NAD 1020 Preamp Problems

[anatech]

Hi GG,
You should completely remove those transistors if you want to bypass them or use another muting method. It appears the signal passes through those fets from past experience with NAD. I would replace them with a signal relay. High value resistors to ground on both sides to reduce any "pop". Or, a high value resistance across the contacts and another to ground for attenuation while in mute mode. Depends on what the circuit is like.
-Chris

[planet10]

Originally posted by GG
have to take that back. Most of the line amp section is similar.

There are at least 3 or 4 different revs of the 3020 on top of all that. The man i got was before any muting circuitry was added.

dave

[planet10]

Here is the map of a 7020 from the volume control to the pre-out. I think the series FET here is a mute.

dave

[anatech]

Hi planet10,
Thanks! Can you post the power supply as well please?

GG,
We can see the mute is triggered by contacts on the power switch. Q509 and 510 control the gates of the fets. Check the switch contacts are working first, short and open yourself and watch the voltages on the "G" terminals.

-Chris

[planet10]

Originally posted by anatech
Can you post the power supply as well please?

[anatech]

Thanks planet10,
Pretty normal supply with a slight twist. Works well.

-Chris

[gmilitano]

I plan to remove the transistors and run the signal to the balance controls. Unfortunately, I will have to let this sit for a few weeks until I get time to work on it. I will post the results.

Then I would like to upgrade capacitors and possible try a conductive plastic pot.

Thanks for all your help.

Also, thank you Dave for the plans.

Originally posted by anatech
Hi GG,
You should completely remove those transistors if you want to bypass them or use another muting method. It appears the signal passes through those fets from past experience with NAD. I would replace them with a signal relay. High value resistors to ground on both sides to reduce any "pop". Or, a high value resistance across the contacts and another to ground for attenuation while in mute mode. Depends on what the circuit is like.
-Chris

[gmilitano]

Back from the northern Canadian Shield and after a nice BBQ steak dinner (and wine) I sat down with the pre-amp again. I could not track down the problem with the gate voltage so I simply removed the muting transistors and conected the source and drain on the PCB and there it was .. sweet music once again. There is -10V where the gates were, but I don't see that as a problem.

Seeing that I have the case open and given the price of fuel, I may poke around this weekend and upgrade some capacitors.

Thank you Chris and Dave for all your help.

[anatech]

So you went up to feed the bugs. Wonderful up there (except for the bugs).

Do me a favour and have another steak in your back yard for me.

-Chris

[gmilitano]

Originally posted by anatech
So you went up to feed the bugs. Wonderful up there (except for the bugs).

-Chris

Feed them I did...they seem to like Italian blood. Those black flies are nasty. I cannot understand how something that you cannot feel when it bites can cause your head to bleed. I am still picking scabs out of my scalp. Fortunately, is was too cold for bulldogs (horse flies).

I get a pop when I power on or switch between sources. I'm not sure if the muting circuit would help when switching between sources? This is not a problem right now as I use the preamp with low power tube gear. I guess I will have to be carefull with a high power amp.

With regards to upgrading capacitors in the signal path, what options do I have to upgrade 4.7uF and 47uF capacitors (25V max)? Poly caps for those values will be expensive and very large in size making it difficult to fit on the board. Would adding some small poly caps in parallel help much?