HP 8557A Spectrum Analyzer in HP 182T display Repair Part 1 (182T power supply)
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- Опубликовано: 7 авг 2024
- In part 1 of this video series I attempt to tackle an intermittent and elusive fault on the 182T CRT display
Part 2 here! • HP 8557A Spectrum Anal...
0:00 - Intro/Overview
8:53 - Visual inspection/teardown
12:55 - 182T power rail testing
14:50 - scratchy intensity pot symptom
15:40 - 3kV test point check
16:25 - A2 control module PCB cleaning
17:16 - New fault symptom, loss of HV supply
18:44 - All low voltage rails found to be low!
20:12 - Isolation of power supply module from unit, LV rails back up
22:30 - Isolating the HV supply
24:00 - small rant about poor design
25:33 - LV rail testing with HV supply & CRT disconnected
27:13 - Reconnecting HV/CRT and CRT powers back on!
30:00 - injecting calibrator signal to ext input - CRT shuts off again…
31:26 - troubleshooting the low LV rails again
36:15 - isolating the power supply module 2nd time
37:24 - checking +100V rail on scope
38:16 - checking filter caps for ripple C1-C4
41:26 - testing series regulator transistors Q1 and Q4 on curve tracer
44:23 - testing VR2 9V reference Zener on curve tracer
47:32 - testing power supply module after Zener replaced with 8.2V
48:52 - adjusting +100V to spec
49:11 - checking all LV rails - all in spec!
49:57 - testing with unit reassembled
53:55 - Outro 
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For intermittent failures like your intensity adjustment, I like to gently tap everything with a screwdriver handle. Sometimes that helps to isolate the fault and locate the bad pot, oxidised contact or bad solder joint.
thanks for the tip!
Interesting... i'm working on a 182t with psu problems also!
Definitely check that 9V zener VR2 before going down a bunch of rabbit holes like I did! 😂 Having a curve tracer definitely helped because I was able to kill the zener for good by increasing the collector voltage. Really helps with those intermittent issues when the source is a failing semi-conductor!
Cool ! Don't leave the beam on in that one spot you don't want to burn the phosphor, de focus or turn it down, of course you know that ! I would love one of these but I would like the low frequency stuff like DC and up. What happens when you press the multiplier button ? and the display buttons ? stuff like this would help diagnose, can you pipe anything into the back of the scope to check that out ? also can you take the outputs from the back of the SA and connect up to another scope to check that out ? not sure if you showed us the back of the units. what do yo have connected to the external input at the moment ?. When you turn the focus up and down does the beam swing to one side or stay put ? have you seen if the calibrator works ? I would imagine that the voltages for the CRT track with the intensity and the focus but for now can you take that pot out and test it on the bench with a few volts ? just to rule it out or hang a meter on there and check the voltage adjusts smoothly. Anyhow lets see what you can figure out as it's early days ....cheers!
Thanks for the tip Andy! I had heard that before (regarding the burn out) but obviously was not keeping it mind for this repair. The magnefier button does not seem to do anything. The horizontal button seems to work but I feel like it should be able to move the trace all the way across the display - it only moves the trace about a divsion or two in length. My foucs was mainly on the 182T so I haven't tried connecting the SA to anything else, but that is a good idea! During focusing, it appears that spot stays in place. The external input is at the front of the unit - I had my CG5011 connected at certained times just delivering a 10Vpp 1kHz square wave. This is the same signal that is available on the unit itself and it does work. It was just easier to connect it via BNC to my CG5011. But the calibrator signal does not seem to appear on the display - it does not look any different when the external input is on or off. It did seem to try to show some signs of life, but barely. I did test the intensity pot for resistance and it gradually increases and decreases between 0-10kohms - so it doesn't test like a bad pot. But I am going to replace it anyway and see if it makes a difference. But I'm leaning towards the problem being on the horizontal amplifier PCB or HV supply/multiplier. Really hoping it is not the CRT that is bad...
Nah I'm confident the CRT is fine and no shift when focusing is good news too :)@@fmashockie
I think that the RF part of the spec An is working. When you shifted the baseline, the band of the noisefloor could be seen.
