I suggest you contact Tim Jarman on

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.

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

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