tektronix 7704a oscilloscope faults (bad waveform) - Restoration - Page 1
Forum section: electronics / testgear

author: circuitravi
author_rank: Contributor
timestamp: January 26, 2016, 01:05:53 pm
content: I input 10mhz to my oscilloscope. but waveform is not good. when i set it to low intensitywhen i set to high intensitywhat is the problem? please help me to correct this. thanks!

author: tautech
author_rank: Super Contributor
timestamp: January 27, 2016, 08:14:30 am
content: Your image with the 2 traces.....could it be a unblanking problem?

author: oldway
author_rank: Super Contributor
timestamp: January 27, 2016, 01:46:15 pm
content: I think you need to read the operator manual to understand how this oscilloscope works and how to operate it.http://bama.edebris.com/manuals/tek/7704a/After, you will explain us what is wrong with your scope !

author: circuitravi
author_rank: Contributor
timestamp: February 03, 2016, 05:16:12 pm
content: thanks to tautech and oldway. I started to restoration my 7704A oscilloscope. I think this is easy to repairable oscilloscope ever made. because all transistors and ics are in socket. also every circuit boards can separate and easy to remove. I found many faults when I start servicing my oscilloscope.step 01: remove all tantalum capacitor and replace with electrolytic capacitors. because I found some tantalum capacitors are short and faulty. they short power supply rails. so firstly We need to get correct voltages in power supply.7704A oscilloscope horizontal output board repair7704A oscilloscope Z-Axis board repairAlso I replace tantalum capacitors in power supply and main interface board.I found some were totally short.

author: oldway
author_rank: Super Contributor
timestamp: February 03, 2016, 05:56:23 pm
content: I really wonder how you could have such a display with shorted capacitors in the power supply.In such a high frequency application, tantalum capacitor can't be replace with only an electrolytic capacitors, you must at least place a 0.1 µF capacitor in = with electrolytic capacitor.

author: circuitravi
author_rank: Contributor
timestamp: February 03, 2016, 11:35:59 pm
content: Quote from: oldway on February 03, 2016, 05:56:23 pmThanks! This all capacitors in power supply line. Are you recommend replacing tantalum capacitor with electronic capacitor? What is your opinion? should I replace with same tantalum capacitor or electrolytic capacitor?Should I place a 0.1 capacitor with electrolytic capacitor?Are you recommend ceramic or mylar capacitors?Sent from my SM-N910C using Tapatalk

author: TAMHAN
author_rank: Frequent Contributor
timestamp: February 04, 2016, 05:40:58 am
content: Definitely replace tantalum with tantalum. I can recall, in the dark corner of my mind, a case where someone replaced elcos with tantys and ended up with a non-working VCO or sth like that.@Oldway. Sorry for being rude. But if you think its an operator's mistake, please in the name of whatever, tell the guy what he is doing wrong. He will end up fooking up the scope bigtime...

author: oldway
author_rank: Super Contributor
timestamp: February 04, 2016, 10:07:32 am
content: I think that circuitravi should have first to describe what is not good with this waveform.The sinus waveform is distorded ...what sinus generator did he used ? Did he verified this waveform with another oscilloscope ?There is also an intensity modulation, but this may occur with some settings of the time base when you take a picture of the crtThe second picture shows that there is an unblanking problem, or even an anormal working of the base time unit.Did he tried with another time base unit ?He said there are some tantalum condensators shorted on power supply rails...It seems that this would not be possible as the 7704A mainframe has a SMPS who would not start if there is a short circuit on the rails.The lack of information let me think that circuitavi is not well aware of working of analog oscilloscopes and this is the reason why I have recomended to him to read the operator manual.

author: circuitravi
author_rank: Contributor
timestamp: February 04, 2016, 04:26:13 pm
content: One capacitor is short in horizontal output board. Others are ok. But I decide to replace others. Is it ok, electrolytic capacitor with ceramic 0.1 ceramic capacitor?I also refer service manual. I found many carbon composite resistors valves are drift.what is the value in below resistor? I think it is 2.4kohms. when it is consider 5 colour resister can't get its valve. what is the last colour band?

