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Penta trimmer adjustment

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Bram
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Bram posted on Sat, Apr 15 2017 1:02 PM

Hello everyone,

After a succesful recap of the filters of my penta Mk3´s I´ve started on the first amp: elco´s replaced, trimpots replaced(copying as much as possible the old settings), the six 0,47 ohm resistors replaced, cooling paste renewed.

  I fired it up for calibration of 30 mV over R103/R104. It starts at around 40 mV and within about 30 seconds goes up to around 50 to 55 mV, at which point I take the power off. Adjusting R59 doesn´t seem to do anything to the voltage. I checked the no load current and it was close to zero: around 7 mV. 

How much adjusting can be done with R59 and up to what point should I be concerned about the voltage rise? I checked it before the makeover and it was a steady 29,5 mV, only slowly increasing like a millivolt a minute.

Any thoughts?

Greetings, Bram. 

 

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Bram
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Bram replied on Tue, Apr 18 2017 10:25 PM

How did it start? Were they always humming or only after something changed in the system or even in the house? No new appliances or something new hooked up to the amplifier? Have you tried using the pentas with a different amp or using the line in´s with a different amp? Just to make sure it´s not an external problem.

I resoldered all the output transistors and some of the smaller ones which looked less solid, but only the output ones had been physically stressed. Also I had to resolder the speaker output cable, it was moved around so much the trace loop was loose on the board, when working on it make sure that cable is still well soldered to the board. Ideally you solder it when the transformer is already in the housing so it won´t move anymore. 

I saw in the post above I wrote R59 where it should obviously say R95 when dealing with the idle current, my mistake. Let me know if you´re 100% certain it´s the amps. Good luck!

Weebyx
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Weebyx replied on Wed, Apr 19 2017 6:46 AM

Bram:

How did it start? Were they always humming or only after something changed in the system or even in the house? No new appliances or something new hooked up to the amplifier? Have you tried using the pentas with a different amp or using the line in´s with a different amp? Just to make sure it´s not an external problem.

I resoldered all the output transistors and some of the smaller ones which looked less solid, but only the output ones had been physically stressed. Also I had to resolder the speaker output cable, it was moved around so much the trace loop was loose on the board, when working on it make sure that cable is still well soldered to the board. Ideally you solder it when the transformer is already in the housing so it won´t move anymore. 

I saw in the post above I wrote R59 where it should obviously say R95 when dealing with the idle current, my mistake. Let me know if you´re 100% certain it´s the amps. Good luck!

It is 100% the amps that make the noise(one more that the other). I didn't use them before I did the re-cap. One did not power up, the other did, so I cannot say if they were humming before.

None of the components, other than the big resister about middle of the board, seems to have been stressed. And after a re-cap and trimmer change, and after having fixed broken traces from the front panel to amplifier pcb, they both started up fine, and "works" fine. Except for the idle current which are drifting, and the one with the "hum".

Dillen thinks that it could be a diode in the power supply rectifier that has died, and so introduces 50hz hum.

I will try to isolate the hum by separating pre-amp from amp, and measure all the voltages around the board, to see if they are correct, or noisy.

I have both amps besides me at the same time, so I can compare voltages and signals between the noisy and the quiet one, that might help :) Will try to use both channels on the oscilloscope to compare the signals and voltages around the 2 boards.

Both of the large caps in the power supply has also been changed, and I have replaced all cooling paste also.

/Jacob

Dillen
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Dillen replied on Wed, Apr 19 2017 7:12 AM

Most Penta amplifiers will hum a bit if the input is not connected to anything or grounded.
Have you tried grounding the input?

Martin

Weebyx
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Weebyx replied on Wed, Apr 19 2017 7:43 AM

Dillen:

Most Penta amplifiers will hum a bit if the input is not connected to anything or grounded.
Have you tried grounding the input?

Martin

The hum in the "silent" one, is very low, and seems normal, and is not hearable during low volume. In the "loud" one, it is very different, and more of a "50hz, without having measured yet". There is also much more "static noise" in the midrange, treble units, on the penta that hums.

The "loud" one, is hearable during low volume. I have not had time to do any measuring yet, but will get to it tonight. For test input I use the line-in socket, and a mp3 player, the "hum" is there regardless of the input source, but seems to increase or decrease with the input level 1-3 switch. Which also leads me to believe that it might be an issue with the input / pre-amp.

Last time I did some "test", I swapped the frontpanel-pcb from the "silent one", and that unfortunately did not make any difference. in which case, it rules out the front panel pcb ?

I have not tried grounding the line-in yet, I will try to just short the wires tonight and see if that makes any difference.

/Jacob

Weebyx
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Weebyx replied on Wed, Apr 19 2017 8:50 PM

Bram:

How did it start? Were they always humming or only after something changed in the system or even in the house? No new appliances or something new hooked up to the amplifier? Have you tried using the pentas with a different amp or using the line in´s with a different amp? Just to make sure it´s not an external problem.

