Dillen wrote:

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?

Nick

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 

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mattjones wrote:

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 

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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

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

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.

Martin

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?

Martin

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?

Nick

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!

Matt