2N2207 replacement in 547.


rbmerckx@...
 

Hi folks, first time posting here. I am a clinical biologist, not an electronics nor scope expert. My hobbies are mainly soccer and Arduino , and on the software side of the latter. I recently got my hands on a package deal of 4 scopes..2 x 547 , 1 scope mobile, a 475 and a TDS 320 with cart ..all for 200..I couldn't resist. Was told all were working...turned out only the TDS 320 was. Managed to get one working 547 out of the 2 by swapping a few broken tubes and failed transistors and repairing knobs/dials, cleaning....( and hours of reading thru articles in this group and online in general). Now I would like to get the second one working. For qualitative measurements, not quantitative. I do not plan on going thru the whole calibration etc.. The working one already has a place into my 60's modern look living room interior because I find the combo scope + 202 model c cart a marvel of industrial design and engineering. I guess we could argue about the scope shape design itself as not being as elegant as the cart :)
The "working" 547 : serial 13996 works great.. but only for 20 minutes...waveform starts to fade ( not really "blooming" , trace remains very sharp..just looses intensity) and disappears all together. I can make it work for 10 minutes longer if from start up I keep trace intensity low. When measuring the voltage as per manual at pin 8 of V800 it starts off at 90 V...it slowly creeps up to 109.5 V and remains there. Pin 2 at V814 measures 161.3 volt. Is this for sure an HV transformer issue ? I am asking because according to a video from Lazyelectrons, that rising voltage symptom on pin 8 can also be due to other parts going bad in the CRT circuitry ( his words..not mine..lol)
The non- working one: serial 10602: What's a modern substitute for the 2N2207 for both Q554 and Q564 on horizontal amplifier. Mr Wise recommended replacing with BC556 or KSA992, but that was in the sweep circuitry. I had to use the good ones of this one to replace the shorted ones of the serial 13996 who came to life after that substitution ( and 5 tubes that had lost vacuum).

Anecdote : Well..i think it's nifty...the working serial 13996 had the CRT replaced on the 6th of January 1975 and was tested and re-calibrated by " R Clark".....2 small decals with the Tektronix Logo on top of the CRT enclosure tell us that part of it's history. And yes..it still contains the little roll of silver solder. I guess Mr Clark had his own supply :).


Morris Odell
 

I'm sorry to say those symptoms are classic for the HV transformer thermal
runaway problem. Unfortunately the only cure is a replacement transformer -
you will have seen endless discussion on this subject if you have been
through the archives. I wouldn't advise swapping the transformer from the
other scope, it's likely to have the same problem if it's an original. You
don't want to go through the complex job of replacing the transformer with
another dud. The only source I know of for replacements is Chuck Harris of
this group and I'm sure he will reply to you. I'm traveling in the US at
the moment so have no easy access to the manual to help you with the
transistor replacement. However you are unlikely to do any harm by trying a
BC556. It's not an expensive experiment.

Good luck,

Morris


Dave Wise
 

I agree that your transformer has lossy epoxy impregnant. Chuck sells a beautiful drop-in replacement, but it's within many people's abilities to wind your own. I did this for my 453 which had the same problem, and wrote it up under "453 HV Transformer Rewind". The 547 transformer is the same order of complexity. If you like making your own stuff, give it a look.

The 2N2207 is notorious for developing tin whisker shorts. Often it's possible to clear them with a pulse of energy. Note that the whiskers frequently short one or more elements to the case, so start there. What do you have to lose?

In the CRT beam unblanking circuits, the ability of the 2N2207 to generate a large, fast transition is exploited to the fullest, and nothing I ever found matches it. The BC556 and several others can be made to work by speeding up the base drive. I doubt that's possible in the horizontal amplifier; then again, maybe they don't need to be so fast there. You have nothing to lose. If they are being employed in a balanced circuit, replace both sides to avoid Si vs Ge Vbe mismatch. Watch the pinout, it's probably not what you expect.

