Gould OS300 focus problem

[ChrisGrahamUK]

I pulled a discarded OS300 out if a skip at the local recycle centre. Spec wise it is slightly better than my OS250 so would like to see if I can save it. Slightly battered (dropped?) but all seems ok except for the focus which at maximum setting is still slightly short of perfectly sharp.
Can I have some suggestions on how I might diagnose and repair this please?
Thanks

[David G4EBT]

Can't help with that, but I do have the manual for it if you don't already have it.

It's 1.68MB, so if you'd like me to forward it to you, please PM me with your email address.

I had one for several years which had been thrown in the skip by the Comms Dept at work as it was 'time expired'. It did an excellent job.

Hope you get it sorted Chris.

[RogerEvans]

I don't know this model of scope so this is rather general.

Best focus often requires alternately adjusting the focus and astigmatism controls.

Typically the focus electrode and focus pot are part of a very high value resistor chain, the resistors can change value with age (usually they go high) or they can acquire dust and grime which makes the resistor chain leaky under high voltage.

The CRT cathode voltage could be low, small errors don't affect the focus very much since it is the ratio of the cathode voltage to the focus electrode that controls the focus. If the cathode voltage is low enough to cause a focus issue then the deflection sensitivities will be wrong in X and Y.

If the scope really has had a bad knock then one or more electrodes internal to the CRT may have moved. Don't jump to this conclusion before testing for faults that are easier to repair.

Regards,

Roger

[AD360 Rob]

In some 'scopes the focus voltage is derived from a resistor chain of high value resistors (1M upwards) it's a possibility that one of these resistors has gone high. It has happened to my Tektronix scope twice now with exactly the same symptoms that you describe. You will need a circuit diagram to see where they are on your scope.

Manual here
http://bama.edebris.com/download/gou...ould_OS300.pdf

[WME_bill]

Advance Gould OS300 focus.
Take heart. This is a classic common fault with many scopes. It is due to the resistors in the EHT chain going high under voltage stress.
Replace them all. If you use common carbon or metal film resistors, expect to replace them every few years, unless you intend to use the scope 8 hours a day every day. They are rated at around 250v.
A proper job is to use light blue metal glaze resistors as Philips VR37 or VR68, which are rated at 2000v.
Maplin and RS used to stock a sufficient selection. You will have to search to find out a supplier.
All high values up in the megohm region. The exact values are not important, as long as the ratio each side of the focus control between the CRT cathode and chassis is about right.

This scope uses thick film hybrid networks in the Y attenuator resistors, timebase timing resistors and the EHT chain. The manuals do not give the resistor values, so the offered manuals will not entirely solve your problem.
I have established the correct values and attach circuit diagrams with the values marked.
I have also made other notes on the circuits. The CRT is a Philips contract type 56825, version of D14-250, a very common tube. If your scope has a different tube, please let us know.
A very useful skip find. Well done.
wme_bill

[ChrisGrahamUK]

Thanks for all the info.
I checked the resistor basis which were all OK.
I then tried the obvious which was contact cleaner in the pot which fixed the issue. Brilliant.
But.... On further checking the mechanical damage means the CRT front mount is completely wrenched and means the CRT is going about and probably unsafe.
Because of this I've decided to scrap the unit.
I'm in Lichfield Staffordshire so if anybody wants this for spares for the cost of postage and packaging please contact me.