Two reasons why there is no horizontal sweep:
1, Timebase (sweep gen) in the spec An is buggered….but low liklihood.
2, Horiz, X-Amp in 141T is buggered….far more likely.
The intensity pot on this had me really stumpped, I could not find the damn thing on the circuit. Followed back from the modulator grid of the tube snd instantly found the 1M “intensity limit” preset pot, but you were talking about a 10K pot, which strongly suggests a “DC restorer” type intensity control … buried in the guts of it somewhere.
Eventually I found it….way back at one of the inputs of the Z-Blanking amp. It is capacitively coupled to the mod grid of the tube by C2, a 22nF (would be high voltage rated) ceramic…probably one of those blue ones. This means that the intensity setting is coming through as A.C. Via C2!
Lots of oscilloscope designs take some high frequency A.C., usually off the EHT transformer via a very high value resistor, 470K or more, so it has a high source impedance. It is also usually capacitively coupled via a small, c10nF cap, then clampped by two diodes, one, reverse biased up to a DC voltage, and another, also reverse biased, down to a lower, or negative DC voltage. These two voltages “clip off” the squarish AC waveform because of its high source impedance, the top, one diode drop above the upper DC voltage, and the bottom, one diode drop below the lower DC voltage…same circuit as used to protect the input of a chip, one diode up to rail, another down to ground or negative rail and a resiator before in series to limit the current of the input swings too far and one or the other diode is forced to conduct.
This “clipped” A.C. I then capacitively coupled WAY DOWN, a frw Kv below ground via a high voltage ceramic cap…the equivalent of C2 in in this circut, there it is rectified back to a DC voltage proportional to its amplitude, but referebce to a point, (usually the cathode or modulator grid of the tube) many Kv below deck. If either of those DC voltages that are clipping off the AC before it passes via “C2” are altered, the AMPLITUDE of yhe clipped AC changes, so the DC recovered from it on the other side of “C2L” also changes proportinately…this changes the viltage between the cathode and modulator grid of the tube and thus the intensity. But the “intensity” pot is thousands of volts above, ground referenced and only needs to change one clipped side of that AC sample “Sniffed” of the EHT transformer.
Look up the circuit diagram of the Hung Chang Model OS 620…the tube and EHT supply part, it uses this technique…called “DC restoration” to “get the intensity pot away from the HV near the tube neck and down to some reasonable voltage that can be pot on a pot on the front pannel. (Some oscilloscopes do the same for the focus pot too, but that is a lot rarer.)
Here, because this is Hewlett-Packard, thay have to do it in that, “Hwelett-Packardey” sort of way that is difficult to get your head around, but basically the principles aire the same. i think the grounded base transistor, Q5, pn the Z-Blanking amp board receives some AC via R13 from the ALT-CHOP oscillator and the intensity pot added a DC bias to it via R15. This is fed into the emitter of Q5 and the “variable amplitude AC” comes out the collector of Q5, goes via CR4, Q6 and Q8, then R28, CR8 and CR9 pnto coax W4 and up to C2 to be injected into the mod grid of the tube. Possibly the mod grid and cathode of the tube make a diode that restores the AC coming via C2 back to a DC voltage that controls the intensity.
Some odd things I noticed….that screen “wash”…the green glow all over the screen…that is NOT NORMAL, it is suggestive that a very intense beam is hitting the funnel of the tube from the inside and some of it is scattering off in secondary emission to foem that green wash. I,d be measuring the voltages on the X-plates of the tube…thsy should be about the same, if thay are drastically different..than there is something amoss with the X-amp and the beam is being bent into the tube funnel way off to pne side or the other.
So, two faults…the intensity and Z-Amp, and the screen wash and X-amp. Take a llo at the X-plate boltages with another oscilloscope…think that will be quite revealing….just make sure
Thise voltages don’t exceed your other oscilloscope’s probe or sttenuator settings.
