Repair: Hitachi v422 - Page 1
Forum section: electronics / repair

author: Gripnook
author_rank: Contributor
timestamp: December 29, 2015, 01:55:34 am
content: I recently picked up an old Hitachi scope. It turns on, but the triggering doesn't work at all. It also has some calibration issues as well, but first I'd like to get proper triggering going.I connected channel 1 to the .5V p-p calibration square wave with jumper wires (I don't have probes yet), and I get a moving wave on auto triggering and nothing at all on normal triggering, regardless of slope selection.I have obtained a pdf of the service manual and looked at the circuit for triggering, and tried turning a few knobs and checking a few connections, to no avail.Any suggestions on where to start looking?

author: tautech
author_rank: Super Contributor
timestamp: December 29, 2015, 02:30:33 am
content: Quote from: Gripnook on December 29, 2015, 01:55:34 amWelcome to the forum.Hitachi model #?Your scope experience? Newbie user errors?Trigger set to the channel of interest?If you wouldn't mind post a link to the SM so we can all be on the same page.Always check the PSU is to spec.Useful pdf:http://www.sphere.bc.ca/test/tek-parts/troubleshooting-scopes.pdf

author: Gripnook
author_rank: Contributor
timestamp: December 29, 2015, 02:51:45 am
content: Model number is Hitachi v422 as in the title.This is my first scope but I have used one in university where I study electrical engineering. This is the first one I take apart though.The trigger is set to channel 1.The waveform does appear on the screen but does not trigger properly and sweeps across the screen.There are also a few other calibration issues:1. The vertical position on channel 1 does almost nothing.2. The vertical position on channel 2 does almost nothing, and channel 2 has a vertical deflection issue and is only focused on the center of the screen.I will have a look at the power supply specs.Link to manual:https://www.dropbox.com/s/iq1hnn2oegk35br/hitachi%20v-211-212-222-422%20oscilloszkop.pdf?dl=0

author: Gripnook
author_rank: Contributor
timestamp: December 29, 2015, 03:26:35 am
content: The output voltages of the power supply all check out, with the exception of a -13V being -15V.

author: KJDS
author_rank: Super Contributor
timestamp: December 29, 2015, 03:46:33 am
content: I'd check to see if the trig level pot, as well as the Ch1 and CH2 position pots actually do anything. Just measure the wiper voltage as you turn them

author: tautech
author_rank: Super Contributor
timestamp: December 29, 2015, 04:17:15 am
content: Quote from: Gripnook on December 29, 2015, 03:26:35 amRipple?PSU is very important for a CRO, all LV rails must meet spec for waveforms to be accurate and the scope to behave as it should.

author: Gripnook
author_rank: Contributor
timestamp: December 29, 2015, 04:21:05 am
content: The trig pot output voltage is a constant 5V measured at capacitor C442.The channel 1 position output voltage is a constant 0V output at R65.The channel 2 position output voltage measured at R165 varies properly between -8V and +8V when turned.So there are issues with the pots for both channel 1 position and the trig level. Since the channel 2 position pot works I guess its an issue with the vertical calibration at the CRT for that channel.

author: Gripnook
author_rank: Contributor
timestamp: December 29, 2015, 04:22:43 am
content: Quote from: tautech on December 29, 2015, 04:17:15 amThis -13V voltage is listed as UNREG in the circuit schematic and seems to be connected to nothing.

author: tautech
author_rank: Super Contributor
timestamp: December 29, 2015, 04:48:35 am
content: Quote from: Gripnook on December 29, 2015, 04:22:43 amYes, I see this now, the -15V measured will be OK then.Did you see the "preliminary control settings"on P11?PSU requirements on P12, however no ripple requirements, but check them anyway. Expect 10mV or less for the LV rails and more for the 120V.Fault finding can be made easier with the DC values on the schematic but the scope must be set as described on P51 for the DC values to be relevant and they are normally referenced to circuit GND.

author: Gripnook
author_rank: Contributor
timestamp: December 29, 2015, 05:16:08 am
content: Quote from: tautech on December 29, 2015, 04:48:35 amAll supply voltages on P12 are within specs. For the high voltage i added 4 10 Mohm 5% resistors in series with my DMM and measured -1.645 kV (No high voltage probes and my DMM only goes to 1kV).I don't know what ripple is. I'm assuming its the AC component of the DC voltages, and i measured 5mV RMS for all voltages except the 120V (actually 75V for this model), and 50mV RMS for the 75V.

