Like New old stock NAD 2700 - works in 4-8ohm setting - stays in protection mode at 8-16 ohm setting

Jump to Latest
#21
1737106357403.png


I think they are TO220 transistors... someone replaced them at some point...
 
#23
Extreme_Boky said:
View attachment 1408578

I think they are TO220 transistors... someone replaced them at some point...
Click to expand...
Hmm ok - I do have an update - I used the variac slow voltage increase method - over 4 days or so to bring up the amp to its rated voltage - increasing about one volt each hour - After that procedure, the first time I switched to higher ohms the amp came off protection - but I tried it again in a few mins a second time and this time it stayed in protection mode at the higher ohms setting - both sides read 72V at the red circled caps for the 8-16 ohm setting so the power filter circuit seems fine. Anyway - planning to leave it as is for now as my speakers are 4 ohms anyways, and it does come off protection at that setting..
 
#25
eswaroop said:
How do I check / test if the amp is properly switching to the higher rails on higher power demand ?
Click to expand...
You will have different rails voltage between the 4 and 16ohm settings, measure at the 100V main caps.
The PE(120V caps) switching is dynamic, you would need to measure voltages at the collectors of the output transistors. I'd be surprised if it's a PE issue since problem occurs with no spkrs connected and by simply selecting the 16ohm setting. Measure 100V cap +/- rails at 4ohm setting then repeat for 16ohm setting. Did you rule out dirty impedence selector
 
#26
Yes the voltages are different - 58-60V on the 100V caps at 4ohm and 72V at 8ohm settings

Haven’t ruled out a dirty switch - I could try switching a few times to see if it goes away - how do we rule it out ? Is there a way to clean with deoxit?
 
#28
eswaroop said:
the voltages are different - 58-60V on the 100V caps at 4ohm and 72V at 8ohm settings
Click to expand...
I have a 2700THX, it measured +/-55V(4ohm) and +/-68V(8ohm) so your results don't seem unreasonable.

Of some interest(?) the unit took about 20seconds to come out of protection after p-on on the 8ohm setting. Amp looks a bit tired and will get a refresh at some time. I will dig into cause of delay, eg, dc offset or...
 
#33
I'd add reaos 3&4, fault in the protection circuit and failed power supply to protection circuit, however the sentiment is correct. Treat it as an everyday protection issue, maybe just one of the outputs breaking down under the higher railes voltage????


First up, determine faulty channel when 8-16ohm selected.

  • left, measure Vdc between TP101 (either pin) and chassis/GND repeat for TP102 and GND. expect xxmV, faulty channel will be >500mV
  • measure voltage at the base of each output transistor for the suspect channel, ie,Left TR313,5,7,9 Right TR314,6,8,320. Expect all to be about +/-0.6Vdc
So only path for faulty TP10x voltage is via c-e breakdown, replace outputs, you could pull one pair to help localize failing TR or just replace the pair.

The 2700THX includes thermal (heat sink sensed) protection, expect same for 2700...
 
Last edited:
#34
Thanks for all the additional suggestions - I’m not sure if I know the correct voltages to the protection circuit IC - I can follow the instructions MBZ laid out and report back. Where are 7815 and 7915 located ? ( are those the speaker relays ? )
 
You must log in or register to reply here.
Top Bottom