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On 12 Apr 2025 5:58 pm, PMF wrote:

They didn't always work well in place of 6DJ8s.

Unfortunately 6DJ8s have gotten horribly expensive because people who think they are audiophiles (but who lack the requisite theory) have been buying them up to use as audio amps, and then sometimes criticizing the results. That is not surprising - they are not audio tubes. They were designed to be cascode amps in VHF TV tuners. Given that design they were a good choice by Tek.
--
Dale H. Cook, GR/HP/Tek Collector, Roanoke/Lynchburg, VA

On Sat, Apr 12, 2025 at 05:58 PM, PMF wrote:

I had gotten some of those very tubes in the 1990s. I didn't use them until I
(tried) to calibrate my 547 a few years ago and found them a little less than
wonderful than actual 6DJ8s to the point that I wondered if they were even
related to 6DJ8s. They didn't always work well in place of 6DJ8s. I hope you
have better luck with yours.

Okay, then it appears that by this time Tek had given up on trying to provide the same careful spares that they used to provide. In this case, I'm glad that this is the tube that was destroyed.

--
Jim Adney
Madison, WI USA

Gordon,

You could have the black slick slime that is on the ceramic part of the funnel on the crt. I have seen this before. That will cause a leakage path and possibly corona. Clean the high voltage area. There is most likely black dust there. I used ammonia and a brush to clean this area, rinsed then let dry. The carbon resistors there could be bad. The arcing could be one or more opening internally. I have had ones like this break as soon as a leg is unsoldered then lifted enough to be away from the place it was soldered to, not bent back enough with force to stress the body.

The high voltage regulation could be off due to R803 being overstressed. I put a 5W in mine. C802, C806 and C802 could be bad. The first two, if replaced, should be a PP type at 600/630V. The high voltage pot wiper could be dirty. Turning it back and forth several times will help as this is a sealed 2W pot. Sometimes the 500mmfd condensers in the high voltage will get leaky. The 453/4 series have this problem.

You may have a bad tunnel diode, D85, in the B sweep. Check the waveform at the junction of D59 and D85. Condensers C76/276 could be open. These two I used ULD types. A 50/63V film such as a Wima could be used. These two original electrolytics were open in mine. You could have leaky 'holdoff capacitors' in the timing circuit. The red body ones I replaced in mine.

The ripple on the 350V supply sounds like any/all electrolytics in that supply are going bad, C701, C730 and C709A. The ,01mfd condensers from the regulated outputs back to the grid of the control valve are known to go bad. These are an automatic replacement when I work on this type of supply after seeing enough bad ones over the years. The cans in the supplies can be restuffed with larger values, e.g. 100mfd to 150mfd. Ones like Nichicon LGR series are great replacements for the larger value and voltage types and ULD/UCY for the lower values such as 10-40mfd.

In mine, I put in 1% tolerance resistors with only a few 5% in areas where the tolerance and 200ppm/deg C would be fine. I restuffed cans. I did find a number of resistors that should be increased in wattage. I was glad I spent the resources after seeing the results. I know it is a labour of love.

Mark

We have no experience with 5xx Tektronix but generally suspects
LV PS check all voltages
HV transformers, multipliers, caps
High value resistors in CRT,and HV ck
Scratchy controls and switches

Jon

My ancient Tektronix 547 oscilloscope with 1A1 plugin was working fine until a few weeks ago when the display suddenly went intermittent and bright, and an arcing sound came from the area of the crt high voltage circuit. No arcing could be seen anywhere, but the sound definitely came from the vicinity of the high voltage assembly. At this point, the Intensity control had no effect, and the intermittent sweeps were very bright. The sweeps only occurred when the arcing sounds were heard. Otherwise, a bright spot stayed on the left side of the display. The focus control also had only a slight effect.

Suspecting the flyback transformer T801, I sourced an assembly from another 547 that included the transformer and the voltage multiplier comprised of the 5642 diodes. Symptoms remained the same, unfortunately. Replacing one of the regulator tubes in the 350V supply (6CW5) with a brand new one strangely made the arcing go away, but still no display. In fact, no more intermittent sweeps, only the ultra bright spot at the left hand side of the display. Currently, I have no way of measuring the high voltages from this assembly (10kV), so I’m not sure if they’re within spec. I also changed out the four 0.01uF caps in the Intensity circuit in case they were arcing, but no improvement.

Scoping the output of the B Sweep Generator section (at tube V91), there was no sign of a sweep waveform (consistent with the stationary bright dot on the left side of the display). I’ve tested every tube in the scope, except for the 6080s (and checked all transistors for opens or shorts). None gave the impression that they would be show stoppers. I also scoped all B+ and B- rails. All were rock steady except for the +350V line, which had some ripple. Even after changing out the 6CW5, the ripple was still there.

It’s obviously generating high voltage as evidenced by the bright spot on the crt. I can’t understand the relationship between the inability to control Intensity and Focus and the lack of a sweep signal. It doesn’t matter whether I’m in A, B or ALT timebases. Could the crt have an internal fault?

Any advice or best guesses as to where I should look next would be much appreciated. Thanks in advance.

Also, note that you cannot operate this chassis out of the case for more that a very short time without external fan on the underside to keep the hybrids cool!

