Oscilloscope flyback trace visible - Page 1
Forum section: electronics / repair

author: Mike99
author_rank: Frequent Contributor
timestamp: May 23, 2025, 04:52:09 pm
content: I'm refurbishing a vintage radio test set and it has this problem in oscilloscope mode:The output of the blanking pulse generator is connected to the cathode of the CRT via a 0.1uF capacitor. This is the waveform on the capacitor input:And this is on the other side of the capacitor (i.e. the CRT cathode):Does this suggest that the capacitor has failed?Mike

author: kreutz
author_rank: Contributor
timestamp: May 23, 2025, 05:17:29 pm
content: There is also the possibility of a short circuit between cathode and filament. It frequently happens when the crt cathode emission has been boosted by increasing the filament voltage.

author: Pentium100
author_rank: Frequent Contributor
timestamp: May 23, 2025, 06:04:34 pm
content: It could also be a bad capacitor. Does the brightness adjustment work or is the trace always stuck on full bightness?I do not know about this particular scope, but I have seen a few oscilloscope circuit diagrams and on those the heater was connected to the cathode because the cathode was at a high negative voltage (unlike a TV where the heater is usually grounded and the cathode is about 100V positive).

author: CaptDon
author_rank: Super Contributor
timestamp: May 23, 2025, 07:31:59 pm
content: Your heater and cathode will be connected together. Look at the scope shot, -1700 volts. Anyway, look for a master brightness control or C.R.T. bias control and turn it down a bit. If the bias is set wrong the blanking circuit can't do it's job properly. Check for any resistors that may be off value in the C.R.T. cathode circuit. Because the blanking is A.C. coupled there needs to be some sort of a 'D.C. Restorer' circuit, often a simple resistor that stops a shift in the coupling capacitor's net charge which will also cause incorrect blanking. Usually, when I see evidence of retrace it is almost always from the master brightness / bias being set wrong to compensate for a weak or dim C.R.T.

author: Mike99
author_rank: Frequent Contributor
timestamp: May 23, 2025, 09:18:59 pm
content: Thanks for all your suggestions. I could post the schematic if that would help.All the controls work normally. The scope bandwidth is 10MHz and the problem is only visible with signals greater than 10kHz if that means anything.Mike

author: Mike99
author_rank: Frequent Contributor
timestamp: May 24, 2025, 06:59:53 am
content: OK here is the schematic. The blanking pulse generator is Q120 and Q121.I don't have much experience of fault finding oscilloscopes as I'm very wary of the high voltages, but my money is on a failed high voltage capacitor C135 (and possibly C137). I will try changing them and see if that helps.Thanks,Mike

author: VK2AZT
author_rank: Newbie
timestamp: May 24, 2025, 07:29:01 am
content: Check the condition of the paper in oil high voltage capacitors C135, C137, C138, C139 & C140. They tend to leak as they age. They're all 0.1uF but two are rated at 1KV and the others are 2KV.C135 may be causing your problem with the retrace line.Cheers,Ian.

author: Mike99
author_rank: Frequent Contributor
timestamp: May 24, 2025, 10:41:03 am
content: Thanks Ian, I will replace them all with 1kV and 2kV polypropylene caps.Mike

author: Mike99
author_rank: Frequent Contributor
timestamp: June 07, 2025, 04:39:54 pm
content: I have spent hours on this now with no improvement to the problem. I replaced all the high voltage oil/paper capacitors with metallised polypropylene and all the electrolytics in the horizontal sweep generator.The cause of the problem can be seen in the attached screenshot. On the highest sweep speed the blanking pulse does not align with the flyback part of the horizontal sweep. It is also 10V lower than the pulses on the lower ranges.Also attached is a partial circuit diagram. The horizontal sweep generator is Q118/Q119 and the blanking pulse generator is Q120/Q121.Q121 is a 2SC1569. I replaced this with a NOS device and on the highest sweep speed the blanking pulse disappeared completely. I deduced from this that the circuit is sensitive to the transistor parameters (input capacitance?) and looked around for any select-on-test components. C150, which is shown as 68pF on the circuit diagram, was 47pF with 100pF tacked on under the PCB. I tried varying this value but it just became unstable.I then found two more 2SC1569s in the circuit that feeds the horizontal sweep to the CRT and swapped one of those with Q121. Not only did the pulse disappear on the highest sweep speed but so did the horizontal sweep!I have concluded that the whole design is marginal and probably can't be fixed but I thought I'd put it out there for any further suggestions.Thanks,Mike

author: D2236
author_rank: Regular Contributor
timestamp: June 09, 2025, 06:03:03 am
content: If it is not too much of a hassle, it will be helpful to see the following plots:D108-cathodeQ118-collectorQ119-collector & emitter

author: Mike99
author_rank: Frequent Contributor
timestamp: June 09, 2025, 07:23:45 am
content: Certainly, I'll do that when I get home this evening.Mike

author: D2236
author_rank: Regular Contributor
timestamp: June 09, 2025, 07:27:36 am
content: Here are the wave-forms I expect to see on the various points.