author: oldway
author_rank: Super Contributor
timestamp: February 05, 2016, 11:37:00 am
content: There are different philosophies regarding the restoration of older devices.My experience has shown that in general, it is best to do as little as possible by replacing only what is really broken.I prefer to check capacitors and does not change if they are within specifications and that they are not visibly damaged (electrolytic capacitors)I repair oscilloscopes, but I've never had a 7704A in my hands.But I read that this model has a temperamental switching power supply particularly difficult to repair.So be careful not to cause a failure of the SMPS.Tektronix used only high quality components.So it is better only fix what is really broken unless it is proven that a component has systematically defects (defects in repeat) in this device.For that, one must have enough skill and experience to diagnose faults.Do you have it?Allow me to doubt and advise you to contact a specialist in this kind of repair.The component values are available in the service manual for this oscilloscope.

author: circuitravi
author_rank: Contributor
timestamp: February 05, 2016, 06:07:13 pm
content: I am appreciate your help oldway! Thank you very much!Power supply is ok.. all power rails within voltage limits.When I check voltage levels referring service manual, I found 51v zener regulated line voltage drift in Z-axis board.. I found one resistor valve drift after troubleshooting.It is 2.4k carbon composite resistor. Colour code is red,yellow,red,gold,and yellow. What is the last colour band mean?Sent from my SM-N910C using Tapatalk

author: oldway
author_rank: Super Contributor
timestamp: February 05, 2016, 07:13:25 pm
content: Quotered, yellow, red, gold...ok, it is a 24.3R resistor, but last colour band should be gold and not yellow.Gold as last colour band means 5%A precision 2.4K 1% resistor would be: red, yellow, black, brown and brown for 1%Look at the nummer of the resistor on the layout and read the specifications of this resistor in the service manual of the scope.http://www.ko4bb.com/manuals/index.php?dir=Tektronix/Tektronix_-_7704_Oscilloscope

author: oldway
author_rank: Super Contributor
timestamp: February 05, 2016, 07:21:54 pm
content: As you have trouble with color code, you can download a free program here:http://www.schematica.com/resistor_color_codes/resistor.html

author: circuitravi
author_rank: Contributor
timestamp: February 05, 2016, 07:35:12 pm
content: Quote from: circuitravi on February 04, 2016, 04:26:13 pmDid you see this pic? Colour code is red,yellow,red,gold, yellow.Sent from my SM-N910C using Tapatalk

author: Cubdriver
author_rank: Unknown
timestamp: February 05, 2016, 08:33:54 pm
content: What does the resistor read when checked with an ohmmeter? That should at least get us to where we can isolate it to an order of magnitude. I've never seen that arrangement with the additional band at the end. I have an older book on passives; I'll try to dig it out later and see if it sheds any light on this.-Pat

author: oldway
author_rank: Super Contributor
timestamp: February 05, 2016, 08:44:15 pm
content: It seems to be a 2.4K 5%.The last colour band may be relacionated with temperature coeficient. (yellow = 25ppm/°C)I remember I read something about it.

author: Addicted2AnalogTek
author_rank: Frequent Contributor
timestamp: February 09, 2016, 04:33:16 am
content: The yellow band at the end of the resistor after the tolerance band means that it is a Mil-Spec resistor. That is typically what I have found, anyway.

author: circuitravi
author_rank: Contributor
timestamp: February 09, 2016, 05:42:53 pm
content: I found good information about this type resistor.http://www.resistorguide.com/resistor-color-code/If 4th band is gold or silver,3rd band become multiplying factor and 5th band became temp coefficient.Thanks Oldway cubdriver and addicted2Analog Tek .Sent from my SM-N910C using Tapatalk