Tek Type 516 Restoration

[MED6753]

The handle that I first purchased won't fit on the top plate properly so I went with this style instead. It's a better match to the original anyway.



The lack of alternate lock at high sweep speeds turned out to be a non-issue. The Trigger has to be taken out of "Auto" and the Trigger control manually adjusted. It locks perfectly.



Next up is the PSU re-cap. The capacitors should arrive Wednesday.

[mansaxel]

The handle that I first purchased won't fit on the top plate properly so I went with this style instead. It's a better match to the original anyway.

Much better. I approve.

[Reddwarf3r]

Thats looks so much better!
Neat how you can't use Auto trigger for everything, that the designers knew auto doesn't work well in some situations and designed accordingly is clever stuff.

[MED6753]

Neat how you can't use Auto trigger for everything, that the designers knew auto doesn't work well in some situations and designed accordingly is clever stuff.

Actually I think it's due to the simpler trigger circuit used in this scope. It lacks the more advanced tunnel diode trigger circuit used in later 500 series scopes as well as follow on 400 series portable scopes. Auto triggers fine in Alternate mode in those scopes.

[Reddwarf3r]

ooooh Tunnel Diodes, now there's a term you don't hear much! Neat little things.

[Zenith]

TDs are very often mentioned in text books, because they are theoretically interesting. They came out around 1960. They could be used to create microwave oscillators and remarkably fast switches. It was thought they could be used to make faster computers. I've seen circuits for discrete logic gates using them. I'm not sure whether they were used in any commercially available computers. The development of transistors, and then ICs, went ahead at such a pace it left them behind. Where they were used a lot was in trigger circuits in high performance oscilloscopes and other test equipment. That seems to have fizzled out by about 1990.

They are still made by American Micro Semiconductor, but the prices will make your eyes water. I doubt there are any new designs using them. At one time there were a lot of Russian TDs on ebay. They are very easily damaged.

A related component is the backwards diode. It has a very low resistance in the reverse direction and so was used as a low level detector. I have a true RMS voltmeter which relies on two backwards diodes to give a true RMS response.

[tggzzz]

ooooh Tunnel Diodes, now there's a term you don't hear much! Neat little things.

It is quite a common subject of discussion in TekScopes and similar groups. Usually in conjunction with the phrase "where can I get".

Apart from that observation, zenith has nailed it.

(And welcome to the forum )

[Reddwarf3r]

ooooh Tunnel Diodes, now there's a term you don't hear much! Neat little things.

It is quite a common subject of discussion in TekScopes and similar groups. Usually in conjunction with the phrase "where can I get".

Apart from that observation, zenith has nailed it.

(And welcome to the forum )

Thank you for the welcome! And thanks Zenith for the detailed answer. Fabulous knowledge gained

Indeed I have seen them mentioned in textbooks, but when I asked the lecturer for more info I just got a blank look.
Subsequently I did find some info on negative resistance and breadboarded various transistors connected backwards with a floating base in a RC circuit to make some oscillators, was quite a fun and interesting experiment observing the frequency range of various transistors. The humble bc547 made a fine LED dinker as it had the lowest frequency range of the transistors I had on hand.

[tggzzz]

The humble bc547 made a fine LED dinker as it had the lowest frequency range of the transistors I had on hand.

With some skill and imagination it is possible to coax remarkable performance out of very unremarkable components. For example, one of my scopes has a 1700MHz bandwidth with only f_T=200MHz transistors (BC107~=BC547) in the signal path. (There are three 2000MHz transistors, in the trigger circuit).

[tggzzz]

A related component is the backwards diode. It has a very low resistance in the reverse direction and so was used as a low level detector. I have a true RMS voltmeter which relies on two backwards diodes to give a true RMS response.

Curious. I wonder how that works.

The other strange diode I've come across is the step recovery diode a.k.a. snap diode. That's typically used to sharpen up pulse edges for use as the strobe to a sampling bridge. According to Prof Thomas H Lee [1] in his rather good[2] book "Planar Microwave Engineering, A Practical Guide to Theory Measurements and Circuits"[3] an SRD with shorted transmission lines suspended in air (for low dispersion) down, 10ps can be achieved for 20GHz scopes. Above that requires shockwave transmission lines.

Others unfamiliar and plain weird diodes he mentions are:

• IMPATT diodes, which even in the 60s were oscillating at 340GHz
• Gunn diodes. I have those in a couple of Marconi radar S and X band radar signal sources. Those two were £5 at a hamfest, much cheaper than a 15GHz TI lmx2954evm evaluation model (£600 at Digikey, or £20 at PPAuctions).
• MIM diodes, another type of tunnelling diode which are capable of rectification at near optical frequencies

[1] to be found contributing to TekScopes

[2] i.e. despite having closed-loop integrals, it is sufficiently easy to read that I can fool myself into believing I could understand the topics - given a decade or two

[3] think of it as the TAoE for RF

[Zenith]

Indeed I have seen them mentioned in textbooks, but when I asked the lecturer for more info I just got a blank look.

