Hi,

my beomaster 5000 has decided to become 'uncooperative'

The sound output is very (very) low, and this is regardless of source, either radio or CD(50). It sounds like the power amp section is not working...maybe.

Turning sound up to full volume still only produces a whisper, only it's distorted as well (probably the pre-am is clipping...but this is a guess).

This occur with either MCP or the front panel switch. In fact when using the MCP the disply correctly reads out the higher level.

And yes, I have the speakers connected to the non-relay output ;-)
(those relays died a long time ago).

I am reasonably handy with electronics, but totally self taught (so I have big blind spots). I have succesfully repaired a CD50, CDX, Beovision8000 and changed this BM5000 from 110 to 240volts (with parts from Dillen), so I can 'follow instructions easily.

Can anyone help...please?

Philippe

I suggest you contact Tim Jarman on www.beocentral.com as I am sure he will be able to help. If you are in the UK and can gat it to him he will fix it for you.

Check the metal links between the line in and line out RCA sockets are still present. If the sockets are not connected the preamp is disconnected from the poweramp and will cause the symptom that you describe. If they are missing, you can use a short stereo RCA cable in their place. If the links are not missing then a repair is needed.

 

OK,
I've done a little bit more work on this. ;-)

After renovating the caps in both the power amp and pre-amp sections, as well as replacing all output darlingtons, and the trim pots....no change.

I tried connecting the pre-amp output to another power amp:

Same result, no sound until level reaches about 4.8 on display, then any increase beyond that becomes increasingly distorted. This is in both channels.


I'm begining to wonder if this is actually a power supply problem, and the pre-amp is simply clipping because it has no supply? (both channels are failing).

Can some-one please help?

Just an idea of where and what to measure in the power supply board, and if there is a supply dedicated to the pre-amp section only?

I've had a few 'brown-outs' in the mains power recently, and the BM had to be unplugged and replugged to re-start. Could this have precipitated the failure?


Every thing else works on this  tuner-amp, and I'm enjoying the connectivity with the CD50. It would be a shame to have to revert to a Sony ;-)


Thanks

Philippe

BTW, I'm no technician, just keen DIY, collector of classic gear.

This could be due to the "muting" system being stuck "on".It's designed to mute the sound between functions,and when tuning between stations.It's controlled by the computer and interface chips.

Philippe,

Did you also replace the caps in the power supply circuits ?
I am thinking about a specific capacitor, 22uF if my memory serves me right (also part of the capacitor kit).
I don't have the manual right here but I remember it as being
orange in color, essential for all digitally controlled
sound settings and located in the power supply area, beneath
the two large cans.

There's a handful of caps in that area, most are black, replace the lot if not done already.

Martin

Thank you for looking in solderon29 and Martin

Martin, yes I have replaced all the caps that were in the kit, and that included a 220uf (radial) in the power supply. Is this the one you're thinking of?

I have done some further testing, and looked up the chip data on the net (TDA 1074, 1028, 1029, and the scary MM5450, which may control the mute).

This is the series of tests and results:

used the 'line out' terminals to drive an external power amp. Sound is clean, and relatively strong (loud). I would say its OK.

used the 'line in' terminal to inject CD player output. fault persists, low volume and distortion when level raised to 'audible' level. Reading 4.8 on the front panel before any sound is audible, then distorts at around 5.2.

 used the 'pre-amp out' terminal  to drive an external power amp. fault persists, need 4.8 before anything is heard in the speakers, then distortion at level 5.0

From staring at the circuit board for a while, it appears to me that the above test places the fault between the 'line in/out' and the pre-amp output, that is TDA1074 (twice). This is an electronic potentiometer, so it looks promising.

What bugs me here is that the fault is in both channels, yet there are two ICs (left and right?) so its seems unusual that both should fail at the same time.

according to the Philips data sheet, pin 11 is the power supply, and they recommend 20 volts. I have 20 volts, so this is not it.

also pin 8 is the reference, and should be 1/2, I have 10.6 volts, which is close enough.

Now I am out of my depth....

I have one last test to try, to run the power amp section from an external pre-amp, just to confirm the problem really is in the pre-amp, but I need to make a 'connector'. Not till next  w/e, I have to go to my real job in the mean time

Does any of this make sense ?

solderon27 where do I look for the 'mute circuit' ? (although would it not just kill all sound ?)

Thanks again, It's nice to know there are others to look over your shoulder at times like these.

Philippe

It does seem as though the interface chip ic5 is at fault,as both channels are affected.It provides the volume control pulses which are converted inside the volume control chip ic3 to a varying dc voltage.If you use a logic probe(available cheaply from hobby shops)you should be able to see a pulse at pin 9 of ic3 when you alter the volume.If it's there ,it's got to be that chip thats dud. If it's not there,it's got to be ic5.MM5450 is a 40 pin ic,so quite fiddly to replace,but ok with patience.Available in the U.K.for about £5.

