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 Post subject: Hitachi V-203F Oscilloscope mystery
PostPosted: Nov Tue 19, 2019 9:25 pm 
Member

Joined: Dec Wed 02, 2015 9:42 pm
Posts: 172
Location: Little Elm, Texas 75068
I have a very clear print of all the boards from this oscilloscope but they are too big to include here. Anyone that wants a good copy can send me a PM with an email address and I'll be happy to forward. I'll post a very blurry copy of the power supply which may or may not prove to be of any value. There is a free download of a very good copy, zip file, of the service manual and prints at this location:
http://everist.org/archives/scans/Hitachi_V202-352.zip

The scope was functional before the move. It was in the shop here for almost 9 months before I finally got to a point where I could turn it on. It found it DOA and when I opened it I found the two pictures I have posted. That burned resistor is R1105, supposed to be a half watt resistor that connects to the collector for the transistor, TR1102. It was burned into two pieces and fell out in two separate pieces when I unsoldered it. Here is the part that has me totally baffled. There was no burn out on my watch. At least there was nothing smoking or smelling like smoke. No one else had access to the scope. I got here and left it packed until I was ready to use it. When I did turned it on it was DOA. I opened it to find it had a smoke out.
After replacing the transistor TR1102, which tested good after I took it out, along with the diode D1103 which also tested good, the 150 volt regulator began to work again. Before repairs it measured 196.7 VDC and now it measures 152.9 VDC. All remaining resistors check within value attached to circuit.

I have a line now, but it is wide and unstable, wiggles a lot and will not adjust up the screen, it is below half and will not center. It extends full across the screen. It seems to be plenty bright, CRT is good I think. FWIW I did not replace R1102, that was done by someone before me. I find it curious it is part of the 150 volt regulator along with R1105, the one I discovered had burned. Perhaps R1105 failed at the same time that R1102 failed and was simply overlooked by whoever did those repairs. You can see they damaged the trace during their work. That may be an indication of their experience. I immediately checked to make certain that their work was properly installed and nothing shorting and everything connected as it should be.
I'm wondering if the problem I found and the current symptoms I have are not connected. The scope was not the most stable that it could have been in California, not that bright and I worried the CRT might be failing, but it worked. Is it possible that it worked with the 150 volt regulator basically not functioning? With that 150 volt power supply measuring 196.7 volts instead?
The 250 VDC power supply is also measuring high, it's 287.5 VDC at R1103. I suspect something is not working down line and without that load the power supply at the rectifier has jumped up.
Curious, inuring old minds would love to know.
My real question is where do I look now to try to find the current issue of it not coming up to center or properly focusing, being stable? Where is that failure likely to be found? I found nothing loose or burned anywhere else on the scope.
I appreciate any advice, suggestions.

Voltage checks I made after my repairs:
C1103=152.9 VDC
R1105/R1103=153.5 VDC
R1105/TR1102=80.1 VDC
Current drop in series with R1105=3.1mA
(Print calls for ½ watt resistor where the one I replaced had burned. My math, .5/150VDC=.003A, my test resistor is a full watt and does not get warm, but isn't that current a bit high for the resistor called for?)
R1103/R1101=287.3
C1101=287.4
C1132=4.96
C1112=11.82
C1131=14.94
IC1131=in 14.77 VDC, out=4.96 VDC


Attachments:
PCBack.JPG
PCBack.JPG [ 354.3 KiB | Viewed 334 times ]
PCBFront.JPG
PCBFront.JPG [ 213.14 KiB | Viewed 334 times ]
Power Supply crp scl.png
Power Supply crp scl.png [ 145.13 KiB | Viewed 334 times ]
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 Post subject: Re: Hitachi V-203F Oscilloscope mystery
PostPosted: Nov Tue 19, 2019 9:42 pm 
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Joined: Sep Tue 30, 2014 6:08 am
Posts: 4893
Location: Norfolk, VA
If something fails after a move, physical damage is a distinct possibility for failure. Do a complete visual inspection (with the power off), looking for cracks, dislodged parts/plugs/sockets, broken boards (at the corners), cracked boards with broken traces, broken pots or switches (a tender spot for Hitachi scopes - I own several and had to junk one that took a spill - the rotary swiches took the hit).

