NAD 2200 power amplifier protection issue.

[anderslober]

Hi forum. Please see picture og protection circuit. I have 27/55/27 vdc on ECB of q401. The power amp sides both measure equally and have 0vdc going into the protection circuit.
I have checked q401/402/403, 2sc2240, out of circuit, all tested good. Ideas welcome.

 

[anderslober]

Got it out of protection again….hopefully. Reversed cap c410……board and scat do NOT correspond to each. Be careful. There are a few other places to.
Also, amp needs more than 180watts bulb dbt power to power up.
Action fotos!

 

[whoaru99]

Original board way normally takes precedence if there is a discrepancy.

 

[pwauk12]

what revision main boards do you have?

 

[anderslober]

No RevNr. on boards. But, board numbers are mal593a/mar593a.

The unit has been playing fine now for a few hours.

Quite a few of the resistors look ‘peeled’……loosing their coating…...they measure fine, but dont look good. Some environmental progress…see picture…….change ‘em?

 

[pwauk12]

The A designates the revision (there are 4). It is "normal" for those resistors to look like that- they can be replaced, however they do usually measure OK and are usually flameproof varieties

 

[Iamstubbb]

Might consider replacing with a higher W equivalent. Sure, they measure fine cold, but it looks like they spend a lot of effort dissipating heat.

 

[anderslober]

I changed all the ‘peeled’ resistors. Anyway……amp works fine, bias and dc-offset is tuned in. When I turned on the (cold) amp this morning, the amp cycled the protect relays 3-4 times before ‘capturing’. Is this normal? After running the (now warm) amp and turning off/on a few times, relays engaged fine.
Just to confirm the behaviour, I put the amp outside to cool down (10 degrees celsius), took the amp back inside and powered up….same problem…relays cycle 3-4 times, and then capture. This amp only likes room temp.

Has anybody had this problem or similar? Can anything be done about it ?

 

[pwauk12]

I changed all the ‘peeled’ resistors. Anyway……amp works fine, bias and dc-offset is tuned in. When I turned on the (cold) amp this morning, the amp cycled the protect relays 3-4 times before ‘capturing’. Is this normal? After running the (now warm) amp and turning off/on a few times, relays engaged fine.
Just to confirm the behaviour, I put the amp outside to cool down (10 degrees celsius), took the amp back inside and powered up….same problem…relays cycle 3-4 times, and then capture. This amp only likes room temp.

Has anybody had this problem or similar? Can anything be done about it ?

the cycling is not normal, from the pictures, it appears the relays the original ones, and these regularly fail. They should be replaced. There is some useful information here:

NAD 2200 mods and upgrades - QuirkAudio

quirkaudio.com

and some excellent threads on AK

The temperature sensitivity could be a number of things, however changing all the electrolytic caps(including the main ones) and the relays generally resolves most problems

 

[anderslober]

All caps and the ‘peely’ resistors were changed. I did not change the relays. I have had the unit on/off for a few times now…..no problems. Fingers crossed!