Hameg HM412 CRT scope - no trace - Page 1
Forum section: electronics / repair

author: TinkeringSteve
author_rank: Frequent Contributor
timestamp: August 14, 2021, 01:10:54 pm
content: It's roughly this, but the front plate does not reveal a variant like 412-4, only 412.EDIT: Link 'corrected'(?). This looks more to be the correct one. Took a while to find!I got it cheap, and do not intend to do stuff like calibration - just want to play a little, not seriously measure.I already saw the warning in the manual about 2 kV being on the CRT socket & PCB. That is somewhat intimidating. I have worked with guitar tube amps, but this is another levelSo, if I turn it on and wait some minutes, nothing is to be seen on the screen,when intensity is turned full up.(the power LED is on)I opened it up and, so far, saw nothing super suspicious (like burst or bulging electrolytics).There are these brown-reddish crusty rings around many solder joints, not sure whether it's just old resin flux or corrosion.How would one go about troubleshooting such a thing?Not sure where to start.Edit: Ok, I saw the manual states expected voltages on CRT socket pins.I don't have anything tomeasure 2+ kV, if that becomes one of the steps. Can oneimprovise some HV probething to measure with a DMM without electrocuting oneself or cause other damage?Pics:The 1st one:Is that black thing to the right one of those pots with some sort of gear inside, to rotate around several times?This thing seems to have no end in either direction... turns forever. On the front panel it says "1x ... 10x" next to delay.The others: "mmmh, crusty..." on the solder joints anyway.Last one: What type of caps(?) are those huge blue ones on the left?

author: TinkeringSteve
author_rank: Frequent Contributor
timestamp: August 21, 2021, 07:17:40 pm
content: Ok, that schematic of the CRT board in that linked manual is the only one I found where the connector does match the one in my scope. So I guess it is that version. (it doesn't say on the front or anywhere).The voltages on that test connector I measured. The ones below 1000V absolute, directly with the DMM probes.The -1900V I did with a (well insulated) DIY probe from a ~ 100:1 resistor divider of 99MOhm + 1MOhm. I also measured the 4 lower voltages with that probe, added that up and divided it by the sum of the 4 directly measured voltages, to get a compensation factor for that inaccurate resistor divider. (couldn't perfectly measure the entire resistance of the thing with my DMM)Nominal:-1900-12.0+24.0+260+140Measured:-1825-11.8+23.7+263+139Need to dig for the acceptable tolerances. The "-1900" rail is almost 4% off.

author: MAS3
author_rank: Regular Contributor
timestamp: August 21, 2021, 09:08:29 pm
content: Hi.The black bar is a multiturn trim potentiometer.That means it is supposed to have 10, 15 or even more turns between either ends.After it reaches an end, the "runner" (by lack of a better word) will run off of the thread, but stay in place so you can start turning the other way.If you listen closely you can hear it clicking while you keep on turning in that direction after it has reached an end.The capacitors look like MKP (Metallized Polypropylene Capacitors), the color reminds me of Philips (that division is now part of Vishay).It will have a value, a type (MKP) and rated voltage printed (in white) on it.

author: BrokenYugo
author_rank: Super Contributor
timestamp: August 21, 2021, 10:55:32 pm
content: Can you see the heater glowing? Have you thoroughly played with the controls, especially the position, sweep (try XY mode?), and intensity controls? Sometimes it's just dirty controls, keep the focus pot cranked one way or the other in the event it suddenly decides to project a dot.

author: bdunham7
author_rank: Super Contributor
timestamp: August 22, 2021, 12:18:00 am
content: Quote from: TinkeringSteve on August 21, 2021, 07:17:40 pmThe exact voltage doesn't matter as much as the difference between pin 7 and 3 of the CRT. Don't try to measure the difference directly! Just measure one then the other, carefully, with your divider.

author: TinkeringSteve
author_rank: Frequent Contributor
timestamp: August 22, 2021, 11:55:24 am
content: Heater glowing: YesIntensity + focus: Fully crankedVoltage pin7-->pin3: 58.5 VI noticed a funny thing.When I held my ~ 100 MegaOhm-to-GND probe against pin3 "WE" of the CRT socket, I heard a very high pitched, barely audible, sweep of perhaps half an octave until the sound went away. I was able to repeat that 1..2 times, but then no more.Now it's back to what it was before all along: I don't hear anything, like I would expect from a TV CRT around 16 kHz (which I still hear very well).Should this thing have any sound to be "healthy" - is that already a clue, if I don't hear a thing?---Edit: Found a video where someone had a similarly old Hameg, and a comment mentions that they have early tantalum caps that are visually not recognizable as defect, with his device it caused X deflection to not work.This thing seems to have some of them here and there: See picture, the brown blob in the center, is that one of them buggers, or did they already have the pearl shape they had in the 90s? (what is that brown thing, in any case?)Edit #2: The HV board has some voltages written as copper on it, I checked them. Top left one is totally off, 267instead of 175 V ! I have yet to look in the schematic what that is. It's worse. In the layout, the solder blob where I thought should be 175V is the end of R550 close to the board edge. That has a trace to the +260V test conenctor, so that can't be it. Must be the one to the right next to it - which measured 530V, argh! (in the schematic, below it, page 21, right half, left upper corner. Big fat R550 is dran as dottet line and horizontal, left to a heatsink. The real point them mus be below the left end of R550, i.e. R536, which is also connected to T512 on that net. T512 is on the bottom of the page, center, above 4-diode rectifier. That would be a "+140V" rail then, on which I measured 530V... why does the PCB label say 135V then, and why is my 140+V test connector pin okay...I'm confused.Ok - I want to slap the guy who made the circuit. There are two different transistors, one TO92 (left edge, slightly below center), and one TO126 with screw hole (right edge of board, higher than center), on page 21 on the layout.Why would anyone call two parts the same designator?! Argh. Need to check this out further, but it's clear that the TO with the screwhole is the 140V refulator and the little one whereI measured > 500V I need to find yet in the schematic. DUH, the "m" in the "mV" on that little "ANENG" DMM was so smallStill don't get that abou the supposed 175V where I measure 267V, though.I saw that the read panel as apparently a deflection signal output on the rear panel. I hooked up my regular scope.All I see is a constant voltage, which I can adjust, with the "X Pos" pot, between close to 0...60V. I played with all the knobs, there never is a sawtooth signal.-If I enable Hor.Ext ( = XY mode ?), the screen is no longer totally dark - it has a gray glow on the whole surface, and I can adjust the brightness of that glow by the left channel's amplitude knob. Then, I can move that glow, as if a dim flashlight with lens was held behind the screen and moved, out of the screen either top or bottom, depending on which direction I turn the left "Y Pos" knob.Also, with above setup (XY mode on but no signal) if I turn the focus knob fully clockwise, the whole screen glows gray. If I turn it fully opposite, only a smaller radius at the right edge of the screen glows - see fotos.From the top foto you can also see the front of the screen glow.

author: shakalnokturn
author_rank: Super Contributor
timestamp: August 25, 2021, 11:22:29 pm
content: It seems you may have multiple problems.I'd start by trying to get the spot back on screen.Check that you have 140V supply to the Y output stages (L501, L502) then measure and report voltages onY deflection plates (points D3, D4).Same for X output stage: 260V at C516?Voltages at X plates (D1, D2 points)?Out of experience I'd check the BF199 transistors in the Y amplifiers but that's looking ahead...