Tektronix 453 vertical channel cross-talk - Page 1
Forum section: electronics / repair

author: jdragoset
author_rank: Frequent Contributor
timestamp: March 10, 2019, 01:34:57 am
content: square wave into channel 1 cross talks to channel 2/ visa/versa, even when input is grounded.Suspect a leaky cap in the downstream mixing.453 seems to be a real performance animal.

author: David Hess
author_rank: Super Contributor
timestamp: March 10, 2019, 05:17:33 pm
content: How much? It is normal for there to be some.Isn't there a specification or verification test in the service manual for crosstalk?

author: JohnPen
author_rank: Regular Contributor
timestamp: March 11, 2019, 09:06:47 am
content: A quick check in the 453 manual it does not quote a crosstalk figure between channels. However the 453A manual does quote a figure of 100:1 up to 20Mhz. The latter changed to FETs at the channel front ends, replacing the Nuvistors, but the remainder of the Y circuitry seems to be relatively minor tweaks. Hope this helps.

author: bd139
author_rank: Super Contributor
timestamp: March 11, 2019, 11:15:00 am
content: I had this on my 453 about ten years ago. Turned out to be leaky diodes in the channel switch. Looking back through my notes, they were 1N3605 which were totally unobtainable. I didn't write down what I replaced them with unfortunately or if I replaced them (the scope was scrapped not long after that as it had multiple other problems). Look like typical Si switching diodes. 1n4148 / 1n914 might be a suitable replacement.It was really really bad on mine though, not just a little bit. Check it against reasonable finger in the air first if there is no spec. I would apply a 10V p-p on low attenuation level and measure from A to B then B to A. If it's the same it's probably ok. If it's different then something is amiss.

author: JohnPen
author_rank: Regular Contributor
timestamp: March 11, 2019, 11:56:10 am
content: The 453A lists the channel switching diodes as 1N4152s. They don't appear to be available on Digikey, Mouser and RS but the attached link does provide a data sheet via google. This at least provides a specification for searching out some alternatives. I would think it advisable to try and match any replacement diodes for best performance.https://www.mouser.com/ds/2/308/1n4152-1190199.pdfBest of luck.

author: David Hess
author_rank: Super Contributor
timestamp: March 11, 2019, 03:40:25 pm
content: The channel switch diodes are a good thing to check if the power supply outputs are not the problem. Before replacing the diodes however, make sure the drive signals to the channel switch from Q215 and Q225 are correct.A 1N4152 is essentially a 1N916/1N4448 which failed its high voltage leakage specification so a 1N916/1N4448 makes a fine replacement. The 1N914/1N4148 has more capacitance but should be acceptable in a slower instrument.The 1N4454 would be good also.Exactly what kind of crosstalk are you seeing? Is it just the square wave edges or is it the DC level? What sweep speed are you using?

author: jdragoset
author_rank: Frequent Contributor
timestamp: March 11, 2019, 06:34:40 pm
content: Cross talk is equal with signal applied to either channel.Scale is 5 divisions 1 k hz square wave signal equals 0.2 divisions square wave in grounded channel.Input scale of grounded channel does not affect the 0.2 division cross talk.Position of the active channel also cross talks to the inactive channel.I will check the drive signals to the channel switch and diodes.Thanks for all the responses

author: jdragoset
author_rank: Frequent Contributor
timestamp: March 11, 2019, 09:02:20 pm
content: My bad,Let the scope warm up for the time it took to write the last response and the cross talk (signal and position)disappear completely.Probably is an off power supply, signal transistor or diode in the switch, but not likely worth tracking to replace.

author: David Hess
author_rank: Super Contributor
timestamp: March 12, 2019, 02:49:55 am
content: Quote from: jdragoset on March 11, 2019, 06:34:40 pmQuote from: jdragoset on March 11, 2019, 09:02:20 pmThis may be caused by a thermal balance issue and have nothing to do with the channel switch.Each channel affects the DC operating point of the vertical signal chain after the channel switch when both traces are displayed. If the vertical signal chain is not thermally balanced well, then the DC offset will change with DC operating point because of the change in temperature caused by the change in dissipation.On an oscilloscope with a readout, this is why you sometimes see the readout slightly "follow" a slow sweep.

author: jdragoset
author_rank: Frequent Contributor
timestamp: March 15, 2019, 06:26:30 pm
content: Surprise,The 453 has a 453A Vertical Preamp Board, No Nuvistors.The cross talk only appears in "alternate" positionTemperature change is most notable in Q234.Thanks for the response.

author: David Hess
author_rank: Super Contributor
timestamp: March 16, 2019, 11:35:09 pm
content: Quote from: jdragoset on March 15, 2019, 06:26:30 pmThe late 453 had a lot of changes which made it into the 453A so both service manuals should be used.Alternate switches at a much lower frequency where thermal balance will be more apparent.Q234 will have no effect. Pair Q284/Q294 and the following pairs are the most likely suspects. You could try swapping them to see if the "crosstalk" reverses.

author: ferdieCX
author_rank: Regular Contributor
timestamp: March 16, 2019, 11:41:12 pm
content: Quote from: jdragoset on March 15, 2019, 06:26:30 pmThis is the correct manual for your tek 453http://chiclassiccomp.org/docs/content/testeq/Tektronix/070-0755-02_Tek453-R453-SN20000+Instruction_1970.pdf

author: jdragoset
author_rank: Frequent Contributor
timestamp: March 25, 2019, 01:08:20 am
content: Many thanks for the correct SN (37114) manual.Touching the tiniest drop of canned air liquid to Q234 made the cross talk ten times magnitude.Put in another 2n4275 and the problem was gone.Thanks.

author: David Hess
author_rank: Super Contributor
timestamp: March 26, 2019, 02:08:53 am
content: Quote from: jdragoset on March 25, 2019, 01:08:20 amSo it was not a thermal balance problem. Q234 might have had low current gain or high leakage causing low or high drive to the current switches.