Nick    

Thanks Nick, I'll get a logic probe tomorow and get back.

Philippe

OK,
got a logic probe, and barring my unfamiliarity with using such a device, I found pin 9 reading constant 'high'. Certainly no pulse.

replaced all 3 100nf decoupling caps, just in case, no change

So I replaced TDA 1074, (IC3) no change.

then replaced MM5450 (IC5) no change.

then I substituted IC4 with the old IC3 (same TDA) still no change.

Then on a guess, replaced IC3 in the power supply, the 20 volt supply LM317 no change

Pretty soon Ill have a 'new' BM5000

Seriously, I'm running out of options?

Cerainly out of my depth...but I can't help thinking its something simple.

The fault was sudden,  one day OK, the next not.
The range in the volume control is narrow approx 4.2 to 5.2 on the display. Less than 4.2 its inaudible, then more than 5.2 and it distorts.
The sound is distorted like a 'crackle, with the base line most affected. (like blown out speaker cones).

???

Philippe

Excuse my ignorance, and I appreciate that I am talking to people far more electronically gifted than myself... but could it possibly be a short in the external circuitry (eg either chewed cables, a stray wire strand at speaker terminals, or internal within speaker)? That would cause the clip circuit to remain "on" would it not?

Very best of luck, Philippe!

Alex

Interesting case and definitely one I would have enjoyed having on the bench, if for nothing else than curiosity and the challenge. 

The 20V supply is essential. Check that this is present and clean at the preamp board. There's a 5V and 28V as well, they also need to be present at the board.

Martin

Your tenacity is admirable,but this is getting to be a real puzzle!

Alex has a point about shorted speaker wires,but this would not affect both channels surely?

I assume that voltages around the power output stage are correct?

I'm still suspicious of the pre-amp/volume control stage,particularly as you have no volume control pulses?

I'll break out the ice bag and study the circuit for further inspiration.

Nick

Hello Nick, Martin and Alex
thank you for your words of encouragment.

I still have a few things to try, like being more systematic with
the power supply measurements, so far I've concentrated with the pre-amp as
this seems to be where the symptoms are, but they may be only the victims of
some other failure. (hard to imagine, as all the supply levels to the IC's
read 20volts?).

Also, as I've mentioned above, we've had a few 'brow outs' lately, maybe the BM was damaged then. I'll have a look around the 'standby' circuit, and see if anything is amiss there.

I've also ordered a TDA 1028 and 1029, as these look like they deal with the
signal also. Lots of IC's to interact.

I'll try a few more things in the next few days and post further on the
board.

Thanks for the encouragement.

Philippe

It might be significant that you have had power interuptions.There may be damage to the pcb which explains why you have 20volt seemingly everywhere on the preamp panel?

The preamp/control section has single ground return to the power supply.Connector P11,pin9 on the preamp panel should connect to connector P9,pin 6 on the power supply panel.P9,pin 6 of the power supply panel,should then connect to the common ground,ie the junction between C6/C7(the two large capacitors that clip into the plastic moulding)

There is a section of print that can rupture in case of flashover,it could be the cause of all these problems?

Good luck

Nick

Check that there is continuity between P11,pin9 on the preamp panel,and the junction between 2C6 and 2C7.These are the two large capacitors that clip into the plastic moulding over the output stage.It should read zero ohms,as it's the ground return for the preamp.If not it may mean that the print is ruptured somewhere,due to flashover and could explain why the preamp voltages are odd?

Nick

OK, I'm still waiting for the TDA 1028/9 to arrive, so I've been looking around some more.

All voltages into the IC are correct, so 20 volts supply seems OK.
I replaced IC1 in the power supply, but this made no change. Amp still distorts.

One last thing I've just discovered, and may be related (How?).

On the tuner board TR34 is too hot to touch.


I see this is a large transistor, BD533, with a metal tag, but it does not seem right to me.

Now how this relates to anything, or maybe it's OK?

I'll update as I find more.

Chers

Philippe

OK

it's finally fixed!

In the end it was something trivial:
a 330nf cap on the IC u741 in the power supply section.

I think it has something to do with the MUTE function, but not very clear how..anyway it works and that is all that matters.


Thank you to all that contributed, especially Martin and Nick.

I thought I would get back to let you know.

(It took a while as Mouser took a long time to send me TDA1074, but that turned out to be a dead end anyway). I reverted to heat gun and freeze spray in the end - brutal but effective

Cheers

Philippe

Philippe,

Thanks for the feedback, always nice to hear what was actually wrong.

Power supply caps can cause the strangest faults.

Martin