Request - information on how to diagnose a dying 547/545B HV ...


tubesnthings@...
 

 
Steve;
The short answer is: the screen voltage of V800 (6AU5), pin 8.
The greater the transformer leakage, the higher the voltage, which is shown as +90V, nominal.
(this voltage could be high because of a tired crt or other fault condition downstream)
 
There is a detailed circuit description in the service manual.
Bernd Schroder


Mike <mikez11@...>
 

I'm not clear whether the issue is inevitable for ALL unmodified 547 and 545B's. I wonder if that's known, at least to the limits of group experience, or if some production units shipped with OK transformers?

My 545B s/n 004651 (ca. 1966) has its share of problems, but not this one (yet). At 3.5 hours running with all covers on, including the metal HV box shield, in a room kept at 77F , V800 screen is steady at 87V (started at 81V cold). T801 sure looks original; wiring seems to have all the original solder, clean bends & lacing etc.

I'd gladly invest in a spare T801 for insurance, but after 44+ years under its belt without symptoms, pulling a working one to salvage the core feels like a serious breach of "if it ain't broke..." Hence I'm curious if days really are _known_ to be numbered.

Mike

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


Steve;
The short answer is: the screen voltage of V800 (6AU5), pin 8.
The greater the transformer leakage, the higher the voltage, which is shown
as +90V, nominal.
(this voltage could be high because of a tired crt or other fault condition
downstream)

There is a detailed circuit description in the service manual.
Bernd Schroder


Dave Wise
 

Like pot metal, there were probably good and bad batches.
They weren't looking for this problem, so process
parameters that affect it weren't controlled.

Dave

-----Original Message-----
From: TekScopes@... [mailto:TekScopes@...]On
Behalf Of Mike
Sent: Wednesday, January 27, 2010 3:48 PM
To: TekScopes@...
Subject: [TekScopes] Re: Request - information on how to diagnose a
dying 547/545B HV ...


I'm not clear whether the issue is inevitable for ALL
unmodified 547 and 545B's. I wonder if that's known, at least
to the limits of group experience, or if some production
units shipped with OK transformers?

My 545B s/n 004651 (ca. 1966) has its share of problems, but
not this one (yet). At 3.5 hours running with all covers on,
including the metal HV box shield, in a room kept at 77F ,
V800 screen is steady at 87V (started at 81V cold). T801 sure


Kevin Wood G7BCS
 

I'm not clear whether the issue is inevitable for ALL unmodified 547 and
545B's. I wonder if that's known, at least to the limits of group
experience, or if some production units shipped with OK transformers?
I'd be interested to know too. As it happens I came across a replacement
HV transformer for my 546 on Ebay last week and it appears to be a good
one (was dated 1972 IIRC). Chuck's post is quite timely and I'm wondering
if I should get my old core rewound.

I guess the answer is to ask if any of these scopes have gone bad in
recent years or whether the ones that have survived this far are generally
reliable?

I know mine has had the issue with the HV supply since I've owned it (over
20 years, thinking about it). If they all died around that time I'd be
confident that the ones that have made it to this age are going to stay
working.

Kevin


ikerooney <ikerooney@...>
 

Hi Folks,

I hired into Rockville June 1967 and my 547 sn was 13xxx and it ran forever, well, at least a year (or more) as I inherited it from another technician that took a hike.

There were no difficulties as far as I recall with transformers in any of the products, specifically associated with potting materials.

Flash forward to September 1970 when I got out of the Army (draftee) and back into the Rockville Service Center (still at 1500 East Jefferson Street).

That brown potting material was everywhere. It cracked, absorbed moisture and became inefficient across the product line. Depending on how conservatively the high voltage for a particular product was designed, it would survive longer.

The transformer in the 611 display was the canary in the coal mine, they were failing in droves and we were installing brown potted transformers right back into them. The 576 started doing the same thing, we replaced transformers with similar brown potted transformers only to see them fail again.

It was probably into 1971 or 1972 that the transformer potting materials were being replaced with silastic material. But it was a tediously slow manufacturing changeover, Tektronix tried a few other resin based (I think you would call them resin based) materials but they suffered from the same failure mode where they would crack, absorb moisture and become inefficient.

Humid areas of the country had far more failures than arid parts of the USA.

So, I think the potting material didn't affect earlier 547's, it was when they switched from the crumbly clear potting to the brown potting that this problem was introduced.

But this was along time ago and I may have overlooked some incipient failures in 1968.