Oh, great to see the curve tracer out in “anger”…that pass transistor with the steeply sloping curves is definitely dodgey and needs replacing, its Hfe looks to have gone low and it will need tons of drive from it’s lottle Darlington driver mate, who will be trying to compensat for the lost gain in the big boy with its own…so both will be pretty “red in the face”.
cheers!
Hey Globe Collector I was waiting patiently for your comment on this one! Knowing CRTs and globes are your wheelhouse! 😂
So the green wash you are referring to I think is the 'Scaling' illumination. The 182T has a feature called scaling which allows you to illuminate the graticules. So it brings the whole display up in brightness. I think this is normal for this unit. But maybe I am wrong!
Always appreciate your analysis of the circuit! So I have made some progress since recording this video. It turns out that intensity pot was bad after all. I temporarily replaced it with a pot I had on hand and now the intensity control is much smoother. I was actually able to find NOS for this pot on eBay which is great because it would be difficult to find a suitable alternative that would fit. But what I'm dealing with now is that when the intensity pot is decreased, the beam spot dips lower vertically and then cuts out. And increasing it, the beam swings up to its original spot. What could be causing that?
The horizontal amp seems to be working now. I applied a 10Vpp 1kHz square wave and I get a trace 10 divisions long which is normal per the service manual. So it seems the Horizontal amp is working properly. But I can still check those deflection voltages to see if they are normal like you suggest.
Another thing I notice is that when adjusting the horizontal positioning from left to right, the beam will pull up and then down (as if it is traveling over an imaginary hump) when it crosses the center vertical graticule. Any idea what could cause that as well?
And also, the beam does not sit on the center horizontal graticule. It sits about 1 div below that. But I have no way to adjust that because the 182T has no vertical amp - that is supplied by the plug-ins. Maybe that is why it is not centered vertically?
Yes I was happy to use the curve tracer for this one! Unfortunately, that -100V series reg transistor with the funky curves is an HP propriety part... Might be difficult to source that one... I can try replacing with alternative that has a matching series of curves though!
I'll check out the Hung Chang OS 620 circuit. I really appreciate it when you offer suggestions on circuits to reference!
Thanks as always for your input and watching!
I’ll go and have a better scan of the circuit to see if I can justify to myself the greenwash function.
The bean spot moving when the intensity is changed is some sort of interaction between the power supplies for the tube (viz intensity) and the power supply to the Y- Amp…by, hey…didn’t you tell me it HAS NO Y-Amp! Is there anything at all connected between the Y-Plates of the tube and the diffirential Y-Inputs on the plug-is edge connectors in the back of the mainframe? Surely there IS SOMETHING….a vertical shit pot maybe….some resistors back to a fixed voltage on some (not too well regulated) power supply.
I’ll tell you something about electrostatic cathode ray tubes that you might not realize until you spend decades working on stuff that contains them. I’ll use this 182 T as an typical example. Notice the tube cathode voltage, it is -3150v. (And there is something I want to say about the heater supply but I willsave it for later.) The very first grid in front of the cathode is called the modulator grid or control grid…this is ALWAYS more negative than the cathode, in some tubes by just a few volts, in others it might be a few tens of volts…this is THE MOST NEGATIVE electrode in the tube.
The next grid is just an accelerator grid with a voltage on it more positive that both the cathode and mod grid.
Oh, Z-Axis, i.e. brightness/beam vpcurrent is controlled buy the magnitide of the nagative voltage on the mod grid. Some designs hold the mod grid fixed and make the cathode more positive with respect to it to decrease the brightness…this was quite common in TV sets…other designs hold the cathode fixed and make the mod grid more nagative with respect to it to dim the spot and I think that is the scenario being used here in this specific piece of equipment.
As you go doen kthe tube neck, all the other electrode voltages in front of the mod grid get ever more positive with respect to the cathode, so here you have that accelerator/shield grid…that shields the mod grid from the focus grid…then the focus grid, astigmatisim grid, etc., etc.
Eventually you come to the deflection plates…in some tubes thay are in shielded cages, in this tube there appear to be none….BUT, the voltages on the deflection plates With the spot in the middle of the screen, MUST be somewhere close to Ov, or whatever DC component of voltage the X and Y amp designs leaves
On them with mo signal input.
