help needed for 325 board in a nac72

[cartman]

Hi, I've just bought a NAC72. It will run fine from the power feed on a 140 but it won't work with a hi-cap.

I've borrowed a friend's Avondaled nac12 and substituting the 325 board from this makes the 72 work perfectly. The problem must be the 325/7 board in the 72 I assume.

I know from reading that this is a relay board, but have little idea which component is likely to have failed or where to get replacements. I can just about solder but have no electronics knowledge. Is it likely to be an expensive repair? Anyone fancy the job?

 

[neiljadman]

It just seems odd that it will work from the single rail of the 140 but not from the dual rails of the Hicap since it works exactly the same way in both instances. One would have thought that any fault with the 325 board would have rendered it inoperative on both power schemes.

But if it is the most likely culprit is the single electrolytic cap - any 47uF/35v cap will do a a replacement and should cost only a few pennies - just ake care with polarity.

 

[MJS]

That 325 is an odd little circuit. It basically compares the voltage across a capacitor with a 20v zener diode. If you take a while to charge the capacitor via a resistor then it forms a nice little time delay circuit.

Unfortunately the caps go leaky over time. This means that the cap is now acting as a potential divider rather than pure capacitance. Some PSUs have higher output than others - but still within range and can just tip it over to work. If the hicap's voltage is a bit lower than the 140 then it might never work.

Change the cap and in all likelihood it'll behave again. The cost should be pennies and any components with the same values will work. They aren't in the audio path so don't worry about getting the exact same ones.

Mark.

 

[misterc6]

The 325 is simply a pair of relays that switch the outputs on after a delay. As Neil says, if your 325 works from the 140 supply then it should work from the HiCap unless there is something marginal with either the 325 or the supply from the HiCap.

I assume that the NAC12 is a typo as the 325 only came along with the NAC32 and later. The fact that your friend's 325 works fine leaves the diagnosis the same, something marginal with your 325 or your HiCap.

Can you measure the output voltages on both rails coming out of the HiCap? Have you access to a different external power supply that you can try?

The capacitor on the 325 is just about the only component on the board that would cause the problem. If you'd like me to change the capacitor for you I'd be happy to do that for the cost of the component and return postage,

malcolm

 

[cartman]

Hi, All.

Thanks ever so much for the help. I should be OK changing the Cap, but if I do have a problem, Malcolm, I'd really appreciate taking you up on your kind offer. Just for info, it wasn't a typo, the 325 board is actually in my friend's Avondale modified NAC12. It's had a lot done to it and comes with a huge Avondale PSU that has 3 frame transformers, 3 regulator boards and 3 6500pf caps. I think he may be selling it if I want it. I don't know how much he wants for it but it is impressive. If anyone is curious and wants to see some pics to satisfy their curiosity, I'd be happy to oblige. It's the least I could do given the help I've had. You guys are the best!

 

[neiljadman]

...If anyone is curious and wants to see some pics to satisfy their curiosity, I'd be happy to oblige.

Pics are always appreciated - especially on mod'd stuff. I know that here were several iterations of the NAC12, my oldest one is actually labeled NAG12 and has the relay on the mother board and bifurcated pins while a later one I have has a 325 card and the more usual style pin mounts.

 

[cartman]

Hi, I hope these work

 

[cartman]

Or better these: