ARCHIVED FORUM -- March 2012 to February 2022READ ONLY FORUM
This is the second Archived Forum which was active between 1st March 2012 and 23rd February 2022
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:
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! Cheers, Matt
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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
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
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
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
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
Hi-res version
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
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 couldbe suspected, and indeed the 4066 could cause noise.
@ 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
Thanks for the tip Nick! My local electronics store has stock of these and for AUD$1 each it's worth a try 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
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
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?
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
Thank you all for your continued help!
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 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.
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.
The small caps are not there to eliminate noise from the power supply to reach the IC, - actually, it's more or less the other way around.Wherever logic circuits are used, switching noise from the IC will be transferred to the power supply lines in the form of very thin but often quite large spikes.The small capacitors catches this noise effectively.They are most often found sitting across the power supply, very close to the IC, occasionally they will have to physically sit on top of the IC to be close enough.Without them, most logic circuits would act unreliably.Occasionally small caps can also be seen across signal pins - to catch switching ringing (fast "ripple").The capacitance value is not as important as their presence, across power lines you'll often see 100nF - one per IC and typically polyester.Across signal input/output pins they are typically smaller - according to whatever signals and frequencies they work with, 47pF-1nF or so and typically ceramic.
Thanks again all for your help. Over the weekend I replaced the 4066 and put those caps back where they were on the new chip. Crackling noises persist, but the output to the speaker has definitely been cleaned up by this replacement (no noises when switching to and from standby, or changing settings on the front panel). I also think that the track behind the crackling is louder and more distinct than it was before.Also, having another listen, the hum really isn't audible to my ear, and only really shows up in recordings (could even be on the track). I think it's negligible enough for me to leave for now, and come back to it if it gets worse/becomes noticeable.I performed some more extensive testing to find out when the crackle kicks in, and it seems to be volume related rather than frequency related. The speaker performed as expected during a frequency sweep with no audible crackle, unless you really turn up the volume. Makes sense as the crackle can appear on both the low end like I sent in the audio clip, and the high-mid end during vocals. I have an ESR meter and a few other tools coming in the mail today, so this week I'll test the big caps and start going over the resistors/diodes/remaining transistors and checking their values.Matt
Do you have a sound source connected - Beomaster perhaps?
Yes indeed,how and what are you connecting to the Penta's?
The crackling could be due to oxidised interconnections,which you should be able to identify by waggling the plugs in the sockets?
Does the crackling change when you alter the sensitivity or bass enhancement switches.
If the noise disappears in a different sensitivity setting,it would suggest a noisy component in the signal route,something that could be located by using "freezer"spray?
Have a BM5500 hooked up for my testing so far. Have tried both amp level inputs and the line input with the same results. Have tried other sources on the line input for verification.
Wiggling the interconnects doesn’t change the performance, and I have tried different ports and interconnects.
Changing the sensitivity settings do nothing other than make the output louder or quieter, no audible change to the quality of the output. The bass switches don’t change anything either, but do have a pop when switching from med to high.
It’s a long weekend this weekend so I’ll conduct some more testing and investigation!
Most Penta amps will hiss and hum a bit on the bench, until you connect an audio source using an original cable.And still then, some amps will hiss ever so slightly. Remember they are quite powerful amps with their volume set constantly at max,so any noise picked up by the cable (read. "antenna") will be amplified a thousand times and sent to the speakers.Perhaps what you are experiencing is actually normal? And it's the other amp that is unusually quiet.
A shortcoming of the "speakerlink"configuration of the Mk1 Penta,is that the signal is taken from the output of the source device,in your case the powerful amplifier of the BM5500.Later versions of Penta used powerlink at line level.
Good (and usually quiet)as the BM5500 amplifier is,your odd crackling noise could be coming from there.
The notorious speaker mute relay contacts could be oxidised?Of course the output stage of the Beomaster will be ready for a re-cap anyway unless you have already done this?
This sort of problem (noises)a re tricky to pin down unless you have an oscilloscope and can trace the signal path to determine where the noise is coming from.
Worth persevering with of course,