The Workbench

Re: Beomaster 5000 very soft sound?

Dillen — Mon, 29 Oct 2007 08:51:18 GMT

Philippe,

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

Power supply caps can cause the strangest faults.

Martin

Re: Beomaster 5000 very soft sound?

phimor — Mon, 29 Oct 2007 07:22:42 GMT

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

Re: Beomaster 5000 very soft sound?

phimor — Mon, 01 Oct 2007 00:28:55 GMT

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

Re: Beomaster 5000 very soft sound?

solderon29 — Tue, 25 Sep 2007 12:20:30 GMT

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

Re: Beomaster 5000 very soft sound?

solderon29 — Tue, 25 Sep 2007 12:11:04 GMT

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

Re: Beomaster 5000 very soft sound?

phimor — Mon, 24 Sep 2007 07:41:07 GMT

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

Re: Beomaster 5000 very soft sound?

solderon29 — Sun, 23 Sep 2007 12:03:49 GMT

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

Re: Beomaster 5000 very soft sound?

Dillen — Sat, 22 Sep 2007 06:57:02 GMT

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

Re: Beomaster 5000 very soft sound?

Alex Barnett — Sat, 22 Sep 2007 03:51:54 GMT

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

Re: Beomaster 5000 very soft sound?

phimor — Sat, 22 Sep 2007 03:14:05 GMT

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

Re: Beomaster 5000 very soft sound?

phimor — Mon, 17 Sep 2007 05:41:07 GMT

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

Philippe

Re: Beomaster 5000 very soft sound?

solderon29 — Sun, 16 Sep 2007 12:52:42 GMT

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

Re: Beomaster 5000 very soft sound?

phimor — Sun, 16 Sep 2007 06:50:35 GMT

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

Re: Beomaster 5000 very soft sound?

Dillen — Thu, 13 Sep 2007 16:05:35 GMT

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

Re: Beomaster 5000 very soft sound?

solderon29 — Thu, 13 Sep 2007 12:35:23 GMT

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.