hello

I read an inspiring post about a dead mx4000 with a happy ending (http://forum.beoworld.org/forums/thread/91170.aspx)

and thought that I should have a go at a similar machine with a similar problem. Its a mx4000 beo4 and it says "type:78/34 EDT" at the back.

symptoms are: no respons when connected to the mains - no standby light, no humming sound or anything like it.

i measured 230V AC over P20-1 and P20-2 when connected - so its not a broken power cord :) - but 0V over C9 (supposed to be 300) and 0V between pin 1 and 6 on T6

 if you have any clue or general tip - please respond!

many thanks

arvid

ps this might be a newbie question - while disconnected from mains I measured resistance between the C and E pin of TR33 (since it was the solution to the problem in the post I referred to) and got something like 600kOhm. A high resistance, but still not totally separated - would this mean that TR33 is broken?

Hi,

if there is 0V over C9, then the fuse  F1 ( T3.15A) is blown, otherwise the filter T6 is blown or the diodes D1-D4.

I think, it's the fuse.

... but if this fuse blowes, then there is a severe bug in the power supply. Usually TR1 BUT12 is defect, caused by a cold spot around C12 6.8nF.

 Check the fuse and tell us more

Martin

thanks for quick reply!!

fuse was blown, diodes D1-D4 ok (is it easy to check the filter?)

next i'll check TR1 and C12

is it plausible that the fuse blows without any other faults?

 thanks again!

hmm, 0.6 ohms between E and C on TR1 - that means that it's short circuited, right?

Certainly is. Shouldn't show much of anything (you're measuring it disconnected, right?).

While you're there, check the small transistors that are driving the BUT12 (can't remember their numbers). In my MX6000, the BUT took a couple of them along when it died...

yes, im measuring disconnected

went through TR1-TR14, so far TR1 and TR11 seems to be out - the rest ok. How about capacitors? Is a broken cap the probable cause?

 thanks once again for all the quick replies!

 arvid

btw i dont think i can measure capacitance with my cheap DMM...

is there any other way to check if a cap is working as it should?

Probably TR1 is dead, if you measure around 0.6 Ohm E-C ... so, replace BUT12. Very classic defect.

TR1 fails either by overvoltage on your powerlines or, much more likely, that C12 or some parts around this cap have a cold spot.

TR1 is part of the switch supply, C12 and the two coils/transformator. If one of these parts fails, TR1 conducts permanent or is permanent open. If TR1 conducts permanent, TR1 will overheat and blowes the fuse... and the LED will not light.

Replace TR1, C12 6.8nF, C29 and C31, 47uf and 100uf, that should be the first step. You could use a 220V lamp instead of a fuse... if it lights very bright, you have saved a fuses life and probably also TR1.

waiting for parts to arrive... i borrowed a better DMM - i can now measure capacitance. is it correct to assume that measurements are unreliable when in circuit? C31 gives 43.5u instead of 47u. could that be the cause?

No... a defect cap looses some capacity, but they start working as a resistor, conducting currents ! This is the reason why transistors fail, the cap starts working as a resistor, the transistor conducts a little bit, becomes too hot and fails.  

DO NOT MEASURE A CAP AND USE IT AGAIN ! Always replace it... saving money by using a used cap again does not work, literally

ok, ive replaced TR1 and TR11 - both where dead, and checked all the caps around them (next time ill replace them instead, promise!!)

so here is my dilemma:

i can borrow an oscilloscope from the laboratory im working at and do the 5V test (described in the service manual) but this will take some time and effort.

 or

just do as you suggested earlier and try with a 220V lamp instead of a fuse. if there is some defect that i missed, will TR1 and TR11 blow instantly?

 

 ps - found this nice post just a while ago - would have saved me some effort earlier on :)

http://forum.beoworld.org/forums/thread/78511.aspx

thanks again for your time and knowledge!

i choose the toilsome way and hooked everything up at the lab... 

i also replaced C29 and C31.

strange thing is that when connected to a 5V DC supply (as according to the service manual) i dont get the described oscilloscope readings at any of the measuring points 1-4

i only get various DC-readings across the bord.

anyone have a clue?

success!! 

i never got the right oscilloscope readings, but decided to give it a go anyway - and it worked! (must have done something wrong with the scope...)

well, thanks to the wonderful knowledge gathered in this forum i got a working mx4000 for 20 euro including spare parts. I also found a partly defect beolink 1000 for 15 euro that i managed to fix.

thanks again everyone!!

arvid