Pintek 60MHz Oscilloscope PS-605 Issues  - Page 1
Forum section: electronics / repair

author: wattsjake
author_rank: Contributor
timestamp: June 19, 2019, 01:56:58 am
content: 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_wThanks for any help!

author: tautech
author_rank: Super Contributor
timestamp: June 19, 2019, 02:24:59 am
content: 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.

author: wattsjake
author_rank: Contributor
timestamp: June 19, 2019, 04:24:50 am
content: 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...

author: tautech
author_rank: Super Contributor
timestamp: June 19, 2019, 05:11:16 am
content: Quote from: wattsjake on June 19, 2019, 04:24:50 amNo 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.

author: wattsjake
author_rank: Contributor
timestamp: June 19, 2019, 10:40:30 pm
content: 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.

author: tautech
author_rank: Super Contributor
timestamp: June 19, 2019, 11:08:03 pm
content: Quote from: wattsjake on June 19, 2019, 10:40:30 pmPages of concern are65 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.

author: wattsjake
author_rank: Contributor
timestamp: June 20, 2019, 01:42:06 am
content: Alright I was able to find some resistor values here they are...Identification Code - Specified Value from Schematic - Actual Value - Color CodeVR813 - 1M - 0.766MR812 - 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 - 230.7KVR819 - 500K - 333.8KVR817 - 500K - 296.7KMy 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.

author: wattsjake
author_rank: Contributor
timestamp: June 20, 2019, 01:45:37 am
content: Here is the schematic for the Pintek 60MHz Oscilloscope

author: wattsjake
author_rank: Contributor
timestamp: June 20, 2019, 03:29:38 am
content: 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 CodeVR519 - 1K - 0.671 ohmsVR516 - 100 ohm - 26.6 ohmsVR523 - 1K - 0.742 ohmsThese 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...

author: wattsjake
author_rank: Contributor
timestamp: June 20, 2019, 04:05:27 am
content: 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

author: tautech
author_rank: Super Contributor
timestamp: June 20, 2019, 04:08:26 am
content: Quote from: wattsjake on June 20, 2019, 03:29:38 amP6 is for one of the CRT output stages so sorry you need look at P5 for the EHT generation from+12V900 # component vales.

author: wattsjake
author_rank: Contributor
timestamp: June 20, 2019, 04:09:51 am
content: BK Precision 2160A Schematic and board layout

author: wattsjake
author_rank: Contributor
timestamp: June 20, 2019, 04:21:58 am
content: Quote from: tautech on June 20, 2019, 04:08:26 amI'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.

author: tautech
author_rank: Super Contributor
timestamp: June 20, 2019, 04:30:43 am
content: Quote from: wattsjake on June 20, 2019, 04:21:58 amNothing 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.

author: wattsjake
author_rank: Contributor
timestamp: June 21, 2019, 12:55:52 am
content: 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.

author: wattsjake
author_rank: Contributor
timestamp: June 21, 2019, 01:02:48 am
content: 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) R4172) R4183) R4244) R4255) R427I'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.

author: wattsjake
author_rank: Contributor
timestamp: June 21, 2019, 02:32:51 am
content: 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 measurement1 - CHOP BLK - Not applicable2 - GND - 0v3 - +5v - 4.7v4 - +12v - 11.83v5 - -12v - -11.79v6 - ALT PLUSE - Not applicable7 - +140v - 175vI'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.

author: tautech
author_rank: Super Contributor
timestamp: June 21, 2019, 10:12:54 am
content: 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.

author: wattsjake
author_rank: Contributor
timestamp: June 21, 2019, 06:17:51 pm
content: Quote from: tautech on June 21, 2019, 10:12:54 amI 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 reading1,2 - 140v - 158.8v *3,4 - 16v - 17.45v3,5 - 16v - 17.47v6,7 - 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: -200VCharacteristics of the 2SB861 bipolar transistorType - PNPCollector-Emitter Voltage: -150 VCollector-Base Voltage: -200 VEmitter-Base Voltage: -6 VCollector Current: -2 ACollector Dissipation - 30 WDC Current Gain (hfe) - 60 to 200Operating and Storage Junction Temperature Range -45 to +150 °CPackage - TO-220

author: tautech
author_rank: Super Contributor
timestamp: June 22, 2019, 02:33:14 am
content: Quote from: wattsjake on June 21, 2019, 06:17:51 pmToo 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.

