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 Post subject: Reparing and tweaking a Philco Junior Scope Model 7019
PostPosted: Jul Wed 31, 2013 6:22 pm 
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I got this tiny little scope (2" crt) recently to play with. A Philco Junior Scope Model 7019, uses a 2AP1-A crt tube. Mine is Serial Number 7590.

I spent a day replacing a bunch of components, mostly caps. Dissasembled a few pots to flush them out with IPA. Now the little guy works pretty well, for what it is!

Attachment:
Philco7019_replaced_parts.jpg
Philco7019_replaced_parts.jpg [ 100.49 KiB | Viewed 2345 times ]


Maybe someone can help me out with a few questions. First, the power supply is putting out a lot less than what the schematic calls for. Transformer secondary puts out 420VAC, instead of the 640VAC the schematic calls for. That's only 210VAC across each half of the CT, where the schematic calls for 320VAC. Of course this means all the circuit voltages are subsequently smaller as well. So, might I have a different version scope/transformer? The schematic figures are in RMS, correct?

Here's a link to the schematic, if you'd like to look at it: http://www.strucktower.com/Philco7019_schematic.jpg

Second question: The trace seems to wooble a bit- looks like maybe a phase problem? Like when it displays a triggered sinewave and only part of it wiggles. Independent of frequency, temperature, or gain settings. I'm wondering if someone could sugggest what I could tweak/investigate to remedy that.

I wasn't sure how to photograph that to show you. Here's a photo of the Lissajous plot for a resistor, first bypassing the HORIZ amplifier by selecting the direct HOR setting, then next to it using the HORIZ amplifier. In the first the trace is a straight line, but you can see (barely) in the last case the trace has become a thin oval rather than a line.

Ideas on how I might adjust/fix that?

Attachment:
Philco7019_trace.jpg
Philco7019_trace.jpg [ 84.07 KiB | Viewed 2345 times ]


Thanks,
Keith Ostertag

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 Post subject: Re: Reparing and tweaking a Philco Junior Scope Model 7019
PostPosted: Jul Wed 31, 2013 7:12 pm 
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This looks a lot like the famous "Waterman" series of scopes. Hmmm, Waterman was based in Philly, just like Philco, guess they could have been an offshoot?

Waterman made a whole series of small compact, and temperatudinally hot scopes, most with advanced features like DC coupling and dual trace. Dual trace at first done by snuggling up two rectangular CRT's top and bottom.

They also got a military contract to make some of those in mil-spec form, plus they made a HUGE AN/USM-24C scope of mediocre design. I guess Tektronix just blew them out of the water in the mid 50's and they were not heard from again.

Anyway, the funny wiggly line is probably due to the power transformer having to be so close to the CRT, like an inch away. Is there a mu-metal CRT shield? Even if there is one, it can lose its shielding properties over time if it gets jostled a lot. Weird.


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 Post subject: Re: Reparing and tweaking a Philco Junior Scope Model 7019
PostPosted: Jul Wed 31, 2013 7:49 pm 
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The crt is shielded, but I don't know if it is mu-metal or not.

Here's a photo of the Lissajous patern of a zener diode- note the loops where there should be straight lines.

Attachment:
Philco7019_zener.jpg
Philco7019_zener.jpg [ 80.6 KiB | Viewed 2333 times ]

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 Post subject: Re: Reparing and tweaking a Philco Junior Scope Model 7019
PostPosted: Jul Wed 31, 2013 9:18 pm 
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How about a look at the trace with Hor-In tied direct to Vert-In and an external audio tone signal driving both?
Try frequencies related to 60/120 HZ and non-harmonics.
then what does it look like if both inputs are driven with 60Hz line?

I imagine your X-Y component curve tracer setup is using line 60Hz?

Can you do the zener plot exactly the same but with X & Y interchanged?

Remember, even the CRT by itself can change its deflection sensitivity if the CRT B+ is variable.

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 Post subject: Re: Reparing and tweaking a Philco Junior Scope Model 7019
PostPosted: Jul Wed 31, 2013 9:55 pm 
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Location: New Albany, IN
Thanks for the ideas!