author: tautech
author_rank: Super Contributor
timestamp: December 29, 2015, 06:55:31 am
content: Quote from: Gripnook on December 29, 2015, 05:16:08 amExcellent. Sounds good.On to the Trigger:Poor interconnects between PCB's? (bad connectors)Faulty mode switch?Faulty coupling switch?Faulty level pot?Those are the easy things to check, in all cases it could just be oxidised contacts/wipers.Clean, rinse and repeat.Following that you need to dive into the trigger circuitry checking for appropriate voltages for IC's etc.Check components in the trigger signal path.....easy with another scope.What other test gear do you have? or have access to?

author: Gripnook
author_rank: Contributor
timestamp: December 29, 2015, 07:59:12 am
content: I've already checked the pots for level and channel 1/2 vertical position, and the level and channel 1 position pots appear to not be connected since they do not influence the voltage at all.So tomorrow I'll have to take out the PCB and check the connections for these pots.I'm afraid I don't have another scope. I basically only have a digital multi-meter and a variable power supply. So I can only check DC and AC voltages in the circuit.

author: tautech
author_rank: Super Contributor
timestamp: December 29, 2015, 08:41:33 am
content: Quote from: Gripnook on December 29, 2015, 07:59:12 amBe sure to check them for correct function too, and all switches to do with the trigger. PCB interconnects do also give problems.....oxidised pins, dry joints (stress cracks).Keep your eyes open for anything amiss. Eliminate the simple and obvious first.

author: Gripnook
author_rank: Contributor
timestamp: December 30, 2015, 03:54:06 am
content: Finally the trigger pot is connected fine to the circuit. So the issue must be elsewhere.After removing and reinserting the PCB, the vertical issue on channel 2 was gone, but now the whole waveform is displayed in the center of the screen horizontally (see pictures).I found a burned connector in the PCB, but checking connections it seems like it doesn't affect the circuit.I will continue checking and repairing after I get access to another scope for troubleshooting.

author: tautech
author_rank: Super Contributor
timestamp: December 30, 2015, 04:21:29 am
content: Quote from: Gripnook on December 30, 2015, 03:54:06 amThis will be a result of insufficient voltage swing on the CRT horizontal plates and therefore failing to divert the electron beam for a full width raster.Check against the voltages and waveforms listed for the H Amp P121

author: Gripnook
author_rank: Contributor
timestamp: January 01, 2016, 04:31:58 am
content: Ok so I managed to do some measurements for the trigger circuit.First picture is the output from R420. The max of 0.2V in the diagram is instead 0.08V, while the minimum is similar. Also, I don't get the same skew in the min/max.The second picture is the input to R445. The V/div and S/div is unspecified in the diagram so I used 1V/div and 0.5mS/div. The min looks ok again but the max is way over, at 4V instead of 1.5V. Also, I don't get the curved drop-off and I get a slight curve in the min.Any ideas what might be causing this?

author: tautech
author_rank: Super Contributor
timestamp: January 01, 2016, 04:55:11 am
content: Bit hard to follow where you are, has the narrow display problem gone?OK on Trig Gen schematic P117The missing roll-off might be a result of a faulty C444, if so be sure to replace it with a 25V cap.

author: Gripnook
author_rank: Contributor
timestamp: January 01, 2016, 05:39:37 am
content: I thought I'd focus on the triggering first since the narrow display is an output problem and probably occurs after the trigger.Ok so progress. I checked C444 with a continuity test and got a beep, so the capacitor is charging correctly. Then I checked C442 and nothing. So I measured the voltage drop across R441 which is in series with C442 and found close to 0 (The voltages at each end of the resistor with respect to ground were ~400 mV square waves centered at -300mV, and the difference was a noisy 40mV pp).This leads me to suspect C442 as broken and causing an open circuit, so I will replace that one. This might also be causing more current to flow through R445 by cutting flow through the capacitor. Am I wrong in my analysis?Should I also check caps near R420 for the missing skew?

author: Gripnook
author_rank: Contributor
timestamp: January 01, 2016, 06:19:56 am
content: Scratch that. R441 does have a square wave type voltage difference, so current is flowing indeed. Remeasuring with the diode test confirms that C442 is indeed not an open (I only measured one way before and the other does beep).Any other ways to test capacitors?

