Penta MK3 6631 Red light slowly to Orange: sound still great (and display ok too)

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Guillaume Posted: Sat, Feb 6 2021 3:27 PM

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Hello

i finally got my second amplifier of my Penta 6631 working... redone ALL solderings and also replace the transformer which was noisy...

It works great but after 1 min (after I connect it to the 110 / 220) I see the light slowly moving from red to orange... really slowly.

when I start by beocenter (9500) the penta goes from red ( if freshly plugged) or from orange, to green and works perfectly, the display works also perfectly.

when shutdown the beocenter the penta goes back to Orange... 

my question is simple (probably not the answer): does anybody got any about how to solve this problem ?

thanks

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Guillaume replied on Sat, Feb 6 2021 4:43 PM

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Just to share with you, I just found this explanation:

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Beolab Penta 3

18th November 2012

This active loudspeaker gave normal results when playing but when the main system was switched off the indicator lamp at the base was sometimes observed to be glowing orange instead of the normal red. Since an orange light here indicates a fault (normally overheating or excessive DC at the output of the amplifier) the symptoms were investigated. Nothing could be found amiss except that even in “off” mode a small voltage drop was still present across the standby relay coil 4RL1. Since this same point also controls the driver transistor 2TR4 (BC557B) for the green section of the indicator lamp this was clearly the reason for the orange light. The relay control transistor 2TR3 (BC337-40) had a small and varying amount of bias present at its base but this was not coming from the preceding stage; instead 2TR3 was found to have had an internal c-b leak. A replacement transistor cured the fault. [Beolab Penta 3

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This looks really the same issue as mine... I ordered the two transistors and will replace.

i will come back with the conclusion of this first step !!!

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Guillaume replied on Sat, Feb 6 2021 4:50 PM

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I located those two in the front control panel...

the guy wrote “2tr4” and “2tr3”... the values are the ones I got in the explanation but on the board I read “tr3” and “tr4” : did the guy made a mistake writing with a “2” , or should I find same transistor on another board ?

thanks !

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Homersaurus replied on Sat, Feb 6 2021 8:57 PM

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The transistors TR3 and TR4 are on PCB 2, the Control board which, as you say, is the front panel.   So 2TR3 and 2TR4 refer to (PCB) 2 TR 3 and (PCB) TR 4.  The transistors you have identified on the board are the ones to change.   TR3 failing is a common fault  and can also be the cause of a Penta always being green and not going into standby (red).

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Homersaurus replied on Sat, Feb 6 2021 8:58 PM

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The transistors TR3 and TR4 are on PCB 2, the Control board which, as you say, is the front panel.   So 2TR3 and 2TR4 refer to (PCB) 2 TR 3 and (PCB) 2 TR 4.  The transistors you have identified on the board are the ones to change.   TR3 failing is a common fault  and can also be the cause of a Penta always being green and not going into standby (red).

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Guillaume replied on Sun, Feb 7 2021 11:19 AM

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Thank you for this confirmation !!! 

Lets have the penta confirming that too

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Guillaume replied on Tue, Feb 9 2021 12:34 PM

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and i confirm : working great !!!!!!

TR3 changed.... (not received TR4 yet)

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Guillaume replied on Tue, Feb 9 2021 12:34 PM

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and i confirm : working great !!!!!!

TR3 changed.... (not received TR4 yet)

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Guillaume replied on Tue, Feb 9 2021 12:34 PM

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and i confirm : working great !!!!!!

TR3 changed.... (not received TR4 yet)

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Guillaume replied on Tue, Feb 9 2021 12:34 PM

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and i confirm : working great !!!!!!

TR3 changed.... (not received TR4 yet)

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Keith replied on Thu, Feb 18 2021 10:20 AM

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Dear Guilaume and other Penta experts,

I got an advice to resolder some of the TR joints to get rid of the big PHOP noise on my Penta MK3.  I am confident that I can do soldering but I have concern about disassemble and reassemble the Penta.  

Can you help direct me to a post/resource that I can learn step by step disassembly?  I did tried to search the forum but there are thousands of post about the Penta. I yet to come across a guideline for Penta novice.


Best,

Keith

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Keith replied on Thu, Feb 18 2021 10:20 AM

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Dear Guilaume and other Penta experts,

I got an advice to resolder some of the TR joints to get rid of the big PHOP noise on my Penta MK3.  I am confident that I can do soldering but I have concern about disassemble and reassemble the Penta.  

Can you help direct me to a post/resource that I can learn step by step disassembly?  I did tried to search the forum but there are thousands of post about the Penta. I yet to come across a guideline for Penta novice.


Best,

Keith

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Homersaurus replied on Thu, Feb 18 2021 7:50 PM

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You can start with downloading a  "Beolab Penta Service Manual", the one for MK3.   This will tell you how to dismantle the frets and separate the base, amplifier and main speaker unit.   You'll need a small socket set and don't forget to label the wires that connect the amplifier to the speaker unit.

The service manual has a lot of useful information in it but it will not talk you through a complete dismantle and service of the amplifier itself; I would suggest you read the "Workbench" section and, if necessary, open a new thread on the work you want to do.

I would suggest that you make sure you have some thermal heatsink compound as you will need it if you are doing any work near the power transistors.  It is critical that the cooling of the transistors is efficient and you might crack old compound off. If you are resoldering them, then clean them

off and reapply thermal compound.

If you're doing work on the amplifier I would also consider a capacitor kit from "beoparts-shop" for the Penta amplifier to replace any aged capacitors; there's also an upgrade included for the signal.  There is also a separate crossover kit.