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Old 24th Dec 2021, 5:03 pm   #1
gridrunner
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Default BO MX3500 repair query

Hi everyone,

I've got hold of an MX3500 with a known fault. Initially it was reported that the set wouldn't come out of standby (the red light is illuminated) to now, no red light and the set appears dead. The remote is now working after I cleared the corrosion from the battery terminals.

I've had the back off and the fuse is still intact.Some poking around showed a capacitor (C12) to have leaked on the board and the nearby deflection transistor (TR8) was short circuit. The transistor is a BU508AF. In my spares box I have a BU508A. Is there any practical difference? The package is slightly different as the BU508A has a metal tab on the rear.

In general, has anyone seen this fault? There's definitely a fault here that would prevent the set from working, but I was surprised to see the fuse intact. I've replaced the capacitors in the power supply module save for the two metal canned beasties.

One observation is clearly that this chassis gets hot in use, the nice 'petals of a flower' arrangement of the modules doesn't make for good air flow!

thanks..
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Old 24th Dec 2021, 6:16 pm   #2
nigelr2000
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Default Re: BO MX3500 repair query

508AF has an insulated tab the 508A has a metal tab connected to the collector so not ideal to mount in this one as it would short the PSU to earth
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Old 24th Dec 2021, 7:09 pm   #3
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Default Re: BO MX3500 repair query

Yep there are some differences on the spec sheets so I’ll order the correct part and put it aside till the new year. Happy Christmas everyone.
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Old 24th Dec 2021, 8:55 pm   #4
its ur aerial
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Default Re: BO MX3500 repair query

If the Line O/P Transistor is S/C it will shut down the switch mode power
supply, so is very unlikely to blow the mains fuse .
I personally would also wait till I had the correct BU508AF.

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Old 31st Dec 2021, 4:31 pm   #5
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Default Re: BO MX3500 repair query

Hi everyone,

I'm getting that 'out of my depth' feeling at this point in the repair.
I replaced the leaking capacitor that had caused the HOT to go short and fitted the correct part for the HOT.

This got me one step along the road with a red standby light as previously the set would not get to standby, however the TV won't come out of standby. The LED goes green and then immediately back to red. I had been following the repair guide on here:

https://archivedforum.beoworld.org/f...px?PageIndex=1

which suggests looking at the 12V supply and R112 and D25. I can't seem to find anything wrong here, however by this point the plastic frames that hold the various boards together are disintegrating in my hands there's a whole lot of wiring and modules strewn across my bench. It's all quite intimidating compared to a single board chassis.

Does anyone have any clues as to what to look for for an MX3500 that won't come out of standby?

thanks!
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Old 31st Dec 2021, 5:16 pm   #6
Maarten
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Default Re: BO MX3500 repair query

Is the +B getting to the LOPT and HOT? There might be a fusible resistor that went o/c.
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Old 11th Jan 2022, 8:29 pm   #7
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Default Re: BO MX3500 repair query

Hi all,

Just a quick update on this repair.

I've now got a working TV which is good news.

As well as suffering a failed horizontal output transistor (this was dead short across all pins and was keeping the power supply shut down), the capacitor responsible - an Elna 220uf capacitor, had a twin that had also leaked all over the Nicam board. I replaced that and also changed the diode packages in the PSU (there are two in heat sink mounted TO220 packages).

Check the boards for broken or loose joints especially on the chassis. All the non metal canned electrolytics in the PSU were changed.

Time hasn't been kind to these sets. The plastic frame that holds the modules together completely disintegrates at first disassembly and so cable ties have had to be used to bind it back together. The speaker surrounds perish and there's clearly a heat dissipation problem owing to the chassis design.
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Old 11th Jan 2022, 8:31 pm   #8
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Default Re: BO MX3500 repair query

Oh, also to mention that some service guides suggest inspecting and resoldering the joints to the deflection coil on the CRT, which I also did.
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Old 11th Jan 2022, 9:14 pm   #9
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Default Re: BO MX3500 repair query

Happy you got it working but wont lie a part of me is sad that I didnt get to nick the Tube from it to restore my 51K5. The search continues...
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Old 12th Jan 2022, 2:30 pm   #10
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Default Re: BO MX3500 repair query

Hi Luke, I'll PM you about this. The chassis is pretty banged up. The tube btw has this label on it..
Attached Thumbnails
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Old 12th Jan 2022, 6:49 pm   #11
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Default Re: BO MX3500 repair query

Quote:
Originally Posted by gridrunner View Post
Hi Luke, I'll PM you about this. The chassis is pretty banged up. The tube btw has this label on it..
That tube will do perfect in my 51K5
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Old 12th Jan 2022, 9:13 pm   #12
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Default Re: BO MX3500 repair query

Hi.
Even while I was a B&O dealer this model was getting to be troublesome in a problematic way. Heat was often an issue, even worse if the set was close to a radiator or ran for hours on end, caps leaking and PCB issues with carbonised sections.
Tubes Incidentally tend to go down too on well used sets, a careful slow boost though works better than a full hard reactivation.
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Old 13th Jan 2022, 12:56 pm   #13
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Default Re: BO MX3500 repair query

I too as a Dealer,sold many of these and other's in the range that used the same chassis.
Being like most B&O designs,a sophisticated chassis,it was rather "marmite" among Engineers!
I liked them,they are a joy to work on,with the fold out modular arrangement,giving good access.
Still capable of good results too,given an external digital source !
I agree that the plastic chassis build has'nt lasted well though.Pcb quality was alway's an issue,needing careful rework when replacing in particular, LSi chips.
Super picture quality and the novel function of being able to use the tv as an audio amplifier.
Designed to be a component in a B&O integrated av system,the tv could be used with say,an ipod or suchlike in it's audio only mode,ie crt off.
Nick
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Old 25th Jan 2022, 11:03 am   #14
murphyv310
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Default Re: BO MX3500 repair query

We were only a service dealer until B&O pulled the plug on us as we didn't spend £250,000 with them annually. The snag was we did many older sets the main outlets wouldn't touch. We saw some pretty bad sets in our time and I was glad to see the back of them, pardon the pin.
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