Forums :: Resources :: Features :: Photo Gallery :: Vintage Radio Shows :: Archives :: Books
Support This Site: Contributors :: Advertise


It is currently Nov Sat 29, 2014 8:26 am


All times are UTC [ DST ]



Post New Topic Post Reply  [ 31 posts ]  Go to page 1, 2  Next
Author Message
 Post subject: DuMont 304H oscillograph interesting observation.
PostPosted: Jan Sun 09, 2011 3:33 am 
Member
User avatar

Joined: Jan Thu 01, 1970 1:00 am
Posts: 11914
Location: Warner Robins, GA
Was just thinking about my DuMont 304H oscillograph and thought of something.

I figure the reason 5ADP* series CRTs are hard to find is because they after I don't know how many hours of use seem to lose their ability to focus properly. It is tempting to say the scope is at fault, but it is not as I have tried several 5ADP* CRTs and even tried them in an unrestored 304H with the same exact results. What is interesting is I can connect another 5ADP1 CRT in parallel with the one in the scope and both focus properly. I could find a way to mount a 5UP1 in the scope as it does work in this scope. Then I could eliminate one 2X2A tube and one HV capacitor which supplies voltage to the side connection on the 5ADP1 CRT. The manual calls for a 5CP1 CRT to be used with the scope. The unrestored 304H has a 5CP1 CRT, but the restored 304H has the 5ADP1 CRT and looks like the scope came from my local air force base as it had a sticker on the front panel stating that the case is grounded and not to use it to measure line voltages.

I don't want to scrap the scope since I have $$$ invested in restoring it which was done before the CRT stopped focusing properly. I wonder if it would be possible to use another 5" CRT that is the same length and uses similar voltages. If I have to rewire the CRT socket I can do that.


Top
 Profile  
 
 Post subject:
PostPosted: Jan Sun 09, 2011 7:19 am 
Member
User avatar

Joined: Apr Sat 11, 2009 4:15 am
Posts: 1192
Location: Portland OR, grid square CN-85
I've never had my hands on a DuMont scope, but from what I hear, they are very basic.

Reportedly, there is very little protection for the CRT, nothing like you'd find in a Tek 500-series instrument, which you could probably leave on permanently if you wanted to. . . . the CRT's in the 535/545/547, etc. would probably last forever if taken care of.

I'd wonder if the primitive circuitry might be a reason for the CRT focusing problems ? Have you checked to see if there are any dirty controls or parts out of tolerance ? A picture or two would be great !!

Good luck !!

Tom

_________________
"PRO PATRIA VIGILANS"
(Watchful for the Country)


Top
 Profile  
 
 Post subject:
PostPosted: Jan Sun 09, 2011 8:19 am 
Member
User avatar

Joined: Jan Thu 01, 1970 1:00 am
Posts: 11914
Location: Warner Robins, GA
The scope is totally restored except for a few resistors. A 5CP1 CRT works properly, but given it does not have a flat face it is no good for oscilloscope work. It is fairly basic, but when the CRT worked the scope was very good for what I used it for. I am not sure what all the 5ADP series tubes were used in, but I have two 5ADP1 CRTs one 5ADP5 CRT and one 5ADP (forgot which phosphor it is) CRT and all exhibit the same problem in two 304H scopes. I have not tried to look for 5ADP series CRTs because unless they are tested in a properly operating device I don't want to take a chance on them when I have to shell out $$$ for each CRT.


Top
 Profile  
 
 Post subject:
PostPosted: Jan Sun 09, 2011 2:52 pm 
Member

Joined: May Sat 22, 2010 4:42 pm
Posts: 2975
It's really unlikely that a CRT loses its focusing ability, unless you rap it really hard.

Have you checked all those high-megohm resistors in the focus voltage divider? These are often at like 250% of their color-coded values by now. There's often dust and mold and grunge on the phenolic terminal strips too which can drain off voltage intended to go to the CRT.

How about all the capacitors hanging on the CRT power supplies? Even a few tens of megohms leakage is waay too much.

