Strange behaviour from scope - Page 1
Forum section: electronics / testgear

author: G7PSK
author_rank: Super Contributor
timestamp: June 02, 2013, 04:15:18 pm
content: I bought a Leader 1041 scope on ebay just to tinker with, I had a good idea when I bid on it that there was something wrong with it, the description was evasif to say the least.I powered it on and the trace on ch 1 only triggers when set to 50 mV position and disappears on all other settings ch 2 works correctly.I thought that the problem might be dirty switches so I cleaned them, after this the trigger would not work on either channel but even stranger when fed with an AC signal ch 2 produced a vertical trace as expected but ch 1 produced a horizontal trace that would only move on the screen by turning the un cal volt/div knob this moved the trace up and down.I now have the trigger working again but the trace on ch 1 will only move up or down within the top third of the screen.It is not the pots as ch 2 works fine and I tried swapping the leads of the two channels pots over and the fault is still on channel 1, this suggests to me that the fault is within the ch 1 amplifier and not the pots or the plate driver amp which is on a separate board and is used by both channels.As I have not been able to find a circuit diagram or manuals for this scope I would welcome suggestions as to where on the board I should concentrate my search and what should I be looking for, I am not very experienced in fault finding on somthing as complex as this, High voltages are no problem I have a 20mm rubber mat etc.

author: robrenz
author_rank: Super Contributor
timestamp: June 02, 2013, 05:00:44 pm
content: Manualherebut probably more than you paid for the scope

author: G7PSK
author_rank: Super Contributor
timestamp: June 02, 2013, 05:08:58 pm
content: Would be after postage and import duty, tax etc. There is a Thurlby Thandar sticker on the back giving an address that is about 45 miles from me I will look them up, I think they were the distributer for the scope back in the 1990's. The scope was made sometime after 1989 as this date appears on a couple of the chips inside.

author: G7PSK
author_rank: Super Contributor
timestamp: June 03, 2013, 04:06:38 pm
content: Took the board out today and found that on one of the switches a leg had never been soldered, it was meant to be as there was a trace going of somewhere that had been tined, I have now soldered that cant test it it yet as I found that the HT board was full of toner, so I have had to wash that out in alcohol and am waiting for it to dry out thoroughly before reassembly.https://www.eevblog.com/forum/Smileys/default/smiliey_scope_broke.gif

author: c4757p
author_rank: Super Contributor
timestamp: June 03, 2013, 04:13:15 pm
content: Toner?

author: G7PSK
author_rank: Super Contributor
timestamp: June 03, 2013, 04:27:17 pm
content: Yes toner as used in photocopiers and some laser printers, must have been in a service area for such machines at one time, it was sticking all along the traces of the HT board under the cover.

author: c4757p
author_rank: Super Contributor
timestamp: June 03, 2013, 04:29:56 pm
content: Ugh... that stuff's a pain in the ass. Did the alcohol get it all off? The toner in my printer doesn't come off with alcohol once it's melted, so I'd have trouble around anything that ran hot.

author: G7PSK
author_rank: Super Contributor
timestamp: June 03, 2013, 05:15:27 pm
content: It got most of it off, what was left came away with a stiff brush, but there is some left on the pins of the crt that is baked on and will need some acetone or some thing to remove it. I hope that it is not conductive if it is all sorts of damage could have happened due to the eht going astray.https://www.eevblog.com/forum/Smileys/default/scared.gif

author: G7PSK
author_rank: Super Contributor
timestamp: June 04, 2013, 09:02:54 pm
content: Put it together but still have problems.There is a trace nice and bright and in focus, horizontal scan does not go all the way across the screen only about three quarters and there dose not seem to be any vertical at all anymore.I have found that the 140 volt rail is down to 57.3 volts the 12,55 and 5 volt are all normal the voltage regulator heat sink is getting hot very quickly, the power supply is linear and all the power transistors share the same heat sink, this suggests to me that the 140 volt line is being pulled down by something, any ideas as to what the 140 volts would drive other than the eht power supply would it be the plate driving amp or would that be the 55 volt.?https://www.eevblog.com/forum/Smileys/default/bangheadonwall.gif

author: PA4TIM
author_rank: Super Contributor
timestamp: June 04, 2013, 09:09:21 pm
content: That is no toner. that is dust and dirt atrracted and fried by the high voltage. Have seen it in a lot of HV circuits.

author: G7PSK
author_rank: Super Contributor
timestamp: June 05, 2013, 11:29:13 am
content: Quote from: PA4TIM on June 04, 2013, 09:09:21 pmDefinitely toner, dust tends to be dull looking and slightly furry this was smooth and shiney also slightly plastic. I have taken many TV's and CRT monitors apart over the years, in the late 60's when I was at school I had a part time job with a local TV repairman, it was my job to take the incoming tv's apart and clean the crap out of them.

author: PA4TIM
author_rank: Super Contributor
timestamp: June 05, 2013, 12:21:19 pm
content: That I did know. So in that case it is save to say you know how black a HV supply can be and in that case this will be something else. But how does toner come in a scope ?I repaired a HP9100 a few months ago that had real black pcbs and components. In the dark you could see the HV crawling through the soot. I removed the HV transformer an some other parts and then put the pcb in my ultrasonic cleaner.But in your case the problem is in something else.Can you decouple the psu and put a resistive load instead. If it stays up the problem is down the line. You can also use an Ohm meter to measure the reistance at that point. If that is not very low probably a cap is leaking. If it is low look for zeners , diodes or resistors. Some manufacturers even give those values. It often also helps to measure the voltdrop over resistors and calculate current.Be aware of former repairs. I had a similair problem and it turned out someone had swapped transistors. The new ones were a different package and he had connected one of them wrong, effective bypassing one of the two transistors. All worked more ore less, but a bit to much ripple until the unit asked more power and then the psu went out of regulation and voltage went down.I always use a scope ( but I have many scopes) while repairing a scope ( or better, i use a scope more as anything else)If i suspect a short I use a 6,5 digit meter and follow the voltage. The short will have the lowest voltage and sometimes the meter leads you direct to the short.If there is more current in one trace as the other, voltdrop wil be higher. If that does not work, or I know the direction, i put the one probe on the psu output and then slide the other probe over the traces ( from solder pad to solderpad) the voltdrop over the traces give me an indication of the trace that carries the most current. This is a mindless thing of just reading the meter, you must know the schematic and interperted the measurement the right way. But I has often helped me to find a fault.I start with measuring psu rails for voltage and ripple. Then Take the schematic, print it out and calculate voltaged and voltdrops from crucial points. Then I measure those points. If one is wrong, i calculate all voltages in that area and measure again. Then I sit down and analyse the numbers and most times that lead me to the fault. I just think, what must be defective to get that wrong voltage. If it is very complex I copy that part to a simulator and try to get that fault by making resistors infinitive or zero Ohm or bridging transistors etc.You probbly know all these tricks but in case you do not I hope they are usefull. (i collect and restore (most rather special) instruments. For me repairing instruments is fun, they are great puzzles)

author: G7PSK
author_rank: Super Contributor
timestamp: June 05, 2013, 01:44:26 pm
content: Thanks for the reply P4TIM, I do have another scope and have checked for ripple the power is dead smooth I have found a component that appears to be dead on one leg but I am not sure if its a diode or transistor I have put it on the what this page.