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 Post subject: OS-8 oscilloscope problem
PostPosted: Jun Mon 30, 2014 4:19 am 
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Location: Warner Robins, GA
I have had an OS-8 oscope for maybe 3-4 years.

I replaced all resistors, paper & electrolytic caps in all sections except the vertical section.

The scope works ok, but does have vertical issues because I haven't done anything to the vertical section.

The horizontal is ok on sweep or external AC input, but on its DC input as I adjust the variable input control I get this and as expected I have to readjust the horizontal position control.

Image

If I add a capacitor from the horizontal output of the scope clock making it AC coupled I have to reduce the setting of the variable input control in order to keep the horizontal from being too wide for the CRT and I get this and I have to readjust the horizontal position to its previous position.

Image

If I AC couple the scope with the horizontal input selector switch set to 1 I get this, but have to reduce the variable horizontal input more. Not sure though if the DC coupling is the same as the AC coupling is on 1 (dc coupling just has one switch position) or if it is lower than when AC coupling is set to 1.

Image

Seems to me like one of the tubes in the horizontal maybe has mismatched sections or perhaps I have a bad tube, but that doesn't really explain why I get the second picture when the scope is externally AC coupled unless something is wrong with the dc balance circuit.

I don't quite get the dc balance circuit either as it uses a 10 meg resistor right after the 2 meg section (AC coupling and sweep uses 3K section) of the dual horizontal gain pot and the DC balance pot connects to the other end of the 10 meg resistor and the dc balance pot adjusts the dc voltage on the grid of the first horizontal amplifier stage. One would think any dc voltage appearing on the dc coupling input terminal would affect the dc balance, but I see where the dc balance pot is fed a relatively high B+ voltage so the dc input voltage wouldn't affect dc balance at all unless it was relatively high which would be way more than enough to drive the beam off the CRT face. Not sure if the dc balance pot is in circuit when using AC coupling or sweep. If it was necessary for the dc coupled input then it would more than likely need to be in circuit for ac coupling and sweep.

I'm using a 3RP1A CRT if that makes any difference.

In the past I think someone had replaced the vertical AC coupled input with a BNC jack. I might do the same to the horizontal AC coupled input jack only if I decide to use this scope as a slave oscope clock display as the clock board is inside a Heath student model oscope which is also the main display.

I will also need to replace the vertical and horizontal position pots as cleaning them didn't help. For that the 2 watt Clarostat/Honeywell pots will work quite nicely.


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 Post subject: Re: OS-8 oscilloscope problem
PostPosted: Jun Mon 30, 2014 11:23 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 100
Location: Colorado Springs, Colorado
I suspect your scope horizontal is running out of gas on the right side. Could be a weak tube as
you suggest. Could be low supply voltage. Mine runs about 350 volts. There is also a full wave
selenium supply that runs about 90 volts. Not sure what this second supply is for. I don't have
a schematic for the B/U scope.

I really suspect your clock circuit driving the scope is not centered around zero volts. It more than likely is
offset to one side of zero. A basic/simple scope like the OS-8 just may not be able to handle this much offset.
This would also explain why AC coupling makes your clock face look better.

OK. I am out of guesses now.
Bob


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 Post subject: Re: OS-8 oscilloscope problem
PostPosted: Jun Mon 30, 2014 11:30 pm 
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Location: Warner Robins, GA
Maybe it cannot handle the dc voltage as it is offset some from zero, but then why have a dc coupled input?

I most likely shouldn't troubleshoot the scope until I get the vertical rebuilt with new resistors and capacitors unless any vertical problems won't affect the horizontal.


The 90 volt supply is full wave and supplies B+ to the 12AT7 vertical driver tube, 12AT7 horizontal driver tube and the 12AT7 blanking/Z axis amplifier stage.

The third rectifier is for the CRT voltage.

I get these voltages.

Voltage at first filter cap of 90 Vdc supply
98 Vdc

Voltage at second filter cap
84 Vdc

Voltage at first HV CRT filter cap
580 Vdc

B+ at first filter cap
357 Vdc


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 Post subject: Re: OS-8 oscilloscope problem
PostPosted: Jul Tue 01, 2014 1:52 am 
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Posts: 100
Location: Colorado Springs, Colorado
Those are very close to the voltages I maesure too. Do you have spare tubes, 6AH6 and 12AT7,
that you can swap into the horizontal amplifier?

