hum problem in NAD 3020 (help wanted)

[garyk]

I have a fairly vintage NAD 3020 amp that recently developed a hum problem.

I am pretty good with tube gear, but not great with S/S!

The amp does a thump and initial loud hum/buzz on switch-on, then settles down ok.

The low level hum is then present on both channels, and doesn't seem to be preamp based...(ie constant regardless of settings including zero volume.)

I replaced the 4 x filter caps in the power supply which seemed to reduce the low level hum for a short while (?) but now seems to be back

Thought it might be in the speaker protection circuit...or..?

All assistance gratefully received (incl. directly to tigga@zip.com)

with thanks

gary

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[Netlist]

Looking at the schematic, I can see a fair amount of caps in the PSU. I suggest you replace them all.

/Hugo

[MikeBettinger]

Originally posted by garyk
I have a fairly vintage NAD 3020 amp that recently developed a hum problem.

I am pretty good with tube gear, but not great with S/S!

The amp does a thump and initial loud hum/buzz on switch-on, then settles down ok.

The low level hum is then present on both channels, and doesn't seem to be preamp based...(ie constant regardless of settings including zero volume.)

I replaced the 4 x filter caps in the power supply which seemed to reduce the low level hum for a short while (?) but now seems to be back

Spend some time looking for the obvious. Get a good light and inspect the circuitry for hot looking areas and bad connections, electrolytics that look bloated or possibly leaking, Look for ground connections to the chassis that are made through screws that clamp the PCBs to the chassis. These can loosen up over the years. See if anything appears to be running hot after an hour or so of normal use( ie. transistors, electrolytics, heatsinks) what is needed is a clue to isolate the section. I don't remember if the 3020 has pre-out/main in jumpers; if it does you can short a set of rca jacks and put them in the main in side and see if you get the turn on sounds you describe.

If it works OK other than the background hum (no glaring power limitations, distortion, etc.) then you have a classic failure that can be a fun puzzle to get to the bottom of. My guess is that with a bit of detective work the problem will stand out.

Regards, Mike.

[garyk]

thanks Mike

I suspect that because the hum seems pretty independent of the front end, and is equal on both channels that it must be in some common rail area, like the power supply. The hum level is independent of the volume control.

will play Sherlock again.

Anyone got thoughts on the "thump" on turn on? Do the caps over the mains switch cause problems like that?

Gary

[AKN]

Hi Gary,

I suggest that you check regulated voltages that comes from regulators built around Q901 and Q902. These regulators have feed from separate winding on transformer and powers both pre amp and voltage amplifying stages in power amp, check electrolytics in this area.

[garyk]

good thought, ta!

[deniduc]

It's a old tread but i read it and i've the same problem as you
did you found the answer to your problem with 3020/1020 amp??

i ve the thumb noise at start up.... and it comes from préamp because, if i enter directly the amp with a mp3 player, it works without any thumb

if you can help me, thanks. If not, never mind!!

[lineup]

Originally posted by Netlist
Looking at the schematic, I can see a fair amount of caps in the PSU. I suggest you replace them all.

/Hugo

very good advice by Netlist
if your NAD 3020 is an old one
you may have to replace some electrolytics

when you get it work with good power supply & potentiometers + front side switches
you may see it is still that very good old amplifier
that 1.000 of people have enjoyed so much