And lastly, the inverter transformer for the 7704 (non-A) used the brown potting material and it never got the "fix."

Joe Rooney

--- In TekScopes@..., "Kevin Wood" <mailinglists@...> wrote:

I'm not clear whether the issue is inevitable for ALL unmodified 547 and
545B's. I wonder if that's known, at least to the limits of group
experience, or if some production units shipped with OK transformers?
I'd be interested to know too. As it happens I came across a replacement
HV transformer for my 546 on Ebay last week and it appears to be a good
one (was dated 1972 IIRC). Chuck's post is quite timely and I'm wondering
if I should get my old core rewound.

I guess the answer is to ask if any of these scopes have gone bad in
recent years or whether the ones that have survived this far are generally
reliable?

I know mine has had the issue with the HV supply since I've owned it (over
20 years, thinking about it). If they all died around that time I'd be
confident that the ones that have made it to this age are going to stay
working.

Kevin


Chuck Harris <cfharris@...>
 

I can't answer that question. I know that there are many of
these scopes that appear to be ok. I hear mostly from guys that have
scopes that have failed.... Most of the bad scopes have won a permanent
spot sitting on a dusty shelf in someone's basement... or have been sent off
to their final reward by now.

Tektronix would know the history better. Perhaps if Deane Kidd would weigh
in, he could lend some of his expertise?

In one of the conversations I had with Deane, back when Bill Schell was
still winding replacements, he told me that the failures started while the
scopes were still in warranty, and that Tektronix found the transformers they
had on the stock shelves were bad. All of them, or just some of them?.. I don't
know.

The epoxy in the bad transformers that I have taken apart seems strange to
me. It is pinkish red, and rubbery... Similar in color to a #2 pencil eraser.
When you heat it up to near the boiling point of water, it loses any strength
it had, and the transformer cores are easily separated and extracted from the
coil... The epoxy can be scraped off like putty. If it wasn't so hot, you
could easily scrape it off with your thumbnail.

I have not taken apart a known good 547 transformer, so I don't know if
its epoxy behaves differently. I do know that the epoxies I use for general
purpose work don't give up this easily at such a low temperature.

Maybe Tektronix was a little too casual about the ratios they used in mixing
the epoxy?... or the degree of mixing?... or the curing temperature? Or the
shelf life?... Or the epoxy manufacture screwed up?... Major wild guessing time.
I just don't know.

If I had a transformer that went 3-1/2 hours with 80V on the screen, I would
probably take the risk and leave it be. I didn't get that lucky with my 545B,
and 547. My 545B wouldn't run 30 minutes. My 547 was almost good. When you
used it after 6 months of sitting, it had the problem, but after using it for
as long as it would let me for a week, the problem got progressively better
until it went away... only to return the next time it sat idle for a few months.

Good, bad, or indifferent, I do know one thing, I am not going to be winding
these transformers forever. I have enough for me. To wind, I have to shovel
off two workbenches, repurpose a couple of power supplies, scopes, and an
environmental chamber. And, the old Sargent vacuum pump has to come off of
the shelf, along the desiccator I use for potting, and some gages...

Maybe I'll go to the trouble of winding some every year, maybe not.

-Chuck Harris



Kevin Wood wrote:

I'm not clear whether the issue is inevitable for ALL unmodified 547 and
545B's. I wonder if that's known, at least to the limits of group
experience, or if some production units shipped with OK transformers?
I'd be interested to know too. As it happens I came across a replacement
HV transformer for my 546 on Ebay last week and it appears to be a good
one (was dated 1972 IIRC). Chuck's post is quite timely and I'm wondering
if I should get my old core rewound.
I guess the answer is to ask if any of these scopes have gone bad in
recent years or whether the ones that have survived this far are generally
reliable?
I know mine has had the issue with the HV supply since I've owned it (over
20 years, thinking about it). If they all died around that time I'd be
confident that the ones that have made it to this age are going to stay
working.
Kevin


Mike <mikez11@...>
 

Good perspective, thanks Chuck.

I hadn't picked up on the possible correlation to idle storage until now. My scope has only run 4 hours in the last 13 years (at least), including the 3.5 today; I knew the last owner. So I'll cross my fingers and let it ride as you suggest.

That said, let me echo; it's a great thing you are doing. I hope someone else can pick up the torch when the time comes.

mike

--- In TekScopes@..., Chuck Harris <cfharris@...> wrote:

I can't answer that question. I know that there are many of
these scopes that appear to be ok. I hear mostly from guys that have
scopes that have failed.... Most of the bad scopes have won a permanent
spot sitting on a dusty shelf in someone's basement... or have been sent off
to their final reward by now.