Funny you should mention "focus".. There's a story in a Tektronix book about how when Tek was buying their CRT's from Dumont, all the CRT's they received were kinda fuzzy-- not enough to be out of spec, but all were real close to the limit. While the CRT's in actual Dumont scopes were all real sharp... Tek also noticed that the CRT's they got were not sequentially serial numbered.... You guessed it-- Dumont "cherry-picked" the real sharp CRT's for their own scopes and sold the oddball ones to Tektronix! That's why Tek bit the bullet and spent many millions starting up their own CRT design and manufacturing plant.


Top
 Profile  
 
 Post subject:
PostPosted: Jan Sun 09, 2011 4:26 pm 
Member
User avatar

Joined: Jan Thu 01, 1970 1:00 am
Posts: 11914
Location: Warner Robins, GA
The only components not replaced are a few resistors in the vertical amp section. Also that would not explain why the CRTs are the same in both the restored and restored scope. What happened is I was using the scope to display a video signal like a TV would and the picture all the sudden went out of focus. Also there is a small space near the top and bottom of the original 5ADP1 CRT in the scope where if a vertical line is moved the line disappears which is another reason I suspect the CRT. Given I can parallel another CRT and have it focus right leads me to believe that maybe an electrode in the CRT is not drawing proper current. Even though the 5ADP1 CRT originally worked I'm this scope it was designed for a 5CP1 and I really need is something designed for.the 5ADP1 so that I can test it properly.


Top
 Profile  
 
 Post subject:
PostPosted: Jan Sun 09, 2011 6:54 pm 
Member

Joined: Jun Fri 19, 2009 6:34 pm
Posts: 4310
Location: Long Island
I've seen a few television waveform monitor CRTs (a scope CRT with a custom graticule) go out of focus after they had accumulated tens of thousands of hours of operating time. It's the same reason old, pooped-out TV picture tubes get dim and out of focus: the cathodes become contaminated and weak, then there isn't enough electron velocity for the other electrodes in the tube to work as designed.

The cure for those--at least temporarily--was the same too, raise up the heater voltage a couple of volts with a "brightener" or heater booster transformer. In the later scopes which had switching power supplies, making boosters unworkable, we simply "cut-n-pasted" different values of focus resistors to give better results.

But cathode contamination is usually a gradual process that happens over months or years. It would not explain why your scope abruptly went out of focus unless a piece of the cathode coating just suddenly flaked off (about as likely as finding a winning Powerball ticket in the street, but not completely unheard of), or there was an arc to the first grid caused by a circuit problem. Cathode coatings damaged by flaking or arcing usually don't show any improvement with boosters.

The fact that you can get the CRTs into focus by connecting two of them in parallel suggests to me that for whatever reason, there is a voltage that is too high somewhere in the scopes. All the electrode currents would double with a second CRT, and that would tend to drag all the voltages down. I would take a suitably equipped HV voltmeter, and using all due precaution, measure the electrode voltages with two tubes working correctly, then start disconnecting the leads until the main one becomes defocused.

Once you know which voltage it is that makes the difference, you can check the circuit out carefully. It's not out of the question that something like a control, switch, or a terminal strip is breaking down under voltage in both scopes and lousing things up. If nothing else, you'll be able to modify the circuit to bring the voltages into line so the CRT focuses again.

_________________
"Hell, there are no rules here--we're trying to accomplish something!"

Thomas A. Edison


Top
 Profile  
 
 Post subject:
PostPosted: Jan Sun 09, 2011 7:13 pm 
Member
User avatar

Joined: Jan Thu 01, 1970 1:00 am
Posts: 11914
Location: Warner Robins, GA
Could be, but a 5CP1 works properly so I doubt it is the scope. Seems like the only 5ADP* CRTs I have got looked like they were used. As I recall all voltages are correct or at least in spec, but I can check them again to be sure. At one time I had the schematic in na k of the service manual, but the schematic dissappeared and I cannot find it. I know there is one schematic online, but it is the one without the extra control in the horizontal amp.


