35 on: July 10, 2019, 12:40:24 am »
I have isolated the power supply circuit found on page 5 of the schematic.

I have taken just one measurement so far. So there's 156.3 V AC coming from the transformer, and there's 208.5 V DC coming off of the rectifier. I'm not sure why there's such a big jump? Also I'm not sure what to test now.

There's 208.5 V DC coming off of the point before R917.

*Also quick question not concerning the project. How do I place an image of the schematic directly into the form so you don't have to open a file to look at it. I see the you (tautech) do this often. Just wondering! Thanks!
 
 

Online tautech

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Re: Pintek 60MHz Oscilloscope PS-605 Issues
« Reply #36 on: July 10, 2019, 01:08:58 am »
I have isolated the power supply circuit found on page 5 of the schematic.

I have taken just one measurement so far. So there's 156.3 V AC coming from the transformer, and there's 208.5 V DC coming off of the rectifier. I'm not sure why there's such a big jump?
Full wave rectification and cap filtering results in a higher voltage, IIRC ~1.4 x AC
Quote
Also I'm not sure what to test now. There's 208.5 V DC coming off of the point before R917.
OK so it doesn't like not having some load to regulate or it's not regulating at all.

We go back to op amp theory to find the answers.
Quote
*Also quick question not concerning the project. How do I place an image of the schematic directly into the form so you don't have to open a file to look at it. I see the you (tautech) do this often. Just wondering! Thanks!
Below the posting box is Attachments and other options.
It's changed in the last few days and more/better features to work with files have been added.
Have a play there and resort to Modify your post if it doesn't come out as expected.

Once we just added files to Attachments to make the thumbnail then copied its URL and enter Modify to add the URL between IMG flags. Use Quote on someones post to see the syntax used.
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Offline bsfeechannel

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Re: Pintek 60MHz Oscilloscope PS-605 Issues
« Reply #37 on: July 10, 2019, 10:31:26 pm »
Here is the schematic for the Pintek 60MHz Oscilloscope

https://www.eevblog.com/forum/repair/pintek-60mhz-oscilloscope-ps-605-issues/?action=dlattach;attach=767028

You must have realized it by now, but I'd like to point out that this schematic seems to be a draft transcribed by an intern to the computer from handwritten notes whose printed version was tossed in the bin and published by someone on the net.

Some parts of the circuit don't make sense at all. Look at the HV supply for instance.



The 100µ/50V has no designation and has the wrong polarity. What is the value of C957? HD950 and HD951 are R4000F. I estimate that the oscillator is running at a frequency from around 50 to 100KHz. That would require fast diodes, which R4000F are not. What is a HV 6.8R resistor (HR959), practically a dead short in this circuit, doing in parallel with a 4n7/3kV capacitor (C959)? C958 is rated for 25V. However if you calculate the voltages in its branch, C958 can be subjected up to more than 100V. The value attributed to VR963 will make the adjustment range from 0.5V to 4kV! (And in the case of the models 200 and 205 up to 6kV!)

Consulting the board layout provided by Pintek, the 100µ capacitor seems to be C962. C957 is 100n. It is certainly a factory adjustment. HD950 and HD951 are not identified by their manufacturer part number. So I can presume they chose a proper high voltage fast rectifier. HR959 is 6M8, not 6.8R! Now, we're talking. And VR963 is 20K, not 20M! C958 is rated for 3kV, not 25V. R955 in the schematic seems to become R963 on the board. Holy crap!



This is what I think the schematic for the HV supply should be.



Now things start to make sense. With those values the HV adjust ranges from -1150V to -1500V for the 605, and from -1600V to -2300V for the 200 and the 205.
 

Offline wattsjake

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Re: Pintek 60MHz Oscilloscope PS-605 Issues
« Reply #38 on: July 12, 2019, 06:18:00 pm »
I would agree that there are many discrepancies between the board and the schematic found. I have been in touch with Pintek and they have generously sent me some original schematics of the boards.

* PS-605 Power Supply.pdf (1293.37 kB - downloaded 68 times.)

* PS-605 vertical.pdf (1263.77 kB - downloaded 42 times.)


