HAMEG 203-7 CRO High Voltage Power Supply Analysis Questions - Page 1
Forum section: electronics / beginners

author: pplaninsky
author_rank: Regular Contributor
timestamp: June 15, 2020, 04:31:36 pm
content: Hello,I hope this is the right place to post this.I would start with a disclaimer:I am not trying to zap myself with high-voltage. I even do not own that particular CRO. I am just curious how things work.This has been driving me crazy. The HV PSU of the HAMEG 203-7 (old 20MHz CRO from the 90s) has 2 rails, created by voltage quadrupler.Please, check the attached screenshot.The following questions are bugging me right now and I am not sure I have the right answers:1. Why the quadrupler uses 2 diodes in series? I am sure it is not the way of handling more voltage or current. Maybe some safety? Or seeking intentaional forward voltage drop of a diode?2. Once the -1975 V DC is created by the quadrupler another rail of -1913 V DC is created with a zenner diode. However, why the feedback regulation between the two lines with LM741? Why the antiparallel diodes attached to the LM741 - protection from ESD from the CRT? How exactly this regulation works? Switching on the 3 transistors and shorting to ground? Why 3 transistors in series?3. And finally, the most misterious thing of all, the third rail of -1890 V DC is created by half rectifying a 31 V~ from the transfromer and feeding back -1913 V DC into it. How exactly this works? Isn't it dangerous to DC bias a transformer with such voltage. Also, from the same tap 6.3 V~ is taken for the cathode heater. What the heck is going on here?I would really appreciate if someone can at least give me some hints, explanations or speculations on that topic.I am attaching a screenshot from the service manual for easy reference.For those interested in details - here is the full manual. It is in German, but page 32 onwards has the schematics in English as well.https://cdn.rohde-schwarz.com/hameg-archive/HM203-7_deutsch.pdfThank you in advance!

author: TimFox
author_rank: Super Contributor
timestamp: June 15, 2020, 05:34:59 pm
content: The EM513 diode is rated at 1600 V PIV. Putting two in series seems reasonable for the voltage here, although I would have added very high value resistors in parallel with each to equalize the reverse voltages.

author: pplaninsky
author_rank: Regular Contributor
timestamp: June 15, 2020, 07:17:46 pm
content: Hi TimFox,thank you for the hint.After your answer - I actually realized that there is really high voltage, so you need more than one diode to handle it.Also, more than one transistor to handle it, which I found out is named a "cascode". Old, thing that goes back to vacuum tubes.* I think they use the CASCODE to short the high voltage to ground, according to feedback coming from the Op Amp. Is that correct? Is it related to the "blanking" of the CRT?* Still, using the 31V~ along with -1913V DC to create the -1890V DC is complete mystery. If you have ideas, please share.Thanks!

author: TimFox
author_rank: Super Contributor
timestamp: June 15, 2020, 09:48:46 pm
content: The three transistors in series form a type of cascode, to make a high-voltage transistor. (The usual cascode has constant voltages on the bases of the upper devices.) The current from the bottom transistor flows through the other two, and the total voltage divides across them with the base resistors.It looks like what we used to call a "bouncer": a 500 V amplifier in series with the bottom of an unregulated high voltage, to allow the op amp circuit and feedback to stabilize the total high voltage. This works, so long as the unregulated high voltage comes from a transformer, so that the bottom end of the rectifier is not grounded.

author: pplaninsky
author_rank: Regular Contributor
timestamp: June 16, 2020, 12:48:58 pm
content: Hello TimFox,thanks once again for your help and continuing engagement on this topic.This whole matter is a bit over my head and I hope I am not pushing my luck with more questions.* Why do they send negative high voltage DC to another transformer coil (the 31V~)?* When you say "stabilize the total high voltage" - what do you mean? The voltage output of the quadrupler referred to ground?

author: TimFox
author_rank: Super Contributor
timestamp: June 16, 2020, 01:26:57 pm
content: The 31V rectifier is connected to the negative high voltage, which goes to the cathode of the CRT. This is probably used to bias the grid negative with respect to the cathode to control the beam intensity and brightness.It is necessary to stabilize the high voltage applied to the CRT, since the deflection of the beam depends on the ratio of the deflection voltage to the accelerating voltage.Note that all voltages shown by the manufacturer on his drawing are referenced to ground.

author: pplaninsky
author_rank: Regular Contributor
timestamp: June 22, 2020, 03:55:32 pm
content: Hello TimFox,Thank you once again for the hint. I took my time to further investigate and try to understand better.Some aspects I have figured out myself, others are still mystery to me.On the attached schematics.Blanking signal comes on the ZUP line.This turns-off T304, which puls down the 12V rail, which turns off the LED in the optocoupler.This turns off T307, because now there is no current through its base supplied anymore.This raises the cathode to -1890V, which becomes more positive than G1 and therefore extinguishthe trace on the CRT.Now, that is all clear however, I am missing some details:1. While T307 is on, it connects both rails (-1913V and -1890V) via R3036 (221K). At this time are they at the same potential? It seems no current is flowing through them.2. What is the role of T308 transistor?3. Is -1890V created just by the Zener D313?4. Why -1890V goes back to the transformer, where it is mixed with 31V~ and the -1913V (Orange and Violet lines)5. Last, but not least, when the LED D316 will ever light up?Thank you in advance!

author: Greybeard
author_rank: Frequent Contributor
timestamp: November 25, 2021, 08:30:30 pm
content: Quote from: pplaninsky on June 22, 2020, 03:55:32 pm1. No, -1913V and -1890V stay constant. R3036 (221K) is just a pull-up resistor between cathode and -1890V line.2. T308 is switched on for a short time at falling edge of ZUP. It improves switching time of the opto coupler and RC-circuit charging time build by cathode capacity and pull-up R3036.3. Exactly.4. ~31V winding is rectified by D311 to generate an aux voltage for z-modulation (delta V ca. 23V)5. The LED never lights up, it's to low energy or to low charge that comes thru C314 from the rising egde of the 0.8-4.4V blanking signal at the beginning of the deflection ramp. I think the LED is used to limit the max. puls B-E voltage of the photo transistor. The LED has a very "sharp" zener curve at about 2V, nearly ideal to limit voltages or used as cheap voltage reference (not metrological).Can anybody check the voltage at "+500V typ." test point (HM203-6 or 203-7)?