author: tautech
author_rank: Super Contributor
timestamp: June 22, 2019, 08:03:09 am
content: 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

author: wattsjake
author_rank: Contributor
timestamp: June 22, 2019, 08:16:02 pm
content: Quote from: tautech on June 22, 2019, 08:03:09 amLooking 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?

author: tautech
author_rank: Super Contributor
timestamp: June 22, 2019, 08:41:00 pm
content: Quote from: wattsjake on June 22, 2019, 08:16:02 pmYes 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.

author: wattsjake
author_rank: Contributor
timestamp: June 24, 2019, 12:33:53 am
content: I've taken some time and checked all of the following components...Identification Number - Specified Value - Actual Values MeasuredR901 - 82 ohm - 85.7 ohmR9 (Replacement for R902 and R903) - Around 1.35k ohm - 1.2k ohmR904 - 4.7 ohm - 5 ohmR905 - 100 ohm - 99.8 ohmR906 - 220 ohm - 219.5 ohmR907 - 560 ohm - 555 ohmR908 - 10k ohm - 9.26k ohmR909 - 121k ohm -16.57k ohm121.0k ohmR917 - 6.8 ohm - 7.1 ohmC901 - 100uF 200v - 95.8 uF 200vC904 - 1uF 200v - 1.052uF *It's a 350v capC909 - 2.2uF 200v - 2.325uF *It's a 250v capD903 - 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?

author: tautech
author_rank: Super Contributor
timestamp: June 24, 2019, 03:10:37 am
content: Quote from: wattsjake on June 24, 2019, 12:33:53 amMost certainly but as a double check what color is the banding ?

author: wattsjake
author_rank: Contributor
timestamp: June 24, 2019, 03:33:58 am
content: Quote from: tautech on June 24, 2019, 03:10:37 amJust 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.

author: tautech
author_rank: Super Contributor
timestamp: June 24, 2019, 07:57:47 am
content: Quote from: wattsjake on June 24, 2019, 03:33:58 amHmmm, 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.

author: wattsjake
author_rank: Contributor
timestamp: June 25, 2019, 12:48:51 am
content: 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...

author: tautech
author_rank: Super Contributor
timestamp: June 25, 2019, 01:32:51 am
content: Quote from: wattsjake on June 25, 2019, 12:48:51 amDoes that make us even ?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.

author: wattsjake
author_rank: Contributor
timestamp: June 25, 2019, 01:59:18 am
content: 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

author: tautech
author_rank: Super Contributor
timestamp: June 25, 2019, 02:34:24 am
content: 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 addIf 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.

author: wattsjake
author_rank: Contributor
timestamp: June 25, 2019, 04:10:10 am
content: 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.

author: tautech
author_rank: Super Contributor
timestamp: June 25, 2019, 04:19:06 am
content: Quote from: wattsjake on June 25, 2019, 04:10:10 amI do as the regulator should adjust for the excess voltage providing its componentry is correct and operating properly.QuoteDisconnect the PSU from the rest of the scope by breaking connector W801.QuoteGood.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.

author: wattsjake
author_rank: Contributor
timestamp: July 06, 2019, 04:14:37 am
content: 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.

author: tautech
author_rank: Super Contributor
timestamp: July 08, 2019, 09:25:27 am
content: 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.

author: wattsjake
author_rank: Contributor
timestamp: July 10, 2019, 12:40:24 am
content: 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!