HTH,
Dave Wise
________________________________________
From: TekScopes@groups.io <TekScopes@groups.io> on behalf of Morris Odell <vilgotch1@gmail.com>
Sent: Tuesday, May 28, 2019 3:56 PM
To: TekScopes@groups.io
Subject: Re: [TekScopes] 2N2207 replacement in 547.

I'm sorry to say those symptoms are classic for the HV transformer thermal
runaway problem. Unfortunately the only cure is a replacement transformer -
you will have seen endless discussion on this subject if you have been
through the archives. I wouldn't advise swapping the transformer from the
other scope, it's likely to have the same problem if it's an original. You
don't want to go through the complex job of replacing the transformer with
another dud. The only source I know of for replacements is Chuck Harris of
this group and I'm sure he will reply to you. I'm traveling in the US at
the moment so have no easy access to the manual to help you with the
transistor replacement. However you are unlikely to do any harm by trying a
BC556. It's not an expensive experiment.

Good luck,

Morris


Bob Koller <testtech@...>
 

In the past I was able to, at least partially, alleviate the HV transformer problem by doing the following, YMMV of course.
Replace the 5642 HV rectifier tubes with fast recovery HV silicon rectifiers, and removing the filament leads, this reduces the load on the HV oscillator circuit.
Replace C808 with a high quality cap, mine was leaky, causing loss.

Also, I have a few NOS 151-0063/ 2N2207 Tek parts available, contact me off list if interested.


fiftythreebuick <ae5i@...>
 

Glad to hear that you have jumped into the vintage Tektronix oscilloscope world! Honestly, once you get spoiled by a 500 series Tek oscilloscope, you're spoiled for good... :-)

The 547 is really a nice scope and I know you will enjoy them. I hate to tell you, but the very high probability is that your HV transformer is the source of the problem. A quick test, if you'd like to run it, is to run the scope until it fails and then hit the HV transformer with a burst of freeze spray that's sufficient to cool it significantly. If the trace returns and then fairly quickly goes away again, then it's the xfmr. The 547 is one of the known bearers of this problem.

There are a couple of folks who rewind the xfmrs, but I don't know who exactly is currently doing it. My buddy who reworked them years ago is out of it now.

I have a theory but have not tried it yet. I suspect that if the transformer is put in a vacuum chamber and held at a deep vacuum for a length of time that is sufficient to pull any moisture out of it and then sealed with something like beeswax, the transformer might be recovered. Planning to give that a try when I have time.

The worst problem with HV xfmr failure that I am aware of was with the 549 mainframe. I've worked on probably 20 or more of them and have never seen one with an original transformer that didn't have the defect. In the case of the 549s that I have, once the coil was removed and replaced with a rewound one, the problem disappeared and the scope worked as new.

Interestingly enough, the problem was extremely rare in the earlier, open frame xfmrs such as those in the Type 555 and 545A, etc.

If it turns out that you do need a xfmr, I bet someone here can point you in the right direction.

Tom


Adrian Nicol
 

I was thinking about that recently when recalling how Parylene conformal coating saved my bacon on high power/ high voltage controller printed circuit boards used in aircraft. It is a vacuum vapour deposition process and it has amazing penetration into the tiniest of gaps. (That made it a nightmare to mask the connectors of course) but I would think it would be the ideal treatment after cooking the transformers?

Adrian

On 5/29/2019 6:04 PM, fiftythreebuick wrote:
I have a theory but have not tried it yet. I suspect that if the transformer is put in a vacuum chamber and held at a deep vacuum for a length of time that is sufficient to pull any moisture out of it and then sealed with something like beeswax, the transformer might be recovered. Planning to give that a try when I have time.


Chuck Harris <cfharris@...>
 

Hi Tom,

Your "theory" has been tried to no avail. The epoxy has changed
into something that is lossy all by itself... more lossy with
moisture infiltrated into it... But, it can only get a tiny little
bit better by baking... Only a tiny little bit better by replacing
the vacuum tube diodes with fast silicon diodes... and only a tiny
little bit better by aggressively cooling it... But never, better.