The Dallas NVRAM batteries have failed. Replace these, you can use the ST Micro M48Z35, drop in replacement.
I strongly suggest that you completely recap the PSU, except for the large bulk mains caps, as these very rarely fail.
Recal and you should be fine.

If CRTs were recently invented and people were trying to improve them, we’d probably see faceplates with cermet bonding to the body using induction heating.

We’d probably see indirectly heated lanthanum hexaboride cathodes or maybe even some sort of diamond or carbon nanotube material possibly as a moderately or even unheated field emitter depending on the beam current density required.

Who knows…with modern high voltage components, electrostatic deflection might be cheaper than large amounts of copper, or even see a hybrid with a horizontal scan via electrostatic and vertical via a yoke for raster displays.

________________________________
From: [email protected] <[email protected]> on behalf of Roy Thistle via groups.io <roy.thistle@...>
Sent: Wednesday, April 16, 2025 3:31:38 PM
To: [email protected] <[email protected]>
Subject: Re: [TekScopes] Damaged 3A3 with Unusual 6DJ8

On Sat, Apr 12, 2025 at 01:49 PM, Jim Adney wrote:

perhaps they were able to control their production carefully enough to make
the ageing and selecting unnecessary.

AFAIK, the 'aging' was significantly about... but not only about, stabilizing the emissivity of the cathode.
Again... AFAIK... despite decades of research ... the exact mechanics of thermionic emission from oxide-coated cathodes... from substances like barium oxide... remain somewhat mysterious.
At least from the published literature.
I'd speculate that give cathode X, constructed with process Y... the engineers knew that Z amount of hours under process alpha... for instance... achieved a stable emissivity...within the desired specification.
I don't know... but I don't think... the scientists... knew... or even now know... what was happening in microscopic regions, and layers, of the thermionic emissve materials that were used in the manufacture of cathodes.
Even if they did... IMO... its akin to heat treating steel... you may know the final microscopic properties of the material; but, you have no way to effect them directly.
--
Roy Thistle

On Wed, Apr 16, 2025 at 10:52?PM Roy Thistle via groups.io <roy.thistle=
[email protected]> wrote:

On Wed, Apr 9, 2025 at 11:34 AM, on6jz wrote:

Does anyone have any tips on where I can look

There is a long thread on the 2712, which was previously posted.
You could search the messages for "tek 2712 freezes during startup"

Here's a link to the thread: /g/TekScopes/topic/7658999.

Dennis, I have a 2432A which suddenly shows Fail codes 7000 & 9000 at startup, drilling a bit deeper: 9210,9220 & 7210,7220,7230,7240.
The scope had already shown a few times CKSUM-NVRAM FAIL but then a restart fixed this every time, now besides the FAIL messages for the above, it shows PASS for this particular check although there is an asterisk markerd like this CKSUM-NVRAM*
Any ideas where to start looking?

On Wed, Apr 9, 2025 at 11:34 AM, on6jz wrote:

Does anyone have any tips on where I can look

There is a long thread on the 2712, which was previously posted.
You could search the messages for "tek 2712 freezes during startup"
There might be some clues there.

--
Roy Thistle

Hello Matthieu:

Unless there has been some Hack I am not aware of, you cannot view the Spectrum Analyzer and Oscilloscope at the same time. So, you are correct, this is reserved for the MDO4XXX Series and later.

Rich

Hi,

I have a Tektronix MDO3024 scope. Working great !

I would like to have on the screen the analog CH1 screen and the RF signal.

Is it possible on the MDO 3000 series or it is only with the MDO 4000 series ? Thank you

Matthieu

On Sat, Apr 12, 2025 at 01:49 PM, Jim Adney wrote:

perhaps they were able to control their production carefully enough to make
the ageing and selecting unnecessary.

AFAIK, the 'aging' was significantly about... but not only about, stabilizing the emissivity of the cathode.
Again... AFAIK... despite decades of research ... the exact mechanics of thermionic emission from oxide-coated cathodes... from substances like barium oxide... remain somewhat mysterious.
At least from the published literature.
I'd speculate that give cathode X, constructed with process Y... the engineers knew that Z amount of hours under process alpha... for instance... achieved a stable emissivity...within the desired specification.
I don't know... but I don't think... the scientists... knew... or even now know... what was happening in microscopic regions, and layers, of the thermionic emissve materials that were used in the manufacture of cathodes.
Even if they did... IMO... its akin to heat treating steel... you may know the final microscopic properties of the material; but, you have no way to effect them directly.
--
Roy Thistle

I read Roy's posts on this tread and I did not find them offensive or aggressive.
However, I do believe that the recapping discussion points have been sufficiently covered so there is no need to continue.
Cheers,
Tom

Posts to this topic are now on moderation.

Roy,

I've had a few complaints about your posts on this thread. If I receive more, I will need to consider banning you from the group, which I'd prefer not to do.

David (Moderator).

Me too ?

Roy

Your tone is highly aggressive, and I think you're just looking for a fight, for some reason which it's above my pay grade to understand.

I'll debate you anytime on technical accuracy, but I decline to engage your ego.

There are enough contradictions in your statements to make mincemeat here.

End of subject.

Menahem

Thanks for this collection, just like a trip down memory lane. Nice to see all the Ampex manuals being saved, I kept hoping to see an ACR-25 collection.
So many shops had a bookcase full of the same items, sadly most have been carted off to the shreader.
Thanks again for all your great work.

Gerry (broadcast engineering retired)