author: ondraN
author_rank: Regular Contributor
timestamp: June 09, 2025, 09:59:42 am
content: Try to display the signal on Q118e and Q121c simultaneously on an oscilloscope. You will probably see a delay in the signal on Q121c. It could be a degradation of the transistors, or some resistor in their circuits. I would try to eliminate or reduce C150 first.

author: Mike99
author_rank: Frequent Contributor
timestamp: June 09, 2025, 02:28:10 pm
content: It looks like you simulated the circuit. Did you use TINA-TI?I'll post the actual measurements this evening.Mike

author: Mike99
author_rank: Frequent Contributor
timestamp: June 09, 2025, 04:59:16 pm
content: OK here are the screenshots, all taken at the highest sweep speed. Sorry I don't have a four channel scope!Q118_b = 1.pngQ118_e and Q121_c = 2.png (Q118_e is the purple trace)Q119_c = 3.pngQ119_e =4.pngondraN you were correct - there is no delay on Q121_c on the lower two sweep speeds, but a slight delay on the third and significant one on the highest.Incidentally I changed Q118, Q119 and Q120 but that made no difference.Mike

author: Mike99
author_rank: Frequent Contributor
timestamp: June 09, 2025, 06:05:14 pm
content: Apologies, it's been a long day and two of the traces I posted above were incorrect.Attached are the correct screenshots for Q119_c = 3.png and Q119_e =4.png.Mike

author: ondraN
author_rank: Regular Contributor
timestamp: June 09, 2025, 06:29:18 pm
content: And what about rezistors R135-137, R198 Are OK?And I would check C117 if it doesn't have high ESR or replace it straight away.

author: D2236
author_rank: Regular Contributor
timestamp: June 09, 2025, 07:23:12 pm
content: Apart from the obvious checks like resistor values and dried electrolytic's, it will be interesting to see if you replace the 2SC1569 with something like a MPSA44/5 for testing. The 2SC1569 seems to have delayed turn-off in your case, and since you already swapped parts, testing the MPSA44 in there will be interesting. This part does not have a high dissipation in this circuit, and it appears it was used there because it was already used elsewhere in the design. The MPSA44 is the model I used in my simulations.

author: Mike99
author_rank: Frequent Contributor
timestamp: June 09, 2025, 08:42:40 pm
content: I checked the resistors in circuit and they all seemed about right. Also replaced all the electrolytics and checked the two diodes (D108 is germanium).I don't have an MPSA44 but I have ordered MJE15030 and BD243C to try as they seem compatible.I will report back.Mike

author: D2236
author_rank: Regular Contributor
timestamp: June 09, 2025, 10:09:50 pm
content: Quote from: Mike99 on June 09, 2025, 08:42:40 pmThese parts are slow and may not work very well. I simulated the circuit with several options, for you to compare. Even the BD139 which is faster than the other two, is too slow for this. If you slow the turn-off through Q120 you get the same effect.One thing you could try while waiting for parts is to play with a resistor between b-e of Q120. Start with around 10 k and work your way down (fit a small pot) to see the effect on the turn-off delay. The idea is to get Q120 out of hard saturation and enable faster turn off times.

author: Mike99
author_rank: Frequent Contributor
timestamp: June 10, 2025, 07:06:38 am
content: Thank you so much for your assistance with this, it's very much appreciated. I will try the resistor and also order an MPSA44.

author: D2236
author_rank: Regular Contributor
timestamp: June 10, 2025, 06:41:20 pm
content: I thought of replacing Q120 with some small PMOS device along the lines of BS250P or similar. It seems to produce fast clean switching in my simulation. It may be worth a try.https://www.diodes.com/datasheet/download/BS250P.pdf

author: Mike99
author_rank: Frequent Contributor
timestamp: June 11, 2025, 07:51:58 am
content: Interesting idea, thank you. I don't have any in my parts bin but will get some and give it a try.Mike

author: D2236
author_rank: Regular Contributor
timestamp: June 11, 2025, 04:07:56 pm
content: You should be able to remove C150 with this, as the MOSFET already has about 60 pF of gate capacitance, and you don't need more loading on that node.