author: tautech
author_rank: Super Contributor
timestamp: July 10, 2019, 01:08:58 am
content: Quote from: wattsjake on July 10, 2019, 12:40:24 amFull wave rectification and cap filtering results in a higher voltage, IIRC ~1.4 x ACQuoteOK 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.QuoteBelow 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.

author: bsfeechannel
author_rank: Super Contributor
timestamp: July 10, 2019, 10:31:26 pm
content: Quote from: wattsjake on June 20, 2019, 01:45:37 amYou 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.

author: wattsjake
author_rank: Contributor
timestamp: July 12, 2019, 06:18:00 pm
content: 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 292 times.)PS-605 vertical.pdf(1263.77 kB - downloaded 238 times.)

author: wattsjake
author_rank: Contributor
timestamp: July 13, 2019, 01:54:13 am
content: 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.

author: tautech
author_rank: Super Contributor
timestamp: July 13, 2019, 07:49:59 am
content: Quote from: wattsjake on July 13, 2019, 01:54:13 amNah, 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.

author: wattsjake
author_rank: Contributor
timestamp: July 20, 2019, 04:38:19 am
content: 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?

author: tautech
author_rank: Super Contributor
timestamp: July 20, 2019, 04:50:14 am
content: 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

author: wattsjake
author_rank: Contributor
timestamp: July 26, 2019, 02:42:50 am
content: 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!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!

author: DavidCG
author_rank: Newbie
timestamp: July 13, 2022, 11:39:55 pm
content: Quote from: wattsjake on July 26, 2019, 02:42:50 amHI jakeI recently measure the power transformer output of the pintek im servicing. (mine only displays a dot, im not able to mess around with controls because something is making a high POP sound after 5 seconds i turn the unit on, sounds like something is arcing inside, no smell of any burned component and no visible burned one)the voltages are:primary on 220vsecondaries:12v15.5 - 0 - 15.5147.530.8 - 0 - 6.2.note: my unit uses another type of voltage regulation(transistors, not LM78XX) and the pcb is coded: PWB 064V.

author: Chris56000
author_rank: Frequent Contributor
timestamp: July 22, 2022, 08:57:18 am
content: Me thinks it's time I gave some input to assist in solving this tale of woe and desperation!the first thing is, don't worry about the 158 VAC from your mains transformer h.t. secondary, I'll come to that later!Repeated failure of R427 means the excess current path can only be via Q404/Q407/Q408 on the Y+ top side of the circuit or Q409/Q410 or Q412 on the Y– lower half of the circuit, or C415 is shorting when the h.t. is applied each time the oscilloscope is switched on!Begin by removing Q411 and Q412 and switch on again – if R427 still burns up your fault can only be C415 or a short in the PCB itself!The two PNP transistors Q407 and Q410 form a constant–current load for the driver stages Q408 and Q410.The quickest and best way to fault–find this type of directly coupled circuit is to invest in one of those little colour screen Chinese TC1 Component Testers, one of these will run through all the transistors in half–an hour or so!How are you for desoldering equipment? – I presume you have access to something as you've mentioned replacing numerous resistors, etc.If you find no fault with the transistors on a low–voltage tester, you can either connect the collector/emitter of each transistor across the scope's +140V h.t. supply via a 100K 2W resistor to see if any are breaking down at high voltage – connect.the collector to h.t. via a 100K 2W and short b/e together and connect to 0V for all the "2SC" NPN devices, and short b/e together, connect to +140V with collector connected to 0V via 100K 2W for the "2SA" or "2SB" PNP types, or alternatively, order new replacements for them of you prefer not to use the oscilloscope's h.t. supply to test them – it's easy to set up a 100K 2W resistor with your DVM across it and two leads plus a jumper for b/e (outer two legs on most Japanese transistors) on a small plug–in type breadboard, into which you can insert the transistors!Either way, the voltage across the 100K resistor should be negiglible if the transistor is a good one – any transistor that shorts out partially or completely will show up as a high voltage across the test resistor, and should be binned!The same check can also be carried out for all the small non–polarised ceramic and mylar film capacitors , again trying them across the +140V supply via your 100K 2W resistor!This is a completely symmetrical circuit from both the Y+ and the Y– sections, so resistance checks between each point and the +140V and earth (0V) in turn should help you locate obvious discrepancies between each half!All the small 1N4148 diodes and zeners, etc., are best replaced in one go rather than trying to check them all one by one in this sort of complex d.c. coupled amplifier arrangment – they're cheap enough!Finally, do make sure you've not got a short to earth on either of the CRT Y–plate connections!Regarding the high voltage from the 145 V h.t. secondary of your mains transformer, it will be wound with very thin wire and the percentage voltage regulation will be fairly poor, so with your vertical circuit faulty, it will rise quite a bit on no load compared with the maker's quoted nominal figure!For a 145 V r.m.s. (nominal) secondary, you can expect(1.4 × 145) = 204 Volts across C901, so if you can get a 250 V rated electrolytic of the same diameter as the 200 V component I would obtain and fit one!Pintek's drawings aren't the last word in clarity, but I cannot fault their helpfulness in providing the information in the first place, and my thanks to them and youself for attaching them so we can help with your queries!Chris Williams

