Barco CRT Monitor doesn't turn on

[decoder_82]

Hi,

I recently repaired a Barco monitor (https://www.eevblog.com/forum/repair/barco-crt-monitor-making-noise/)
It worked well for maybe 50 hours spread over several days but now when I switch it on, nothing happens.

Service manual can be found here: https://archive.org/details/manual_DCD2740PAL_SM_BARCO_EN

What I have checked:

- Fuse are not blown
- I only have voltage on the primary side of T1

Then I looked more carefully at the TDA4601 IC:

- pin 1 Vref 4.2v
- pin 3 overload amplifier 2.1v
- pin 7 DC output for coupling capacitors 1.5 v
- pin 9 supply voltage 10v

More info on the IC Here: https://pdf1.alldatasheet.com/datasheet-pdf/view/45811/SIEMENS/TDA4601.html

It mentions "current based shutdown is activate pin 7 when 1.6V". Does it mean I should look for a short in the circuit after T1 ?

[Uunoctium]

my procedure would be:
check housekeeping-voltage at IC pin 9
check coupling C at base BU208
check for secondary short by opening the DC-line +210V and applying proper load. At no success, open all secondarys but remain load (bulb)

[shakalnokturn]

I second all Uunoctium's advice.

Many SMPS faults will show as a "hiccuping" PSU (several possible causes), if no adequate scoping method available I'd at least making ripple voltage measurement on TDA 4601 power supply pin.

[decoder_82]

Alright so I have disconnected D1 and plugged a light bulb. It glows when I turn it on. My guess is that I can rule out a short on the primary side.
What is the best approach to identify shorts? I have checked usual suspects like the HOT.

[decoder_82]

Something strange the bulb is glowing but I measure 0 volt DC and 200 volt AC...should I have +210 ?

[Uunoctium]

direct at transformer out it's 15...25kHz AC. After rectifier & capacitor it should be +210DC.

But all in all this are good news. PSU is running. If you did not disconnected the lower Voltages from transformer, they are working and should be measurable with their default values. So the fault is in horizontal deflection / HV stage.

I would further check BU508A (Q52), output transformer (T51), cascade & blue 1.5kV polypropylene Capacitor (C64? 1100pF).
Opening collector Q52 with bulb installed lets you observe correctness of pulses at Q51/T50/base of Q52

[decoder_82]

I checked BU508A (Q52), output transformer (T51) and the capacitors in the area but did not find anything so far.
What do you mean by "Opening collector Q52 with bulb installed" ? Can you show me on the diagram where you would plug the bulb?

[Uunoctium]

i suggest, that the load (bulb) remains in PSU to make sure, PWM primary IC is in its duty-cycle window. With no load and overload it immediate went in shutdown.
So resolder Diode D1. Maybe with all secondarys reconnected the load (bulb) is better placed in 40W region instead of 60..75. Main load in CRT set is cathode current plus deflection (~60W). If you disconnect horizontal stage (with open deflection-transistor) for measurement, PSU turns off. So an minimal load is a must.

I read your previous post and the issue with arcing. My first impression tends to that FKP Capacitor wich is essential (value!) for picture width and HV-tension. So plase check for deformation, coloration and inspect his solder joints. In doubt, measure it

A quick check for an faulty cascade: set unit to original state (without additional load) and unsolder wiring to cascade. Unit should run - of course without picture

[decoder_82]

Thanks for the explanation. Looks like BU508A (Q52) was faulty. I will order a new one and post update here once replaced. I only noticed it was shorted once removed. I guess I oxidation was making bad contact when I initially checking.

[shakalnokturn]

They occasionally fail without apparent reason, most of the time there is a cause though. In my experience about 60% weak solders, 35% bad HV transformers, 4% capacitors or other transistors in horizontal section. (The remaining % is voodoo...)
Check solders in horizontal section including base drive transformer.