author: Mike99
author_rank: Frequent Contributor
timestamp: June 11, 2025, 06:53:46 pm
content: Thanks, I'll report back in a few days.Mike

author: Mike99
author_rank: Frequent Contributor
timestamp: June 13, 2025, 10:48:40 am
content: I have some results for the blanking pulse with a selection of transistors for Q121. I left C150 at 47pF in each case.The transistors are:2SC1569 (the original device)MJE15030BD243MPSA44The first three were as expected from the advice I've been given here. The last one was a surprise ...I don't have any BS250s for Q120 yet.Mike

author: D2236
author_rank: Regular Contributor
timestamp: June 13, 2025, 02:18:42 pm
content: It seems like Q120 may be still slow to turn-off. In my sim I used BC212 for Q120. It will be interesting to see what the P-FET will do. Have you played around with the resistor yet?

author: Mike99
author_rank: Frequent Contributor
timestamp: June 13, 2025, 04:09:12 pm
content: Yes I tried the 10k pot but that didn't do anything.Mike

author: D2236
author_rank: Regular Contributor
timestamp: June 13, 2025, 04:32:43 pm
content: You should only start seeing any real effect with the value below 2k between b_e. Below 1k will be too low and might stop the sweep circuit. I am surprised that you saw no effect though.

author: Mike99
author_rank: Frequent Contributor
timestamp: June 13, 2025, 07:14:08 pm
content: OK I will try it again.Mike

author: Mike99
author_rank: Frequent Contributor
timestamp: June 14, 2025, 04:23:24 pm
content: I fitted a 2k pot across Q120 b-e. No change at maximum value. As I decreased it the amplitude of the horizontal sweep sawtooth increased but the blanking pulse did not change.Mike

author: D2236
author_rank: Regular Contributor
timestamp: June 15, 2025, 05:59:26 am
content: Hopefully, the PFET gives us the fast turnoff that is required at the start of the flyback period.

author: Mike99
author_rank: Frequent Contributor
timestamp: June 16, 2025, 07:13:23 pm
content: OK here's the latest. I left the MPSA44 in for Q121, removed C150 and fitted a BS250 to C120.Now the blanking pulse is in the correct place for all the sweep settings, but the amplitude measured at the collector of C121 is negligible ...Mike

author: D2236
author_rank: Regular Contributor
timestamp: June 16, 2025, 07:51:39 pm
content: Mike,Please make double sure that the PFET is connected correctly. It appears that it stays in off condition now. I will study it a bit more and come back to you. I may even knock up a test circuit to verify as soon as I have some spare time.The gate threshold voltage for the BS250 can be anywhere from -1 to -3.5 V. Looking at the amplitude of your sawtooth wave form; it is quite a bit less than the simulation shows, so it may not be enough to cross the threshold of the particular device you are testing with, keeping it off. I may have some devices with a smaller threshold here in the lab that I can test and compare with.

author: Mike99
author_rank: Frequent Contributor
timestamp: June 16, 2025, 09:45:01 pm
content: Quote from: D2236 on June 16, 2025, 07:51:39 pmSorry I measured the sawtooth in the wrong place for that picture. Here it is measured on the CRT, same as the previous ones:As for the BS250 I did note that there are two different pin configurations. I used the one for a TO-92 package but I can always try turning it round.Thanks again for being generous with your time. I feel that we are getting somewhere!Mike

author: D2236
author_rank: Regular Contributor
timestamp: June 16, 2025, 09:59:48 pm
content: I put a quick circuit together. I am still looking for the different FET's to continue testing, but at least I can do some real circuit comparisons. I used BC212 for the circuit, as in my simulations. I also used a MPSA42 for Q121 and BC212 for Q120. The same slow turn-off is visible in my circuit, although there is rapid switching on node VQ118_e.The plots show:(Ch1) VQ119_e(Ch2) VQ121_c(Ch3) VQ118_e

author: D2236
author_rank: Regular Contributor
timestamp: June 17, 2025, 12:24:53 am
content: Still working with just the BJT's. I could reduce the turn-off time by inserting a resistor (27k-33k) in the base lead of Q120. I think it helps it from not going into hard saturation and thus faster turn-off. I will do more testing, but I like what I see. I will be testing with the FET's later. My supply voltage to Q121 is a bit low at the moment. It is running of a 100 V rail, but need about 200 V. I will test at the full voltage later when things look good with the rest of the circuit.Simulation show the same result with the 33k.

