Author Topic: Repair: Hitachi v422  (Read 8818 times)

0 Members and 1 Guest are viewing this topic.

Online tautech

  • Super Contributor
  • ***
  • Posts: 17458
  • Country: nz
  • Taupaki Technologies Ltd. NZ Siglent Distributor
    • Taupaki Technologies Ltd.
Re: Repair: Hitachi v422
« Reply #25 on: January 01, 2016, 06:23:22 pm »
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?
No.

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 diodes
Power transistors, but more so HV output types
Signal transistors
Bridge rectifiers

Most 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.php

Find one you like, print it and keep it handy.

Avid Rabid Hobbyist
 

Offline Gripnook

  • Contributor
  • Posts: 33
  • Country: us
Re: Repair: Hitachi v422
« Reply #26 on: April 24, 2016, 01:19:45 pm »
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.
 

Online tautech

  • Super Contributor
  • ***
  • Posts: 17458
  • Country: nz
  • Taupaki Technologies Ltd. NZ Siglent Distributor
    • Taupaki Technologies Ltd.
Re: Repair: Hitachi v422
« Reply #27 on: April 24, 2016, 08:44:18 pm »
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.
Avid Rabid Hobbyist
 

Offline Gripnook

  • Contributor
  • Posts: 33
  • Country: us
Re: Repair: Hitachi v422
« Reply #28 on: April 25, 2016, 12:32:22 am »
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.
« Last Edit: April 25, 2016, 12:41:41 am by Gripnook »
 

Online tautech

  • Super Contributor
  • ***
  • Posts: 17458
  • Country: nz
  • Taupaki Technologies Ltd. NZ Siglent Distributor
    • Taupaki Technologies Ltd.
Re: Repair: Hitachi v422
« Reply #29 on: April 25, 2016, 12:59:55 am »
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.


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.
Check 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.
Avid Rabid Hobbyist
 

Offline Gripnook

  • Contributor
  • Posts: 33
  • Country: us
Re: Repair: Hitachi v422
« Reply #30 on: April 25, 2016, 02:29:31 am »
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?
 

Online tautech

  • Super Contributor
  • ***
  • Posts: 17458
  • Country: nz
  • Taupaki Technologies Ltd. NZ Siglent Distributor
    • Taupaki Technologies Ltd.
Re: Repair: Hitachi v422
« Reply #31 on: April 25, 2016, 02:47:48 am »
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?
Can'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?
Avid Rabid Hobbyist
 

Offline Gripnook

  • Contributor
  • Posts: 33
  • Country: us
Re: Repair: Hitachi v422
« Reply #32 on: April 25, 2016, 02:50:35 am »
Here's the relevant part of the schematic:
 

Online tautech

  • Super Contributor
  • ***
  • Posts: 17458
  • Country: nz
  • Taupaki Technologies Ltd. NZ Siglent Distributor
    • Taupaki Technologies Ltd.
Re: Repair: Hitachi v422
« Reply #33 on: April 25, 2016, 03:26:48 am »
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?
« Last Edit: April 25, 2016, 03:30:12 am by tautech »
Avid Rabid Hobbyist
 

Offline Gripnook

  • Contributor
  • Posts: 33
  • Country: us
Re: Repair: Hitachi v422
« Reply #34 on: April 25, 2016, 03:31:48 am »
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
 

Online tautech

  • Super Contributor
  • ***
  • Posts: 17458
  • Country: nz
  • Taupaki Technologies Ltd. NZ Siglent Distributor
    • Taupaki Technologies Ltd.
Re: Repair: Hitachi v422
« Reply #35 on: April 25, 2016, 03:40:04 am »
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.  ;)
Avid Rabid Hobbyist
 

Offline Gripnook

  • Contributor
  • Posts: 33
  • Country: us
Re: Repair: Hitachi v422
« Reply #36 on: April 25, 2016, 03:46:36 am »
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.
 

Online tautech

  • Super Contributor
  • ***
  • Posts: 17458
  • Country: nz
  • Taupaki Technologies Ltd. NZ Siglent Distributor
    • Taupaki Technologies Ltd.
Re: Repair: Hitachi v422
« Reply #37 on: April 25, 2016, 03:50:36 am »
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.
:-+
Avid Rabid Hobbyist
 


Share me

Digg  Facebook  SlashDot  Delicious  Technorati  Twitter  Google  Yahoo
Smf