Top
 Profile  
 
 Post subject:
PostPosted: Jan Sun 09, 2011 7:31 pm 
Member

Joined: May Sat 22, 2010 4:42 pm
Posts: 2975
I've had great success rejuvenating old CRT's using an old TV CRT rejuvenator. basically it discharges a capacitor from cathode to grid. The grid on a CRT is actually a very sturdy can with a pinhole in it, so the grid can stand the discharge, and quite often the cathode works much better afterwards too!

I've used it on several old HP 200 CRTs that had a very weak spot-- they brightened right up after a few rejuv-blasts.


Top
 Profile  
 
 Post subject:
PostPosted: Jan Sun 09, 2011 8:38 pm 
Member

Joined: Jun Fri 19, 2009 6:34 pm
Posts: 4310
Location: Long Island
Tube Radio wrote:
Could be, but a 5CP1 works properly so I doubt it is the scope. Seems like the only 5ADP* CRTs I have got looked like they were used. As I recall all voltages are correct or at least in spec, but I can check them again to be sure. At one time I had the schematic in na k of the service manual, but the schematic dissappeared and I cannot find it. I know there is one schematic online, but it is the one without the extra control in the horizontal amp.


Well then, comparing the data sheets for the 5CP1 and 5ADP1 (available online), it looks like the tubes are extremely close, and probably interchangeable in many instances. But they are not identical. Specifically, it appears to me that the 5CP1 likes to run at a slightly higher focus voltage than the 5ADP1 does. As with all CRTs, the focus voltage required depends on the final anode voltage.

It is therefore possible that the 5ADP1 tubes will work better if you modify the scope for them. Specifically, it looks as though the focus voltage probably needs to be lower by perhaps 60 volts. Might be worth experimenting with different value resistors in the focus circuit to see what happens.

_________________
"Hell, there are no rules here--we're trying to accomplish something!"

Thomas A. Edison


Top
 Profile  
 
 Post subject:
PostPosted: Jan Sun 09, 2011 8:44 pm 
Member
User avatar

Joined: Jan Thu 01, 1970 1:00 am
Posts: 11914
Location: Warner Robins, GA
I could, but the scope had a 5ADP1 in it when I got it nearly 10 years ago and worked property till about three years ago.
what I could do is wire up a 5UP1 CRT in a defunct service grade oscilloscope and connect the deflection plates directly to the deflection plate terminal board of the DuMont and still get use of the scope.


Top
 Profile  
 
 Post subject:
PostPosted: Jan Mon 10, 2011 3:30 am 
Member

Joined: Jun Fri 19, 2009 6:34 pm
Posts: 4310
Location: Long Island
When you get to your mid-50s, you begin to notice a lot of things that don't work the way they did ten years ago! :wink:

I was playing with DuMont 304 oscilloscopes 35 years ago in high school. They were about 20 years old at the time and quite "long in the tooth" even then, but the New York City school system never throws anything away, nor do they buy anything new except maybe once every 80 years or so when they can actually get a budget for it. So that's what we had.

Even back then, those scopes were difficult to keep running. Built like battleships, but very unreliable with resistors and capacitors failing constantly. I did indeed have one that made a snapping sound and then lost focus. But it wasn't the CRT, it was a bad focus control. Not having a proper replacement, I put the scope back in the school storeroom diagnosed but unfixed, where it undoubtedly still sits to this day.

Look, I'm not saying that both of your 5ADP_'s couldn't be bad, but the odds of getting two with exactly the same defect make it very unlikely. On the other hand, getting two DuMont 304 scopes with the same problem is not only possible, you can bet on it!

_________________
"Hell, there are no rules here--we're trying to accomplish something!"

Thomas A. Edison


Top
 Profile  
 
 Post subject:
PostPosted: Jan Mon 10, 2011 4:27 am 
Member
User avatar

Joined: Jan Thu 01, 1970 1:00 am
Posts: 11914
Location: Warner Robins, GA
The focus control in the restored 304H is new along with all the other variable resistors except the variable sync control. I can measure voltages soon as I clean up enough to get to the scope.


