Cossor 1071K 'scope - cracked valve bases in 'scopes.

[Phil G4SPZ]

I have been given one of these 1960's era units and after removing a number of non-standard mods carried out by a previous owner, I'm pleased with its performance.

However, one fault still puzzles me. The range of available Y-shift differs between Y1 and Y2. The Y2 shift control can move the trace right off the screen both top and bottom, but the Y1 shift will only move the trace down to just below the centre line with the Y1 shift fully clockwise. The HT voltages are spot on and I've set the common Y shift control to give 150 volts at the slider as described in the manual. I've measured the range of DC voltages at the anodes of both Y amplifiers, and find that Y2 anode can be made to vary from 220 volts right down to 100 volts with the Y2 shift pot fully clockwise, whereas Y1 won't go below 130 volts. I've changed the EF80 valves but that made no difference.

I suspect one of the series resistors in the grid circuit, particularly R89 which is 1.5 megohms and may have gone high. However, this component is hardly stressed. Alternatively, the 100 ohm screen grid resistor may have gone high - I haven't checked the screen voltage yet, but would this reduce the valve's DC gain? Is anyone else familiar with this 'scope and could advise, please?

Finally, I have had three instances of valves cracking at their bases, an ECF80 in the timebase, an EZ81 HT rectifier, and lastly the EF80 Y1 amplifier, despite no known mechanical shock! Any ideas why?

Regards,

Phil

[YC-156]

Originally Posted by G4SPZ

Finally, I have had three instances of valves cracking at their bases, an ECF80 in the timebase, an EZ81 HT rectifier, and lastly the EF80 Y1 amplifier, despite no known mechanical shock! Any ideas why?

This is a fault you often see with large glass base transmitter valves if they are suddently exposed to very cold air when heated to full working temperature. The thermal stresses simply fracture the bases as the pins violently cools the hot glass.

I don't recall seeing the phenomenon with small Noval valves, but if the suspects are located relatively close to each other, it might be a possible explanation.

Hope this helps a bit.

Frank N.

[Nickthedentist]

Originally Posted by YC-156

This is a fault you often see with large glass base transmitter valves if they are suddently exposed to very cold air when heated to full working temperature. The thermal stresses simply fracture the bases as the pins violently cools the hot glass.

Someone been fault-finding with freezer spray, maybe?

[YC-156]

Originally Posted by Nickthedentist

Someone been fault-finding with freezer spray, maybe?

Actually that sounds like a very plausible explanation.

[jim_beacon]

Thermal shock and corrosion are common causes of cracking in small all glass valves, I've often seen it in Tek scopes.

Check your Y amps carefully, they are DC coupled, so a small error early on will cause a large error at the Y plates. From your symptoms, I would suspect that a differential (long-tailed pair) stage, or a balanced amp is out of adjustment (or even different gains from the two valves in the section - one of the times when a vlve tester is REALLY useful).

Jim.

[Sam]

Originally Posted by jim_beacon

Thermal shock and corrosion are common causes of cracking in small all glass valves, I've often seen it in Tek scopes.

I had to replace two ECC88s in my Tek 547 because they had cracked like this - I think it must have been thermal as there are no signs of corrosion inside the 'scope.

Sam

[Duke_Nukem]

Finally, I have had three instances of valves cracking at their bases, an ECF80 in the timebase, an EZ81 HT rectifier, and lastly the EF80 Y1 amplifier, despite no known mechanical shock! Any ideas why?

I've had a TV62 with an RF chassis in which a third of the valves were broken. The chassis had seen some damp so I suspect corrosion of pins as the prime suspect.

I suspect one of the series resistors in the grid circuit, particularly R89 which is 1.5 megohms and may have gone high. However, this component is hardly stressed.

When a resistor is relatively new then failure is usually due to stress. However as they get really old environmental effects take over. High value resistors love to go higher / open circuit.

Regarding the difference in the two Y-Shift controls, are they equally sensitive e.g. 1/4 turn of each moves both traces the same distance ?

TTFN,
Jon

PS : Any chance of a scan of the manual ?

[Brased]

This seems to be quite a common problem with Teks. There was some discussion on the TekScopes yahoo group some time back. Perhaps it is particularly noticed with them because of the large number of valves used.

My own 585A was rescued from a shed where it had been stored for ten years or more. Six of the 6DJ8s (ECC88) were cracked (there are 33 of them!). There was no particular sign of corrosion but I guess that even a small amount of moisture getting trapped could then expand as it froze and thus start a crack from one of the pin seals.

Edward

[Phil G4SPZ]

Thanks for the replies and advice. The scope certainly had been stored for some years in a damp-ish garage... however, since last writing, I've been switching the scope on every day and no more faults have occurred, and no more valves have cracked either... yet!

Both Y-shift controls are about the same sensitivity, it's just that the Y1 shift control needs to be almost fully clockwise to get the Y1 trace at mid-screen. I'll make a few cold resistance checks which should identify the culprit soon enough, time permitting.

As for the manual, mine is a copy/scan itself, but I can tell you where to find the original. It's in Australia... but if you join Charles Edmonds's fantastic library of service manuals you can loan it out for the cost of return postage

Visit http://electronictrader.webserver.com.au/handbooks.html

Kind regards,

Phil