Not surprising. They were always an expensive part with few applications. It wasn't as if you could make a much better TV by using TDs so they were produced in hundreds of millions. They are also electrically very delicate.

There are a few other things covered in books that you aren't likely to come across, such as Traveling Wave Tubes.

Subsequently I did find some info on negative resistance and breadboarded various transistors connected backwards with a floating base in a RC circuit to make some oscillators, was quite a fun and interesting experiment observing the frequency range of various transistors. The humble bc547 made a fine LED dinker as it had the lowest frequency range of the transistors I had on hand.

Tetrode valves have a negative resistance region in their characteristic, which is why pentodes and beam tetrodes were developed. KT88 ( a beam tetrode) is Kinkless Tetrode 88.

The most practical way to investigate negative resistance is with a Lamda diode, which is made from an N channel JFET and a P channel JFET. I don't know whether they were ever sold as Lamda diodes, but they are easy enough to make up. They are much cheaper and more robust than TDs. Unfortunately, you can't simply replace a TD with a Lamda diode.

https://en.wikipedia.org/wiki/Lambda_diode

[Zenith]

A related component is the backwards diode. It has a very low resistance in the reverse direction and so was used as a low level detector. I have a true RMS voltmeter which relies on two backwards diodes to give a true RMS response.

Curious. I wonder how that works.

Through the magic of Ballantine. They have these two diodes in the signal path, which they say are back diodes, which give a non-linear response, and they have a very unusual proprietary moving coil meter which is supposed to have a logarithmic response, and together they give a true RMS response.

They have been making the same design with refinements since the 60s and they seem to be well regarded.

http://www.ballantinelabs.com/323/

[MED6753]

UPS is getting close to the top of my shit list. For the 2nd or 3rd time the paid for 2 day shipping has become 3 day shipping. The capacitors were supposed to arrive today. Instead they are sitting in a distribution center no more than 20 miles away. For delivery tomorrow. WTF? According to the tracking they arrived there at 4:45AM this morning. Sounds like plenty of time to get on a truck for delivery.

I suppose I shouldn't complain too much. USPS is 10X worse.

[Zenith]

In my experience, it doesn't pay to take tracking information from any of these delivery companies as gospel.

[MED6753]

My experience is that UPS and FedEx tracking is usually accurate. USPS tracking on the other hand is usually nursery rhymes.

[Reddwarf3r]

With some skill and imagination it is possible to coax remarkable performance out of very unremarkable components. For example, one of my scopes has a 1700MHz bandwidth with only f_T=200MHz transistors (BC107~=BC547) in the signal path. (There are three 2000MHz transistors, in the trigger circuit).

That's pretty darn impressive, and handy when it comes to repair time.

A related component is the backwards diode. It has a very low resistance in the reverse direction and so was used as a low level detector. I have a true RMS voltmeter which relies on two backwards diodes to give a true RMS response.

Curious. I wonder how that works.

The other strange diode I've come across is the step recovery diode a.k.a. snap diode. That's typically used to sharpen up pulse edges for use as the strobe to a sampling bridge. According to Prof Thomas H Lee [1] in his rather good[2] book "Planar Microwave Engineering, A Practical Guide to Theory Measurements and Circuits"[3] an SRD with shorted transmission lines suspended in air (for low dispersion) down, 10ps can be achieved for 20GHz scopes. Above that requires shockwave transmission lines.

Others unfamiliar and plain weird diodes he mentions are:

• IMPATT diodes, which even in the 60s were oscillating at 340GHz
• Gunn diodes. I have those in a couple of Marconi radar S and X band radar signal sources. Those two were £5 at a hamfest, much cheaper than a 15GHz TI lmx2954evm evaluation model (£600 at Digikey, or £20 at PPAuctions).
• MIM diodes, another type of tunnelling diode which are capable of rectification at near optical frequencies

[1] to be found contributing to TekScopes

[2] i.e. despite having closed-loop integrals, it is sufficiently easy to read that I can fool myself into believing I could understand the topics - given a decade or two

[3] think of it as the TAoE for RF

I'd heard of Gunn diodes, but the others are new to me. Might have to look those books up, especially if they're not heavy on the brain buster maths and anything in TAoE style sounds good to me.

[Reddwarf3r]

The most practical way to investigate negative resistance is with a Lamda diode, which is made from an N channel JFET and a P channel JFET. I don't know whether they were ever sold as Lamda diodes, but they are easy enough to make up. They are much cheaper and more robust than TDs. Unfortunately, you can't simply replace a TD with a Lamda diode.

https://en.wikipedia.org/wiki/Lambda_diode

Wheeeee! More rabbit holes! Fetch the coffee I'm going in. Don't wait up for me

[tggzzz]

With some skill and imagination it is possible to coax remarkable performance out of very unremarkable components. For example, one of my scopes has a 1700MHz bandwidth with only f_T=200MHz transistors (BC107~=BC547) in the signal path. (There are three 2000MHz transistors, in the trigger circuit).