author: Mike99
author_rank: Frequent Contributor
timestamp: June 17, 2025, 07:22:22 am
content: I did have the BS250 the right way round. So the blanking pulses are correctly timed and the right width. We just need to make them bigger 😀.Mike

author: D2236
author_rank: Regular Contributor
timestamp: June 17, 2025, 03:00:08 pm
content: The BS250 is not switching. I will see how it is different when I test FET's with lower Vgs thresholds.

author: Mike99
author_rank: Frequent Contributor
timestamp: June 18, 2025, 12:22:53 pm
content: I've made some progress ...Supply = +175VQ120 = MPSA92 with 33k in base leadQ121 = MPSA44C150 = 82pFThe pulse timing is perfect, just need to increase the amplitude.Mike

author: D2236
author_rank: Regular Contributor
timestamp: June 18, 2025, 02:32:52 pm
content: You can make the 33k lower and see how much you can increase the amplitude while retaining the timing. The MPSA92 may be slower and low gain being a high-voltage transistor. Q120 doesn't have to be the high-voltage type. You can try something like a common 2N3906 for Q120.

author: Mike99
author_rank: Frequent Contributor
timestamp: June 18, 2025, 04:24:03 pm
content: Thank you for nudging me in the right directionHere is the blanking pulse with a 2N3906 and 27k base resistor:And here is the resulting trace, much improved![ Specified attachment is not available ]Now I just need to flatten the top of that pulse somehow.Mike

author: D2236
author_rank: Regular Contributor
timestamp: June 18, 2025, 05:23:25 pm
content: The pulse should be flatter at the top if you decrease the resistor a bit more. How is the retrace on the CRT now?

author: Mike99
author_rank: Frequent Contributor
timestamp: June 18, 2025, 05:56:33 pm
content: That was the unavailable attachment 😀.It's much better now.

author: D2236
author_rank: Regular Contributor
timestamp: June 18, 2025, 06:14:55 pm
content: Yes, it is getting there. Compared to the other picture most of the flyback retrace is blanked out.

author: Sorama
author_rank: Frequent Contributor
timestamp: June 18, 2025, 06:18:27 pm
content: Well done, both of you!

author: D2236
author_rank: Regular Contributor
timestamp: June 18, 2025, 06:33:47 pm
content: Mike, you may want to play around with the value of C150, making it smaller I would think.

author: Mike99
author_rank: Frequent Contributor
timestamp: June 18, 2025, 07:33:33 pm
content: I think it's well done D2236, they have been an inspiration.I did play with C150. Removing it completely didn't change the sweep picture.I think I'm happy with the outcome unless there is anything else I can try?Mike

author: D2236
author_rank: Regular Contributor
timestamp: June 18, 2025, 07:50:10 pm
content: Quote from: Mike99 on June 18, 2025, 07:33:33 pmI will call it a team effort. If you don't mind, I am waiting for some FET's to be delivered (could not find the ones I was looking for), to explore this option. I had to order some parts for the lab so I included the FET's to experiment with. It will be nice to see if this can perform better than the BJT for Q120.

author: Mike99
author_rank: Frequent Contributor
timestamp: June 18, 2025, 09:19:56 pm
content: I shall await the outcome of your FET experiments.The visible flyback is now so short that it can't be seen when the trace is centred 😀😀.Mike

author: Mike99
author_rank: Frequent Contributor
timestamp: June 20, 2025, 04:06:29 pm
content: Out of idle curiosity I replaced the MPSA44 with the orginal 2SC1569 for Q121 and it made no difference to the flyback:The advantage to this is that the PCB looks more original than with a TO92 with its legs crossed.The blanking pulse looks similar too:So I guess that Q120 is the critical component here.Incidentally, the spec for the CRT blanking voltage is -28.5 to -67.5V.Mike

author: D2236
author_rank: Regular Contributor
timestamp: June 20, 2025, 04:22:54 pm
content: I think the problem was caused by the slow turn-off from Q120. I should get the FET's by the weekend to test with.

