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Why don't you save yourself the trouble and buy a new one for $12?



Malcolm

On 23 March 2016 at 04:15, snapdiode@... [TekScopes] <
TekScopes@...> wrote:

Sounds good but at 45$ for 47mL it's eating into my scotch budget...

Hi again Dennis ..I had a close look at your trace and from what I can tell a 0.3dB Peak to peak is quite possible from just before 750Mhz to around 820 Mhz.

The aberration I see in David's head is about that. It's just I am running at an order of magnitude more precision so it shows up.

I'll still pull it down and resolder everything and do continuity and short tests on the head components.

This has me curious.

Hi Dennis:
Yes...well you can see from the graphs something is amiss for sure in David's head. That 0.5dB dip jumps out at you. Tricky thing with that, without doing these flatness tests I could have not noticed the head is outside spec. as its only 0.3dB out in that band.

Both the SA and the Boonton saw it.
Really strange...I might do a tear down on it and reassemble.

Otherwise how does the graph of my head look compared to your data on a properly operating head?

On Tue, 22 Mar 2016, snapdiode@... [TekScopes] wrote:

Ammonium bifluoride + _ANY_ relatively strong acid => HF + ammonium salt of
that acid. It is simply more convenient and safer to store HF in form of its
salt.

According to the MSDS, etching cream is actually ammonium bifluoride and some other weird chemicals. This has a "severe effect" on Nylon and Delrin.



I wouldn't go near HF with 15 space suits on.




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It's about $20 with shipping on Amazon.

Or even better: $6 with shipping on ebay:

I just found this in the manual:

Insufficient Horizontal Deflection
If the horizontal trace starts at the left side of the graticule, but does not extend to the right side, the Hold-Off circuit is causing V135A to conduct too soon after the triggering pulse had forced it into cutoff. If the trace cannot be expanded the full length of the graticule with the Swp. Length control R176, check the resistance values in the cathode circuit of V173.


All the resistors in the tube circuit are 47 Ohms, as they should be. Should I check if the resistors are bad? They look fine from a visual inspection.


However, I do see what to seems like a paper capacitor connected to a pin of V152, which is connected to a pin of V173. It is squishy if that is any indication. Picture: Imgur: The most awesome images on the Internet



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Not sure if this is actually a paper capacitor. It seems from the schematics this one should be ceramic or mica.


I also found something in the troubleshooting guide about No Horizontal Sweep (Timebase A). The manual says (the important parts):


If the Timebase Generator is not producing a sawtooth sweep voltage when the STABILITY control is adjusted for a free-running sweep, some defect in the generator is hanging up the Miller circuit... The manner in which it is hung up may be determined by measuring the voltage at the SAWTOOTH A binding post...


Albert, I think this is what you were alluding to. The voltage at the binding post is about -3 Volts. The manual:


... If hung up at the low end, the voltage at this point (SAWTOOTH A) will measure anywhere between ground and -20 to -30 volts...


So the Miller circuit is hung up in the low end. The manual goes on to say:


If the Miller circuit is hung up in the low end of the sawtooth...a check of the voltage at the plate of the Miller tube will offer a clue to the cause. If this voltage is quite high (+300 Volts), check the neon lamp B167 and the Runup CF tube V173.


Checked the plate voltage, it's aroun 41 Volts. The manual:


However, if the plate voltage of the Miller tube is near the quiescent level (+45 Volts) the trouble will generally lie ahead of the Miller tube. The result of the trouble is that the On-Off Diodes V152 cannot be gated off; they are conducting heavily and clamping the grid of the Miller tube near ground. If all the tubes have been checked, then check the open plate and cathode resistors in the Sweep-Gating Multivibrator circuit, the Hold-Off circuit and the Runup CF circuit (where V173 is). Also check that the stability control can vary the voltage at the grid of V125.


Okay, so I looked In the circuits they mentioned and everything seems fine. It seems that both issues originate in the same circuits and I'm thinking it's a dead capacitor... or maybe a few.


Related: There is a capacitor C180 that is in the Hold-Off circuit that is paper. However, in the parts list, Tektronix lists C180A through C180E. Which is the C180 that is used in the circuit? Is there a way to know?


