Repair Analog Oscilloscope Z Amplifier - Page 1
Forum section: electronics / repair

author: darkness_electro
author_rank: Contributor
timestamp: May 25, 2015, 05:36:48 pm
content: Hello, I'm new to this forum, and I wonder if someone would be glad to help me. I have this old Tesla BM566A oscilloscope, you can find the service manual online, if you want i can post it. I have a problem trying to repair the Z amplifier. It shows no trace on the screen. Resistor R176(910r) keeps burning(i replace it and the new one smokes immediately). I tried disconnecting the resistor and then resistor R175 (1k) burns. I replaced C167 (100n) and tested diode E167. Any help would be greatly appreciated appreciated.ftp://84.242.89.252/files/tesla_bm566a_-_osciloscope.pdffor manual.

author: Grapsus
author_rank: Regular Contributor
timestamp: May 25, 2015, 07:06:59 pm
content: Hi,Have you checked transistors E164 and E165 ? They look like they could provide a low-impedance path for R176 to burn.

author: darkness_electro
author_rank: Contributor
timestamp: May 25, 2015, 08:11:45 pm
content: I checked e164 with a multimeter on diode test and it seems OK. I will try again tomorrow. Thank you very much.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: May 25, 2015, 09:56:56 pm
content: Any number of possibilities;C171, C166 leaky or shortedE166 openE164, E165 shortedDoes U211 measure within spec?Tim

author: darkness_electro
author_rank: Contributor
timestamp: May 25, 2015, 10:00:08 pm
content: I couldn't find what voltages should be at the test points. Also, as soon as i apply power smoke comes out of the resistors, so i can't actually measure.

author: darkness_electro
author_rank: Contributor
timestamp: May 26, 2015, 05:14:15 pm
content: Quote from: T3sl4co1l on May 25, 2015, 09:56:56 pmThank you very much. indeed, half of the transistors were damaged, the resistors don't burn anymore, but still no trace.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: May 26, 2015, 09:45:07 pm
content: Hmm, possible to check other stuff related to the CRT -- maybe it arced over internally and fried a bunch of things? (You think ESD from squishy meat bags walking on carpet is bad? Try metal-on-metal at direct kV+ charges!) Examples: measure all HV supplies (if possible!), any circuits that connect with the tube (Z drive is one, both X and Y deflection are more; anything else?), and, maybe it was the power supply that cacked instead, so check those (voltages and ripple) too?Tim

author: darkness_electro
author_rank: Contributor
timestamp: May 27, 2015, 07:33:59 am
content: I don't know if it's a crt problem, because when i turn off the scope it appears like a discharge that projects a wierd beam on the screen. Also, across the 1k resistor there are 0V. I shall investigate and see what is in there

author: Grapsus
author_rank: Regular Contributor
timestamp: May 27, 2015, 11:03:00 am
content: A possible way to find out what's wrong is to measure everything at the CRT level. To get something on screen you need at least- current in the filament (between 1 and 14)- a few hundred volts of potential difference between the heater (1 and 14) and the cathode (3)- a high voltage at the anode

author: darkness_electro
author_rank: Contributor
timestamp: May 27, 2015, 11:18:20 am
content: Do you know any easy method to measure the 14KV at the anode with a generic multimeter? A 1:100 divider would be ok?

author: w2aew
author_rank: Super Contributor
timestamp: May 27, 2015, 01:45:09 pm
content: Quote from: darkness_electro on May 27, 2015, 11:18:20 amAbsolutely NOT! It is unlikely that you'll be able to put together a 100:1 divider with enough voltage rating to be safe! This is dangerous stuff, and you really need to be using the right equipment for it. At the very least, it can kill your meter. At the worst, it can kill you!Are you sure it is a Z-amp problem? Is there a beam finder on the scope? Maybe there's a deflection problem which is pushing the beam off the screen...FYI - here's a video I did on High Voltage probes:

author: Grapsus
author_rank: Regular Contributor
timestamp: May 27, 2015, 02:41:17 pm
content: Quote from: darkness_electro on May 27, 2015, 11:18:20 amSorry if my previous message somehow implied that you have to measure the anode voltage. You don't have to, it's way too dangerous, but the other CRT signals can be checked if you're careful.Apart from all safety considerations, 1:100 divider won't work. If you want to measure 14 kV, the total resistance must be very high in order not to load the high-voltage circuit too much. For example, with a 100 megaohm resistor the current drawn would be 0.14 mA and the power around 2 Watts! So 1 gigaohm (14 uA and 0.2 Watts) would be an absolute minimum and with 1:100 ratio that would imply a 10 megaohms sense resistor, only 10 megaohms is the resistance of a standard DMM! Therefore a passive probe for measuring 14 kV with a 10 Meg DMM should be at least 1:1000 (a few percent droop) or better 1:10000 (practically no droop).

author: darkness_electro
author_rank: Contributor
timestamp: May 27, 2015, 02:54:07 pm
content: Indeed, it has a beamfinder button, but still no trace. The only time it shows something is when powering off, it's a green beam doing wied stuff. I will check the pins sugested. Probably sonething broke when the transistors were shorted.

author: w2aew
author_rank: Super Contributor
timestamp: May 27, 2015, 03:31:52 pm
content: Quote from: darkness_electro on May 27, 2015, 02:54:07 pmIf you're getting a green beam that "does wierd things" when you turn the scope off - that tells you that the CRT is likely OK (apart from possible deflection plate damage), and that the HV is ok. It might be a deflection or triggering problem that is keeping the beam from sweeping or being pinned off-screen somewhere.

author: darkness_electro
author_rank: Contributor
timestamp: May 27, 2015, 03:47:48 pm
content: That's what i was thinking. Seeing on the schematic that point 1 ( in a circle) has a connection to the circuit that supplies the filament i belive that the problem is in this general area. Some other transistors (TO3) get awfully hot after a few minutes makes me belive that there still is a short or something wrong in yhe supply section. Also, is it normal to have 0V voltage drop on the 1k resistor R175?