Beomaster 6000 (2702) restoration: recapping output PCB + trimmers

After the complete dismantling of the Beomaster 6000 quad it is time to start the restoration work. I decided to tackle the output PCB first (not the darlingtons yet). This PCB houses the 4 power output driver networks and the bank of push buttons for different actions like: front and rear speakers, loudness, hi/low filters, mono, stereo, SQ decoder, ambio sound and AFC / silent tuning. This bord is long and small and sits in a metal subframe. It is not very well fixed in place and therefore, due to warping of the board, the risk of a short circuit between the print side of the board and the metal chassis is high. NEVER attempt to power up the board while is sits in the metal frame without properly isolating the board from the chassis. I usually put a piece of carton in between (that stays in there permanently). But since the whole unit is now disassembled, a more permanent solution will need to be found (see my next blog about this).

Replacing electrolyte capacitors and tantalum's in a vintage electronic unit is always a good idea. They start drying out, leak or loose there value. As you will see on the picture below, one of the ROE caps had a serious crack and was leaking. Very common. I always replace electrolyte capacitors with quality 105C ones that have a longer lifespan. The tantalum's are also replaced with quality electrolytes. I like to use MKS's, but they did not fit in the narrow space on this board. I have seen exploded tantalum's in the past (in a Beocord 5000) and several ones that where short circuit. This failure mode, specifically on older types, is called Field Crystallization and is the major reason for degradation and catastrophic failures of solid tantalum capacitors. It causes a dielectric breakdown and happens very fast (milliseconds).

The recapped output PCB

As you may notice, I also changed the trimmers for the bias current setting (quiescent current). The originals are small 1-turn types that are a bit difficult to reach and not easy to set at the correct value. Moreover, the risk of making a short circuit when setting them with a non isolated screwdriver is high (some readers will probably smile now...!). I replaced them with encapsulated 22 turn trimmers. I prefer to use the long type since they give a bit more stability and makes it easier to reach the small turning knob. Off course, one needs to redirect the wires to make them fit into the 3 existing holes from the old trimmers. It's a good idea to make sure the turning direction of the new ones versus the old ones is the same (fully anti-clockwise for lowest current setting).

And here's a view of how the trimmers are rewired. Make sure the wires are not touching each other. Use some isolating tape or heat shrink tubes.

Earlier on I noted already that one of the front right output stage darlington transistors was fried. It was time now to check if the driver section was still OK. And it was not. The emitter resistor of 0,12 ohm was "open" and the BC312 was also fried. I have spare original ones available so replaced them as well. 

This is a picture of all the caps, trimmers, resistor and transistor that where replaced on this board. Old en new together !

And here are the 4 tantalum's that where replaced with electrolytes.

Next I will tackle the subframe with the 8 darlington transistor and there cooling plates.