 
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Offline wattsjake

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Re: Pintek 60MHz Oscilloscope PS-605 Issues
« Reply #39 on: July 13, 2019, 01:54:13 am »
I have removed the power supply board from the unit. I have decided that it would be quicker and easier to just replace all of the components on the board. I'll be replacing all of the components for the 140v power rail.

I was wondering if I should replace all of the capacitors on the board even the ones not involved in the issue. Also what other components would be good to replace at this time since I have the board removed. Should I replace coils, and diodes? I'm thinking about replacing all of the resistors too. Any thoughts?

There is also a discrepancy with the diode located on the power supply schematic. It's D903 which is specified as a 4108 on the schematic from Pintek and 4148 on the other schematic I got from the internet. Well when I when to replace D903 a few weeks ago it was a 4148 diode. I'm not sure if I should replace it with the same diode or follow the schematic from Pintek.
« Last Edit: July 13, 2019, 02:51:05 am by wattsjake »
 

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Re: Pintek 60MHz Oscilloscope PS-605 Issues
« Reply #40 on: July 13, 2019, 07:49:59 am »
There is also a discrepancy with the diode located on the power supply schematic. It's D903 which is specified as a 4108 on the schematic from Pintek and 4148 on the other schematic I got from the internet. Well when I when to replace D903 a few weeks ago it was a 4148 diode. I'm not sure if I should replace it with the same diode or follow the schematic from Pintek.
Nah, look again when you blow the hand drawn schematic up, it's certainly a 1N4148.

The 140V rail relies on the accuracy of the +12V and -12V rails and they must be right for the regulation to operate correctly however only the +12V rail allows for adjustment which implies the -12V absolute value is not critical.
Check each LV rail for ripple with your DMM on AC mode, before and after the regulators before throwing money at the main smoothing caps.

Don't worry about the -2000V EHT supply just yet. The chokes (coils), if they both pass a resistance test with similar values will be fine. They're there to restrict the EHT switching noise from entering the 12V rails.

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Offline wattsjake

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Re: Pintek 60MHz Oscilloscope PS-605 Issues
« Reply #41 on: July 20, 2019, 04:38:19 am »
The replacement capacitors as well as resistors and other parts have shown up. I'm going to go ahead and start replacing the old parts. I just have a quick question.

Would it be wise to use a variac to reduce the 120 V AC to something lower as a way to reduce the risk of burning parts? I'm just worried that I'll replace everything and have the same thing happen again. R427 will start to burn. I've always wanted a variac so this would be a good excuse to get one. Just interested in knowing if it would help at all?
 

Online tautech

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Re: Pintek 60MHz Oscilloscope PS-605 Issues
« Reply #42 on: July 20, 2019, 04:50:14 am »
Some tips, never fully trust overlays WRT component polarity and double check connections against the schematics.

Instead of the cost or a variac you can easy build yourself a Dim Bulb Tester, see this post:
https://www.eevblog.com/forum/repair/suggested-for-a-sticky-part-one-comments-or-additions-please/msg470686/#msg470686
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Offline wattsjake

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Re: Pintek 60MHz Oscilloscope PS-605 Issues
« Reply #43 on: July 26, 2019, 02:42:50 am »
Alright I have replaced all of the capacitors on the board and most of the resistors associated with the power supply and the Vertical Final Amplifier circuits. I replaced the resistors with 1% tolerance since they were around the same price and I replace all of the electrolytic capacitors and a couple film caps. I also replace the voltage regulators (5V, +/- 12V)and the B861 as well as the UA741.

Well I haven't purchased a variac so I decided just to test it out again and see what would happen. Well you guessed it! R427 started to smoke again!  |O

I honestly do know what I should do now. I'm happy that even after replacing all of the components and doing so much rework on the board nothing awful happened. It's kind of good that the resistor smoked because I still know that it's not completely ruined.

So I haven't done any voltage checks to see if any voltages have changed. I didn't blow the fuse in the back since I switched off the machine quickly after turning it on. I checked the board to see if there were any short circuits and I wasn't able to see anything that could cause a short.

It's been a long time coming on this project and I'm not about to give up on fixing the issue. I know I can look into seeing what the voltages from the regulators are but I have a feeling they are just fine since I just replaced them.

Please any advice or help would be greatly appreciated!
 


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