The scopes generally have 5 or 6 key voltages - a positive and negative source (+/- 12V, check on the outer pins of the trace rotation pot) for the vertical, +5V for the logic, and the necessary voltages (+150V and +250V) for the CRT. Only when all are in spec does one go elsewhere. The service manual has the list and specs for each.

_________________
Brian
"Capacitor Cosmetologist since 1979"
USN Retired 1984-2006 (Avionics/Cal)


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 Post subject: Re: Hitachi V-203F Oscilloscope mystery
PostPosted: Nov Tue 19, 2019 10:29 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 1489
Location: Hartford, KY
I have the Hitachi V-212 scope. And if I had issues with it, I believe I would start at the power supply and see if the voltages are what they are suppose to be going to all the various cards. Once that is determined then you can branch off and try to divide and conquer. Is the vertical working yes/no then horizontal yes no. Then decide from there. Some good advice from above me.


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 Post subject: Re: Hitachi V-203F Oscilloscope mystery
PostPosted: Nov Wed 20, 2019 3:00 am 
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Joined: Nov Wed 30, 2016 7:35 pm
Posts: 5390
Location: Sunbury, Ohio 43074
From your description of what you see on the scope face, it would seem that once you verify the power supply voltages, the Vertical deflection amplifiers would be the next place to check

_________________
Preserving the hist. of electronics, one boat anchor at a time! :)
https://www.bbtvtestequipment.com


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 Post subject: Re: Hitachi V-203F Oscilloscope mystery
PostPosted: Nov Sun 24, 2019 12:33 am 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 1489
Location: Hartford, KY
Tomie- Make sure you let us know what you find with your trouble shooting. I would like to know? Thanks.


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 Post subject: Re: Hitachi V-203F Oscilloscope mystery
PostPosted: Nov Mon 25, 2019 12:44 am 
Member

Joined: Dec Wed 02, 2015 9:42 pm
Posts: 172
Location: Little Elm, Texas 75068
I'm still in the dark but looking. Following Brian's suggestion I determined to follow the voltage supplies as far as I had information to do so. The 12V+ and –, and the 5V+ and –, are both stable. After my repairs the 150 VDC supply is also right on. But the 250 VDC is running high, about 290 VDC or roughly 15% high. So I started on it.
On the power supply board it shows the 250 VDC power supply being established and then going off of that board via the plug P1102 which comes out on plug P902 which attaches to the Horizontal Deflection board PEF-516. On the print I found 250 VDC present in 3 places on that board and my determination was to simply check it there. On the right side of my print it shows it is attached to the collector of transistors TR856, TR857, left and right channels. It travels first via 22K ohm, 5 watt wire wound resistors, one for each transistor/channel. I attached my DVOM on that resistor and I had voltages all over the places. The 250 VDC was stable at about 289 VDC, but on the working end of those resistors I had unstable voltage readings. On R854/TR856 I was as high as 150 VDC and as low as 93 VDC. R855/TR857 they were 107 VDC for a high and 50 VDC for a low.
And then suddenly while I was testing it all began to work. I had a good line on both channels and the voltage stabilized. There are no voltage measurements printed on the schematic so I have no way of knowing if what I measured is anywhere near where it should be. I note that on the Horizontal Deflection board schematic which can be downloaded from the address I provided earlier, someone has drawn in oscilloscope readings for the collector on both transistors, TR856 and TR857. I don't know what those voltages mean or how they were obtained but I have nothing like that on this scope. My measurement for TR856 on the collector is 98VDC, and on TR857 it is 102VDC also on the collector. Both measurement were taken with the negative probe of the meter attached to the common ground and were confirmed by checking the measurements with the scope set for both channel 1 and channel 2.
I was puzzled about how simply checking the measurements could have, “cured,” the problem, until I noticed what is evident in the picture below. The Horizontal Deflection has two large, metal heat sinks located toward the end of that board. The plastic mounts for that board do not extend all the way to the end. I have the scope standing on it's back for these pictures and while I am making these tests. Over time, possibly with the scope being stored in a hot environment, the board has warped. Maybe those heat sinks provided enough heat to cause the warp. Simply by applying the meter probe to a contact on the board, with just enough pressure to get the meter to react and give me a measurement, I was inadvertently deflecting the board. Now I know how I was accidentally able to get the symptoms to change. But what actually changed I don't know. Moving the board with it energized and the meter connected does not appear to make it change. It is hard to look at the screen and tap at the same time. There is no change in the voltage measurements while tapping or movement. Temperature seems to effect it, in as much as the scope seems to improve the longer it is on and the hotter it gets. But it didn't seem to change until I accidentally deflected the board. I have visually inspected the whole board and can find no evidence of a broken trace. I am now checking for loose components while the scope is energized, by tapping with a chop stick.
I am guessing there must be a break and since it effects both channels it has to be somewhere that is relative to both channels. I just haven't found it yet.
I made a short video of the warp deflection of the board. I have devised a way to straighten that board and hold it in place but I won't make that repair until I have found the break and repaired it.
Thanks for the interest. Happy Holidays.

