View Full Version : Scope question


Tube Radio
10-22-2013, 09:05 PM
I have a question about a DuMont 304H scope that has a 5ADP1 CRT installed in it.

Up until about 5 years ago the scope (mostly restored) worked great and the CRT always focused properly.

One day while using the scope to watch TV (required two pother scopes to do that) the picture got out of focus.

When displaying a trace the focus control now has to be nearly full CCW for proper focus, but when displaying a waveform the focus control has to be at least 3/4 CW to somewhat resemble proper focus (never focuses as sharp as it does when just displaying a trace) which I know is not right.

I've tried another 5ADP1, a 5ADP2 and two 5ADP5 CRTs and all have the same problem. I even got another 304H scope with a 5CP1A CRT installed and tried the 5ADP* CRTs in it with the same problem while the 5CP1A CRT works properly in my original 304H.

On another forum I have had at least two topics over the years and have one going now without any answer as to why I have this problem.

I cannot use a 5CP1 CRT as I did use this scope at times to measure waveforms.

bob91343
10-23-2013, 12:19 AM
Apparently there is a problem in the potentials supplied to the tube. Compare them on the good and bad units and you will track it down easily.

6GH8cowboy
10-23-2013, 01:18 PM
Been my experience with most scope fails in the past that the HV caps cause a lot of grief for focus, brite, etc..
I'm not familliar with this scope but if it is an old one, the caps cost more than the scope is worth. Often oil filled, 2kv or better...cha ching$.
Comparing voltages is a good idea, just be VERY careful.

N2IXK
10-23-2013, 01:49 PM
I agree, I would be looking at the various CRT gun potentials, and comparing them to the schematic or the CRT datasheets. Is the 5ADPx even a valid sub for a 5CPx?

The DuMont 304 was one of the first scopes I ever played around with. They were probably DuMont's best selling scope, and a pretty rudimentary circuit to repair. Check the HV caps for leakage, and the 2X2 tubes. Check the divider resistors in the focus/intensity control circuits, and replace any that are out of tolerance.

Be VERY careful measuring HV potentials in this circuit. The voltages are lower than a typical TV, but the 60Hz supply is capable of delivering a lot more current.

Tube Radio
10-23-2013, 03:22 PM
Here's the voltage readings I took back in 2009

Meter negative connected to cathode of CRT.

Anode 3
Maximum intensity +2.94 KV
Minimum intensity +2.89 KV

Anode 2
Maximum intensity +1.542 KV
Minimum intensity +1.504 KV
Intensity control adjusted to where image just appears on CRT
Anode 2 voltage + 1.597 KV

Anode 1
Intensity minimum
Focus control fully CCW +500 V
Focus control fully CW +253 V
Intensity maximum
Focus control fully CCW +592 V
Focus control fully CW +356 V

The 5ADP1 CRT was working in the scope when I restored it and there's no telling how many decades the CRT has been in that scope.

Main reason I don't understand this is because the scope is fine as the 5CP1A CRT focuses properly, but the few 5ADP* series CRTS don't. Same exact thing happens in the unrestored scope.

bob91343
10-23-2013, 04:55 PM
Wait - you have me confused. I thought one 'scope malfunctioned regardless of which CRT you used, and all CRTs work fine in other 'scopes. Now you say something else.

I also started my 'scope career with a Dumont 304A. Then I "upgraded" to a 401. Once I got my hands on Tektronix I never looked back. I had several 545s and 535s and a delightful 547 and a Fairchild clone. Now I have four Tek and one HP, all solid state. One of the Teks is surplus to my needs so let me know if you are interested.

N2IXK
10-23-2013, 05:34 PM
Comparing the datasheets for the 5ADP- and 5CP- shows MULTIPLE differences in required element voltages. The 5CP- uses a post-accelerating electrode, while the 5ADP- doesn't. They ARE NOT the same tube, or even close substitutes for each other. I can't see how a single scope could ever accommodate both tube types. The differences are just too great. Are you just leaving the accelerating anode lead hanging loose when installing the 5ADP-?

According to the manual I have here, the 304H is designed for a 5CP- type tube. The "H" designates High Brightness, thanks to the post accelerator. You need a 5CP- tube. Fortunately, they are VERY common, and relatively cheap. Here's one for $25 Buy-It-Now:

http://www.ebay.com/itm/Vintage-RCA-5-CRT-Vacuum-Tube-JAN-5CP1-A-/121194498206?pt=Vintage_Electronics_R2&hash=item1c37c1489e

Tube Radio
10-23-2013, 06:22 PM
the 5ADP* series of CRTs do have a post accelerator and are the CRTs which don't work right in either scope. the 5CP1A CRT does work right in both scopes.

the restored 304H had the 5ADP1 CRT in it when I got the scope and did indeed work great with proper focus.

