NAD 7020i - pre-amp power supply issue

[mashley_nad]

Hi all

I've been give a NAD 7020i that hasn't been used for while.

Powers up fine, but will only tune into stations up to about 100MHz. AM is fine.

After a bit of open heart surgery I found that the pre amp power supply, one - 28V line of which runs to the tuner, is sick.

+ 10.3 V instead of + 28 V, and - 5.1 V instead of - 28 V !!!

Input voltages to the regulator are correct (about + & - 50V).

I took a punt and replaced all the electrolytics --- but no change!

Getting 0.6 V across B-E for all the transistors - so they are probably all OK. Only weirdness is that the 24 V zener (D513 in the schematic attached) only has about 3.5 V across it.

Been a while since I've done this stuff ... can anyone give me a clue here???

Cheers

[h_a]

No schematic here

Only weirdness is that the 24 V zener (D513 in the schematic attached) only has about 3.5 V across it.

What about fitting a new 24V Zener? Or is it a clamp?

Have fun, Hannes

[analog_sa]

Don't have the circuit but assuming the regulators are similar to the 3020 there is a 4u7 capacitor filtering the base voltage of the BC559. If that cap is shorted you may experience similar problems. Of course if you have replaced all the caps already... Still, an extremely simple and easy to troubleshoot circuit. If the negative supply doesn't work, neither will the positive.

[planet10]

I would suspect that the 7020i has the same circuitry as the 3020i, regulated PS section arrached. I have the entire map but it is too bug to post.

dave

[analog_sa]

C523 on Dave's circuit

[mashley_nad]

Thanks for all your tips --- turned out to be a combination of a blown 24V zener plus a bad solder joint on one of the main regulator transistors.

Soldering on the whole is pretty crappy - these units aren't that well put together

Anyhow, now the tuner is working just fine

[mashley_nad]

power supply diagram for future ref