author: tautech
author_rank: Super Contributor
timestamp: January 01, 2016, 06:35:51 am
content: Quote from: Gripnook on January 01, 2016, 05:39:37 amThat's the first time I've heard of using a continuity test for testing caps and it can only reliably tell you if the cap has shorted or not. Absolute value will be the important spec and I'd want to measure ~45 uF or more for C444. C442 is a 1 uF bipolar, be sure to use the same.You have a misunderstanding of coupling caps in the signal flow, they prevent the pass of DC but not AC and your DC measurement of 0V across R441 would be as expected.At some point you'd be well advised to acquire a DMM with Capacitance measurement and maybe an ESR meter, though for basic repair you can often get away without one. Experience testing caps is important to gauge results but with good tools.....trust them.Until I got these I foundered too, expensive I know but if you dogeneral repairs and especially SMD you won't regret the purchase.http://www.advancedevices.com/

author: Gripnook
author_rank: Contributor
timestamp: January 01, 2016, 07:04:39 am
content: 0V across R441 was measured with the scope, not a DC multi-meter, but after zooming in sufficiently I noticed a square wave pattern.I have a capacitance tester on my multi-meter but it only goes up to 20uF. So I was checking for shorts/opens since a discharged capacitor that's working correctly should act like a short initially and then block current.For C442 I get .9240 uF as an in-circuit measurement.For C444 I put the capacitor in series with a 10.64 uF capacitor and measured a combined 8.23 uF. So that puts the cap in question at 36.3 uF, well below spec. So then this is one capacitor to replace. (Note that my multi-meter has 4% error on capacitance, which means that the capacitor is still within spec within this error considering the measurement I made)I'll do some more capacitance measurements in this part of the circuit between trigger SOURCE and MODE.What do you mean by "C442 is a 1 uF bipolar, be sure to use the same."The same what?

author: tautech
author_rank: Super Contributor
timestamp: January 01, 2016, 07:33:34 am
content: Quote from: Gripnook on January 01, 2016, 07:04:39 amGood work on C444.C442 is Bipolar as opposed to Polarised as most Electrolytics are. Note on the schematic there is no + to identify polarisation and BP next to the stated value of 1 uF.

author: Gripnook
author_rank: Contributor
timestamp: January 01, 2016, 07:37:35 am
content: Would the 0.9249uF for C442 be a reason to replace it? What's the usual tolerance on capacitors before they need to be replaced?

author: tautech
author_rank: Super Contributor
timestamp: January 01, 2016, 07:55:25 am
content: Quote from: Gripnook on January 01, 2016, 07:37:35 amHmmm, if you have to get others then replace it, it seems a little low IMO but I don't know your meter like you do.I normally work to 5%, 10% MAX entirely depending on the application. As this is a coupling cap via 1K for edge coupling not audio you can probably leave it in.

author: Gripnook
author_rank: Contributor
timestamp: January 01, 2016, 09:09:26 am
content: Ok so I tried adding a 10uF capacitor in parallel to the C444 to bring it up to spec, and the measured waveform did not change. Still too big in p-p and without the curvature. Same thing with adding a 0.1uF cap to the C442.Could there be a problem with the transistors amplifying the signal too much and causing the too large p-p voltages?Tomorrow I'll remeasure all the power supply voltages and every DC voltage in the trigger circuit to make sure its not that. Any good way to check the proper operation of a transistor?

author: tautech
author_rank: Super Contributor
timestamp: January 01, 2016, 06:23:22 pm
content: Quote from: Gripnook on January 01, 2016, 09:09:26 amNo.Go to a point earlier in the signal path and check for the correct waveform, say C405.Component failures to expect when troubleshooting: (my order of suspicion)Resistors OC or drifted high.Caps, electrolytic, other dipped types, cracked coatings allowing moisture ingress.Signal diodesPower transistors, but more so HV output typesSignal transistorsBridge rectifiersMost stuff is easy to check with a few tools and datasheets, however in circuit measurements can be deceiving, thought and experience must be applied.A cheap DMM HFE tester is usually all that's required to check transistors against datasheets for correct gain.Same with diodes, cheap DMM diode tester.There are many guides for transistor tests in circuit, a quick look found this one:http://www.electronics-radio.com/articles/test-methods/meters/transistor-circuit-fault-finding.phpFind one you like, print it and keep it handy.

author: Gripnook
author_rank: Contributor
timestamp: April 24, 2016, 01:19:45 pm
content: Finally found all the issues (I hope).One blown fuse resistor R859.One missing contact to +8V power rail for TRACE ROT pot.All pull-able pots on the front panel seem to have rusted and only make intermittent contacts.With alligator cables shorting out the switches and power rail, and adding a 220 ohm resistor in parallel with R859, the scope works as it should (aside from calibration).Now I would like to make these repairs more permanent. Can I replace R859 with a 1/4W resistor or do I need to use a fusible resistor? Also, how would I repair the switches? Is there a way to clean them up to make them work again?Any help is appreciated.

