Slow failure of NAD 7225PE. Suggestions?

[riotpack]

Would a 3.9R in series with the base sort this out?

 

[mightydub]

Jaycee is correct. A few years back I fixed a NAD receiver with a similar power amp circuit. You need emitter resistors to make it thermally stable with the new generation of transistors. I also had intermittent crackling sounds out of the amp, cured it by replacing all the electrolytics and reflowing every solder joint.

 

[riotpack]

What is the lowest resistance I can get away with? .22R?

I will pick up a bunch of 0.5W and join 8 together in parallel per transistor (A 5W will not fit underneath).

It is working fine and I am feeling the heatsink every few minutes and it is only warm, I will still add the resistors tomorrow. Here she is with the new transistors. M2U00275 - YouTube

 

[jaycee]

2W resistors will do and you can get those quite small
BTW, those "ISC" branded devices are almost certainly fakes.

 

[riotpack]

Yes those ISCs are there for a test, Jaycar Electronics sell them now as a cheap Chinese alternative. I wouldn't call them fake as they make their brand known, unless you mean they probably have a smaller die?

 

[riotpack]

I inserted .35R per emitter, set the Q current and offset, and hooked speakers up and it did it again! Then I noticed someone has replaced Q412 2SD669A with a BD139!! while Q414 is still the old 2SB649A. This replacement is only on the channel with problems. This is starting to get annoying now!

 

[Mooly]

These transistors are still my first suspect as mentioned at the start. Fit a BD140.

 

[riotpack]

I have ordered 10 each of the 2SD669 and 2SB649A from Futurlec, I have pulled 6 of the main transistors for that channel and hit them with my cap leakage meter between C-E at close to their rated max voltages but no leakage at all. A few electros show leakage in the microamps but this may be normal, they will be replaced anyway. I will also test the 1n4148 by hitting it in reverse with my cap leakage meter after doing the usual forward/reverse tests with a DMM. Apart from that there could only be a resistor starting to fail.

I might replace components one by one just to see what actually is causing this, I would say you are right about it being the drivers too.

 

[Mooly]

Those transistors fail by going open circuit base to emitter when in actual use... why that type of Japanese device does that I don't know but it is a classic failure mode. The only way to be sure is to either catch the amp in the act when it is faulty (and that might be impossible as it might only be momentary) or replace those particular devices.

 

[riotpack]

I have a feeling whoever done the repair tested the PNP driver and thought it was okay so they only replaced the NPN (with another part mind you).

Once I changed to the stronger output transistors the problem didn't show itself, but when I added the emitter resistors, I had to bump up the Q current trimpot R444 back up (which was now showing around 18mv due to the added resistance) to 28mv and that seemed to make the problem worse.

It will take around 2 weeks for the parts to come and I will conclude this thread.

 

[riotpack]

Just to close this thread - It was the driver transistors causing the problem, just as Mooly suggested in the first response to my post! So thank you very much Mooly for lending me your expertise in this matter, it is great to have geniuses like you in the DIY audio community.

I ordered 10 sets of 2SD669A/2SB649A from Futurlec and used one set to replace the drivers - Setting the quiescent current now meant the trimpot was at full tilt to get close to the specified 28 +- 2mv across the resistor but after days of continuous abuse I have only heard music!

Well this beast now has upgraded transistors 15004/15003 and 3x1R 1W emitter resistors so I guess I will enjoy it as an amp in my guest room.

Next project - Luxman L85V - Symptom, Dead (hope it's not the mains TX!) but that's in the mail coming next week.

 

[Mooly]

That's great to hear... thanks for the update