Thank you,


Alex Brinister

Hi Ancel,
In the test I performed there was no anomaly in David Partridge's
replacement head as compared to George Steen's original head when I compared
them including at 800MHz. There was a very slight 0.4dB rise between the two
heads across the spectrum from 500MHz to 1000MHz but it was so gradual that
it was essentially a straight line except for a very slight dip (~0.1dB)
centered from 875MHz to 930MHz.

The gradual 0.4dB rise I saw could have been due to instrument drift (not
warmup). I made every effort to eliminate any warmup effects from the SG-504
and 7603 / 7L14 by allowing them to warmup for 2 hours before running my
tests. But the test itself takes 6 minutes to cover the entire spectrum and
that may have accounted for the slight increase.

Tek used to be famous for their ultra-sharp trace width. But a side effect
of the Expansion Mesh, which was necessary in the 7000 series CRTs, was that
the beam became thicker. The inherent limitation of my methodology is ~0.1dB
due to the trace thickness of the 7603 CRT.

The next time I perform this test (with all three heads) I will run it in
both directions at 6 minutes in each direction. That should eliminate any
instrument drift from the results.

Dennis Tillman W7PF

Sounds good but at 45$ for 47mL it's eating into my scotch budget...

Try Devcon Plastic Welder II. It's a methacrylate structural adhesive. It does not require any etching or special cleaning. Link:

Here's the link again for convenience to the PICS and the EXCEL sheet.

.

I started a new thread as I did some changes to the PCB layout and changed a component value to optimize the results. Version 1.1 head!
Also, the Rigol DSA interpolation of the the error correction constants leads to some potential precision gaps between the fixed frequency samples used in the first set of tests.

This time I used the Boonton 4210 RF meter directly (far more stable results then the Rigol SA) and added the 6Mhz reference signal into the comparison with some interesting results.Like Dennis did I exercised my arm slowly sweeping the SG504 to see ALL the issues.

In summary, perhaps DavidP's head has some kind of problem as it's out of spec around 800Mhz. Of course that doesn't help me, so now I created my own.

Anyhow have a look at the new Excel sheet as I charted the two heads data (across the SG504 span), logging the data once they showed any flatness amplitude variance >0.02dB from the previous sample.

BTW, I am doing absolute sampling, NOT sampling referenced to the 50Khz or 6 Mhz signal. That could lead to a hidden measurement offset should there be a problem in the SG504 or the cable connections.

So 6 dBm is exactly that, not referenced to anything but the Boonton.

Going back to read the theory section was helpful. Improper
neutralization can apparently cause this exact problem which is a side
effect of bootstrapping the amplifying input stage to increase common
mode rejection. The theory section on page 3-6 of the manual covers
it.

The 7A13 uses a different design where the input stage has a gain of 1
so no neutralization is required. I think this might have been
mentioned here in the past.

What I worry about though is how did the neutralization get so screwed
up? Did it get misadjusted or has there been a component failure?

Given that neutralization is so easy to calibrate, I don't think you
can do any harm by messing with it. If the adjustment range of C141
and C241 is enough to correct it and the rest of the specifications
can be met after finishing the input calibration procedures, then
someone just fiddled with it. No special equipment is required to
adjust the neutralization other than a clean signal source.

According to the MSDS, etching cream is actually ammonium bifluoride and some other weird chemicals. This has a "severe effect" on Nylon and Delrin.



I wouldn't go near HF with 15 space suits on.

@Albert: I ran the sawtooth signal through both Timebase A and B. There is still nothing on Timebase A. On Timebase B, it shows up nice and clear (albeit still on half the screen). However, when I change the time division on Timebase A, the sawtooth signal gets very badly distorted, sometimes to the point where it stays distorted. The working sawtooth and distorted sawtooth: Imgur: The most awesome images on the Internet



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I took out the CRT out of its socket to see if there were any bad connections and whatnot. I also looked at the CRT circuit to see if there was anything going on there. Everything seemed to check out. Looking at the circuit diagram in the manual, it seems there is a very high number of oil paper caps in the CRT circuit. Perhaps one or multiple are dead and need replacing?


