Hi all,

I have just finished restoring a BM2400, almost identical to the BM1900. Complete recap, driver transistors, some diodes, etc. Full writeup to follow.

All adjustments according to the Service Manual.

There remains one problem.

See the picture below.

The is a momentary (1/2 second) worrling gargle when switching between the FM presets, in a certain way.

In order to replicate the problem consistently, I set up the FM presets progressively with FM1 at the low end of the FM scale, and FM5 at the upper end. That means that FM1 is a low-voltage setting, and FM5 is a high-voltage setting.

Green is silence, and Red is gargling noise.

So, when I switch UP from FM1 to FM2, etc, there is no noise. But when switching DOWN, from FM4 to FM3, there is noise. And if I switch from FM5 to FM1 (a large voltage change), the noise lasts a bit longer.

However, if I set two stations, say FM1 and FM5, to the SAME frequency, there is no noise switching up or down.

After the momentary noise, all is OK.

There is no noise when switching from Phono to Tape to FM, and vice versa.

So this is definitely related to the changing voltage when switching stations, but only when switching down - ie going from high-voltage to low-voltage.

Is this related to the muting circuit, or perhaps something else?

Any ideas?

Menahem

I've done some tracing with the scope, but it's not conclusive.

At all these points, with the scope set to AC - 20mV - 10ns, on the sine wave display, the tiny bump when switching UP, and the large jump when switching DOWN, is clearly evident.

Components in (brackets) depending on PCB versions.

I started on PCB3 at 3TR6 (3TR29) - voltages are spot-on.

At either side of 3D11 (3D21) - the jump is clearly evident.

Then I measured further back in the switching circuit on the main PCB2 at 2D20 - again the jump is clearly evident at both sides of this 2.7V zener. This is the 12.5V source at pin 8 of 2IC5 and 2IC6 SAS570S sensor IC's.

Then I went further back to the main 15V PSU decoupling cap (on this unit now 3300uF instead of 2200uF), and again the jump is clearly evident.

So the switching jump shows all the way from the 15V source, through the sensor circuit, and to the Muting/IL1 Fault circuit.

I've still not found a solution.

I'm jumping between the BM1990 and BM2400 Service manuals.

This unit has the BM2400 main PCB, but BM1900 small PCB's, so it must have been one of the first BM2400's made.

I disconnected the preamp one channel at a time, and the spike remained on the other channel. So it's not from the preamp section.

Then I disconnected the preamp/tuner PCB completely (can't hear anything, of course). This means that the preamp and poweramp are completely disconnected.

I put the scope probe at the cathode of 3D11 (D21), and switched between FM presets - the spike is still there. So the problem is not in the Pre/power-amps. 

Then I disconnected the FM tuner red wire to see if the FM box was the problem;(of course I can't hear any stations). I put the scope probe at the cathode of 3D11 (D21), and switched between FM presets - the spike is still there.

There are TWO voltage discrepancies:

Discrepancy ONE

On PCB3 (the little one under the volume lamps - at 3D5 cathode should be 12V (I think??) - I measure only 0.7V.

On 3D5 anode, the voltage jumps between 11-15V. The diagram does not specify a voltage for that. I replaced 5 1N4148 diodes around this, but no improvement. I didn't think there would be - B&O had a habit of installing incredibly reliable 4148's - I've never found one failed.

At 3TP1, I have a zigzag scope pattern, not a nice square wave...

I removed 3 TR1 and 3TR2, measured them - exactly the same hFe's, and replaced them again.

I replaced 3TR8 and 3TR9 with a matched pair.

All other voltages check OK on PCB3, within a few millivolts.

Discrepancy TWO:

On the main PCB at 2TR30 collector, I have -280mVDC. It should be 0V. I'm used to seeing less than 20mV at the places where 0V is marked on the diagram, so this stands out as excessive. I replaced 2TR30 and 2TR31 - no improvement.

All other voltages are OK.

Any comments or help would be greatly appreciated.

It's hard to believe that I'm breaking new ground here, and nobody's jumping in with some pointers....

Perhaps a faulty SAS570S???