Beolab Penta Mk III amplifier problem

[filip_kbh]

After recapping my Beolab Penta amplifiers, one of them is not working correctly.

The idle current has been set at 30 mV. However, I cannot measure any changes in DC voltage on the speaker cable. The On button is also behaving strangely – sometimes it switches to green but most of the time it stays at red when selecting either Auto, R or L.

I checked all solderins, and I am afraid that I need a little help to pin point where the problem could be. Any help is appreciated.

[Glitch]

I assume it worked before the recap?

Post some pictures of the top and bottoms of your boards. Someone might be able to spot the problem visually.

Glitch

[filip_kbh]

Thanks. I hope so.

Indeed it did work before the recap but was getting luke-warm at even low volumes so I presume the idle current was too high (or the dense layer of dust in it caused it). I have re-attached the black ground wire (hopefully to the right place – the solderpoint to the left of D9).

 

 

 

 

 

 

[filip_kbh]

I hang my head in shame. I used the wrong setting on my multimeter, choosing ADC instead of VDC. So there was no problem at all. The DC current is now between 0 and -1 mV. Sorry for the thread.

[filip_kbh]

However, one problem remains. The displays do not work in either of the speakers. I have them connected to a Beomaster 7000 with a Powerlink Mk2 cable, and the selector at the bottom of the speaker is on Audio. The display in at least one of the speakers worked before I recapped the amplifier and the crossover boards. I did not test the other before I started the rejuvenation.

[Glitch]

It is good to hear that the main problem was an easy fix ;-).

It is normal for the two BF858 transistors to run a bit warm. Based on the discoloration on your circuit board it looks like the amp was pushed pretty hard at some point.

I don’t use the displays on my Pentas, but I have noticed that the wires on the lower display connectors have a tendency to break.

Hopefully fixing the displays will also be easy for you…

Glitch

[filip_kbh]

Yes, that area appears to have been hot sometime in the past. It was the same area in both speaker amps. I don’t know the history of the speakers. I bought them from an elderly lady, and she threw in a Beolab 3500 plus cables and connectors for the quite modest price of 263 dollars for all of it. I wasn’t exactly thrilled about getting the Beolab 3500 and immediately put it up for sale. However, it does not appear that it is in demand, so I guess I will mount it in my bedroom. I just need a 1611 converter box.

Glitch – can you tell me where the “lower display connectors” are found? Do you mean in the lower board of the two, or is it found in another place than where the displays are mounted?

[Glitch]

filip_kbh wrote:

can you tell me where the “lower display connectors” are found? Do you mean in the lower board of the two, or is it found in another place than where the displays are mounted?

A better description of the location is on the bottom cover of the speaker. This cover is the access point to the crossovers. There is a round 8 (or 9?) pin connector on it. You will see the wire connections that I’m talking about if you remove the bottom plate.

On my Pentas, the wires are small and don’t have any strain relief. The other end of the cable (by the displays) has a better connection (crimped versus soldered)

Glitch

[filip_kbh]

Clear – thanks. Will check the soldering on that connector.

[filip_kbh]

So, it was the soldering but not in the actual connector. It was the damn Powerlink cable where the Data Ground had snatched off. After repairing the cable, the display works! Thank you for all the help.

[filip_kbh]

I now had time to test everything. So the good news is that the display works on one of the pentas. Using the same cable, making sure that the audio/video button is set to audio, the display of the other penta is still not working. Argh.

I made a continuity test from the pins of the attached powerlink MKII cable to the connectors on the powerlink of the actual speaker (with the amplifier completely disconnected). All connections worked out fine.

When connecting the powerlink cable to the amp, the continuity test is a bit strange. I get no continuity on pins 1, 4 and 7 in the Auto mode. I believe pin 4 is data. Any ideas where I should be looking next?

Any help is appreciated.

 

[filip_kbh]

The error is somewhere in that amp.

I just switched the two amps around, and then the display does not work in the other speaker where it worked before.

[Author]

Posts