I resoldered all the output transistors and some of the smaller ones which looked less solid, but only the output ones had been physically stressed. Also I had to resolder the speaker output cable, it was moved around so much the trace loop was loose on the board, when working on it make sure that cable is still well soldered to the board. Ideally you solder it when the transformer is already in the housing so it won´t move anymore. 

I saw in the post above I wrote R59 where it should obviously say R95 when dealing with the idle current, my mistake. Let me know if you´re 100% certain it´s the amps. Good luck!

So...

Did some measurements on the amp today, voltages seem ok, they differ a bit around the +-68V, I measure 72v, don't know if that is too high ?

Re-soldered the front panel and all the output transistors, actually found 2 small resistors where there could have been a bad solder , and it seems to have helped a bit on the floating idle current.

It is set to stabilise around 34mV after 3-4 minutes now, this is the best I can do. The amp plays fine, and are not getting hot or anything, so this is probably just the best it can get.

What and how fast does your amp settle on when measuring idle current ?

Regarding the hum. I actually does not hum when using Line-in, only when using powerlink cables from my Moment. They are Beolab 5 Powerlink cables, don't know if MKII cables are better in handling noise for the Penta.

Unfortunately I don't have any to test with..

/Jacob

 

Weebyx
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Weebyx replied on Fri, Apr 21 2017 7:14 AM

Bram:

Sorry for the abscence, easter holiday got in the way. After resoldering the output transistors and making sure the ntc made good contact with the body I tried again..: and all is well! During installment of the caps, etc caution is advised when laying the pcb on the transistors; flipping it over so many times caused old soldering to break.

Let me know what you find with the scope next week. Even though my problem in the end was just soldering, while on holiday I´ve been studying up on the schematic and may be able to help you!

Thanks so far, greetings, Bram. 

 

Hi again..

My amp is still acting up, but now I have found the source of the hum at least. Found but not solved...

When the trafo is inserted all the way into the housing, the buzz(it's more a buzz in the midrange/tweeter) is very noticeable. Pulling the trafo out 2-3 cm removes the buzzing.

I have tried to relocate wires, and resoldered one of the 2 transistors with cooling plate on top (TR23 if I remember). It seemed that it had a loose trace, and it had, so now this is fixed, but that did not make any difference.

Something from the trafo is "buzzing" into the circuit, and I suspect maybe the small std.by trafo, as it is located very close to PCB 7 when mounted.

Any ideas ?

Regarding the idle current, I really don't understand what is going on. I now have it adjusted so it settles on 30mV after 3 minutes. It starts on 16, and then rises 14mV on 3 minutes ??

What are your start current, and how much does it float ?

/Jacob

Weebyx
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Weebyx replied on Fri, Apr 21 2017 8:29 PM

Ok, So I am giving up..

The amp's are now set to reach 30mV after 2 minutes, and they stay there +- 1mV.

Regarding the buzz/hum. If I set the input level to minimum, together with relocating some of my wire-fixes for broken traces, it is almost gone.

So... if/when they at some point die or act more up, they will be exchanged for new speakers :)

/Jacob

Bram
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Bram replied on Mon, Apr 24 2017 11:19 AM

Sorry Jacob for the late reply, I didn´t get any notifications by email for some reason.

My idle current settles within a couple of seconds to 29,5 mV and does not move much from then on which I imagine is how it should be.

Happy to see you may have found a cause, to isolate it further you might switch the different transformers, but if they´re sufficiently listenable now that my be a hassle. Is your transformer set to 220V? I set mine to 240 but voltage on the rails does not go up more than 63V with about 225 line voltage. 

I can´t see an obvious relation between the idle current and the transformers though...

Greetings, Bram.

Weebyx
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Weebyx replied on Mon, Apr 24 2017 12:54 PM

I really don't know why the idle current behaves as it does, it is very strange, and I am a bit worried that it is not as I would like it to be.

Wish I did a test before doing the re-cap, so I could have isolated the problem a bit more. I have used my scope to also verify that it is not my DMM that is acting up, and that shows the same.

I even swapped R95 back to the original one, just to check if there was a malfunction in the new trimmer, but the old one behaves exactly as the new.

Do you have enough knowledge of the circuit to determine what part is controlling the idle current ? Something must be wrong, but at least it settles and does not continue to rise.

The part that puzzles me most, is that both speakers behave exactly the same. The time to settle and the voltages are 95% the same, so it is not a problem with one of them. I had a thought that I might have changed a cap to a wrong value, but have not looked further into it, and don't know if it could be the cause of this.

The transformers are both still set to 220V, meant to switch them to 240, but I forgot :( And I am not keen on disassembling the speakers again..

The speakers sound fine, the hum is still noticeable at low volume in a silent room, but if I keep the input level on 1, it is livable. just hates having something that does not behave as I expect it to :)

/Jacob

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