That's pretty darn impressive, and handy when it comes to repair time.

Ideally, yes, but in practice not so much.

The single transistor driving the SRDs is unidentified an unidentifiable and operating in avalanche mode.

More worryingly the passivation on the front panel pots has just about run out of puff. Don't like the resulting powder,.and zero chance of finding replacements.

Others unfamiliar and plain weird diodes he mentions are:

• IMPATT diodes, which even in the 60s were oscillating at 340GHz
• Gunn diodes. I have those in a couple of Marconi radar S and X band radar signal sources. Those two were £5 at a hamfest, much cheaper than a 15GHz TI lmx2954evm evaluation model (£600 at Digikey, or £20 at PPAuctions).
• MIM diodes, another type of tunnelling diode which are capable of rectification at near optical frequencies

[1] to be found contributing to TekScopes

[2] i.e. despite having closed-loop integrals, it is sufficiently easy to read that I can fool myself into believing I could understand the topics - given a decade or two

[3] think of it as the TAoE for RF

I'd heard of Gunn diodes, but the others are new to me. Might have to look those books up, especially if they're not heavy on the brain buster maths and anything in TAoE style sounds good to me.

I hadn't heard of MIM diodes.

There is notably more maths than TAoE, and no tables of components.

I was lucky to get my copy cheap, because the spine is in poor condition. After beating them down, it cost £58

[Reddwarf3r]

There is notably more maths than TAoE, and no tables of components.

I was lucky to get my copy cheap, because the spine is in poor condition. After beating them down, it cost £58

I will have to keep an eye out for a copy, I found a preview copy on google books and liked enough of what I allowed to peek at. Be nice to have on hand to thumb through. But oooof at the price.

[Zenith]

More worryingly the passivation on the front panel pots has just about run out of puff. Don't like the resulting powder,.and zero chance of finding replacements.

It's hard to believe they were a Philips designed special. I suppose since it's a specialised instrument, Philips may have worked with Bournes, or someone like that, to co-develop something not in the catalogues.

Why not put up a photo? Some of us may have a match in our collections, or come across them on our travels.

[tggzzz]

More worryingly the passivation on the front panel pots has just about run out of puff. Don't like the resulting powder,.and zero chance of finding replacements.

It's hard to believe they were a Philips designed special. I suppose since it's a specialised instrument, Philips may have worked with Bournes, or someone like that, to co-develop something not in the catalogues.

Why not put up a photo? Some of us may have a match in our collections, or come across them on our travels.

Yeah, photos are long overdue.

The source problem is that which plagues Tek & HP scopes from ~1970: cramped front panel controls with custom arrangements of pots and switches behind.

IMG_7716.JPG

IMG_7451a.JPG

IMG_7438a.JPG

[Zenith]

It looks as if it's been kept somewhere damp. The passivation is probably zinc but I suppose it could be cadmium, although I have a vague idea that cadmium salts are yellowish. I'd have a go at getting rid of the loose dust with a toothbrush and vacuum cleaner, and then give them a wipe with a lightly oiled rag.

If they are not causing problems why worry about them? If they do play up it's a pain. I have a selection of pots from old scopes including double ones with concentric shafts and there may be something which could replace your grotty ones. You'd need to say what the resistance needed was and the size, and diameter of the shaft.

[tggzzz]

It looks as if it's been kept somewhere damp. The passivation is probably zinc but I suppose it could be cadmium, although I have a vague idea that cadmium salts are yellowish. I'd have a go at getting rid of the loose dust with a toothbrush and vacuum cleaner, and then give them a wipe with a lightly oiled rag.

If they are not causing problems why worry about them? If they do play up it's a pain. I have a selection of pots from old scopes including double ones with concentric shafts and there may be something which could replace your grotty ones. You'd need to say what the resistance needed was and the size, and diameter of the shaft.

That's what I did, and that's my attitude too. I presume Cd, without evidence.

The "gap" in the pots next to the connections faces downwards, which is comforting. The pots are better behaved than most Tek 4x5 pots

[MED6753]

The replacement capacitors finally showed up late yesterday. I won't have time to work on the scope today as I am getting a new addition to the lab later.

[MED6753]

Started the PSU re-cap.

This is a prior repair which the owner told me about. C640 in the -150V reference circuit. The original can was 125uf/350V and this replacement was 100uf/250V which seems under rated. When I checked how the repair was done I got a big surprise. This replacement cap was tacked across the original can which was still in circuit. This dumb ass practice was common among old school TV/radio techs. Simply tack a replacement cap across the old, leaky cap. So I had to unscrew this mess.




Removed the 100uf/250V cap. Took the original can out of circuit but left it in place. The replacement cap is 150uf/350V.



Before proceeding with any additional work made sure the -150V reference was OK. Initially it read -148V. Adjusted it to spec. All is well. And there does not appear to be any additional surprises in the PSU. It all looks original.