Repairing a Hitachi V-422 - Page 1
Forum section: electronics / repair

author: LoyalServant
author_rank: Regular Contributor
timestamp: October 10, 2012, 08:39:37 am
content: Anyone that's been around for 20+ years is sure to have seen these entry level/tech scopes or even used one.Perhaps his little brother the V-222.Damn good scopes and a nice display.I loved the Hitachi scopes when I was in the service business.... simple and to the point but no holdoff which is sort of nice to have but oh well.These were great for working on the electronics of the day like amplifiers, televisions, tape decks...Hard to believe that I used to actually fix that stuff... man have the times changed....Anyhow.... nostalgia aside...I have acquired a second one of these scopes and it was dead.A few power supply issues resolved and it's back from the dead.It had HV section issues (no hv to the crt), one rail on the +/- 8 volt supply missing, the channel 2 trigger balance circuit let out theblue smoke and actually burned a 10k trimpot down into a pile of carbon... etc....Poor guy was in bad shape!Now.. for the issues I am not sure how to proceed on.Channel 1 is in need of some love.When the trace is centered on the screen and you rotate your volts/division knob the vertical position of the trace jumps around.Not the dirty control kind of jumping but going from say 2 volts/div to 5 volts/div the trace will jump upward 2 whole divisions.Some jump a half a division, some jump several divisions and it's either positive or negative of center.It is doing this with no probe attached and coupled to ground.With a probe attached and viewing the cal square wave it jumps around the same way.The amplitude of the displayed waveform does not change, just it's vertical position.Channel 2 stays more or less flat.. it does move but ever so slightly - and this was the channel that let out the blue smoke....The second issue is the internal 1Khz square wave generator.It's not 1Khz... it's 1.1 Khz... so it's off a fair bitIt's pretty obvious when your looking at the waveform.There is no adjustment for the frequency, just the amplitude.I am sure some components have drifted off.The one I have had for many years measures 999.998Khz (not bad!)They used a 7474 flip-flop for the generator.Not sure if I should care about this or not?I guess the good thing about this is that I have a second one that altho is certainly not calibrated it's in far better shape.Anyone have any tips?

author: PA0PBZ
author_rank: Super Contributor
timestamp: October 10, 2012, 10:13:57 am
content: I still have one of those!There is a small trimpot for each channel on the front panel, I think it is called "DC Balance". I was able to cure the jumping trace using these.

author: LoyalServant
author_rank: Regular Contributor
timestamp: October 13, 2012, 04:03:57 am
content: It was the DC balance that was causing the trace to jump.New pot:http://imageshack.us/a/img26/9112/imag0039tp.jpgNote the scorched board..... took a good heating from that pot.The original pots were on a ceramic base with a plastic head but I think the new one will do fine.There are some sellers on amazon that ship these parts out of Kong Kong for free.I got 25 of these pots for $5 USD... I guess you can't really beat it.I just have to figure out what's going on with the square wave gen and it will be restored to it's former glory.

author: PA0PBZ
author_rank: Super Contributor
timestamp: October 13, 2012, 10:00:08 am
content: That board looks really cooked.I wouldn't worry too much about the square wave generator, it will not make any difference for the probe compensation if it's 1KHz or 1.1KHz. Maybe the one you measured before was just a lucky shot? I'll measure mine and see what it does.Ok, it says 1.013 KHz, close enough.