Top
 Profile  
 
 Post subject:
PostPosted: Jan Mon 10, 2011 4:43 am 
Member
User avatar

Joined: Jan Thu 01, 1970 1:00 am
Posts: 17316
Location: Somers, CT
What is interesting is I can connect another 5ADP1 CRT in parallel with the one in the scope and both focus properly.


How does connecting two supposedly defective, worn out CRTs in parallel produce a sharper beam and clean up the focusing??

I'd be comparing the voltages to see what is changing when the second CRT is added. It sounds like a voltage problem.

Pete


Last edited by Peter Bertini on Jan Mon 10, 2011 4:47 am, edited 1 time in total.

Top
 Profile  
 
 Post subject:
PostPosted: Jan Mon 10, 2011 4:46 am 
Member
User avatar

Joined: Jan Thu 01, 1970 1:00 am
Posts: 11914
Location: Warner Robins, GA
I cannot explain it either. Other than having a focus problem the CRTs work great in all other respects.


Top
 Profile  
 
 Post subject:
PostPosted: Jan Mon 10, 2011 4:48 am 
Member
User avatar

Joined: Jan Thu 01, 1970 1:00 am
Posts: 17316
Location: Somers, CT
Compare the CRT voltages with one and two CRTs. It has to be in the scope.


Top
 Profile  
 
 Post subject:
PostPosted: Jan Mon 10, 2011 4:49 am 
Member
User avatar

Joined: Jan Thu 01, 1970 1:00 am
Posts: 11914
Location: Warner Robins, GA
If it were the scope then a 5CP1 would have the same problem.


Top
 Profile  
 
 Post subject:
PostPosted: Jan Mon 10, 2011 5:02 am 
Member
User avatar

Joined: Jan Thu 01, 1970 1:00 am
Posts: 17316
Location: Somers, CT
You've been flogging this problem for a few years. Six months ago you had a new 5ADP2 on the way. I assume it didn't fix the problem.

The scope is designed to use a 5ADPx tube. I really doubt, given the collection of jugs you've amassed, that they are all bad.

I would compare the voltages on pin 2 (cathode), the three grids, and the anode, and see what changes when you go between one and two in tubes parallel.


Top
 Profile  
 
 Post subject:
PostPosted: Jan Mon 10, 2011 5:05 am 
Member
User avatar

Joined: Jan Thu 01, 1970 1:00 am
Posts: 11914
Location: Warner Robins, GA
Actually according to the schematic the scope uses a 5CP1 CRT. I guess I am so frustrated with it that I put it aside for a long time then think about it later on. If the scope was indeed at fault then a 5CP1 would have the same or similar problem. Maybe I can post some pics tomorrow if I clean up enough to be able to get to the scope.


Top
 Profile  
 
 Post subject:
PostPosted: Jan Mon 10, 2011 5:11 am 
Member
User avatar

Joined: Jan Thu 01, 1970 1:00 am
Posts: 17316
Location: Somers, CT
304A did use the 5ADPx series, but the astigmatism and focus controls were both adjustable in that version.

If you are trying to use a different CRT than the chassis was designed for, you may have to modify the circuit to incorporate an adjustable astigmatism control. The fact that the display is sharp with two tubes in parallel is a good indication that the CRTs are good, and that a voltage is changing in the right direction when both are paralleled.

I could be wrong, but that's my suggestion.


Top
 Profile  
 
 Post subject:
PostPosted: Jan Mon 10, 2011 5:12 am 
Member
User avatar

Joined: Jan Thu 01, 1970 1:00 am
Posts: 11914
Location: Warner Robins, GA
Could be, but the restored 304H had a 5ADP1 in it when I got it which worked good up until about three years ago IIRC.


Top
 Profile  
 
Post New Topic Post Reply  [ 31 posts ]  Go to page 1, 2  Next

All times are UTC [ DST ]


Who is online

Users browsing this forum: No registered users and 6 guests



Search for:
Jump to:  
















Privacy Policy :: Powered by phpBB