All,

I have seen mention on the forum of a 'fix' for the power-down 'pop' or thump.  Can someone tell me what that fix involves?  I can't actually seem to find the thread where it is explained...

I'm certain these particular set of Penta 3's are possessed - I'm sure it wasn't doing it before I managed to repair the 'bad' one, and yet it is the 'good' one which is now doing the pop!

Cheers, Steve

Steve,

There were two situations where you could get this type of problem as I recall and they were:-

1. Using a Beosound like Ouverture or BS9000 as a master, the amplifier will be turned on a few seconds before the sound is plugged in through the relay; causing a blop in the speakers if they haven't been turned on for while, say an hour.
2. When Beolab Penta is connected to Beovision AV 9000 or Avant, a plop could be heard in the speakers when the speakers were muted.

The solution to the first problem was apparently to:-

• Keep a continious small load on the big 10000µF electrolytic capacitors 6C1 and 6C2 by means of two diodes and a resistor: On PCB1 at the bottom of the amplifier/speaker. You need to put two IN4002 diodes in series with a 2.7 K ohm resister. Place the end of the string with the anodes of the diodes at the plus (+) of 1C18 which is 22 Volts  Place the end of the string with the cathodes (and resistor) of the diodes at the minus (-) of 1C20 which is -22 Volts.

The second issue was resolved by:-

• The control voltages connecting the speaker signal in Beolab Penta in/out on 22IC2 pins 5/12 are determined by the voltage dividers 22R13 to 22R16. As the control voltages are higher than the supply voltage of 22IC2, a positive DC voltage will occur on the Power Link input. When the Beovision is muted, the speaker signal is pulled to chassis. Thereby a DC loop occurs which results in a plop in the Beolab Penta speakers. The solution is to change 22R13 and 22R15 from 390 Kohms to 470 Kohms.
  

Thanks Keith for your (yet another) comprehensive answer!

Situation 2 sounds similar to what's happening with my system - except I am using a Beocenter 9500. 

It is interesting that PCB22 in the popping penta also has a fault which has caused the display to stop working.. I have swapped over that PCB between the 2 pentas and whichever it is installed in, the display does not work...  I wonder whether there is more extensive damage on that board which is also causeing the pop?  I guess I will find out when the new PCB22 arrives.

Unfortunately when I swapped the boards I was concentrating on the display problem, and can't recall whether the speaker was popping on the other penta.

I did notice that both input control boards have been 'modified (looks like it was at the factory) by isolating pin 1 of the powerlink sockets (tiny grind mark cutting the track, PCB21) and soldering 2 wires of the ribbon connector together (I have reassembled the units, so I can't say which two wires - the one which would have connected pin 1 powerlink to PCB22...?)

Pin1 is the 'power-up' signal so I suspect this was joined to pin4 the 'loudspeaker-on' signal...?  I guess B&O must have had a reason for this...

Anyhow I think I'll wait and see whether the new PCB22 fixes the display issue and the popping issue before diving in with the soldering iron again!

Cheers, Steve

Steve,

If PCB22 has a problem which is making it draw more current from the power supply, then it could be the cause of the popping because the power will likely rise a few milli seconds slower. What is the actual fault with the display ? Based on your comment it sounds like you have ordered a new one anyway.

With regards to the Powerlink modification and the cutting of the track on pin 1, not sure about this apparent change, but the circuit should be as below:-

Keith,

The problem with the display is that it does not light up at all - nothing.  When I swapped the PCB22 with the other one the display issue followed the board, so I guess it is the board which is faulty.  I have ordered a new one which will hopefully 1) turn up in a week or so, and 2) fix the problem with the display.  If your suggestion about the popping being related to pcb22 is correct, which I suspect it is, then perhaps replacing it will fix the two problems... one can only hope!

And yes, the circuit diagram you have posted is the one I'm looking at - but on both units there is a very neat little grinding mark cutting the track (PCB21) leading from pin 1 of the powerlink sockets.  And that wire on the ribbon connector between pcb21 and 22 is solder-bridged to the wire alongside it, again on both units.  It does not look like some amateur botch, and it does not appear to affect operation at all...

Regards, Steve 

Hi Keith & other Penta people,

 I'm sure you'll be pleased to know that I have finally fixed all issues with the Pentas.  New PCB22 arrived today and I fitted it - display now works and the "Penta pop" is gone!

Now just waiting on a handfull of capacitors to re-cap the crossover network boards and they should be like new!  I fitted new mid drivers last weekend, too.

Which is a good thing, since I'm getting quite sick of dismantling and reassembling the damn things - I really bought them to listen to music...

Cheers, Steve

It will be worth it in the end!

I'll never understand why the Pentas were discontinued, one of the best things to come out of Struer!