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Tek 2215 vertical gain

 

Hi,
I recently picked up a well used Tektronix 2215 late serial number scope (with A18 pre-regulator board) which has an issue with vertical gain. Same occurs on Ch1 and Ch2 so must be in a common circuit. The symptom is when displaying square wave (off the front panel), the height of the square wave varies wildly as the trace is moved up and down using the Ch1 or Ch2 vertical shift control. When moved to the bottom the height is around 2.5 squares, when moved to the top the height is around 4 squares.

The only check I have made so far is to R378,R379,R388,R389 which are meant to be 340 ohm but all measure very closely at 315 ohm each (in circuit). I don't think these would be the cause as they are the same in + and - sides of the push-pull output circuit.

Any suggestions on what to check please? Hopefully a pot tweak will help balance the vertical.
Thanks.
John

Michael W. Lynch
 

The symptom is when displaying square wave (off the front panel), the height of the square wave varies wildly as the trace is moved up and down using the Ch1 or Ch2 vertical shift control. When moved to the bottom the height is around >2.5 squares, when moved to the top the height is around 4 squares.
John,

I do not know if this is relevant, but I have a 485 that did a similar thing. The square wave supplied by the calibrator was fine when the trace was centered and as you moved the trace away from center, the trace shrank vertically, to about 10-20% of its original amplitude. This 485 scope had been in storage for a very long time and I traced the problem to a very dirty beam finder switch. I used DEOXIT, IPA and a lot of exercise to clean that switch and the scope returned to normal and signals now display correctly, regardless of vertical position.

--
Michael Lynch
Dardanelle, Arkansas

 

Tom Jobe (RIP) was the resident expert on these 2200 series scopes.

Considering the problem appears on both channels, the first place I would look at is the channel switch and vertical delay line driver. See schematic 3 in the 2215 service manual. You should make some voltage measurements and compare to the schematic readings. Start with Q341, 345, 331, and 335.

If you have another scope, that would be a help.

Regards

On 6/26/2020 3:03 PM, John Shadbolt wrote:
Hi,
I recently picked up a well used Tektronix 2215 late serial number scope (with A18 pre-regulator board) which has an issue with vertical gain. Same occurs on Ch1 and Ch2 so must be in a common circuit. The symptom is when displaying square wave (off the front panel), the height of the square wave varies wildly as the trace is moved up and down using the Ch1 or Ch2 vertical shift control. When moved to the bottom the height is around 2.5 squares, when moved to the top the height is around 4 squares.

The only check I have made so far is to R378,R379,R388,R389 which are meant to be 340 ohm but all measure very closely at 315 ohm each (in circuit). I don't think these would be the cause as they are the same in + and - sides of the push-pull output circuit.

Any suggestions on what to check please? Hopefully a pot tweak will help balance the vertical.
Thanks.
John

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On Sat, Jun 27, 2020 at 02:37 AM, Michael W. Lynch wrote:

I do not know if this is relevant, but I have a 485 that did a similar thing.
The square wave supplied by the calibrator was fine when the trace was
centered and as you moved the trace away from center, the trace shrank
vertically .. the problem ...a very dirty
beam finder switch.
Thanks Mike, very relevant, I cleaned up the beam finder switch and the difference between bottom and top height is now 1.8 and 2.5 squares (1:1.4) so an improvment over 2.5 and 4 squares (1:1.6). Beam finder switches clearly don't get used much!

Regards,
John

Michael W. Lynch
 

On Mon, Jun 29, 2020 at 05:30 PM, John Shadbolt wrote:


Thanks Mike, very relevant, I cleaned up the beam finder switch and the
difference between bottom and top height is now 1.8 and 2.5 squares (1:1.4) so
an improvment over 2.5 and 4 squares (1:1.6). Beam finder switches clearly
don't get used much!

Regards,
John
John,

Glad to hear that this helped you.

Exercise all those switches vigorously. My 485 took quite some time to become fully functional. I cleaned and used DEOXIT on the thing about 3 or 4 different times before the trace came fully back as it should be. Make sure to use a cleaner that does not leave residue or attract dust.

There are a lot of these switches on TEK scopes and they all suffer from the same malady. Clean everyone that you can find on your 2215.

I find dirty switches and contacts are the number one cause of scope "failures".

Good Luck with your scope.

--
Michael Lynch
Dardanelle, Arkansas

 

Hi again, same 2215 is playing up again. Now both traces have moved down by about 8 squares, so can't move them up above the middle of the display, even with beam finder pressed. Time to open it up again and check voltages.
Regards,
John

Bert Haskins
 

On 7/30/2020 1:43 PM, John Shadbolt wrote:
Hi again, same 2215 is playing up again. Now both traces have moved down by about 8 squares, so can't move them up above the middle of the display, even with beam finder pressed. Time to open it up again and check voltages.
Regards,
John
Sounds like you  have a break between the main pcb and the deflection plate pins.

Common on these scopes.

-- Bert