I'm beginning to wonder if there is am inherent problem with the 5ADP* series of CRT that shows up with age which if so could maybe account for why there are not many of those CRTs around.

Tube Radio
10-23-2013, 06:24 PM
Wait - you have me confused. I thought one 'scope malfunctioned regardless of which CRT you used, and all CRTs work fine in other 'scopes. Now you say something else.

I also started my 'scope career with a Dumont 304A. Then I "upgraded" to a 401. Once I got my hands on Tektronix I never looked back. I had several 545s and 535s and a delightful 547 and a Fairchild clone. Now I have four Tek and one HP, all solid state. One of the Teks is surplus to my needs so let me know if you are interested.

what model Tek is the surplus one and how heavy is it?

N2IXK
10-23-2013, 07:01 PM
the 5ADP* series of CRTs do have a post accelerator.


Yes, you are right. Looking at the wrong outline diagram in my datasheet.

But the voltages are still somewhat different. Perhaps the 5ADP- that you say was once working was actually a somewhat worn-out tube that was subbed in sometime previously, and the weakened characteristics allowed it to focus properly with 5CP- voltages applied. Then you replaced it with GOOD 5ADP-types which will not.

Tube Radio
10-23-2013, 07:07 PM
best I can tell the scope always had the 5ADP1 CRT installed.

I've even tried a well worn 5ADP5 which has light burn marks where the trace was for years in whatever the CRT came from and it didn't focus right either.

I cannot seem to find anything about the 5ADP* series of CRT except some for sale and datasheets.

N2IXK
10-23-2013, 07:22 PM
I have a 304H manual here in my library, it shows a 5CP-

The 2 304H scopes I have worked on in the past both used 5CP- tubes.

I have a Rider Encyclopedia on Oscilloscopes, and the 304H schematic shows a 5CP-

A Google search for DuMont 304H schematics shows 2 separate copies of the schematic, both showing 5CP- tubes.

I can't find a single reference to a 5ADP- in a 304H, besides your posts about this on various forums over the years.

All this would lead one to believe that the 5ADP- is NOT original to your scope, and coupled with the differences in gun voltage requirements and the fact that your scope works properly when you try a 5CP- in it would seem to make the conclusion obvious.

Tube Radio
10-23-2013, 08:35 PM
only thing I can figure is judging by the warning sticker about not measuring line voltage the scope came from the local air force base where I work which may explain the 5ADP1 CRT given the inherent issues a non flat faced tube has with distorting the displayed waveform.

also the 5ADP1 CRT worked properly for how ever long it was in there until the focus messed up. when I got it I used it maybe 3-4 years before the focus problem happened and it always had proper focus.

I am begoning to suspect it is some defect with the CRT that maybe shows itself with age. only way to be sure is with a CRT tester or even better something designed for that CRT.

N2IXK
10-23-2013, 09:03 PM
Heathkit made a 5MHz "Professional Oscilloscope" model OP-1 that used a 5ADP- tube, IIRC. Styled to somewhat resemble a Tektronix unit of the era. One of their best tube scopes, IMO.

Not a very common model, though.

Tube Radio
10-23-2013, 09:10 PM
one thing I just remembered is I can connect another 5ADP* series CRT in parallel with the one in the scope and the focus then works correctly.

N2IXK
10-23-2013, 09:27 PM
This could be explained by the tubes drawing twice as much current on the various anodes, and shifting the voltages on the resistive divider network accordingly. Not really a meaningful data point, I'm afraid...

Tube Radio
10-23-2013, 10:27 PM
I suspect that is what's happening. One way is to confirm by trying that experiment again and measuring the voltages.

In a way that leads me to suspect that if the 5ADP* CRTs are indeed bad it may have something to do with the focus electrode.

bob91343
10-23-2013, 11:03 PM
I have what I believe to be a good 5ADP1 if anyone is interested.

earlyfilm
10-24-2013, 11:52 AM
This is a very long shot, but I ran in to it on a DC-coupled scope many years ago.

Measure your CRT voltages both with and without a signal applied to the vertical input.

If anything except for the deflection plate voltages change, suspect something is amiss in the DC coupled sweep amplifier that causes it to pull too much current and create a power supply voltage drop that is causing the CRT to go out of focus.

Jas.

Tube Radio
10-24-2013, 03:53 PM
I can try that, but why would the 5CP1A CRT not go out of focus and why would two identical scopes have the same exact problem?