author: D2236
author_rank: Regular Contributor
timestamp: June 21, 2025, 05:47:09 am
content: So, I revisited the BS250. I connected the gate to VQ118_b and got a nice-looking pulse. You can see how it performs in your circuit.I will do more testing with the other FET's. It is more of a pain with the breadboard since they are SMT devices.

author: Mike99
author_rank: Frequent Contributor
timestamp: June 21, 2025, 09:43:17 am
content: Breadboarding SMDs can be tricky but there are adapter boards available:https://www.ebay.co.uk/itm/235406316627I tried connecting the BS250 gate to Q118 base. The pulse shape does look good, but the amplitude has dropped from 40V to 15V. Could that be due to RDS(on)?The scope trace looks the same as the 2N3906, except if I crank up the intensity when I get this:Mike

author: D2236
author_rank: Regular Contributor
timestamp: June 21, 2025, 04:57:20 pm
content: It is obvious that the voltage drop over D107 (orig diagram), influences the amplitude available at Q121, since it will affect the gate threshold voltage via the b_e of Q121. I have added another 1N914 (2 x 1N914's in my case) in series and the amplitude difference is clearly visible in the two plots. Your circuit and mine may behave different since we also have the production spread (-1 to -3.5 V) on Vgs threshold that can affect things as well.Those adapters look very handy!

author: D2236
author_rank: Regular Contributor
timestamp: June 21, 2025, 06:46:59 pm
content: So I tried with two other devices (DMP2035 and IRLML6402). The first one did not work at all. The last one sort of work (see plot) with the gate position at Q118_e, but timing and amplitude are not good. So as a conclusion,the 2N3906 with base resistor works very well, and the BS250 may work with modification around D107, and the gate connected to Q118_b.

author: Mike99
author_rank: Frequent Contributor
timestamp: June 22, 2025, 10:17:47 am
content: I tried the BS250 again with its gate connected to Q118 base and a 1N4148 in series with D107.As you predicted this has increased the amplitude of the blanking pulse nicely:Here is the trace at normal intensity:And here it is at maximum intensity:So I would say that it is probably the better of the two solutions, although the 2N3906 is good enough.My next challenge is finding a neat way to incorporate the mod on the PCB as I plan to sell this when I have finished aligning it.Mike

author: D2236
author_rank: Regular Contributor
timestamp: June 22, 2025, 02:32:49 pm
content: It is amazing what one extra diode voltage drop can make. It was an interesting exercise, and I am glad we could resolve the problem. I am curious how another SM-220's measurements in this area will compare. Maybe someone with a similar unit can take some measurements for us to look at. Then only may we determine if the original design is marginal or not.Thanks for playing along!

author: Sorama
author_rank: Frequent Contributor
timestamp: June 22, 2025, 03:01:16 pm
content: Quote from: D2236 on June 22, 2025, 02:32:49 pmI guess we all have to thank you!

author: Mike99
author_rank: Frequent Contributor
timestamp: June 22, 2025, 03:38:00 pm
content: Quote from: D2236 on June 22, 2025, 02:32:49 pmAnd thank you for all the time and effort you've spent helping me resolve this. I have learnt a lot.I decided to go down the 2N3906 route as it was the easiest to implement neatly. Removing C105 made it much easier.The final job is to replace the electrolytics in the power supply. On audio frequencies the lower part of the sine wave fades out before the upper part when the intensity is reduced. I'm guessing it's hum on the CRT supplies.Mike

author: Mike99
author_rank: Frequent Contributor
timestamp: June 22, 2025, 03:39:37 pm
content: I forgot to say I've asked on a Kenwood forum for any other owners to see if their units are the same. I'll report back if I get any replies.Mike

author: Mike99
author_rank: Frequent Contributor
timestamp: August 10, 2025, 06:46:46 pm
content: I sold the station monitor to a chap who already had one and he sent me a photograph of the two of them in action (mine is at the top).So you can see after all the effort that went into trying to fix the bug it's actually a featureMike

author: D2236
author_rank: Regular Contributor
timestamp: August 10, 2025, 07:12:57 pm
content: I thought your initial observations reported more severe flyback lines? Thanks for the update.

author: Mike99
author_rank: Frequent Contributor
timestamp: August 12, 2025, 04:20:11 pm
content: Yes it did, although it was frequency dependant. I also found that the severity varied when I replaced the original transistors with NOS devices. I think the design is marginal at best and very dependant on device characteristics.Mike