I can not believe my OS-8 works. Today is the first day I have applied power to it. The thing has
been sitting in my barn for 15 years and it appears all original. I saw a few parts with a 53 date code.
It seems to be fully functional except I notice some nonlinearity on the right side of the CRT. Could
this be a common problem with these scopes? It has a major mechanical problem though. The back has
been smashed forward so that all the sheet metal and frame are buckled and moved forward. The front
panel is even twisted somewhat. I never thought it would display a waveform on the CRT.

Tomorrow I will dig out some new spare tubes and try them and will report back to you.


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 Post subject: Re: OS-8 oscilloscope problem
PostPosted: Jul Tue 01, 2014 2:28 am 
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Not sure about spare tubes, but I can swap the 12AT7 tubes around and see what I get.

Also I strongly suspect that if the triode halves are not closely balanced that will affect things as well.


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 Post subject: Re: OS-8 oscilloscope problem
PostPosted: Jul Tue 01, 2014 3:08 am 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 100
Location: Colorado Springs, Colorado
In reference to your AC coupling picture and comment. The horizontal AC gain and DC gain are the same.
In the DC mode, the switch just shorts around the AC coupling capacitor. The verticle AC / DC is entirely different.

The fact that you had to decrease the horizontal gain when you switched to AC tells me you had to overdrive
the horizontal in DC mode to try to overcome whatever was causing the nonlinearity. DC offset or tube problem
or design limitation.

You asked why they provided DC coupling. Who knows what they were thinking or were required to do for the
mil contract. In this case DC coupling was free. The amplifier had to respond to near DC for very slow sweep speeds.
DC is also required if the horizontal is driven with low frequency square waves or even low frequency sine waves.
You must realize too. The DC voltage applied to the input has to be within the scope's amplifier capability to bring
the trace back on screen. This applies to both horizontal input and verticle input.


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 Post subject: Re: OS-8 oscilloscope problem
PostPosted: Jul Tue 01, 2014 3:23 am 
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The schematic I have shows the horizontal dc input going to the other section of the horizontal gain pot and looking at the scope the dc input jack connects directly to the 2 meg section of that pot.

The center terminal of the pot then connects to the switch and a 10 meg resistor. The other end of the 10 meg resistor connects to the 5 meg dc balance pot which varies a dc voltage between zero and some higher voltage.

If the scope was used to measure DC voltage or a waveform riding on a dc voltage wouldn't the dc voltage on the input affect the dc balance?


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 Post subject: Re: OS-8 oscilloscope problem
PostPosted: Jul Tue 01, 2014 4:16 am 
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Location: Colorado Springs, Colorado
I am not quite following you because we are working with two different schematics. I admit my
schematic is different from my scope too. My schematic shows the external horizontal signal,
both AC and DC, going through the gain pot and then into the grid of the first amplifier tube.
Then the position and balance pots and circuits are between the first and second amplifier tubes.

So that is different from what you describe.

You asked about balance changing position or position changing balance. Yes they do interact!
My manual mentions the balance being an internal adjustment that was set at the factory. I will
fiddle with mine tomorrow but at first glance I am not sure where it is located. It is such a simple scope
I am sure I will find it. Typically the balance is adjusted so that the trace doesn't move horizontally
as the gain control is varied.

By the way. I like your scope clock. Where did it come from?


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 Post subject: Re: OS-8 oscilloscope problem
PostPosted: Jul Tue 01, 2014 6:04 am 
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Location: Warner Robins, GA
The clock board is from Sparkfun https://www.sparkfun.com/products/9306

Can you post a picture of yours?

Maybe yours is a different version?

I can send you a schematic VIA email, but it is in the djvu format.

Too bad this scope is so compact. Would make a nice stand alone scope clock, but the clock board wouldn't fit in the case I don't think without removing some stuff. Plus due to the heat of the tubes and such a compact chassis the board might not last as long.

I may try to feed the horizontal to the scope's horizontal dc coupling input tomorrow and this time use a smaller external coupling cap as I used a pretty large one initially so the charging current could still have let the DC voltage pass through. The scope also may not have been designed for a fixed dc voltage to be present on its dc coupled inputs.

