HP 120B update
Check:
1. Resistance across R304.
2. Resistance from V301 #1 to V301 #6.
R304 is good (enough), measures 500k for 560k.
Then between the plates of the two triodes I measure as expected the sum of the two plate resistors, 650k or so.
So looks good to me...
So now for the update. Spent all day on the thing, as I found it a convenient excuse for not going outside working on the front yard, to do what I was supposed to have already done yesterday....
Schematic below, I hope you like colours (and aren't colour blind...) , it's made to make your life less miserable as you will be reading my essay below. Well I mean, for the one person in a million that might actually do that. One in a trillion more like it.
Inspecting the BoardAccess was not that bad, feeling relieved... 4 screws and then there is enough slack in the wiring to tilt the board almost upside down, away from the edge of the cabinet. So that's good. I could then access the bottom of the board, redid all joints no questions asked. Result ? No joy.
Better access meant easier to clean. So I had another go at cleaning all the blakc dust cooked all over it. A challenge... lots of IPA and a brush is not good enough. Not even one with stiff nylon hair. You do need to rub the board with a paper towel to get that black stuff off.
More pleasing to wok on that board now.
Component testingI first tried to figure out what part of the circuit were not relevant to my HV problem, so I don't waste too much time...
I think 2 sections are irrelevant as they don't seem to interfere at all, be involved in, the generation of the HV. I circled these two in blue :
1) Top right : the voltage divider that feeds the Astigmatism control grid on the CRT.
2) the bottom right part of the schematic : all the circuitry fed by the second secondary winding of the transformer : it takes one input : "Gate" and has one single output : the first grid of the CRT. That's all. zero connection to the rest of the board. I guess it's responsible for blanking the CRT during retracing.
Still, I noted that the string of x3 beefy resistor R323 / 324 / 325 were all way off, +50%. They all measure 12+ Mohms instead of 8.2. So that would need "fixing" later, but it's not gonna cause my lack of HV so I moved on for now.
HV feedback voltage(Area circled in yellow.)
So the first area actually involved in the generation of the HV, that I looked at, was the long voltage divider that sets the feedback voltage to the grid of that triode, V301B, since it gets 150mV instead of negative 6.1V. There are no less than 5 resistors in series + 2 trim pots. So I looked at them all and found one of the x3 beefy resistor string (R309/310/311), to be way off. They are 5.6M. Two are spot on, but the third measures at 25M !
So I replaced it, no luck, zero change to the DC levels I measure, as if I did nothing. Still, it needed doing for sure, so that's one step in the right direction, but the core of the problem must be somewhere else.
So what other component do we have, between the grid of the triode and the output of the voltage divider ? Well we have a couple disc ceramic caps. One at the output of the divider, C305 then another cap directly at the grid of the triode, that's C304. How to test them ?
My reasoning is that all these ceramic caps all over the place on that board are not fundamentally required... I mean, other than the smoothing cap to rectify the voltage after the HV rectifiers, all the other caps are seeing only stable DC voltages. So I don't see how they could be required to generate the HV. They serve a purpose of course, maybe removing noise, handling transients, I don't know... but not essential. So I though well I can just disconnect them and it should not keep HV from being generated.
So I did that. Disconnect first the one at the grid of the tube, because it looked very black / suspicious... though I thought it was probably a red herring and just again that black dust cooked over it, nothing more. Indeed it had no effect on my problem whatsoever. So then disconnected the other cap right at the output of the divider, C305. Again zero change one way or the other.
So not much luck there.
Brightness control(Area circled in Green)
So next area of interest is the brightness control circuitry. As one can see, it is in series with the return path of the HV transformer winding, so obviously it's open circuit there won't be any HV.... But there is a quick test one can do to rule all of that out : switch S3C, the " Beam Finder" switch, shorts the entire circuitry and connect the winding straight to ground. So one just needs to press that switch and if the HV comes back you know the problem is the brightness control circuitry. But pressing that switch didn't change anything to my problem.... so that's not it. Oh well...
Oscillator(Area circled in Red)
So the only part left for me to tinker with was the oscillator itself.
Again because it's all a closed loop system, you never know what part of the loop is keeping the oscillations from happening but.... I thought hey, let's break that loop and work on the oscillator, standalone. I mean, what does the oscillator care if its control voltage comes from the feedback path or... from me just setting the grid at a fixed voltage of my choice ? Eh ?? Schematic says that the grid should be at 92V when it's working so... well I will give it 92V and it's supposed to oscillate eh ?!
So I pulled the 12AU7 tube, the two triodes in the error amplifier, to break the feedback path. Instead, I soldered a resistor at the grid of the oscillator tube to replace the triode. So now I get about 90V. 87.2V to be exact, not 92 but hey should be good enough for something to happen, at the least...
Result ? Still no joy. The oscillator... does not oscillate. The feedback voltage from the transformer / RC network (C202 / R302 / R301 ), still won't budge, still 1 volt or so, instead of negative 123V according to the schematic....
Checking the primary winding of the transformer: it's got a middle tap (pin #7), where the unregulated 400+ V supply is connected to. Then one side of the winding (pin #8] goes to the plate of the oscillator tube, and the other side (pin #6) goes to the RC feedback network. I do see 400V at both ends of the primary winding. That means that at the least, the transformer primary does get its power, and the two parts of the windings are not open circuit, they can convey DC. Now OK that doesn't mean they are good. They may be shorted, they would still pass DC... so I measured the overall resistance of the primary, I get 7+ ohms. Sounds good... that's not a short. Well I guess you could still have one half of the winding shorted and the other good, I don't know... but at least it's not 100% shorted...
Yes both resistors are good. The cap ? A small "domino" style mica cap, 390pF. I measured it, spot on. Of course it might be leaky as they did in my Tek 180A time mark generator... so I did a leak test. It does leak, about 5 or 6 uA. That I guess explains why I have a small voltage at the grid... that leakage current goes through R302 which is 100k, so that makes it drop a small voltage. I replaced the cap with a film cap, and now grid voltage is 0.000V..... no leak any more.
Still, it refuses to oscillate.
Starting to drive me nuts. Winding has power. Tube has appropriate driving voltage at its relevant grid, also has 400V at its plate, all is good. So why the fuck doesn't it oscillate... well bad tube perhaps but again, the point this week-end is we are trying to see if anything other than the tube, which I can't replace right now short of having one on hand... might be at fault.
So I wanted to see the capacitor charge, the voltage evolve at the grid of the tube, i.e across resistor R302.
Because at this point what we have is now simple : cap C302 charging through resistor R302, between ground and 400V.
So even if it does not oscillate, at the least I should be able to see the transient stage, where at power up, the 400V rail comes up and charges the capacitor, once.
This should produce a positive pulse/spike across the resistor. R is 100k and C is 390pF, so that's a 39us time constant. So setting the sweep speed of my scope to 50us say, should let me see the shape of that pulse / exponential just fine.
But.... for the life of me I can't capture that transient ! For all I see, there is no pulse, it's at zero volt right from the start, no charging of the capacitor going on ?!
I am baffled, does not make sense..... ground on one side, 400V at the other, this cap needs to charge FFS how it could it not do that ?!
So that's where I am at...I am a bit stuck now... waiting to receive a new oscillator tube or two from evil bay.
Baffled I am, a freaking cap that won't charge ?! What has the world come to !
Hope you like this video. If you did please like share and subscribe and get some merch and donate on Patreon please. Every donation will go toward the old boat anchor restoration fund. Thanks in advance.
Yours Truly.
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