NAD 7020 , small problem...

[pat allen]

Hi, someone brought me a nad7020. it was dead per the last diagnostic of the last person who wrenched in it
there was also a list of bad components, among them the power transfo was claimed to be defective.

Well, after a bit of investigation, one output was dead Q618, as well as its pre driver Q614, and the bootstrap cap C616 was leaking mad.

I replaced C626 since i was in.

I replaced theses and now it is playing very well. The only issue is the pre-driver Q614 gets real hot.

I measured dc output and idle current, all is within specs.
There is no noticeable distortion.

All static voltages are equal on both channels. Only Q614 of this channel gets real hot

I was not having the exact same transistors, but i matched the hfe and icmax. Maybe Q614 got a bit less gain, but i think it is within min/max expected since the 2n6556 got a very wide tolerance on this, like min 80 max 300. Maybe i am not interpreting the data sheet well enough...

Any idea ?

thanks to the ones who sent me the schematic.

[pat allen]

hi, i found the problem.

there is a resistor on the collector of the pnp output, to set the idle current. under normal operation, this resistance is shorted with solder under the pcb. When someone what to set idle current, the short is removed.
The solder was out, leaving only the pre driver to realy generate some music, and the resistor was burned. I believe i made the mistake while cleaning up the pcb and maybe found this as a potential wrong short...

So i shorted it as it should and nows it is 100% top notch. Cool ! this litle amp got a lot of pep !!

Only issue now is the power switch, it is in real bad shape.

And i have to adjust the FM tuner, stereo mode lock on side band

I cleaned a bit its inards, the wiring is such a mess, looks like it is a diy kit.

Other than that, it seems to be a real good litle amp. the owner willl be happy i think.

thanks.

[ginsner]

Was your 7020 a real mess?
I am trying to clean one up that i picked up at a thrift store and it seems as if someone spilled what i first thought was chocolate on the output board but now i am thinking its adhesive of some kind that was very poorly put on. Is yours the same? Also the soldering looks like what you would expect from a cheap amp.
I expected better workmanship from NAD.
When i took apart the power switch i found the contacts were thick with carbon aswell.

[planet10]

Originally posted by pat allen
Only issue now is the power switch, it is in real bad shape.

Power switch failure is common. Think of it as an opportunity to mount a toggle on the back panel right next to the transfprmer.

The 3020/7020 were very cheaply built. Excellent design, and intelligent parts choice to give something that punched way above there weight class sonically. These amps are now pushing 30 years old.

dave