Quote:
Remember, even the CRT by itself can change its deflection sensitivity if the CRT B+ is variable.

Do you mean ripple, or line supply fluxuation?

I was thinking it might be phase related, because it _looks_ like I'm getting a slightly different trace on the positive and negative supply swing.

Yes, for these Lissajous patterns I am using the 60Hz line. However, I see a similar effect just running sinewaves of various frequencies in time mode- it is just harder to show in a photo because I can't show which part of the waveform is "wiggling".

Here is the same zener Lissajous pattern with X & Y interchanged, albeit with a little tweaking of gain for each:

Attachment:
Philco7019_zener2.jpg
Philco7019_zener2.jpg [ 46.88 KiB | Viewed 2322 times ]


I now find I have a problem with the HORIZ gain/sync pot, so I am not getting as much range as I should- need to fix that.

As far as trying an external audio signal to drive both signals in the XY mode, I will try that later tonight if I get a chance.

Thanks,
Keith

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 Post subject: Re: Reparing and tweaking a Philco Junior Scope Model 7019
PostPosted: Jul Wed 31, 2013 11:12 pm 
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OK, I was able to drive the X & Y with 400Hz and then with 4KHz. See photos. Doesn't seem to matter if the frequency is a multiple of 60Hz, as I varied the generator ouput.


Attachments:
Philco7019_zener3.jpg
Philco7019_zener3.jpg [ 60.59 KiB | Viewed 2314 times ]

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 Post subject: Re: Reparing and tweaking a Philco Junior Scope Model 7019
PostPosted: Aug Thu 01, 2013 5:20 pm 
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The Philco 7019...
http://www.oscilloscopemuseum.com/scope ... 7019-s6133
..and the Waterman Pocketscope S-10-A...
http://www.oscilloscopemuseum.com/scope ... 10-a-s1676
have pretty much identical circuits.
The Waterman manual lists the voltages for each tube pin.
Not sure how much the lower voltage from the power supply affects subsequently the amplifiers. Could you replace the 6X4 with diodes and see if the increased voltage improves things? Any ripples on the supply?

Good Luck with this, it is a nice little scope,
Volker

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 Post subject: Re: Reparing and tweaking a Philco Junior Scope Model 7019
PostPosted: Aug Thu 01, 2013 9:34 pm 
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The loops are probably due to phase-shift-- as you can see from the schematic they did not take any pains to use identical X and Y amplifiers, so the 20 degrees of phase shift is perhaps normal.

On their later scopes like the OS-8 they used real push-pull CRT driving tubes, with little trimmer caps to adjust the neutralization and phase shifts. On this one they were just going for extreme simplicity, at the expense of just about everything else. It's really a bit of a miracle that it's a semi-usable scope.


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 Post subject: Re: Reparing and tweaking a Philco Junior Scope Model 7019
PostPosted: Aug Thu 01, 2013 9:51 pm 
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Location: New Albany, IN
Yea.

Is there a simple way to tweak it? Some way to balance/neutralize the phase shift?

This little scope is cute, but isn't really much good for anything except very low frequency work, so I thought I maybe could use it as a dedicated component tester in XY mode.

Maybe you have other thoughts of some way to use it, just for fun? Not fast enough (I assume) to turn into a scope clock...

Thanks,
Keith

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 Post subject: Re: Reparing and tweaking a Philco Junior Scope Model 7019
PostPosted: Aug Fri 02, 2013 4:20 am 
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Quote:
Do you mean ripple, or line supply fluxuation?

Both actually, since there is no regulation in the PS.

About the 'loops' on the zener traces; to me, the pictures look like the first zener photo shows Y displacement, and the second zener photo shows X displacement, making the loops (but is hard to be certain).
Based on that change from Y to X the displacement causing the 'loops' would be entering the scope from the X/Y curve tracer.

On the pair of 400/4000 Hz photos, the fact that the higher frequency shows the larger 'loop' area, it does sound like phase mismatch between Hor and Vert.