Mine unfortunately doesn't have the nameplate on it and it was a forum member here who identified the model back when I got the scope.


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 Post subject: Re: OS-8 oscilloscope problem
PostPosted: Jul Tue 01, 2014 3:41 pm 
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Posts: 100
Location: Colorado Springs, Colorado
OK, I will try to pm you with my email address. I have a djvu viewer program somewhere.

I think if you get your scope working properly it will handle the DC input just fine. Scope DC
input means just that as long as the DC offset is within it's capability.

From what I understand in your last post, you are running your clock output signal through
a capacitor into the scope DC terminals. This tells me you are not driving the scope with
too much DC offset voltage and that the scope really does have a balance problem. Just like
mine had until I fixed it this morning!

I decided to check the DC balance by mechanically centering the horizontal position control and
then observing where the spot was. With correct balance it would be center screen. It was way, way off.
I knew it was off yesterday but it didn't register how much until I checked it today. The internal
balance adjust pot was sealed at the factory to prevent someone from messing with it. So before
I broke the seal to adjust it, I decided to swap tubes. The 6J6 driving the CRT had very little effect.
The 12AT7 driving the 6J6 had a huge effect. In fact the spot was at center screen with the H position
control centered. I didn't have to mess with the internal balance pot.

One of the 12AT7 triode sections must have been weak.

I then went back to sweep mode and applied a sine wave to the verticle input and the nonlinearity I
mentioned in a previous post was entirely gone.

I suggest you check your DC balance as above or just swap out the tubes and see what happens.


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 Post subject: Re: OS-8 oscilloscope problem
PostPosted: Jul Tue 01, 2014 4:56 pm 
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Location: Warner Robins, GA
I used the capacitor as a test to see if there was something wrong with the dc coupling input since it is separate from the AC coupling.

Forget what value of capacitor I used as I just grabbed one and it might have been too high a value for the scope's input impedance which I think means the charging current would be enough to make it appear as though I had directly dc coupled the clock. I'll try again this afternoon with a small value capacitor and see if I get the same results.

If using the capacitor then straightens out the display on dc coupling then I'll know the scope just cannot handle the clock board having dc on its outputs.

I think the main reason the scope clock has dc on its outputs is because it has a burnin prevention feature that shifts the display slightly at a user set interval from 1-6 minutes or the feature can just be turned off.

EDIT: Just looked at the manual and saw where the BNC jack for the AC coupled vertical input was stock.

Also reading the manual AC voltage can be applied to the dc coupled inputs, but its bandwidth is determined by the vertical or horizontal gain controls so it could be the controls having something tio do with the display issues when using dc coupling.

I can adjust the refresh rate of the scope clock board (special firmware available on request from the designer) so I may try a low refresh rate and see if the image improves any on dc coupling.

Tried a smaller capacitor and that didn't help anything and I tried the cap I used yesterday and there was no dc on the output of the cap.

With the clock directly connected to the horizontal dc coupled input if I have the horizontal gain set to full the horizontal position is affected.

Pretty sure my vertical issues are because it has all original components in the vertical section and that is further evidenced by the vertical looking better the longer I leave the scope on.

I need to at least replace the electrolytic and paper caps in the vertical section before using the scope any more though.

I'll replace the resistors as well. Been putting it off as things are crammed in there.

For the 90 volt supply if I replace the seleniums will I need to add a voltage dropping resistor? For now I will leave them in as they work just fine unless there is good reason to replace them.

The selenium for the HV is a long tubular one looking much like a capacitor. Should that be replaced with a diode or left alone?


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 Post subject: Re: OS-8 oscilloscope problem
PostPosted: Jul Wed 02, 2014 1:52 am 
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Swapped some tubes and got some improvement so when I get done with the vertical section I will be ordering a new set of tubes.


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 Post subject: Re: OS-8 oscilloscope problem
PostPosted: Jul Wed 02, 2014 2:19 am 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 100
Location: Colorado Springs, Colorado
I would be inclined to leave the seleniums until they cause problems. What I have noticed is that in very low current
applications, such as your scope, they last a lot longer or their problems are not as noticable.

With the horizontal gain set to maximum you are likely driving some tube into cutoff and or saturation and this
could affect position.

