Troubleshooting a scope bought off ebay[Solved] - Page 1
Forum section: electronics / repair

author: fletch
author_rank: Newbie
timestamp: June 20, 2018, 11:34:58 pm
content: Hi all.... Just a bit of a beginner here who was looking to advance into the world of scopes. So I got a Hitachi V665A a while ago. Seemed to work fine until I downloaded the user guide and found out it was supposed to have a text character display along with the trace. I have no idea how or even if it is possible to get the cursor display working. The trace is crisp and clear and everything else seems to work fine. On power on, there is supposed to be a message displayed on the screen. However on this scope, it seems to go through the power on routine ok, but nothing is displayed except the trace line. Shows waveforms fine and probe compensation is not a problem. I opened up the cover and the inside is relatively dust free and there are no obvious problems or loose connections. I haven't really dug into it as fixing a scope is a little above my skill level. I am willing to try though. I have looked into a few posts relating to this series of scopes here but nothing really deals with the problem I am having.Any advice on where the problem lies or am I looking at having to be happy with just a trace showing?

author: Haatveit
author_rank: Regular Contributor
timestamp: June 20, 2018, 11:51:38 pm
content: As far as I can tell the only time any text would display is if you are in the Measurement mode, and it'll show the time, or voltage, between two cursors. Are you sure they aren't visible in Measurement mode, maybe they have been moved off-screen (if that's possible)? do the indicator lamps around the Cursor/Measurement area work?This is something that could prove difficult to troubleshoot, however it's not critical for the functionality of the oscilloscope. It's a handy way to measure time and voltage, but you can always do that by looking at the graticule and your scope settings as well, it's just less convenient.

author: alsetalokin4017
author_rank: Super Contributor
timestamp: June 21, 2018, 02:05:09 am
content: Well... you could try turning the readouts on....

author: fletch
author_rank: Newbie
timestamp: June 21, 2018, 02:23:57 am
content: Yes... That part of the manual I read first. Still no readout...Note the first line says Readout displays on power on. Not so with mine anyway...

author: alsetalokin4017
author_rank: Super Contributor
timestamp: June 21, 2018, 02:29:13 am
content: Quote from: fletch on June 21, 2018, 02:23:57 amHmm.... I figured it was too obvious. Although the instructions are a little complicated.People have reported trouble with the SELECTOR switch, you might try spraying it with some Deoxit or other good contact cleaner.It is strange that there isn't a front panel intensity knob for the readout.

author: fletch
author_rank: Newbie
timestamp: June 21, 2018, 02:34:49 am
content: Quote from: Haatveit on June 20, 2018, 11:51:38 pmI did make an attempt at adjusting the cursors as per the service manual but they are still not visible either.Yes, indicator lamps work ... Not sure about the variables encoder.Quote from: Haatveit on June 20, 2018, 11:51:38 pmTrue

author: fletch
author_rank: Newbie
timestamp: June 21, 2018, 02:37:14 am
content: Quote from: alsetalokin4017 on June 21, 2018, 02:29:13 amMaybe the encoder while I'm at it too, eh?There is... Right beside the trace rotation

author: alsetalokin4017
author_rank: Super Contributor
timestamp: June 21, 2018, 03:00:05 am
content: Oh yeah, there it is, right in front. I suppose you've rotated the heck out of that control with no effect. I haven't looked at the schematic but this may be a HV circuit similar to the trace intensity function. You could check the pot and the divider resistors for continuity/value drift.This thread may be helpful:https://www.eevblog.com/forum/testgear/teardown-hitachi-v1065/

author: fletch
author_rank: Newbie
timestamp: June 21, 2018, 04:08:01 am
content: Thanks for the link to that thread...I keep trying to find it, sometimes unsuccessfully.On re-reading it I notice there is a mention of a cursor display not working. Now where would I find the "gain bias"?

author: alsetalokin4017
author_rank: Super Contributor
timestamp: June 21, 2018, 05:30:16 am
content: OK so I've been studying the schematic, and I see I was wrong about the intensity controls being HV. It looks like the readout intensity control depends on the -12V supply. Could you please check to see if the -12V supply is working up to snuff.

author: fletch
author_rank: Newbie
timestamp: June 23, 2018, 01:56:32 am
content: Seems to be... Measured -11.88V on the-12V at the main board on top and 11.84 on the PEF-785B board.Other voltages seem to be good too.

author: alsetalokin4017
author_rank: Super Contributor
timestamp: June 24, 2018, 05:03:49 pm
content: Well, at this point I'd start at the readout intensity pot, check to see if that's doing anything electrically (check to see if voltage varies on the center pin as you rotate the control) and follow that -12V line and check all the resistors in the line back to the logic circuitry.

author: factory
author_rank: Super Contributor
timestamp: June 24, 2018, 05:31:01 pm
content: I'm guessing the time-base readout doesn't show either, which is kind of important at it uses two buttons for the that function instead of a rotary switch.The one I used to use at work had a very dim readout which got worse over the years, the intensity control was maxed out, it got retired & swapped for similar age Tek scope. Unfortunately we aren't allowed to use company time to try to fix anything.David

author: fletch
author_rank: Newbie
timestamp: June 24, 2018, 05:50:12 pm
content: I'll check it out.Quote from: factory on June 24, 2018, 05:31:01 pmLOL.. Maybe that is the one I ended up with...who knows?On a side note, the trace intensity has to be turned up to the top end of the pot to show a trace but is still nice and bright at full.

author: factory
author_rank: Super Contributor
timestamp: June 24, 2018, 09:26:00 pm
content: Quote from: fletch on June 24, 2018, 05:50:12 pmI'd forgotten about that, the trace intensity control also need to be maxed out for higher timebase speeds, but that wasn't too much of a problem unlike the readout.I would first be checking all the main power supply rails are good, then checking the high value resistors (which can drift high with age), as well as the adjustment pot, as someone else said.I think the one at work is still with all the other faulty/surplus equipment in a storeroom, the problem is they prefer to dump it rather than sell it. I remember some very nice stuff got skipped about 10 years ago.David

author: fletch
author_rank: Newbie
timestamp: June 24, 2018, 10:37:50 pm
content: Finally found the problem... The "gain bias" I mentioned before must have been the CRT Bias adjustment. I went through the first part of the calibration instructions in the service manual (Having finally obtained it). Adjusted the +12V rail to a solid 12v (was at 11.77V...out from the 11.97-12.02v stated in the manual) and on following the procedure for the CRT Bias adjustment. Low and behold... the readout appeared!!Now the trace intensity nearly washes out the CRT at full. Cursors and readout all work as expected now.Thanks to you guys for helping me out. Got some ideas on how to proceed through this, it really helped me to figure out how to solve things!