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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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Dillen
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Dillen replied on Sat, Apr 15 2017 1:34 PM

You are messing things up. The idle current is the one you measure across the emitter resistors.

If it keeps increasing, check your work.
And check the thermal connection of the output stage transistors to the cooling fins.
(Never power amplifiers up without the cooling fin - long story, but it's the path to problems).

Martin

Bram
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Bram replied on Sat, Apr 15 2017 5:49 PM

That´s the same point in the circuit: between emitter TR27 and R103 over to between R104 and emitter TR32. 

I´m going over the print right now and check all soldering, traces and connections. All transistors are well connected to the cooling body with new paste. 

What I didn´t understand is how the NTC resistor is supposed to make good contact with the cooling body, it´s slightly away from the body, and the plastic "flap" from the PCB 2 housing doesn´t quite reach it. Any thoughts on that?

Thanks! Greetings, Bram.

Bram
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Bram replied on Sat, Apr 15 2017 5:49 PM

That´s the same point in the circuit: between emitter TR27 and R103 over to between R104 and emitter TR32. 

I´m going over the print right now and check all soldering, traces and connections. All transistors are well connected to the cooling body with new paste. 

What I didn´t understand is how the NTC resistor is supposed to make good contact with the cooling body, it´s slightly away from the body, and the plastic "flap" from the PCB 2 housing doesn´t quite reach it. Any thoughts on that?

Thanks! Greetings, Bram.

Weebyx
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Weebyx replied on Sat, Apr 15 2017 7:17 PM

Bram:

That´s the same point in the circuit: between emitter TR27 and R103 over to between R104 and emitter TR32. 

I´m going over the print right now and check all soldering, traces and connections. All transistors are well connected to the cooling body with new paste. 

What I didn´t understand is how the NTC resistor is supposed to make good contact with the cooling body, it´s slightly away from the body, and the plastic "flap" from the PCB 2 housing doesn´t quite reach it. Any thoughts on that?

Thanks! Greetings, Bram.

I have the exactly same issues with mine. They also drift in voltage for 4-5 minutes until they stabilise. I decided to adjust to 30mV after 2 minutes, and let them be there.

The plastic tap will pull the resistor to housing using the screw that holds the plastic front together. This screw actually reaches the plastic flap and pulls it in.

If you make any progress with the floating, please enlighten me :)

/jacob

 

Bram
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Also, I read somewhere about the cooling of IC1, so I took a picture. Is the little rubber tube supposed to make contact with the cooling body?


Bram
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Bram replied on Sat, Apr 15 2017 7:27 PM

Hello Jacob, let´s try and help each other out then! What is the top voltage you see and after how long? When it´s rising do you notice any alteration when turning R59? Because I do not notice a controlled drop or rise.  Then again, so far I´ve been pulling the plug when it reaches +50 mV for fear of fireworks. 

I´ll have a look what´s happening with the NTC first.

Greetings, Bram , the Netherlands. 

Bram
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Here´s a pic of the NTC. They left it too short and most likely bent it against the housing. Pretty risky to have bad measurements here I´d think... But could it be related to drifting idle current? I think a bad soldering is still more likely, but so far nothing has come up. 


Weebyx
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Weebyx replied on Sat, Apr 15 2017 10:35 PM

Bram:

Hello Jacob, let´s try and help each other out then! What is the top voltage you see and after how long? When it´s rising do you notice any alteration when turning R59? Because I do not notice a controlled drop or rise.  Then again, so far I´ve been pulling the plug when it reaches +50 mV for fear of fireworks. 

I´ll have a look what´s happening with the NTC first.

Greetings, Bram , the Netherlands. 

In mine the NTC is pressed to the cooling case, so yours is shorter than mine.

I cannot remember how high the voltage was, but I have just bought an oscilloscope, and need to look at one of the amps again, since it is humming in the speakers. But when I turn the resistor it is very noticeable on the mV right away. I will measure again in a week or so  when I get to it.

Are you sure your amp is powered on while measuring ? I just attach a phono cable to line-in, this makes enough noice for the auto power on to kick in.

/Jacob

 

Bram
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Bram replied on Sat, Apr 15 2017 10:55 PM

So even with the rising idle current you are using the speaker?

Very strange indeed then that my trimmer R59 seems to have little or no effect, something seems amiss on the negative side then. 

The amp is on while measuring, on auto. It switches off automatically after a while because no input, but long enough to measure.

I´d really like to exclude a cooling issue with IC1 and the NTC, but can find little info on it. 

Weebyx
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Weebyx replied on Sun, Apr 16 2017 12:09 AM

Yes, the current stabilises after approximately  5 minutes, but as I wrote, I adjusted to 30 after 2 minutes if I remember correctly.

They work just fine, and does not get hot, even when playing loud for some time.

I just need to figure out why one has a "loud" hum that is noticeable during low volume.

thats where the new oscilloscope comes in:) 

I will do a measure during next week, and let you know what it stabilises on.

I bought the kit from Dillen and everything fits perfectly. But... they have a big soft spot with the connectors of the wide flat ribbon cable where it connects on the amplifier board.. I had a lot of lifted tracks and some where broken all together in 1 of them.

Had to use wires to connect to the board. Don't know if this can cause the hum ?

 

/Jacob

 

 

Bram
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Bram replied on Sun, Apr 16 2017 12:29 AM

If done in a neat way and without shorting anywhere or passing close to some powerful field or oscillating current, then not likely. I kind of liked the board in comparison to other B&O boards; the boards in my 4500´s were much more sensitive.

Does it sound like 50Hz powerline hum? Have you switched cables? Did you switch left and right with the mode button? There´s no possible hum source near cable or amp? I gather it´s a continuous hum? I had a faulty cable from my cd4500 to the beomaster that introduced noises when communicating through datalink.

Do you remember what the maximum voltage was when stabilizing?  

Weebyx
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Weebyx replied on Sun, Apr 16 2017 8:23 AM

Bram:

If done in a neat way and without shorting anywhere or passing close to some powerful field or oscillating current, then not likely. I kind of liked the board in comparison to other B&O boards; the boards in my 4500´s were much more sensitive.

Does it sound like 50Hz powerline hum? Have you switched cables? Did you switch left and right with the mode button? There´s no possible hum source near cable or amp? I gather it´s a continuous hum? I had a faulty cable from my cd4500 to the beomaster that introduced noises when communicating through datalink.

Do you remember what the maximum voltage was when stabilizing?  

Can't remember, but I am sure that that adjusted down when it got to about 40.. will look at it again next week to find the hum :) it is continuous, and also without anything connected but power. It seems like 50hz

/Jacob

Bram
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Bram replied on Tue, Apr 18 2017 6:25 PM

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. 

 


Weebyx
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Weebyx replied on Tue, Apr 18 2017 8:35 PM

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. 

Thanks... I will give it all a good soldering again. I have the problem with both my amp's, they behave exactly the same regarding idle current(one buzzes more than the others though), and that puzzles me a bit. Did you re solder all components, or just the big output transistors ?

I will test all the voltages according to the service manual, and see if something is wrong. And try to isolate where the buzzing is coming from.

I am a bit new in measuring amplifiers and ripple and so on, but I will give it a ago :)

/Jacob

 

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