14" Matsushita TM-153 CRT Flyback Testing? - Page 1
Forum section: electronics / repair

author: NivagSwerdna
author_rank: Super Contributor
timestamp: February 25, 2021, 10:49:30 am
content: I have a very old CRT monitor in my arcade table...14" Matsushita TM-153https://www.arcade-museum.com/manuals-monitors/Atari%20Monitor%20TM-153%202nd%20Printing%20Matsushita%2014in%20Color%20Raster%20.pdfThis is the type of monitor you might have seen in the 1980s with Mission Command and the like... very much associated with Atari (although that's not what my game is)Anyway the monitor is dead and I have replaced the HOT and some caps and now rather than blowing fuses it (the chassis) makes a terrible screeching sound that can be heard two rooms awaySo I feel I should test the flyback somehow? (I really don't want the flyback to be the problem as they are unobtainium and if that is the problem the CRT is destined for CRT heaven)I don't have a "flyback winding tester"... but I do have a DE-5000 and a DMM... and a scope and a sig gen....How would I test it to see if we are really at the end of the roadThanksPSCould there be a non-flyback related cause to the problem?

author: ambrosia heart
author_rank: Regular Contributor
timestamp: February 25, 2021, 12:28:14 pm
content: use oscilloscope and signal generator to test it.You can search for Ringing test transformer or flybacktransformer for further details.

author: CJay
author_rank: Super Contributor
timestamp: February 25, 2021, 01:12:25 pm
content: Quote from: NivagSwerdna on February 25, 2021, 10:49:30 amYou need to 'ring' the transformer as already mentioned, it's a pretty simple test and you can even throw together a tester for not much money but as a one off it's probably not worth it.It is entirely possible it might not be the LOPTx at fault, it *could* be a tuning cap, one of the high voltage ones in the HOT/LOPTx circuit, it could the the LOPTx as you fear, worse still it could be the scan coils. Even worse, it could be all three.LOPT tester here:https://worldradiohistory.com/UK/Television-Servicing-UK/90s/Electronic-Servicing-1998-02.pdf(You don't need the TBA920, anything that can generate line frequency and enough current to switch the transistor will do)

author: perieanuo
author_rank: Frequent Contributor
timestamp: February 25, 2021, 02:24:09 pm
content: yes, may be power supply relatedtest the ps by disconnecting flyback and using some load like a 60w bulb, if the screeching sound dissapears, then you ps is good and enters in current limitation due to bad flyback transformerwhen i was repairing such monitors, usually when the fbt was dead (short between loops in primary for example), the flyback transistor driving the fbt was dead before hearing any sound...so my bet is on collateral flyback problems or ps(power supply)hope i'm clear enough

author: fzabkar
author_rank: Super Contributor
timestamp: February 25, 2021, 06:46:39 pm
content: You can ring test an FBT by driving the windings with the capacitance test function of a DMM. Monitor the other windings on a scope,

author: NivagSwerdna
author_rank: Super Contributor
timestamp: March 30, 2021, 01:31:40 pm
content: I have been trying to build a Ring Tester based on the design herehttp://bobparker.net.au/lopt_tester/ringer_instructions.pdfwith very limited success.I think the principle of operation here is that C4 is charged to 2V and then Q1 is driven to short the +ve side of C4 to GND and the C4 and the DUT form a series LC circuit which oscillates around a bit...I think D2 and D3 are intended to clamp the output voltage going into the high impedance part to 6V8...Anyway with a random test inductor I had lying around I am not getting many oscillations...My test inductor is...https://tscomponents.com/9856.pdfMaybe I problem is that it has too high an inductance for this test? i.e. it can be 10H and then f0=1/2pi*sqrt(LC) means it just oscillates too slowly?What would be a good test inductor that is representative of a flyback primary?

author: andy2000
author_rank: Frequent Contributor
timestamp: March 30, 2021, 04:39:37 pm
content: A simple ring tester doesn't have to be that complex. Just connect your signal generator (low frequency square wave) across any winding through a resistor (value isn't critical, 1 k should work). Monitor the signal on the the primary winding with your scope to see if it rings. Stray capacitance is enough to make it ring. With a good flyback, even a single shorted turn will kill most of the ringing. You can simulate that by wrapping a piece of wire around the core and shorting the ends together. Obviously, do this with the flyback completely disconnected from the monitor.

