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Beolab Penta MK1 Amp Issues

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mattjones
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mattjones posted on Fri, Jan 5 2018 5:25 AM

Hi all, 

First time poster here at Beoworld!

I came into possession of a pair of Penta MK1's (Australian version, 6605?) around 6 months ago and had them refoamed, but have only recently got stuck into their failing amps. I only tried them briefly to check their state, but intended to use Martin's recap kit before I started proper diagnosis (might as well give them a fighting chance, right?). During that initial test, one flicked from red to component protection mode (yellow) when a signal was applied, and the other exhibited a crackling sound with what sounded a very muffled version of the track playing at a low volume.

In the last couple of weeks I refreshed both units using Martin's fantastic kit (caps, IC2, pots), and the same issues persisted. After giving the crackling one a good going over (I'll have a look at the other one this weekend as I anticipate it may be a little easier to diagnose), here's where I'm at right now: 

  • The speakers work great sans amps, and I have double checked that the crossovers are in good condition until I can recap these later. We can rule them out as a point of failure.
  • The large transistors around the outside test fine, I have re soldered them to ensure that there is no cracked joints. They have a good and fresh thermal paste application and have never been run not attached to the heatsink.
  • I've resoldered the pins for the front panel based on the recommendations of some threads
  • I've also tried swapping the front panels of amps to rule them out as an issue
  • There is no visual indication of damage to any component on the board

Are there any other common failure points that could be causing this crackling audio? At the moment I'm just going through and testing every component on the board so any advice would be greatly appreciated!!

And yes, I'm probably a little nuts not to just ditch these amps but I am a purist and won't let them die! Stick out tongue

Cheers, Matt 

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Weebyx
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Have you re-soldered the 2 "big" transistors in the middle of the board ? They are bend downwards with a heat sink on. These can be pushed down by the wires just above them when the amp is put together, and may cause them stress over time. I have seen many amps where these have bad connections. It is very easy to see if you look at the bottom of the PCB while carefully wiggling them. But be careful, the PCB traces might also be loosing their grip to the PCB.

Try re-soldering them.

Also, a re-solder of the connections might not be enough. Many times the traces on the PCB are actually broken, so you need to test with a multi-meter if there are connection between each pin and to the first(or any) component it is connected to.

/Jacob

mattjones
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Great suggestion Jacob, thank you! I have not yet re-soldered or tested those transistors, I'll give that a go shortly and report back with my results.

I'll also go over the traces this weekend, I have noticed while replacing components on both boards that they have been losing their grip as you say.

Matt

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mattjones
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I checked those transistors over the weekend and their solder was definitely stressed. I renewed the solder on them and tested them out of circuit, everything checked out ok. 

I also went over pretty much every trace on the board and couldn’t find a fault there either. Plugged in to retest and the same fault persisted. I recorded a snippet this evening of the issue to give a clearer picture. I used the start of Hotel California (Hell Freezes Over) as a reference track. https://drive.google.com/open?id=1XRwtuXa_rvURGjCp4lanlzK1pBNinJ8a

Recorded that connected through coax, but it’s the same on every input. That’s pretty much max volume on the source, and the recording device held right next to the mid range (although the crackle doesn’t seem channel specific). That constant buzz is coming from the speaker also, but it isn’t noticeable in person unless you have your ear to it. 

At this point I think the only components I haven’t really covered are the resistors and some diodes, so I will go over these when I have the chance.

Matt

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Dillen
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Dillen replied on Wed, Jan 10 2018 10:11 PM

Sounds to me like a grounding problem. Check ground continuity.
Check also for cracked solder joints (reflow them all) where the ribbon cable from the board inside the frontpanel is soldered to the main amplifier board. 

Martin

mattjones
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Thank you Martin, we have good continuity between the negative/ground input terminals and the chassis of the amp, as well as on the negative output. Would you recommend I work through the schematic and check every ground or would that be sufficient? I reflowed those joints a couple of weeks ago based on a recommendation of yours in an older thread, and I've also tried exchanging the front panels between the two amps just to *probably* rule those out.

I've attached a picture of my amp for reference, just in case there is a glaring failure that I have overlooked Wink 

Hi-res version

The correct number of B&O products to own is n+1, where n is the number of B&O products currently owned

solderon29
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Did you change the pre-amp integrated circuits and the signal router i.c.(4066)

I often used to put these Penta amp's back together only to find intermittent noises and nasties afterwards,so for what they cost,I alway's replace those chips too.

Does Martins kit include all the electrolytic caps?The standby supply decouplers are often out of spec,and can cause humming etc.

Nick

Dillen
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Dillen replied on Sun, Jan 14 2018 6:45 PM

The kit includes all electrolytics except the two large main supply cans in the transformer holder.

I never experienced a bad 4066 IC in a Penta.
The 4066, being a CMOS chip, is very sensitive to static electricity, so if work was done in a non-ESD environment any and all CMOS could
be suspected, and indeed the 4066 could cause noise.

Martin

Rob
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Rob replied on Sun, Jan 14 2018 6:51 PM

@ Solderon29, 

 

what do you mean by The standby supply decouplers, i'm trying to repair 6 beolab 150 amps and need all

the info i can get :-)

Recap with Nichicon UFW and UFG and BIAS tune is what i'm doing right now.

