Two Tek 475s

[Zenith]

These two were bought at different swapmeets, one last September and one last Sunday. They both cost £5 and I couldn't resist them for that much.

The first was made in the Netherlands. It came with the front cover, and a document pouch with nine probes complete with grabber hoods etc. Five were 250MHz.

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It's clean with no missing knobs, heatsinks or signs of being neglected or knocked about. It blows the mains fuse when it's switched on. I'm assuming it's a shorted electrolytic in the power supply section. It will need to be investigated methodically.

The second was made in Guernsey. It's been kept somewhere damp. There's aluminium corrosion in parts. All knobs and the feet are present and in good condition. The cover is rough and was corroded to the front panel. The vertical gain fine control has a broken linkage and the delay vernier knob is stiff. It has corrosion inside. It's disgraceful the way some people treat these beautiful things.

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The corrosion on the top of the front panel has caused the paint to bubble up slightly.

When powered on it produced a display. After I'd adjusted focus and brightness, smoke appeared. I unplugged the mains immediately. It isn't clear where the smoke came from. There are no obviously burned parts. I'd expect a shorted component to show signs of damage. I would have thought it would blow the mains fuse. These don't have the notorious Rifa caps.

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Here you can see that some cables have become blackened with mould. There are other signs of damp storage and slight corrosion.

I think I'll concentrate on the Netherlands produced one first. There isn't corrosion which can cause all sorts of hard-to-find problems. The Guernsey produced one may be reserved as a parts mule, but that seems a shame.

[MED6753]

Nope, they don't have the notorious RIFA's but they are loaded with the notorious beaded tants. Guaranteed that's what smoked.

[tggzzz]

Nope, they don't have the notorious RIFA's but they are loaded with the notorious beaded tants. Guaranteed that's what smoked.

Yes, but...

There's always the big 5-pin smoothing capacitors

Or, IIRC, the 475 has a bridge rectifier that gets uncomfortably hot to the touch. (Not as bad as the 2901, but hot enough)

[Zenith]

Thanks both.

My experience of failing tantalum bead caps is they sometimes explode, they usually get hot and smoke, and they sometimes go dead short and so don't generate any heat. I couldn't see any darkened tants in Guernsey, or anything else that appeared heat damaged, so where did the smoke come from? I suppose it could be the transformer, but the secondary windings appear to be low resistance. I'd have thought a load sufficient to cause the transformer to smoke would be enough to blow the mains fuse. I didn't check the fuse to make sure it was the correct rating, I should. The transformers seem to be adequately rated and don't have a reputation for failing.

Netherlands blows the mains fuse straight away. I'm guessing it's either an electrolytic or a tant that's failed short circuit. It could be a failed mains transformer.

As I'm sure you both know, a problem with these scopes is that they are not very modular, and unless you have to replace a part which is socketed, or happens to be near the edge of the board, so there's access for soldering, there's a lot of dismantling and reassembly involved.

[tggzzz]

Previous experiences with my 475: https://entertaininghacks.wordpress.com ... -response/

[Zenith]

Of the two scopes, Guernsey did at least produce a display, before it started to emit smoke. It was probably going to be the most approachable. I thought about what MED said about tants in these scopes, and decided to look more carefully with a magnifying glass. A tant across the +15V line was slightly brown on one side, which I'd thought was dirt, but it wouldn't clean off. On the other side was a spot of metal. I wonder if it's tantalum or solder from an internal connection. I suspect solder, as tantalum has a very high melting point. It's a 10µF 25V tant which I replaced with a similarly rated electrolytic. Some of the tants in these scopes are operated right on the edge of their ratings, and so might be damaged by excessive ripple. This one isn't, but tants across power lines have a reputation for causing trouble.

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Since it was likely I'd want to turn the scope off very quickly, I tried the on/off switch. It was very stiff and worked better with the scope laying on some sides than others. I loosened the switch/voltage selector assembly and turned it so the switch operated smoothly and tightened it up.

I powered up the scope. There was no smoke, it had been that tant. There were no lines displayed with the trigger on auto. Fiddling with the controls produced a spot sometimes. I went back to basics and checked the power lines for voltage and ripple. The 50V line was nearer 51V and well outside the figure in the manual. I adjusted it to be as close to 50V as I could get it. The other power lines were measured and all found to be well within spec - the 50V line is used as a reference for the others. I checked for ripple with another scope and it was low on all lines.

I looked and the scope was now displaying two lines. Either it is that sensitive to the correct voltages, or maybe being powered on for a time had dried something out.

It more or less works, but there are some very quirky things about it that are probably down to pots and contacts suffering from damp storage.

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It only shows a display when the 100Mhz or 20MHz filters, or display finder are selected. The channel 1 display vanishes when the vertical shift is moved past a certain point. The attenuators act strangely. A lot of the controls are erratic, e.g. it's hard to get the display brightness right. It easily jumps from barely visible, to over bright with a halo.

The next stage will be cleaning contacts and cleaning or replacing controls. Generally it will be eliminating the quirky behaviour.

[tggzzz]

Just what you need: a 475, clean case, cover, manual, currently no bids £1.75, collection only from Hereford. See the Mappa Mundi and chained library while collecting it

https://www.ebay.co.uk/itm/256848781297

[Zenith]

Oh you wicked tempter. You're trying to get your own back for that Marconi Bridge at Weston-super-mare. You may succeed.

