Did some more work on my B&O3000 today and it has been quite fruitful, although the R ch is still dead.

I have a basic signal injector which I used systematically to test each transistor stage. A very effective piece of equipment that saves a lot of time checking the individual components, both passive and active. About 4 or 5 transistors were replaced as well as a few resistors and the diodes. I applied the signal to the base of each transistor starting from the output stage. Some curious results occured:

1) Base of TR50 (lower 3055) did not transmit the signal. Replaced it and was now OK. However, a diode test of the old transistor shows it is still OK (as reported yesterday). Strange!!!

2) Injecting the same signal to the same transistor on each channel produces different noise intensity.    For example, TR40-42 signal is louder on the left (good) channel than on the right.

3) TR37-38 (used BC182B) proved the most difficult. Signal is heard faintly on the L channel, nothing on the R ch.  The end result is that there is no output from the R ch with FM switched on.

Can someone please explain the problem with point 3)??? What should be my next step???

Many thanks in advance. Regards,

f_b

flejguta_beo:

1) Base of TR50 (lower 3055) did not transmit the signal. Replaced it and was now OK. However, a diode test of the old transistor shows it is still OK (as reported yesterday). Strange!!!

The diode test can't prove that a transistor is OK - it's only a quick check to find out if it's broken for sure in a certain way; shorted or open. Those are the most common failures for a transistor, but it can also for example lose its gain and you'll need a transistor tester to measure that.

I'm afraid I'll need to leave point 3) for somebody more familiar with these particular sets, but note that those stages are before the volume and balance controls; you may simply have problems there.

flejguta_beo:

test points F,G & H  

Now that is strange as those are points with voltages well above 0V.

Now let's clear some points.

- What is the device you measure the voltages with? A digital voltmeter or a moving-coil instrument? (I assume a DVM from your fractional values...)

- as the voltage values on the working channel also seem strange:

  - voltage on C420/500? (47V)

  - voltage on C447/533? (18V)

As it's a DC-coupled amplifier voltages depend on a lot of transistors. But even the pre-stages are way-off.

If the left channel is functioning properly we have to find out about the strange voltage numbers you report

flejguta_beo:

Signal is heard faintly on the L channel, nothing on the R ch

As long as the DC voltage values aren't even close to the correct ones in the schematics everything is just guessing

Thanks tournedos and hemenex for your input. Much appreciated. 

I didn't know that transistors can "age" (much like valves)! Today I changed the other 3055 (SGS) to match the other one. On tournedos' suggestion I cleaned the bass, treble and volume controls. In the case of the volume slider pot I opened it and cleaned it. There was rust on on the centre strip, cleaned and lubricated it. I also changed the balance control with a new 47K (the nearest available) pot. The one installed was 100K. After changing practically all (not all) transistors I still have the right channel dead on FM (and possibly other inputs too).

Hemenex, thanks again for your support. Yes, my measurements are taken with a DVM. I'll check again on C420/500 and C447/533. I agree with you that the voltages at F, G&H, at least on the functioning channel, should be close to those specified. Meanwhile, I have again checked the 11 pins at the back of the pcb. With the new transistors I still can't see why the discrepancies in the 11 pins at the back of the pcb between the two channels. But I'll leave that for later.

Hope to report soon.

f_b

Hi hermenex.

Sorry for misleading you with my previous voltages for F, G & H. I took new voltage readings with another DMM and these are now much closer to spec. F: 3.4V / 3.56; G: 8.05V / 8.85V; H: 24.82V / 24.77 (Left / Right).

Using my signal injector  things seem OK up to the Balance control, so I eliminated the non-working channel from the volume pot and connected it to another (rotary pot). With the volume fully open there was a small, distorted reproduction of the radio signal. I disconnected the left (working) ch and connected the right ch. to the original volume; turned the volume full on and I could hear the same result on the non-woerking channel. This convinced me the volume control was fully functional.

Then I again took readings at the 11 pins at the back of the pcb and all voltages on the left were close to spec.  On the right (non-working) ch. voltages were again close EXCEPT for the pins connected to TR45. These are given as the full supply rail (56.4V) on all pins (e-b-c).

I hope this gives you a good picture of the unit as it is now, still without the right ch. Your advice is much appreciated. 

flejguta_beo:

On the right (non-working) ch. voltages were again close

So Pins "3", "4","8","9" right channel are all around 28 Volts? Can't really see how C of TR45 could be 54V then which would kill TR44 and/or TR49 immediately