I've 3 amps with standby issue and 2 getting really hot.

Greetz,

Rob

 

mattjones
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Thanks for the tip Nick! My local electronics store has stock of these and for AUD$1 each it's worth a try Stick out tongue
Who knows what/who may have touched these amps in their life before me! 

Would failing standby decouplers produce a hum in the amp itself or in the speakers?

Matt

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hemenex
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Dillen:
I never experienced a bad 4066 IC in a Penta.

Martin, well I did.

The Penta did a frightening loud "plop" on switching to standby.

Took me some time to trace it down to the 4066.

  hx

solderon29
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I can only speak as I've found,but  I've had enough problems with op amps and particularly 4066 switches,to include them in my "usual suspects" list.

The 33uf caps in the standby supply can easily be installed reversed polarity,but looking at your photo,I think they are ok?

Can you set up the amplifier quiescent current and centre point zero voltage as per the manual?

What do you mean when you say "is the hum in the amplifier or the speaker"?

Are your main supply cap's (the two big can's on the transformer)Nichicon or the yellow type?

 

Nick

mattjones
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Today I was finally able to get into the shop after work and pick up some 4066 switches (TI CD4066BE) and sockets to be safe.

Nick, the 33uf caps are in the correct orientation according to the silkscreen and my initial reference photos. I have also followed the manual's guidelines to set the pots. The two main supply caps are black ELNA CE-W 80v10000uf.

Excuse my wording, I meant to ask whether the hum you speak of would be emitted through the speaker when attached, or come from the transformer inside the amp. The hum recorded in my audio clip was being produced by the connected drivers.

When I went to replace the 4066 IC's, I came across some strange findings. Firstly, it appears that one of my amps (not the crackling one) has likely had its main amplifier board replaced at some point, as it has slightly different components that the other, and a slightly different silkscreen. I had suspected this for a time, as this amp had a poor and messy thermal paste application compared to the other. However, upon inspecting the 4066 on the crackling amp, I noticed two of what look to be capacitors (excuse me if I am mistaken) across pins 5-7 and 12-14. These look to be a recent addition as the solder is shiny and not oxidised like the other pins, and I haven't reflowed them. Picture attached for reference, current IC1 is type MC14066B, silkscreen is correctly oriented.

I would not have questioned this, but these (capacitors) do not appear on the other amp. Can't see any reference to them in the service manual so I am a little confused Unsure 

Thank you all for your continued help!

Matt 

The correct number of B&O products to own is n+1, where n is the number of B&O products currently owned

Weebyx
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Weebyx replied on Thu, Jan 18 2018 12:24 PM

mattjones:

Today I was finally able to get into the shop after work and pick up some 4066 switches (TI CD4066BE) and sockets to be safe.

Nick, the 33uf caps are in the correct orientation according to the silkscreen and my initial reference photos. I have also followed the manual's guidelines to set the pots. The two main supply caps are black ELNA CE-W 80v10000uf.

Excuse my wording, I meant to ask whether the hum you speak of would be emitted through the speaker when attached, or come from the transformer inside the amp. The hum recorded in my audio clip was being produced by the connected drivers.

When I went to replace the 4066 IC's, I came across some strange findings. Firstly, it appears that one of my amps (not the crackling one) has likely had its main amplifier board replaced at some point, as it has slightly different components that the other, and a slightly different silkscreen. I had suspected this for a time, as this amp had a poor and messy thermal paste application compared to the other. However, upon inspecting the 4066 on the crackling amp, I noticed two of what look to be capacitors (excuse me if I am mistaken) across pins 5-7 and 12-14. These look to be a recent addition as the solder is shiny and not oxidised like the other pins, and I haven't reflowed them. Picture attached for reference, current IC1 is type MC14066B, silkscreen is correctly oriented.

I would not have questioned this, but these (capacitors) do not appear on the other amp. Can't see any reference to them in the service manual so I am a little confused Unsure 

Thank you all for your continued help!

Matt 

Those caps are normally found on MK2 and MK3 penta's have seen them a lot there, but not on the 2 MK1's I have seen. They are from factory production. and most likely some improvements made over time, but didn't want to layout a new PCB.

/Jacob

solderon29
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The caps mentioned are actually across the supply lines to the chip,so I can only assume that they are decouplers to improve stability?A different value to those fitted to other later Penta variants oddly.Might just be to compensate for a particular make/spec of ic that was used in the production at that time?

If the hum is coming via the speakers there must be some "ripple" somewhere?Are all the supplies correct (+15/-15 etc)?

The large yellow main supply cap's are usually suspect but the Elna ones are usually ok,but they will be close to 30 years old now I guess,so again for what they cost,it might be worth changing them too.

I think you will need to trace where the hum is coming from by isolating sections.C25 is the signal coupler for the main amplifier,you could try disconnecting it to see if the hum remains afterwards.If it does,the problem is to do with the main amp.If it stops it's in the pre-amp somewhere.

I don't expect that changing the pre-amp chips will have any influence on the hum,but it could overcome the mystery noises.

Nick

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