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Beolab Penta MK2 - learning by doing

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dahiyas
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dahiyas posted on Tue, May 14 2019 2:24 AM

Help needed !!

Last month on a peaceful Sunday evening I decided to take on the challenge of recapping my Penta mk2's. Ordered capacitor kits from Martin and took a week to complete the task. I had 50% success with my effort.

One speaker worked perfectly fine (until today) and sounded much better than before.

The second speaker has a loud hum though the music plays without any issues. To eliminate the crossovers as a source of hum, I connected the working amp to the speaker and it worked fine. So looks like crossovers are ok. While music was playing on this speaker I noticed that the display was not working, so I pushed display connector a bit and there was loud noise from the speakers, some smell and orange light.

result - one amp has hum and the other has orange light.

For the hum problem, Martin suggested that I look for cracked solder joints and I intend doing that tonight. 

Any additional suggestions for troubleshooting the two issues ?

Considering the fact that I managed to get one amp working (initially) makes me confident that with guidance I should be able to troubleshoot the issues. 

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dahiyas
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So one problem fixed...used my iPhone as a magnifier and found the issue..one of the capacitors I soldered had a broken trace on one of the legs. Had to connect a jumper and the hum is gone. So one speaker fixed now.

Thanks Martin !!

Now need suggestions for the orange light issue.

dahiyas
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So one problem fixed...used my iPhone as a magnifier and found the issue..one of the capacitors I soldered had a broken trace on one of the legs. Had to connect a jumper and the hum is gone. So one speaker fixed now.

Thanks Martin !!

Now need suggestions for the orange light issue.

Weebyx
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Weebyx replied on Tue, May 14 2019 8:11 AM

dahiyas:

So one problem fixed...used my iPhone as a magnifier and found the issue..one of the capacitors I soldered had a broken trace on one of the legs. Had to connect a jumper and the hum is gone. So one speaker fixed now.

Thanks Martin !!

Now need suggestions for the orange light issue.

Have you changed caps in amplifiers, or just crossovers ? I have restored many Penta amps (I am not an electrical engineer at all, just good hands and willing to learn), and what I have done with all Penta amps, is to disassemble them completely, that meaning removing PCB and de-solder all power transistors, cleaning everything, installing new caps on PCB, looking with magnifier for broken traces/bad solder joints, and fitting new trimmers for idle current. Then new paste between transistors and then case, re assemble everything, and adjust idle current to specs.

This normally works. I have had a few where there was a problem with voltages to the PCB, that required a lot of measuring, and since it is easy to blow up these amps, it is really important to be carefull.

Normally when doing fixes, it is recommended to NOT just change components without actually fixing the issue first, since it is very easy to introduce bad solders/broken traces and so on without actually having fixed the problem.

I am sorry if this sounds like a big job, but it also kind of is, and especially not something to do unless you are very careful and know how to solder and has the right stuff for the job. If you do not have thermal paste, it is possible to leave the amplifer transistors in place, and just remove the solder so the PCB can be lifted up. Dont remote transistors without having new paste.

/Weebyx

dahiyas
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Weebyx - Thanks for the words of wisdom !! There is structured learning and then there is unstructured learning. Unstructured learning leaves a lot of scars but you "really" learn Big Smile

Yes, I did recap both the amps and the crossovers.

Looking for guidance on next steps for the orange light issue now.

BEOVOX141
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Hi there.

First step,- do you have a readable hard copy of the correct schematic and the applicable parts list?

The good news is the old school B&O stuff usually is well documented and fairly straightforward.

dahiyas
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Hi..Yes, I do have the service manual...
BEOVOX141
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A paper version you can use to make notes on?

It might sound trivial, but today people tend to believe everything needs to be "virtual", for this though nothing beats paper, a pencil and a marker for signal paths.

What kind of equipment do you have? 

dahiyas
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Paper/Pencil/Marker - Check

DMM/Soldering station - Check

Equipment ?
BEOVOX141
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Spot on!

A signal generator would have been nice, however you do have a finger and and it usually works just fine Smile

Which one goes first the "orange light" ?

Did you "recap" the two big 80V/1000uF caps?

If I am correct something burnt on that one, disconnect the display for now.

Since you already did the damage when powering up its most likely okay to fire it up again as is (not entirely correct though).

Check the 80V DC supplies, the 15 V DC supplies and the the 10V AC to the display, are they all present?

 

BEOVOX141
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Do be careful! 80/160V DC is VERY uncomfortable....

dahiyas
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dahiyas replied on Wed, May 15 2019 12:19 AM
No, I did not recap the 10000uf caps.

I get 73v on p5-1 and p5-4.

nothing on T2. Both the fuses are ok.

BEOVOX141
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Is the voltmeter in AC mode when you are measuring the T2 output ?

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