Tektronix 475 vertical position knob problem - Page 1
Forum section: electronics / repair

author: israel
author_rank: Contributor
timestamp: September 05, 2014, 01:54:42 pm
content: Hi,It has come to my hands an scope with so many years in st-by. It is a Tektronix 475 is in very good condition but has several problems.Necessary repairs in the power supply have been performed successfully, but there is a problem with one of the channels (2).The problem is that the vertical knob does not work. As time goes up the trace to get lost in the top of the screen.It's like there DC voltage at the amplifier channel 2.I have removed the first amplifier IC, and trace can view and move perfectly. Thinking that the problem could be the amplifier IC, I've exchanged. But the result is the same, I havealso disconnected the resistance goes (solder aerially) to one of the pins of the IC (I guess that is the entrance). But still unsuccessful.In a quick check the voltages, I have seen are all correct and identical to the channel that works well.I rule out a problem in the power section of the vertical deflection, when removing the IC amplifier channel 2, the vertical knob responds perfectly.Any ideas or suggestions?They will be very well receivedSee video...http://youtu.be/DMu_bnzYmv0Sorry for bad my english.Best regards,

author: David Hess
author_rank: Super Contributor
timestamp: September 06, 2014, 02:15:03 am
content: Quote from: israel on September 05, 2014, 01:54:42 pmDoes the vertical position control do anything?The only place a gradual drift can originate is one of the FET inputs which have low input bias current.Does it still drift when the input coupling is set to GND?QuoteMy guess is that R39 on the attenuator board is open. This resistor sinks the FET input bias current to ground. Otherwise it would gradually charge the input capacitance.QuoteAsk over at the TekScopes@yahoogroups.com email list.

author: oldway
author_rank: Super Contributor
timestamp: September 06, 2014, 09:28:43 am
content: Measure the input voltage of the first amplifier of channel 2 and compare with channel 1. (pin 13 of U220 or R62 / R204)Seems to be a problem of faulty diode(s) CR203/204/205.NB: back side of the board.See also R219 (30R).

author: David Hess
author_rank: Super Contributor
timestamp: September 06, 2014, 02:59:56 pm
content: I agree about the input protection diodes. I did not see any examples of "drift" in the video. If swapping U120 does not change anything, then the input protection diodes are the next likely cause of the problem.If the diodes are bad, then I would grade some small signal transistors for high base-emitter reverse breakdown voltage and use them. If the diodes are changed, then the input compensation will need to be adjusted.

author: israel
author_rank: Contributor
timestamp: September 06, 2014, 04:15:57 pm
content: Thank you very much all for your invaluable help. Eventually I was able to find the fault, it was a broken one end resistance.It was almost imperceptible to the eye, so I pass unnoticed.The problem is that I could see that one of the amplifiers IC is faulty.Now the adventure is to find one of these Is (1St cascode amplifier U220).Best regards for all.

author: David Hess
author_rank: Super Contributor
timestamp: September 06, 2014, 04:49:15 pm
content: That would do it.I recognize the part. It is a 470k 1/8 watt carbon composition resistor which was selected because they have good overload capability and tend to fail open. 1/8 watt carbon composition resistors are practically extinct and I do not believe anybody makes them anymore.I do not have any good suggestions for an ideal replacement. A 1/4 watt carbon composition resistor will certainly work but so will any 1/8 watt film resistor. The best option might be a thick film 1/8 watt or lower chip resistor with a high voltage rating like 500 volts.

author: israel
author_rank: Contributor
timestamp: September 06, 2014, 07:01:01 pm
content: any idea where to get this "little boy"?

author: David Hess
author_rank: Super Contributor
timestamp: September 06, 2014, 07:12:09 pm
content: Quote from: israel on September 06, 2014, 07:01:01 pmDid you decide it is bad?I would start with the Tektronix part number which is 155-0085-00. Offhand I do not know which instruments used it so besides finding it in another 475, I would check places that have Tektronix custom parts.Sphere has the 155-0085-01 available which I suspect would work but you might want to ask someone over at the TekScopes@yahoogroups.com email list. QService has it also.http://www.sphere.bc.ca/test/tek-parts/tekparts3.htmlhttp://www.qservice.tv/vpasp/shopexd.asp?id=12565&bc=noThey show up on Ebay periodically.

author: israel
author_rank: Contributor
timestamp: September 06, 2014, 11:50:09 pm
content: QuoteOnce the fault is cleared, I proceeded to exchange the amplifiers. I could clearly see that one of them was bad, possibly the lack of resistor that caused some kind of self-oscillation in the IC and this broke down.Did not know these supply websites , many thanks for the info.a big hug for everyone

author: iDevice
author_rank: Regular Contributor
timestamp: September 07, 2014, 10:01:18 pm
content: Quote from: David Hess on September 06, 2014, 04:49:15 pmCarbon composite resistors are also sometimes chosen for their low inductance which could be the reason here in a high frequency circuit so a film resistor could change it's characteristics.That's why these resistors are still widely available at the usual suppliers like Mouser etc.

author: David Hess
author_rank: Super Contributor
timestamp: September 07, 2014, 11:28:33 pm
content: Quote from: iDevice on September 07, 2014, 10:01:18 pmThat is often the case but here, the resistor is bypassed by a capacitor and its self inductance has a negligible effect because it is part of a high impedance circuit. It was chosen for its overload tolerance and propensity to fail open.

author: SoundTech-LG
author_rank: Frequent Contributor
timestamp: September 11, 2014, 07:46:38 pm
content: My 475 may have a related problem. Ch 1 INTERMITTENTLY jumpy (vert) and fuzzy (with signal). Moving the input jack around seems to affect it, clears it up for a while. I suspect same areas shown, since a lot of those components might be subject to vibration. I'll have to take a look sometime soon.Thanks for the great photos, and info!!!

author: tombi
author_rank: Regular Contributor
timestamp: September 24, 2014, 04:47:42 am
content: Hello,With the jumpy channel 1 - mine too has these sort of issues. If you open the cans where the attenuators live you can see a bunch of gold switch contacts in there. Turn the volts/div dial until a switch opens, stuff a piece of isopropyl alcohol soaked paper between the switch and the contact, gently(!) move it around to clean the contact and pull it out.Other issues I had were the AC coupling capacitor is not soldered but socketed. Tapping it changed the waveform. Pulling it out and pushing it back in seemed to make lots of this go away.Finally, there are a few of these gold contacts underneath the grey volts/div selector barrels. I think the ones underneath set the gain for the cascode amps so if these switch contacts are dirty it makes a *big* effect on the signal. Same procedure can be applied without removing the unit to clean them. You can get to them from the side between that lump of metal and the underside of the attenuator can.Tom

author: SoundTech-LG
author_rank: Frequent Contributor
timestamp: October 08, 2014, 07:49:19 pm
content: Hi,Thanks for that info. When I get the chance to go in it, I'll be looking at all that. I think IPA is much safer alternative to other cleaners for this work. You want something that evaporates completey, does not leave any film, and doesn't melt plastic!!!Lots of places for intermittents!!!