Beeswax/paraffin mix is not any more resistant to moisture than the
epoxy. Its infiltration rate is almost identical. Its benefit over
epoxy is it is lower loss... by a factor of almost 10x... than the
epoxy even when the epoxy is new. Its other benefit is if moisture
makes it get hot, it melts, releasing the moisture.

Tektronix knew all of this, which is why they designed the HV box
to accept a heatsink under the core. They were married to the idea
of a high temperature scope (647), and a sealed, modular HV compartment,
which is why the stuck with epoxy, warts and all.

I have wound hundreds of these transformers... until I was cross eyed,
and numb, from the highly repetitive task. At my best, I had to wind
4 transformers to get 3 that worked... which is not that good.

To ease the extreme toll winding causes me, I designed a computerized
coil winder, and after I finished the hardware, but before I could write
the software, I got old... Suddenly, other tasks demand higher priority.

I can foresee a day when I will be winding again, but unless I break my
leg, it won't be anytime soon.

-Chuck Harris

fiftythreebuick wrote:

Glad to hear that you have jumped into the vintage Tektronix oscilloscope world! Honestly, once you get spoiled by a 500 series Tek oscilloscope, you're spoiled for good... :-)

The 547 is really a nice scope and I know you will enjoy them. I hate to tell you, but the very high probability is that your HV transformer is the source of the problem. A quick test, if you'd like to run it, is to run the scope until it fails and then hit the HV transformer with a burst of freeze spray that's sufficient to cool it significantly. If the trace returns and then fairly quickly goes away again, then it's the xfmr. The 547 is one of the known bearers of this problem.

There are a couple of folks who rewind the xfmrs, but I don't know who exactly is currently doing it. My buddy who reworked them years ago is out of it now.

I have a theory but have not tried it yet. I suspect that if the transformer is put in a vacuum chamber and held at a deep vacuum for a length of time that is sufficient to pull any moisture out of it and then sealed with something like beeswax, the transformer might be recovered. Planning to give that a try when I have time.

The worst problem with HV xfmr failure that I am aware of was with the 549 mainframe. I've worked on probably 20 or more of them and have never seen one with an original transformer that didn't have the defect. In the case of the 549s that I have, once the coil was removed and replaced with a rewound one, the problem disappeared and the scope worked as new.

Interestingly enough, the problem was extremely rare in the earlier, open frame xfmrs such as those in the Type 555 and 545A, etc.

If it turns out that you do need a xfmr, I bet someone here can point you in the right direction.

Tom




Chuck Harris <cfharris@...>
 

Tektronix tried:

Beeswax/paraffin
Epoxy varnish (brown)
Epoxy potting (brown)
Epoxy potting (black)
Epoxy varnish (brown) potted in clear RTV silicone, in a metal can...

The black Epoxy was the only one of these variations that worked.
What the epoxy was, I haven't a clue.

There isn't more than a couple of hundred volts between any two wires,
so the epoxy/beeswax/??? isn't there improve the HV resilience. It is
more to keep the winding together.

The brown epoxy is about 10x more lossy than beeswax/paraffin... even
when new.

Even in the hayday of flyback transformers in TV sets, etc.. they still
failed due to potting failure.

-Chuck Harris

Adrian wrote:

I was thinking about that recently when recalling how Parylene conformal coating
saved my bacon on high power/ high voltage controller printed circuit boards used in
aircraft. It is a vacuum vapour deposition process and it has amazing penetration
into the tiniest of gaps. (That made it a nightmare to mask the connectors of course)
but I would think it would be the ideal treatment after cooking the transformers?

Adrian

On 5/29/2019 6:04 PM, fiftythreebuick wrote:
I have a theory but have not tried it yet. I suspect that if the transformer is
put in a vacuum chamber and held at a deep vacuum for a length of time that is
sufficient to pull any moisture out of it and then sealed with something like
beeswax, the transformer might be recovered. Planning to give that a try when I
have time.