author: DavidCG
author_rank: Newbie
timestamp: February 07, 2023, 09:39:12 am
content: hello. I know is an old post but...maybe this is the problem:2SC2570, Check the pinout.probably the ones you bought from ebay are fake ones with the standart BASE-COLLECTOR-EMITTER or COLLECTOR-BASE-EMITTER pinout. I replaced those on my pintek with fake ones(wrong pinout) and haved the same Resistor burned (it also cause to burn the power transistors). I put cheaps 2n3904 taking care on the pinout and now the scope is running...(maybe with reduced bandwith but...well ...)

author: harry dalek
author_rank: Newbie
timestamp: February 24, 2023, 10:55:09 pm
content: Hi i have a rebranded Dick smith scope pintek 200 pretty much the same scope having no vertical position control on the trace and waveform looks distorted /The power supply transformer went and i replaced it with 2 transformers to get the correct AC voltages ....all DC voltages look correct and scope powers up fine .I think the HV wire to the CRT was to close to other wiring and arced causing damage to the vertical position circuits ..i have a schematic

author: harry dalek
author_rank: Newbie
timestamp: February 24, 2023, 11:25:47 pm
content: The added on new transformers and PCB for hook the scope to the new transformers secondariesThe original scopes transfomer died 3 years ago it was over heating blowing fuses taking it out i think it was to small for the job even tried rewinding the primary nope pretty much rat S never to work again ,the new transformers are larger and happly no magnetic problems so thats fine.I think that white wire near the CRT is the HIGH NEGATIVE that caused all the problems apart from the transformer killed off . I will place spacer rubber hose over it should help isolation KV from sparking over close cables .

author: DavidCG
author_rank: Newbie
timestamp: February 27, 2023, 04:58:46 am
content: Quote from: harry dalek on February 24, 2023, 10:55:09 pmHi larry, are you able to measure HV? the first problem i had with my scope was severe arcing (scary)I checked the flyback section components, HV diodes, Transistor and High ohm Resistors (photo1). One of those High ohm (15Meg) resistors in the feedback path of the flyback circuit was dead, so i replaced it. I didn't have an HV probe at the time, so I needed to make one., 10M x9 and 9M x1 in series (99Meg) inside acleanpbc tube, in order to use it with a cheap 1Meg impedance voltmeter. (THIS IS PRETTY DANGEROUS TO USE) i managed to measure the HV with the probe using fixed connections so that I could be located far away from the device or any posible arc (yeah i do exaggerated, didn´t want to take any risk)-1900v was setting and then the arcs dissapearbad news was: several transistors were destroyed by those previous arcsMy suggestion is to check HV first and then go for the vertical section to check if the transistors in there are still alive. ¿do you have another oscilloscope to check the signal path on the vertical section? (maybe the problem is on the final section: power amp)