B760 power supply

[b0190]

I do not speak German so do allow me to write in English.

Recently I got a B760 as a part machine. I disassembled the boards and spent a lot of time cleaning, reassembling.

It appeared that this B760 had been fixed by the previous owner, but with no success. The power supply board miss one 10uf tantalum capacitor.

The first time I plug in power the display lit and respond to the dail, but nothing else. A little while later there was a smoke from the power supply area.

It was a 3.3ohm resistor got burning. I then look more carefully on the 32V section of this power supply. BC446, BC546 were replaced by 2N5400 and MPSA06.

I checked and found BD177 and 2N5400 was fine. MPSA06 got short! So a new replacement seemed to be unavoidable.

As I inspected this area a more shocking fact was revealed: 2N5400 and MPSA06 was soldered in a mis-configured E-B-C, C-B-E! Previous owner did had the experience to find the replacement transistors but failed to recognize that the pin position of european transistor is different from that of most U.S./Japanese transistors.

Do you have any suggestion on the possible equivalent replacement for BC546?

Thank you!

[sebaki]

AW: B760 power supply

Hello B0190,

a suitable replacement for all small signal transistors used in Revox equipment would be for NPN transistors to use BC550, which is the low noise type of the 546-547-548-549-550 family. Be aware though of the maximum applicable collector voltage, which is 100 V for BC546 and 40V for BC550 AFAIR.

for PNP the same applies to BC560.

If current capabilities (PWR supply) or high frequency (mixer) is an issue, replace with the originally specified types althoug this can mean to pull(unsolder) them from a donator unit. Same goes for FET and the like.

I personally do avoid the Motorola small signal types as they, in my 30 years' experience, exhibit thermal interrups as well as 2nd breakdown too early. I replace them where i meet 'em, just like old electrolytics.

BC550C and BC560C are still being manufactured.

good luck

[b0190]

Re: B760 power supply

Hello sebaki:

Thank you for your quick information. One store I visited only have 547-548.
So I end up buying a 2N5551. Now the power supply do stay on but I only get about 26V (VR turning fully clockwise) from the circuit. Maybe I should try 547 (50V max)?

Without 32V B760 can not function properly. When I pulg in the singal meter always have good reading for any frequency under 100MHz, but no audio output. Later the stereo pilot light sometimes lit. Without muting I can hear the background noise. Mono switch seemed to work. The dial seemed to function properly: I can rotate the dial to get 87~107 continuously with 50kHz step. It also can memorize stations.

Then something happened... I accidentally shorted the display lamp to ground. Both 630mA fuses blown. Now the stereo pilot light and muting light was on all the time. Then there is no audio output. Totally dead?

I pull out the RF, IF, FM demodulator, stereo decoder boards from B760 and put them onto my B780. They all works. The other four boards are local oscillator, sytheizer, logic board and audio board.

I will still hunt for BC550C, BC560C.

Best Regards!

Y.-J. Wu