There is a bias set pot for the 6J6 that drives the horizontal plates. You might try adjusting this to see if you can
get more drive ability and see if the clock display looks better. Mark the position of the pot wiper so you can get
it back to original if the adjusting doesn't help.


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 Post subject: Re: OS-8 oscilloscope problem
PostPosted: Jul Wed 02, 2014 2:43 am 
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I set the bias pot per the manual.

I did not replace the two power resistors that supply the plate voltage for the 6J6 tube yet so if one was out of tolerance that could cause a problem.


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 Post subject: OS-8 Blows Fuses
PostPosted: Feb Tue 06, 2018 5:20 am 
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Joined: Feb Mon 13, 2017 5:42 pm
Posts: 110
Location: Gualala, CA, 95445
I just came across an OS-8. It seemed to power up OK at first, but then it blows both 3/4 Amp line fuses.

Can anyone provide some advice on what I should look for?

Richard

(Sorry, posted entered twice.)


Last edited by rchrdm on Feb Tue 06, 2018 5:24 am, edited 1 time in total.

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 Post subject: OS-8 Blows Fuses
PostPosted: Feb Tue 06, 2018 5:22 am 
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Joined: Feb Mon 13, 2017 5:42 pm
Posts: 110
Location: Gualala, CA, 95445
I just came across an OS-8. It seemed to power up OK at first, but it blew both 3/4 Amp line fuses.

Can anyone suggest what I should look for? Replace electrolytic capacitors?

Richard


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 Post subject: Re: OS-8 oscilloscope problem
PostPosted: Feb Tue 06, 2018 3:58 pm 
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Before you do anything you will need the service manual. I have the manual. PM me your email and I'll send it to you.

Replace all electrolytic capacitors, all paper capacitors and because it is a piece of test equipment I would advise replacing all carbon comp resistors.

The two capacitor cans at the front it is best to remove the cans from the power supply chassis, cut the cans right above the base drill small holes next to the existing terminals and install the new capacitors on the top of the base with the new wires going through the small holes drilled in the base. You then reattach the bases to the power supply chassis and rewire the caps in circuit. Have to do it that way because there's no room for the replacement capacitors otherwise nor is there room for the necessary terminal strips to mount the new capacitors if not using the original can bases.

Provided I can find mine I'll take pictures of how I did the can capacitors and post them here or email the full size pictures to you.

Also test all tubes.

If you have not had any experience working on an extremely compact chassis then I do not recommend restoring this yourself and would then suggest a local electronics tech that is good with tubes.


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 Post subject: Re: OS-8 oscilloscope problem
PostPosted: Feb Tue 06, 2018 7:02 pm 
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Joined: Feb Mon 13, 2017 5:42 pm
Posts: 110
Location: Gualala, CA, 95445
Thanks! I have worked on tube equipment before, but not super compact ones. I will take a look at the chassis and see how they look.

Please go ahead and send the manual. I found three on BAMA, but you no doubt have the best one.

Richard
rchrdm@earthlink.net


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 Post subject: Re: OS-8 oscilloscope problem
PostPosted: Feb Tue 06, 2018 7:30 pm 
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Joined: Feb Mon 13, 2017 5:42 pm
Posts: 110
Location: Gualala, CA, 95445
In the manual I have, there is no parts list, but I see a picture of the power supply and two rather large "can" type capacitors. It looks like, from the schematic, that one is 20/20/20/20 mF and the other is 20/20/20 mF, correct? What voltage did you use for replacements?

There are some sources of new ones, such as at Antique Radio Supply.

Richard


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 Post subject: Re: OS-8 oscilloscope problem
PostPosted: Feb Tue 06, 2018 8:10 pm 
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You're welcome. The chassis are extremely compact. They basically crammed what is normally found inside a full size tube scope into something extremely small.

At first I was hesitant to work on mine, but after I started working on it I found it wasn't really as hard as I thought.

Think I may have used 450 volt capacitors.

Forget exactly why I did not use the can capacitors.

Either size or cost.

If you do use individual electrolytic capacitors use ones rated at 105C if possible. They will last longer.

Will be tomorrow before I can send the service manual.

If yours uses a regular 3RP1 CRT with the curved face look for a 3RP1A which has a flat face.


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