Alex Brinister

Hi Mitch,
It is not clear which message you are responding to about melting points -
Snapdiodes or mine.

Teflon won't melt with any soldering iron I have ever had in my 45 year
electronics career. I have Thermal Wire Strippers that melt the Teflon
insulation off wire but the blades on that wire stripper glow cherry red.

Dennis Tillman W7PF

On Tue, Mar 22, 2016 at 7:48 PM, <snapdiode@...> wrote:

It melts quite nicely at a temperature that Nylon melts at. I wonder who told me it was Delrin.
Anyhow, I'll try glass etching cream, there's a local store that has it in little bottles.

I'm sure Chuck Harris will be posting any minute, but that's the last
thing I'd use. The active ingredient is HF, which will etch glass and
your fingers, but not most plastics.

That would depend on how "Butch" the soldering iron is... ;-)

Mitch
N4MF

It melts quite nicely at a temperature that Nylon melts at. I wonder who told me it was Delrin.
Anyhow, I'll try glass etching cream, there's a local store that has it in little bottles.

The idea is still to etch the surface so epoxy will grab it.

Chromic acid will etch Delrin and Nylon, but I don't see how I can make the acid easily locally.

The grey end cap is glued to the white part, so I'd guess it isn't Teflon...

Hi Snapdiode,
There actually is a glue or adhesive (I'm not sure what the difference is in
the terms) for Teflon. I have a roll of Teflon tape with glue on one side of
it that I can stick down on a surface. It is the only time I have ever seen
something stuck to Teflon.

Dennis Tillman W7PF

Hi All,

Just wanted to share my victory repairing my 2225. Using a bamboo skewer, I was able to locate a spot that induced the failure at will, when I pressed on the printed circuit board.



It ended up being the final output transistors in the vertical deflection amp. (Q256 and Q257).

These transistors have neat little heat syncs attached to the frame, and I noted that when the board slightly flexed under the mechanical pressure of the bamboo skewer, the transistor did not budge. I suspected a broken leg or solder joint, and indeed that was the case. The collector pin of BOTH transistors were broken off at the can of the transistor.





I assume heat induced mechanical stress over the years caused the metal to crack. I had to remove the big 7W 330 Ohm resistors to access these two transistors, but that was still easier than removing the whole pc board from the frame. Plus it made testing those resistors a piece of cake, and they were good.

I purchased a pair of 2N3866A off eBay and the scope is now working like a champ!

Thanks for helping me isolate the circuitry based on the symptoms. I don’t know if this is a common failure mode for this model ( 2225 ) but it was definitely the reason my scope was acting up. And I hope this post is useful to someone else facing a similar issue.


Cheers,

Steve

On Mar 9, 2016, at 1:30 AM, stvn.czrt@... [TekScopes] <TekScopes@...> wrote:

Hi Dave, I don't see any discoloration in that corner of the board. It kinda looks like some of the rosin flux hasn't been cleaned fully.

Tonight when I powered it up, it was in failure mode for a short while. Probing with my Rigol scope shows the the voltage at the node that connects R272, R266, and Q256 at roughly 35v DC (flat, not much ripple).


The schematic states "+12.5v Centered" and sure enough, later, when the scope had magically fixed itself, that same node was at roughly 12v with copy of my input waveform on it (roughly 3v peak to peak).


If memory serves me, the node that connects R267, R273, and Q257 was at roughly 12v with in inverted copy of my input waveform on it while the scope was in both failure mode and operational mode.



So I know I'm in the right ball park. And this seems to rule out a failure inside the CRT, and the wiring from the main board to the CRT deflection plate. Like you mentioned earlier, it could be an intermittent open in one of those big 7watt 330 ohm resistors. I wish I had measured them before powering up.


So I gotta start working my way back.



Using a bamboo skewer (thanks for the idea) I poke and prodded the area. I can make the waves jump a bit, but not go into serious failure mode. No spot seem to be extra sensitive to light pressure or tapping with the bamboo skewer.


Will try agin tomorrow. I have my Rigol, and multi meter all ready to go. Just need the 2225 to fail again.


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