7704A, smaller/shrinked image on CRT - Page 1
Forum section: electronics / repair

author: JPortici
author_rank: Super Contributor
timestamp: September 24, 2016, 04:56:15 pm
content: How much am i screwed right now?Vertical is right, but horizontal and readout are shrinked.

author: David Hess
author_rank: Super Contributor
timestamp: September 24, 2016, 06:13:48 pm
content: The horizontal linearity looks good so assuming that *only* the horizontal gain is low, it should be an easy repair. Trimmer potentiometer R4310 shown on schematic 13 which adjusts the overall horizontal gain is probably open. Mechanical parts like trimmer potentiometers are always suspects because of their low reliability. R4310 is easy to access on the horizontal CRT amplifier board once the cover is removed.I am a little concerned though that this may not be the case because the readout is vertically out of position. Did you verify that the vertical deflection is accurate?

author: JPortici
author_rank: Super Contributor
timestamp: September 25, 2016, 08:18:22 am
content: the readout is *not* really out of position, it is shrinked. I actually thought the whole image was shrinked but the vertical deflection is correctthe readout is also wobbling a bit... do i make a video?

author: David Hess
author_rank: Super Contributor
timestamp: September 25, 2016, 08:05:45 pm
content: Quote from: JPortici on September 25, 2016, 08:18:22 amThe wobbling is usually normal; it comes about because of thermal imbalance in the amplifiers.So with the horizontal gain incorrect *and* the readout vertically out of position, there are two likely unrelated problems. Maybe someone misadjusted the horizontal gain and the vertical readout position adjustment.Do you have another timebase or vertical amplifier? A vertical amplifier can be used in the horizontal slot to check the horizontal gain and make sure the horizontal gain problem is not with the timebase you are using. You could swap your existing vertical amplifier and timebase to check the horizontal gain.

author: JPortici
author_rank: Super Contributor
timestamp: September 26, 2016, 06:19:46 am
content: It's been a long weekwnd, have yet to check anything.This ain't good.No signal in, the horizontal is using the magnifier to i can see the problem better. I think i'll start with checking the power supplyTo add to the crt image problem, the trace is also bending slightly to the corners..so after i check the PS i'll try and go throug the crt calibration. What do you think?

author: JPortici
author_rank: Super Contributor
timestamp: September 26, 2016, 06:24:09 am
content: Vertical in the horizontal slot: 3V square calibrator from a 1054z is exactly 5 divisions at 500 mV/div

author: dom0
author_rank: Super Contributor
timestamp: September 26, 2016, 06:38:01 am
content: Low post-deflection acceleration voltage?

author: David Hess
author_rank: Super Contributor
timestamp: September 26, 2016, 02:45:14 pm
content: Quote from: JPortici on September 26, 2016, 06:19:46 amIf you set the timebase to line triggering, then you will be able to see if this is from AC line ripple or something else. These oscilloscopes are old enough that it is likely the aluminum electrolytic capacitors in the power supply are worn out.QuoteThe CRT calibration may affect this but it is not serious. CRTs usually have some geometry distortion at their edges.There are actually *two* adjustments which affect CRT geometry. R41171 adjusted the CRT geometry but the vertical deflection plate shield voltage adjusted by R41176 also has an effect on CRT geometry.Quote from: JPortici on September 26, 2016, 06:24:09 amSo the lack of horizontal deflection is probably what I suggested. Check the gain potentiometer (R4310) on the horizontal CRT amplifier first. It may be bad or a previous owner may have misadjusted it.Quote from: dom0 on September 26, 2016, 06:38:01 amThis would affect both horizontal and vertical deflection. Vertical deflection is apparently accurate.

author: JPortici
author_rank: Super Contributor
timestamp: September 26, 2016, 04:40:54 pm
content: it's been in my hands for about a couple of months now.It was supposedly recapped and recalibrated before i purchased it, but i didn't check.other stuff i noticed:noise amplitude variates with vertical position (at midpoint no distortion)noise seems to be a 10 kHz square wave (chop signal?)reaching and passing the edges brings in severe distortion, like if i was bending a string attached to the corners (it never did that)

author: N2IXK
author_rank: Frequent Contributor
timestamp: September 26, 2016, 07:28:00 pm
content: Quote from: dom0 on September 26, 2016, 06:38:01 amLow accelerating voltage on a CRT will result in an EXPANDED image, because the electron beam becomes easier to deflect at lower velocities. To produce a shrunken image, you would need a HIGHER accelerating voltage applied to the tube, resulting in higher beam velocity and less time spent traveling through the deflection plate region.

author: David Hess
author_rank: Super Contributor
timestamp: September 26, 2016, 09:20:52 pm
content: Quote from: N2IXK on September 26, 2016, 07:28:00 pmThis is the case for the cathode acceleration voltage but not necessarily for the PDA (post deflection acceleration) voltage.I ran tests a couple months ago for TekScopes@yahoo.com and determined that lowering the PDA on the 7000 series oscilloscopes lowered the deflection. On a 7904, no PDA resulted in about 50% deflection and raising the PDA from zero resulted in a roughly proportional increase in deflection. The test involved using the PDA output from my 7603 to drive my 7904 which was fun. The 7603 itself which uses a very different CRT behaves similarly.