Hi all,

We have a mx4000 that turns off all on its own!!! as it gets hoter it turns off more regualy. has anyone got any idears on how to fix it.

 Many Thanks Paul

Hi Paul,

replace the capacitors in the power supply. That's a standard fault after this long time...

Martin

I would be happy to take a look at it if you can get it to me.  I am in Farnham in Surrey.

Regards, Neil
 

Hi,

My MX4000 just went dead  - after it has been turning off randomly for a few days. Now there is not even a stand-by light. I can hear some noise from Transformer T2 when switching on the TV - that's all.

I heard that C29 and C31 was the ones to check in the power supply, but they are ok in my set. I have alse checked C102, C103, C61.

Does anyone know what to check next?

Regards,

Peter

 

Peter,

Whilst capacitors can be a problem and I know a lot of knowledgeable members suggest this as their first action, but given you say that even the Standby light is not on, which means that it is likely that you do not have the 300 volt line at pin 5 of T2 or it is low. I suggest, that being very careful you check the voltage across C9 which should be around 300 Volts with a multi-meter

If the 300 volts is OK, then I would turn to the Oscillator circuit around TR4 & TR5 which must be running for any other voltages to be generated via T2. There is an overload protection circuit here which stops everything if there is any kind of overload. Check the collector pin of TR4 where it joins R19 and R21 which should be about 2 volts in Standby, in normal running it will be about 4 volts and if it is 7 volts or above something is overloading the power supply.

These are difficult circuits to fault find because they require feedback and also +/- 5 volts to be generated to keep the circuits working, so even if a component is marginal it can stop the circuit working.

Do remember, if you are not experienced with electronics or the high voltages associated with any TV, then leave it to the repair shop.

Hi Keith,

Thanks for your suggestions. I have 330V on pin5 of T2 (measured from pin2 which is GND, I guess). Across C9 I measure 320V.

On Collector of TR4 I have the 2V. So everything seems ok.

However, you mention that marginal components can stop the circuit and actually I measured C31 to 80µF in stead of 100µF - so my next step will be to replace C31 and C29 with new.

Is there anything else you can think of then please let me know.

Thanks,

/Peter

 

@Holly: i usually start with checking the 6,8 nF cap across the HT transistor, sooner or later the soldering of these go bad ending with a short circuited HT transistor.

Thanku Guys

Will have a look and report back.

Thanks again Paul

Hi Paul,

Yes, the voltage across C52 "+8v st. by power" is only 2V.

Could you please tell which one is the HT transistor and maybe also the numbers for the 6.8nF caps you're mentioning?
I don't have a circuit diagram, so I'm reading the numbers directly from the PCB.

I just measured the T2 primary side which is:
Pin1,2,3,4 = 0V
Pin 5 = 330V
Pin 6,7 = 750V
Pin 9,10 = 143V
Does this make sense?

Thanks, Peter

Peter,

Sorry for the delay in getting back to you on your problem.

Well, your feedback looks fine, however the key piece of data you stated at the begining of the thread is that the "Standby" light is not on, so we need to focus in on that.

So, check the following:-

• On module 15 check the voltage across C1 and also check the 315ma fuse, the voltage here should be about 8 volts. If that is OK then
• Check the voltage across C3 on the same module which should be 5 volts, if that's OK then
• Check the voltage across C80 on the Microprocessor module which is PCB 6 which should be 5 Volts

Then, Peter I suggest you provide feedback of the results and I think one of the above is going to be missing

Thanks for taking the time, Keith,

The voltage across C1 on module 15 is 2 VDC (3.8V AC!!). The fuse is ok. C3 has 0V.

I upgraded my profile so now I have the diagrams (not that it helps ). However I measure some strange voltages, I think. For instance:

•  on top of C9, module 4 I have 300VDC as it should be, but also 737VAC?!
• Across C29 I measure 5.7VDC (ok) and 12VAC (?!)
• Across C31 I get -5.8VDC (ok) and 0VAC (ok).

But I might be fighting two things, because first the set just automatically went into stand by mode and after several on->auto st.by->on->auto st.by also the st.by LED switched off and everything was dead. So I might also have a PowerFailure to deal with...

Regards, Peter

Peter,

You may be correct in having two problems, but I am focused on the lack of "Standby" light and it is clear from your voltage results that having 0 volts across C3 is the reason you do not have the standby light and why it will not switch on. The voltage across C1 needs to be 8 volts to allow the regulator IC1 to produce the 5 volts.

So, we need to track back from the lack of 8 volts at plug P79-1. This comes from the Power supply module PCB 4 at plug P13-1.

Given that the voltages across C29 & C 31 are correct means that we are starting to narrow it down to around D35 & D36. I suggest your next steps on the Power Supply module is to check the following:-

• Measure the DC voltage across C53 it should be 8.5 volts
• Measure the AC voltage between R58 and ground. This goes into a complexed Power fail circuit, so if this is less than 5 volts AC, then you may wish to un-solder one end of this resister, as this power fail circuit may be pulling the line down.
• Test the two diodes D35 & D36 on a meter with the power disconnected

 

Keith,

The mystery continues

• Voltage across C53 is 0.9V DC
• AC voltage R58<->GND is varying 6 V - 9V. This is the same when I removed C50 (R58 is difficult to get to)
• D35 and D36 seems to be working as diodes

...I'm begining to loose hope, here. Anyhow, I need to pause my work for a couple of days since I will be traveling.

Regards,

Peter

 

Hi Peter, don't bother making things more complicated than they are: the HT transistor is easy to locate, mounted on the metal piece next to the HT transformer there are 3 transistor looking components: 2 of them only have 2 legs (they are diodes) and the biggest one is the HT transistor.

Great post ipaul!

Hi Paul,

You are right!! the HT transistor (which is called TR33) has a short circuit. Type is btw BU 2508 AF. I will hunt one of these down and let you know how things come along after I replaced it.

Thanks for the detailed explanation  

Regards, Peter 

YES, YES, YES, now it works.
 

Thank you for great support.

I bought the BU2508AF at RS Components (3 euros).

Regards,

Peter 

Peter,

That's great, well done...