Help with older NVA A60 power amp

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Hi everyone,
I was recently given an older NVA A60 power amp that had 2 faulty transistors and 3 burned out resistors on one channel so I have replaced the resistors and decided to replace the transistors on the working channel also with new ones.
I powered it up and connected the NVA P50 Passive pre-amp that came with it and it worked perfectly for about 20 minutes and then I started getting distortion on the right channel which is the channel that was not damaged when i got it.
I have been told that NVA use a resistor to set the bias so i need help with this please as my knowledge is fairly basic.
Do I need to reset the bias or someting like that?
Any help and advice to guide me would be great.
Thanks very much!
 

passive420

Member
2013-06-11 3:39 pm
Years ago I had an early AP20 integrated, I was kind of shocked at the hand built quality inside but it sounded very good. Is the case still bonded instead of mechanically fixed?

Yes Bias should be adjusted and measurements taken. Hopefully there will be an adjustable potentiometer but he might use a fixed resistor.

You should take some pics of the board to help. Have you tried contacting Richard at NVA?

You could also try Pinkfish forum as I think Richard of NVA used to hang out there so there will be some owners who could help.
 
Hi Yes it does look very hand built! the case has screws on all corners so it can be taken apart if that's what you mean?
I have not tried contacting NVA as of yet.
There is not a potentiometer on the pcb at all, i have uploaded some pics here for you to look at:
http://i1218.photobucket.com/albums/dd420/Chaggy78/NVA Power Amp/P4020630.jpg
http://i1218.photobucket.com/albums/dd420/Chaggy78/NVA Power Amp/P4020631.jpg
http://i1218.photobucket.com/albums/dd420/Chaggy78/NVA Power Amp/P4020633.jpg
 

Ian Finch

Member
Paid Member
2010-04-11 4:22 am
Coffs Harbour, NSW
You certainly need to reset bias whenever you replace output stage transistors. NVA's production techniques are basic in the extreme but having fixed resistors to adjust values for rather than screw a miniature pot. adjustor, seems a bit slow as a hand built production method. 'Probably a tenet of "purist" belief.

It would be quicker to contact NVA for advice of where to measure the current and what resistor(s) value to alter. You will need the relevant circuit details or precise description of where they are though, as there is nothing like a silkscreened parts overlay.
 

Ian Finch

Member
Paid Member
2010-04-11 4:22 am
Coffs Harbour, NSW
Note that the schematic I linked to (#5) shows the bias and measurement points. If one channel is still working, you can use that diagram to estimate bias current without cutting into the wiring, by using Ohms law and the mV reading across either large 0.68 ohm emitter resistor.

Unfortunately, the original bias setting is lost with the original transistors fitted but I assume the current will calculate to something less than 50 mA, varying considerably with temperature. Using the example of 0.68R, that would equate to a reading of 34 mV or less. The string of 3 diodes would be the place to increase bias with a low value series resistor, determined from the actual current flowing through the existing resistor, that is only represented by the 3 diodes used for simulation purposes. I don't think though, that adjustment would be precise or ever made, since all amps seem to be fitted with the same parts and there would be reasons why this was so.
 
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