Very old oscilloscopes simply applied the test signals DIRECTLY to the deflection plates, so thay had to sit at 0v under no-signal conditions….if they drift TOO FAR away from this equlibrium voltage then odd things start to happen to the display, can’t focus it, can’t get a round spot ot odd movments when you adjust unrelated things…like the interaction between the spot vertical position and the intensity settings.
Ot in front of the deflection plates there are often more electrodes, often applied to the inside of the funnel part of the bulb by aluminium sputtering ot graphitic paint of some sort. Some tubes have “helix” electrodes, basically a helical strip of grapgitic coating
Starting at the post plate acceleration structire and spiralling up the inside of the funnel to the ultor cap. These typically have resistances of 10-30Meg..and go from a few hundred voltts above ground right up to 10-15 Kv at the ultor cap.
So the whole tube can be seen sort of like a seesaw/teeter-totter in a kids’ play park, the fulcrum in the middle representing the deflection plates, the cathode and mod grid way up the end that sits on the ground and the ultor cap represented by the other end that is up in the air! If something upsets this conger-line of almost ever increasing voltages…or the point where they cross zero, confusing and befuddling crap can happen!
O.K. Something I really don’t like sbout this 182 T circuit…the tube’s 6.3 volt heater supply. In those oscilloscopes that derive the EHT off a Royer or Flyback type “high freqiency” suppl with a ferrite cored transformer…generally the heater supply comes from an overwind on this same ferrite cored transformer…why? Because it needs to be only a few turns at the high-volt-per-turn ratios of high frequency transformers…and since it is referenced to a point 3150 volts below deck, it can be pretty heavily insulated wire.
Some older cposcilloscope circuits derive the EHT straight off the 50/60Hz mains transformer…this is “potty” because not only do you need thousands of turns to furnish the EHT itself, you slso need to put that little 6.3 volt heater winding…but the insulation in the transformer around it..be that shellac soaked paper, resin soaked paper, wax soaked paper, captan tape…whatever..it has to float 3150 volts away from every other winding in there…add the peak value of the high AC voltage the EHT comes from and you can have points 5Kv or more aoart in the same small transformer…sure, it’s OK down at the local switchyard, where the cores a huge and the turns-per-volt are low because the cores a so big….but in a sub 200VA transformer at 350mv per turn? The designer needs his “head read”!
This oscolloscope us trying to do both things, derive its EHT off a ferrite cored transformer, (which you can wtap a croc-clip lead arroubd the core of a few times and connect the ends to your oscilloscipe and get a “safe” look st what it is doing)
…but, get the floating tube heater supply off the main 60Hz ower transformer. That is why there is that 100K resistor from the cathode to one end of the heater….if that has been “red in the face”….something very nasty has happened between that elevated/depressed heater winding and one of the other windings on that transformer!
O.K., “Rant” over! Now for that pass transistor in the power supply.
It is just a power supply, it is not going into a sattelite to be shot into geosynchronous orbit…it is just a pass transistor, chuck an MJ15022 pr MJ15002 in there….good enough for “Straylia” as Dave would say.
Yes, if it is the first local oscillator transistor, I’d be a bit circumspect and careful, but a bloody pass transistor in a 100v power supply, AND in a feedback loop….hells bells, the zener was dead and you could not get one exactly like it.
Oh, ine thing about that zener, the original was probably some swanky low thermal drift one, but there is a simple solution…put TWO 5v1 zeners in series to replace it….why 5v1…because those with lower voltages are actually BREAKDOWN diodes, which thermally drift one way….and those above 5v1 are true ZENER diodes that thermally drift the other way…the 5v1 zener sits right in the middle and barely drifts either way…do it is a “poor man’s” swanky voltage reference.
The “hump” behavor you refer to…I have no real idea, but I would look at tracking between main supplies ans EHT supply…is the main supply driving the EHT transformer dipping as the brightness is turned up, does this supply effect the one setting the “no signal float” voltages on the Y-Plates?
And, oh, could I go on about the spectrophotometer anology you used….but another day!
Cheers