author: NivagSwerdna
author_rank: Super Contributor
timestamp: March 31, 2021, 05:14:51 pm
content: I got it to work... the flyback transformer from a PSU have lying around rings 68 times (>5mV) and if you short the secondary that falls to near zero so looks like I have a tester!The original is shown here...and should be available here...https://evbesrmeter.pt/ring%20tester.htm

author: megajocke
author_rank: Regular Contributor
timestamp: April 13, 2021, 08:07:58 pm
content: Screeching, assuming it comes from the monitor, sounds like it's either operating at a too low horizontal frequency or cycling on/off for some reason.Have you checked the regulator Q801 after the HOT blew? If it is shorted the power supply voltage will be too high and it should shut down or possibly cycle on/off (which might sound like a screech).Could there be some bad component or solder joint around the control IC?Or maybe open circuit? A bad connection or bad component related to the horizontal deflection yoke could also mess things up.What is it powered from? Mains frequency isolation transformer or SMPS?I helped a friend repair the monitor in a Defender cabinet where the HOT and power supply regulator transistor had blown. Actually it blew up while I was playing the gameAfter replacing the HOT I powered it up with a lab power supply feeding its regulated ~110V rail but observed abnormal consumption of almost 100W and a funky stretched-out waveform on the collector of the HOT. Quickly shut it down.The problem in this case was a shorted film cap in the horizontal deflection circuit. Verified operation on a lab power supply after replacing the cap, now drawing about 30W. After replacing the power supply regulator, actually a hybrid in this unit, it worked fine.Bad guy:Testing waveforms:Diff probes to measure HOT Vce and Vbe (Vce is about 1 kV pk so a normal scope probe won't usually stand this).Works again

author: Renate
author_rank: Super Contributor
timestamp: April 15, 2021, 06:39:59 pm
content: Step #1: Look at the base of the HOT and see if it's a reasonable signal at a reasonable (and steady) frequency.Oh! You do have this on an isolation transormer?!?!?

author: NivagSwerdna
author_rank: Super Contributor
timestamp: April 15, 2021, 07:43:05 pm
content: I got rather sidetracked, as I tend to do, and built a clone of Bob's tester...I haven't had a chance to get back to the monitor yet but when I do I will extract the flyback for my testing

author: NivagSwerdna
author_rank: Super Contributor
timestamp: April 16, 2021, 08:27:39 am
content: Quote from: megajocke on April 13, 2021, 08:07:58 pmIt is attached to the cocktail arcade table which is a japanese import (from 1980s); the table has a very large transformer which takes 240V mains and outputs 110V to the monitor. I have been considering this as an isolation transformer although I haven't been brave enough to probe anything when turned on yet. I am not used to anything above 12V so trying to be very careful!

author: NivagSwerdna
author_rank: Super Contributor
timestamp: April 16, 2021, 09:45:51 am
content: One thing i did notice is that only one of the capacitors near the HOT is populated... I pretty sure it isn't shorted (because I measured across the HOT when I last had the monitor out) but it is weird how there are multiple caps on the schematic but only one in the board... however the solder looks like it is original.

author: Renate
author_rank: Super Contributor
timestamp: April 16, 2021, 12:28:59 pm
content: Quote from: NivagSwerdna on April 16, 2021, 09:45:51 amI've seen this often.The capacity affects peak flyback voltage and retrace time.Some multisync monitors even had relays on extra caps.

author: megajocke
author_rank: Regular Contributor
timestamp: April 18, 2021, 10:36:04 am
content: The capacitors being like that make sense for the reasons Renate mentions. Either to allow for options on horizontal sweep frequency or maybe to allow using the same board for other CRTs/screen sizes. If it was shorted it would be blowing fuses.It makes sense it is powered by a transformer in the game. Without isolation it would have been difficult to get the video signals from the game to the monitor unless the whole game circuitry were live which would be unusal.An easy non-powered measurement is to check Q801. If the HOT was shorted and it was blowing fuses, this regulator transistor would be very likely to also have died. It should read 150 ohms across C-E because of resistor R803 in parallel. If much lower it has also shorted and needs to be replaced.You could check resistance across C516 also which seems to be the main flyback capacitor.The base drive signal to the HOT is a good measurement like Renate says. This is low voltage and can be checked with a normal oscilloscope probe. The sawtooth at pin 4 of IC401 could also be interesting to check. There are lots of waveforms in the service manual. The ones marked (H) for horizontal frequency are interesting.Except for the voltage at the HOT collector and horizontal deflection yoke (1000 V pulses), a normal oscilloscope probe should work fine.