Tektronix would know the history better. Perhaps if Deane Kidd would weigh
in, he could lend some of his expertise?

In one of the conversations I had with Deane, back when Bill Schell was
still winding replacements, he told me that the failures started while the
scopes were still in warranty, and that Tektronix found the transformers they
had on the stock shelves were bad. All of them, or just some of them?.. I don't
know.

The epoxy in the bad transformers that I have taken apart seems strange to
me. It is pinkish red, and rubbery... Similar in color to a #2 pencil eraser.
When you heat it up to near the boiling point of water, it loses any strength
it had, and the transformer cores are easily separated and extracted from the
coil... The epoxy can be scraped off like putty. If it wasn't so hot, you
could easily scrape it off with your thumbnail.

I have not taken apart a known good 547 transformer, so I don't know if
its epoxy behaves differently. I do know that the epoxies I use for general
purpose work don't give up this easily at such a low temperature.

Maybe Tektronix was a little too casual about the ratios they used in mixing
the epoxy?... or the degree of mixing?... or the curing temperature? Or the
shelf life?... Or the epoxy manufacture screwed up?... Major wild guessing time.
I just don't know.

If I had a transformer that went 3-1/2 hours with 80V on the screen, I would
probably take the risk and leave it be. I didn't get that lucky with my 545B,
and 547. My 545B wouldn't run 30 minutes. My 547 was almost good. When you
used it after 6 months of sitting, it had the problem, but after using it for
as long as it would let me for a week, the problem got progressively better
until it went away... only to return the next time it sat idle for a few months.

Good, bad, or indifferent, I do know one thing, I am not going to be winding
these transformers forever. I have enough for me. To wind, I have to shovel
off two workbenches, repurpose a couple of power supplies, scopes, and an
environmental chamber. And, the old Sargent vacuum pump has to come off of
the shelf, along the desiccator I use for potting, and some gages...

Maybe I'll go to the trouble of winding some every year, maybe not.

-Chuck Harris



Kevin Wood wrote:
I'm not clear whether the issue is inevitable for ALL unmodified 547 and
545B's. I wonder if that's known, at least to the limits of group
experience, or if some production units shipped with OK transformers?
I'd be interested to know too. As it happens I came across a replacement
HV transformer for my 546 on Ebay last week and it appears to be a good
one (was dated 1972 IIRC). Chuck's post is quite timely and I'm wondering
if I should get my old core rewound.

I guess the answer is to ask if any of these scopes have gone bad in
recent years or whether the ones that have survived this far are generally
reliable?

I know mine has had the issue with the HV supply since I've owned it (over
20 years, thinking about it). If they all died around that time I'd be
confident that the ones that have made it to this age are going to stay
working.

Kevin


Kevin Wood G7BCS
 

Good, bad, or indifferent, I do know one thing, I am not going to be
winding
these transformers forever. I have enough for me.

Maybe I'll go to the trouble of winding some every year, maybe not.

-Chuck Harris
I appreciate that, Chuck. Since I have a spare core sitting around anyway
it is probably a no-brainer to turn it into a known good spare while the
opportunity exists.

Kevin


John Sykes
 

Re Chuck's observations on removing the old brown compound:

I'm no chemist, but I was taught that plastics fell into two broad groups: quoting from Wikipedia

 " thermoplastics and thermosetting polymers.

Thermoplastics will soften and melt if enough heat is applied; examples are polyethylene, polystyrene, polyvinyl chloride and polytetrafluoroethylene (PTFE)[4].

Thermosets can melt and take shape once; after they have solidified, they stay solid."

I thought the generic class known as epoxies fell into the thermoset category?

John


-



stefan_trethan
 

I don't think this classification is very useful.

Epoxy will not melt as such, but it will get a whole lot softer and weaker.

ST


On Sat, Jan 30, 2010 at 5:27 PM, John Sykes <John@...> wrote:


Re Chuck's observations on removing the old brown compound:

I'm no chemist, but I was taught that plastics fell into two broad groups: quoting from Wikipedia

 " thermoplastics and thermosetting polymers.

Thermoplastics will soften and melt if enough heat is applied; examples are polyethylene, polystyrene, polyvinyl chloride and polytetrafluoroethylene (PTFE)[4].

Thermosets can melt and take shape once; after they have solidified, they stay solid."

I thought the generic class known as epoxies fell into the thermoset category?

John