https://www.youtube.com/watch?v=fDEIrhM8mEo


Attachments:
Warp.JPG
Warp.JPG [ 322.55 KiB | Viewed 129 times ]


Last edited by Tomie on Nov Mon 25, 2019 3:48 am, edited 1 time in total.
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 Post subject: Re: Hitachi V-203F Oscilloscope mystery
PostPosted: Nov Mon 25, 2019 1:14 am 
Member

Joined: Nov Wed 30, 2016 7:35 pm
Posts: 5390
Location: Sunbury, Ohio 43074
Welcome to "problems that are common with antique electronic equipment". And sometimes not even all that antique.

I would, at this point, re-solder every connection of any components that get hot (power resistors and transistors) .. are heavy (power resistors, transistors with heat sinks, transformers) and any connection that might have strain on it (plugs, sockets etc). You do not need to remove the old solder ... just reheat the joint and add a tiny bit of new solder.

I have occasionally seen a power resistor lead that just gets to the point it won't take solder and becomes intermittent. You can, in those cases, UNsolder the lead, scrape it with an xacto knife or burnishing tool, and then resolder it.

Hopefully that will cure it. If not, my next guess would be a micro crack in one of the traces on the board. Those can be the devil itself to track down sometimes. They usually occur where a large component's via (the round part of the printed board with the hole in it) and the trace it's attached to, but in reality can occur anywhere.

For these, sometimes a heat gun can at least narrow the issue down to one section of a board, so you're not chasing the entire board.

I have very seldom seen an actual component defect cause this, but it has happened. An example would be a mechanically intermittent resistor or power transistor. RARE in my experience, which is why the above items should be done first.

Best of luck.

_________________
Preserving the hist. of electronics, one boat anchor at a time! :)
https://www.bbtvtestequipment.com


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 Post subject: Re: Hitachi V-203F Oscilloscope mystery
PostPosted: Nov Mon 25, 2019 3:56 am 
Member

Joined: Dec Wed 02, 2015 9:42 pm
Posts: 172
Location: Little Elm, Texas 75068
I can't fix what's not broken. I went down a while ago and the scope fired up perfectly. I did nothing but turn it on. I do notice a difference between the two channels, 1 is not as sharp a line as channel 2. But the scope works. I can't chase a problem that's not there.
Not sure what I'm going to do next. I want to chase the high voltage on the 250 VDC power supply if I can. That's an unregulated power source. There is one diode off the transformer secondary and an electrolytic capacitor. That is followed by a 10 ohm resistor which checks at 11 ohms. From there it heads off of the power supply board.
I'm going to have to study that print more to understand where the load could be light causing a voltage spike, if that's even possible.
At this point it's working and I want to thank everyone for all the help.
I'll check back in if anything changes.

Tomie


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