author: tautech
author_rank: Super Contributor
timestamp: April 24, 2016, 08:44:18 pm
content: Nice progress.Fusible resistor current values are normally denoted by their color codes, Google them for the correct value.220 ohm is quite possibly incorrect. I have in the past bodged fuseable links with a miniature fuse with extended leads.Switch contacts are probably oxidised, exercise can help even some IPA while doing so too, although the most preferred product for cleaning tired pots and switches is Deoxit.

author: Gripnook
author_rank: Contributor
timestamp: April 25, 2016, 12:32:22 am
content: The resistor is indeed 220 ohm. See picture attached. From the BOM it is a 1/4W 220 ohm fusible resistor. For now I have soldered a 1/8W 1% 220 ohm resistor I had in its place, which should be enough for the standard current of 17mA mentioned on the datasheet and also measured by me. I'll see if I can get a fusible one at a later time.As for the switches, I decided to try re-soldering the joints under them, to both heat the switches and let the rosin flux clean the contacts somewhat. It seems to have done the job. After doing this to all switches they all work, and I was able to repair everything and calibrate the scope as well as I could with the equipment I have. It now works very well, with only some screws and knobs missing. See pictures for some wave-forms.Thank you everyone for helping me out. I really appreciate all the advice and assistance I got in repairing this scope.EDIT: Is a fusible resistor really necessary? I can't find the part on DigiKey or Mouser. The part description says PF-25S on the datasheet. Does this mean 25mA, or .25W? The typical current through it is 17mA according to the datasheet.

author: tautech
author_rank: Super Contributor
timestamp: April 25, 2016, 12:59:55 am
content: Quote from: Gripnook on April 25, 2016, 12:32:22 amCheck the BOM for a manufacturer's code and Google them to see if there's a datasheet.25mA sounds correct for a 17mA load but if you can only get say 50mA go with that.Just check that part of the schematic for possible failure modes so that a higher fuse value won't do more damage than expected to other componentry.Enjoy.

author: Gripnook
author_rank: Contributor
timestamp: April 25, 2016, 02:29:31 am
content: I can't get any current ratings directly from distributors. They all seem to be rated for power instead, and based on the part numbers like "FRM-25JT" which is similar to "PF-25S", it seems it's just a power rating of 1/4W. So now the options I have are 200? 1/4W or 220? 1/3 or 1/2W, since I can't find the exact 220?, 1/4W. Given that the 200? 1/4W would only make for a nominal max current difference of 5%, I guess that would be the better choice. From the circuit diagram it doesn't seem like the resistor value affects the bias current for the amplifier, so this would be all right. Am I right in assuming this?

author: tautech
author_rank: Super Contributor
timestamp: April 25, 2016, 02:47:48 am
content: Quote from: Gripnook on April 25, 2016, 02:29:31 amCan't check the schematic from Dropbox anymore, seems it's expired so going on your comments another solution might be use the 220 ohm plus a 25mA fuse in series. Bodge time?

author: Gripnook
author_rank: Contributor
timestamp: April 25, 2016, 02:50:35 am
content: Here's the relevant part of the schematic:

author: tautech
author_rank: Super Contributor
timestamp: April 25, 2016, 03:26:48 am
content: OK, horizontal plate output stage HV supply.I wonder why it blew?Slip on your part?I don't see any overwhelming reason why the fuse is included other than to protect the PSU.Is that rail fused?

author: Gripnook
author_rank: Contributor
timestamp: April 25, 2016, 03:31:48 am
content: Its probably my fault. Initially the horizontal was fine and after I went messing around I shorted a few things through the ground plane by accident.The rail does not appear fused apart from that resistor.Here's an updated link to the schematic:https://www.dropbox.com/s/jzsz7vrngojveq2/Hitachi%20v-211-212-222-422.pdf?dl=0

author: tautech
author_rank: Super Contributor
timestamp: April 25, 2016, 03:40:04 am
content: Quickest remedy will be the bodge I've described previously or take the risk and just install a 220 R.Don't forget to document it in your notes.

author: Gripnook
author_rank: Contributor
timestamp: April 25, 2016, 03:46:36 am
content: I don't actually have such fuses available so I'd have to buy something anyway. I think I'll just buy one of the two options I mentioned above to be safe. 1/3W or 1/2W should still be fine to blow in case of a short, and I think I'd prefer keeping 220 ohms for consistency with the datasheet in case I ever sell it.

author: tautech
author_rank: Super Contributor
timestamp: April 25, 2016, 03:50:36 am
content: Quote from: Gripnook on April 25, 2016, 03:46:36 am