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Old 6th Jul 2017, 8:39 pm   #1
chriswood1900
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Default Fixing the focus on a Tektronix 2210.

Along with a few other items I recently acquired a Tek 2210, this is the low-end storage scope of the 22XX series with 50Mhz analogue bandwidth and slow speed storage. The symptom was completely out of focus with a spot of around 20mm diameter and only slight change from the focus pot.
There appears to be two common layouts in the 2200 range the older ones with the integral switch mode PSU like the 2225a/35/36 and the slightly earlier or later (not sure which) 2210/11/12/25 the external distinguishing features with the later ones seems to be a plastic handle and slide switches replacing small toggle ones. Inside from the limited number I have seen the main boards seem to have two common patterns early ones have an integral Switching PSU and the later designed ones a large toroid on the back panel and a vertical PSU board at the back.
I looked on the web for a manual without luck I knew from previous work it was different to layout to the 2235 I had worked on before so I looked and found a 2225 which whilst having different specs seem to have the same main board, I followed the circuit and saw a very similar layout to what I had seen before on the 2235 with a tapping off the multiplier and a chain of resistors, R876 to R884 in this model they were 1M carbon composite instead of the usual 510K. To find them I traced the focus control with its long shaft back to the PSU area which is under a clip-on black plastic cover, the focus pot is visible near the back with a small ribbon cable dropping on to the board, with another cover over the underside of the main board. With this off I could locate the chain of resistors. With the power off I checked their reading with a DMM most were in tolerance except R879 and R880 which had changed to 1.5M and 5.6M respectively, hopefully that was the problem so a look in the parts bin found some NOS Philips composite resistors which but that had all gone high in value, I selected the lowest which was about 1.1M and tacked that on the underside and powered up success, I could now get a good focus over a reasonable range.
Some Googling indicated the correct replacement is 1/2W carbon composite, the only ones I could find in stock were the RS 386-020 in 25s, I took advantage of the free postage and ordered them and they arrived as usual the next day.
To fit them was a bit fiddly, whilst you can get to the bottom to unsolder you canít reach inside as the PSU capacitor is in the way. I found the best route in was to remove the toroid which unplugs from the PSU board, that board is then removed by taking out the screws holding the IEC connector and a screw at the top of the board. To remove this screw you need to remove the nut first separately as it is a locknut, you can then take out the screw and angle the board out of the way whilst it dangles on its leads. I stood the unit on its side and then with a hot iron and solder wick removed the solder and used a length of wire and a hook to pull out the two defective resistors. I know some people would say replace the lot but my philosophy is to replace only known defective parts, I put the 2 new resistors in and soldered them in place, reinstalled the PSU and did a quick power on to confirm things were working which pleasingly they were. The new resistors were indistinguishable from the ones fitted by Tek. Then I put the case back together and put it through its paces checking out the functions followed by a good clean, I now have a nice spare scope for the garage workshop.
I hope this helps others with the same or similar problem.
Regards
Chris
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Old 7th Jul 2017, 7:22 am   #2
RogerEvans
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Default Re: Fixing the focus on a Tektronix 2210.

When I saw just the title of the thread my first thought was '510k resistors drifted high', it seems to be a common problem even if yours are 1M!

I have a 2221 with the increased 100MHz bandwidth of later serial numbers, it is a delightful scope with the convenience of analogue mode but push one button and you have the digitised trace, download over GPIB and save on the computer, best of all worlds (almost). I have never found a manual, the analogue boards are almost identical to the 2235 but the digital and memory board is different to any of the analogue/digital 22xx scopes.

Hope you enjoy the new scope,

Roger
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Old 7th Jul 2017, 9:45 am   #3
MrBungle
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Default Re: Fixing the focus on a Tektronix 2210.

Nice work. I've found surgical gear is great for working inside equipment like this. You can usually it into places that standard hand tools don't reach. I have some surgical snips for removing the old parts by snipping the leads off, clean up the holes from underside and then use forceps to get the new parts in again. Has saved me hours of dismantling.

My sister is a vet so she gets the stuff for me
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Old 7th Jul 2017, 11:01 am   #4
RogerEvans
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Default Re: Fixing the focus on a Tektronix 2210.

I haven't looked for ages but you sometimes found 'dental' tool sets in the £1 shops. A couple of odd shaped brushes, a mirror on a stick, and a double ended toothpick with one end hooked. Very useful!

Roger
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Old 7th Jul 2017, 2:14 pm   #5
G6Tanuki
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Default Re: Fixing the focus on a Tektronix 2210.

A nice simple fix! (even if getting in there to do it was less than easy). Those kind of high-value resistors seem to enjoy drifting high even if they're not being subjected to any current/voltage/heat.

As to using surgical instruments - I've found locking "Artery Forceps" to be hellish useful in these sorts of situations: you can lock them on to a component lead before unsoldering, then give the forceps a quick pull to wangle the lead out of the PCB. Much less risk of damage than trying to coordinate soldering-iron abd ordinary snipe-nose pliers in a confined space.

One of the small traders that visits radio rallies in the South/South-West usually has vast trays of these kind of forceps in different sizes for a quid or so each. Seems that in times-past the used surgical-instruments would be collected up, autoclaved and re-used but in these days of heightened infection-control they have standard radiation-sterilised "packs" of all the instruments you need for a particular operation and once these packs are opened any unneeded contents are no longer sterile so cannot be used in-theatre. Hence they end up being disposed-of, to be used by us radio-types!
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Old 7th Jul 2017, 2:29 pm   #6
MrBungle
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Default Re: Fixing the focus on a Tektronix 2210.

I think they're quite cheap anyway. They have an expiry date on the boxes I understand so you're probably getting sterile new ones that just can't be used.
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Old 7th Jul 2017, 3:23 pm   #7
chriswood1900
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Default Re: Fixing the focus on a Tektronix 2210.

Thanks for the kind words, I do use some surgical tools and indeed used forceps to get the resistors back in, even with the board and transformer out the way you can't get your hand in long tweezers forceps and a 6" length of wire with a hook formed on the end were required!
Chris
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