The handles on some scopes can be damaged/bent and get hung up on the chassis rear plate as the case slides out.  Sometimes you can feel the handles bind when the case stops moving.  Try pushing the case back in slightly and then pull the handles out as you try to remove it again.  You might be able to feel if the handles bind up as the case slides. 

 

To my knowledge, there are no "good" 547/545B transformers made
by tektronix. They all will fail given time, and the right environmental
conditions... unconditioned space, basements, etc. are really bad
for 547 transformers... as is lack of use.

There are quite a few 547's that have transformers made by Bill Schell,
and by me. They should be good for the long haul. There are so many
out there that surely some have changed hands by now.

-Chuck Harris

David wrote:
> Your hypothesis fits with other design choices that Tektronix made and most
> other companies make. Minimizing R&D expense by reusing existing designs with a
> minimum of alteration is great up until the loss of institutional knowledge
> causes problems. It is easier to quantify an initial hard dollar expense in
> time and money versus a later speculative loss that can only be estimated until
> it is too late by which time the loss has already occurred and is ongoing.
>
> Maintaining institutional knowledge is an expense with no immediately
> quantifiable benefit.
>
> I have not attempted to refurbish my 547 yet because I have no immediate need to
> use it until I get a uniquely useful plug-in for it like a sampler, TDR, or
> spectrum analyzer. Did any 547 oscilloscopes *not* have the bad epoxy
> transformer?
>

 

Hi Cliff,

  Wow, that's a neat test.  Seeing the primary voltage and the secondary change in lockstep sure seems to vindicate the transformer.  The voltages should remain in the same ratio as the turns ratio, unless perhaps you saturate the core and then it deviates further from an Ideal transformer.

One other thing to try might be to remove intensity circuit by removing V842 (at least partially and connecting an 8MOhm resistor across that transformer winding to give it a similar load as it has now.

That will leave nothing but the CRT anode circuit on the HV side.   On that note, is the anode clean, without dust around the connection point?  Are we are sure there are no discharge paths over the surface of the CRT?

Before you do that, measure the voltage across the intensity pot as Albert recommended.  

What have we done to vindicate R804, R805?  Still assuming all the caps are good.

Dan

---In TekScopes@..., wrote:

I ran another wire through the transformer once like you mentioned. I used another scope and calculated the RMS value. It started at 1.9 and went down to 1.0. Note that the circuit is still cut at R855, which might be causing the higher voltage. Also I think my winding is closer to a full turn than the real filament winding.

I then put a scope probe on the primary side of the transformer (AC-coupled). I set up the monitor scope such that both signals were stacked (appeared to be only one trace) by adjusting the variable v/cm control on one channel. When the fade out happened, they went down exactly together. I *think* this means the transformer's efficiency isn't changing. True? If so would that mean the transformer is not at fault, I think, although I'm not to certain about this part. Can anyone offer insight to that?

Then I added a diode clamp from pin 6 of V816 to the +100 supply. Note that the circuit is still cut at R855. Here's the values in that scenario.

Primary (pin 5, V800): 415ish to 325 (p-p)
"Filament" (my "winding"): 1.7 to 1.3 (rms) (compare that to the first time w/o clamp above)
DC Output (junction of R855 and C855): -950 to -630 (dc)
R807 (referenced to transformer end): 10.03 to 9.67 (dc) Should I measure this with the scope also? I can use my Type W differentially.

Respectfully,
Cliff White, W5CNW
w5cnw@...

On 12/05/2013 05:30 AM, aodiversen@... wrote:

 

Well, that rules out a possible tube fault at the primary side.
The diode heaters nominally receive 1.25 V AC from the single-turn "windings" through the transformer. This is 50 kHz AC (no DC component), so your DMM must be suitable for that frequency. It would be nicer, if possible, to guide an additional well insulated wire through the transformer and then view the induced voltage at an other 'scope. In my opinion you can only find a decrease to some 75% of nominal.
I don't know whether V800 can deliver more and more power when the screen voltage rises so extreme. Hence my suggestion to clamp the screen with a diode to the +100V supply. Then the change in primary AC swing during warming up will indicate whether the secundary load increases or decreases. Perhaps the AC swing reduces like before or even further now, for sure indicating now an increasing load at V800. But then the secundary load decreases  (less heater power and less power at the DC outputs, assuming again you already eliminated possible DC leakage by cap replacements). Hence that would imply leakage in the the secondary windings or leads. Again the reasoning is that the DC outputs collapse because of the reduced heater voltages, not because of the reduced secundary HV voltages.
I would also monitor the voltage across R807. At higher load the anode current will increase. (BTW I think R807 would burn out when its resistance increases dramatically.)

BTW did you measure DC across the Intensity pot? Should normally be slightly over 100 V.

Albert

 

----- Original Message -----

From: alexeisenhut@...

To: TekScopes@...

Sent: Saturday, December 07, 2013 11:12 AM

Subject: [TekScopes] bama edebris manual uploads

 

Has anyone uploaded a manual there and notified ed? I did and never heard back. My upload is still sitting in the uploads directory so the file is there.

Too bad, it's a good idea.

Most of the time with the 545, the problem is the black beauty oil/paper
tubular capacitors used for the HV filters.  They always go bad, and if
they haven't been replaced, you must replace them.  Ceramic is fine.

There is also a BB cap that is in series with the grid feedback winding,
and another in parallel with the plate winding.  Both must go.

-Chuck Harris

Daniel Koller wrote:

Howdy,

The recent discussions on HV repairs has me thinking I should look into the issues
in my 545 (not A or B, old style square case).   It works very very well, but
about half of the time, increasingly more, the HV circuit will not oscillate.  If
I pull the plug-in out and drop it back in, it will work.  I know this is not a
recommended option!  Alternatively, if I shut it off and wait for it to cool, it
will work when I turn it on with about the same probability, or a little less.
Once the scope is on it's up to spec.

The 6AU5 is not brand new, but is good.  It was replaced recently with no
improvement.

So, suggestions as to where to start on this intermittant problem are welcome.  Is
there anything I should look for or measure that might indicate what is getting
tired?

Dan

----- Original Message -----

From: gmail

To: TekScopes@...

Sent: Friday, December 06, 2013 10:49 PM

Subject: [TekScopes] How to open a scope?

 

Someone brought a Tek 2221 along tonight to do some measurements, which hadn't been used for a while. After some 15 minutes on, magic smoke escaped from the back. I'm guessing it is the usual suspect. So we tried to open the scope and we could only slide it out of the cover for nearly half the distance. Then it got really stuck. The scope was not opened up lately. Anyone with tips and tricks for this, or is it just brute force?

 

Thanks,

Guido

Howdy,

The recent discussions on HV repairs has me thinking I should look into the issues
in my 545 (not A or B, old style square case). It works very very well, but
about half of the time, increasingly more, the HV circuit will not oscillate. If
I pull the plug-in out and drop it back in, it will work. I know this is not a
recommended option! Alternatively, if I shut it off and wait for it to cool, it
will work when I turn it on with about the same probability, or a little less.
Once the scope is on it's up to spec.

The 6AU5 is not brand new, but is good. It was replaced recently with no
improvement.

So, suggestions as to where to start on this intermittant problem are welcome. Is
there anything I should look for or measure that might indicate what is getting
tired?

Dan