I go with other post(s) suggesting that the power supply voltages need to be more correct (and free of line ripple), before fine tweeking of how the amplifier stages work.

You also want to check the B+ voltages and eliminate any ripple caused by the sweep circuit running. If the sweep circuit loads the PS and causes ripple it can put kinks in the trace.

I see this is another one of those 'split' power supplies with the transformer, rectifier, and first filter cap floating off of ground and the ground reference is tapped on the string of resistors. There has been much discussion of this type of PS in the Heathkit C-3 Cap tester threads.

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 Post subject: Re: Reparing and tweaking a Philco Junior Scope Model 7019
PostPosted: Aug Fri 02, 2013 1:45 pm 
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Willitwork wrote:
Quote:
I go with other post(s) suggesting that the power supply voltages need to be more correct (and free of line ripple), before fine tweeking of how the amplifier stages work.

Yes, the power supply voltages are a mystery to me.... The output of the transformer secondary is about 420VAC, according to the schematic it should be 640VAC. However, the filament voltage is a little high, running around 7VAC, so doesn't that suggest there isn't a short in the transformer? Also, the CT is actually centered.

I guess I _could_ disconnect the filament winding replacing it with an external one, then run the line up a bit with the variac to see if an increase in line and subsequent increase in the secondary will change much... Hmm. That's why I asked originally if anyone thought perhaps this might be a case of the design being different from the published schematic?

The plates on the 6J6 are running around 60V, the schematic calls for them to be around 100 (90 & 110). The plate on the 6AU6 is running around 151V, the schematic calls for it to be about 242V. Suggestions of what could account for that? Despite that, the scope seems to work at low frequencies just fine. Displays a 1Khz square wave fine, though it can't do a 25Khz square wave (integrated).

I see in the datasheet for the 2AP1-2 crt a note that says: "In general, anode-No.2 voltage should not be less than 500V." The schematic calls for it to be 360V, and I measure it at 236V.

Thanks for the reference to the C-3 thread, if I get a chance this weekend I will look it over.

Keith

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 Post subject: Re: Reparing and tweaking a Philco Junior Scope Model 7019
PostPosted: Aug Fri 02, 2013 7:18 pm 
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Are the transformer secondary voltages much different with the rectifier tube out of the socket?
Also, I notice the schematic lists winding resistances for the transformer which could be checked.

Speculating about WHY would there be a different power transformer (that has low output on your line power) gets me two LOGICAL ideas;
1. Higher primary voltage rating - could it be for a power source like line-to-neutral on a 208V L-L 3phase delta source?
2. Different power frequency spec - could it be designed for 50 or even 25Hz 'hydro' power (like some Swiss trains)?

Is there anything about the scope that suggests a special operating environment, or conversely settles these questions?
Is there a line voltage and frequency label for 117VAC 60C ? Does anything suggest it was Navy or Air Corps?

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 Post subject: Re: Reparing and tweaking a Philco Junior Scope Model 7019
PostPosted: Aug Fri 02, 2013 7:45 pm 
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I'll check the transformer windings over the weekend, both resistances and open circuit voltages.

I previously thought the transformer to be the original (it just has that look of being old), but now I notice the secondary leads were spliced and covered with shrink wrap- which would be weird for a factory job.

This is a regular consumer scope, the schematic shows the line supply for it as 117V 60cycles.

Thanks,
Keith

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 Post subject: Re: Reparing and tweaking a Philco Junior Scope Model 7019
PostPosted: Aug Fri 02, 2013 9:45 pm 
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Location: New Albany, IN
So, assuming now, that this is not the original transformer.... I doubt it would be very easy (or inexpensive) to find a better substitute.

Would raising the B+ voltages to spec (by installing a better transformer) improve the phase problem at all? My guess is it might improve the trace sharpness... but I don't know about the phase problem.

Keith

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 Post subject: Re: Reparing and tweaking a Philco Junior Scope Model 7019
PostPosted: Jan Mon 14, 2019 4:31 am 
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Hello Guys,
I have this scope and I am going to start restoring it so I can put it to use.

Sincerely rich


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