I'd have thought in a tidy state, and with the front cover and a service manual, it would sell for much more than I've been paying for these things of late, but it is collection only and from a rural location. A rough one with missing knobs and no display, sold for £51 plus £17 shipping, a time back. I'll keep an eye on it.

Do I need a seventh 475? Is it time to graduate to 485s?

[tggzzz]

Oh you wicked tempter. You're trying to get your own back for that Marconi Bridge at Weston-super-mare. You may succeed.
...
Do I need a seventh 475? Is it time to graduate to 485s?

Tee hee.

Got to have 7 before you can have 8

I've got a few CMOS ICs that might or might not be of interest to you.

[Zenith]

Oh you wicked tempter. You're trying to get your own back for that Marconi Bridge at Weston-super-mare. You may succeed.
...
Do I need a seventh 475? Is it time to graduate to 485s?

Tee hee.

Got to have 7 before you can have 8

Not necessarily. A pair or more could be bought at the same time, so jumping past 7. The problem is I now have three properly working ones,
Guernsey, which is well on its way and Netherlands which I have high hopes for. That leaves me with only one parts mule, and that's rescuable, so I'm looking for another parts mule.

I've just tried Guernsey again. It's easier not to get confused when it's the right way up, as it's intended to be used. It seems to get better with powered on hours, maybe because of traces of damp being removed by warmth, and I assume the controls self clean somewhat with use. The problem with it not working on full bandwidth has disappeared - that was likely the finger switches. It definitely needs contacts cleaning and the calibration checked. The Channel 2 attenuator seems a bit dodgy.

I'd guess when it was put in a garage 10 years back it was a properly working scope, and damp and age have taken a toll.

[tggzzz]

Oh you wicked tempter. You're trying to get your own back for that Marconi Bridge at Weston-super-mare. You may succeed.
...
Do I need a seventh 475? Is it time to graduate to 485s?

Tee hee.

Got to have 7 before you can have 8

Not necessarily. A pair or more could be bought at the same time, so jumping past 7.

If you have 9 pesos, you also have 8 pesos (and 7 etc).

The problem is I now have three properly working ones,

Now there's a first world "problem" if ever I saw one

Cue "When I were a lad...." anecdote.

The problem with it not working on full bandwidth has disappeared - that was likely the finger switches. It definitely needs contacts cleaning and the calibration checked. The Channel 2 attenuator seems a bit dodgy.

Probably, and "aren't they all?".

[Zenith]

The problem is I now have three properly working ones,

Now there's a first world "problem" if ever I saw one

Cue "When I were a lad...." anecdote.

In 1973 they cost $2,500. Depending where you were in the UK that would buy a small house or be a large part of the price of one. For most amateurs a scope would have been unattainable, or possibly a used Solartron or Telequipment valve scope with a bandwidth of 15MHz at most. I don't know what they would have cost, maybe £50 and up. A Tek 475 would have been like something from Star Trek. ISTR that around 1990 they were around £400, used from Stewart of Reading.

Probably, and "aren't they all?".

Yeah. The contacts in these are a well known source of problems, picking up dust and crud from the atmosphere. With obvious signs of being kept somewhere slightly damp, this seems a bit worse than usual. For instance the delay time position multiturn control is sticky and needs to be taken apart, cleaned and lubricated.

[tggzzz]

In 1973 they cost $2,500.

Basically a years salary.

[MED6753]

I've had my fill of 465/475's. Too many tants and the attenuator switch decks are either always dirty or the main shaft is worn causing poor contacts. But I will consider more 485's if they come cheap. (Already have 2)

[tggzzz]

But I will consider more 485's if they come cheap. (Already have 2)

Good man.

But they also have the golden fingers, and getting to the EHT is .... prolongued.

[Zenith]

Of the Tek 400 series non-storage, in my experience in the UK, 475 is the most common, 465 less so, then 465B. I haven't come across a 475A or 485 on my travels. I don't see them on ebay either, although I don't particularly look out for them. I have seen some of the lower bandwidth ones rarely, but didn't take much notice, the same for the storage scopes.

I'd have thought that being from the same era, and the same stable, a lot of the problems would be the same across all of them, including the 485.

The youngest 400 series scope is 40 years old. If you are going to use one regularly it will fail occasionally, and it will possibly be a special part. You could try some specialist supplier, such as Sphere, or ebay, but it makes sense to have a parts mule.

.

[MED6753]

Of the Tek 400 series non-storage, in my experience in the UK, 475 is the most common, 465 less so, then 465B. I haven't come across a 475A or 485 on my travels. I don't see them on ebay either, although I don't particularly look out for them. I have seen some of the lower bandwidth ones rarely, but didn't take much notice, the same for the storage scopes.

I'd have thought that being from the same era, and the same stable, a lot of the problems would be the same across all of them, including the 485.

The youngest 400 series scope is 40 years old. If you are going to use one regularly it will fail occasionally, and it will possibly be a special part. You could try some specialist supplier, such as Sphere, or ebay, but it makes sense to have a parts mule.

.

If you look at the design of a 485 vs a typical 465/475 you would come to same conclusion as I did that they were designed by different engineering teams. The main PSU is switch mode vs linear. And for the most part easily accessible. The attenuator switch decks are compact and easily removable in about 5 minutes. And in my experience much more stable even though they do use the golden fingers.