NAD 2150 Power Amp Output Transistor Question

[madtech]

Just completed a full re-cap on a friends NAD 2150 and 1020A preamp. This amp is about 30 years old. Before I give it back to him should I replace all the mica insulators and heat sink compound? No obvious heating issues but since I've taken it this far......? tks

[url=https://www.flickr.com/photos/144056422@N07/][/url]

 

[Binkman]

Greetings; Nope... under magnified pic.. look fine.

 

[madtech]

Replaced the single turn pots VR 601, 602 (offset), 603 and 604 (idle) with Bourns pots. Before I took them out the dc offset was fairly close and the idle current alignment voltage was 0 mv on both sides. It is supposed to be between 4.4 mv and 11 mv. After putting in the Bourns I can get the DC offset to 0 mv for both channels but I can't get the idle current alignment up to anything above 0 mv. The last 7 or so turns on VR 603 or VR 604 does nothing to bring it into the positive 4.4 mv range. Should I start measuring resistors in that section or not worry about it?

 

[mbz]

Apologies for the basic question but assume you had speaker A selected when attempting to set bias/idle?
Bit strange that both channels are affected. Protection relay clicks after 3-8 seconds of power ON?
Amp plays audio?

0mv bias/bias not responding to trimmer normally means something has failed (transistor/resistor/...)
Start at the bias transistor Q609, both the emitter and collector voltages should vary when the trimmer
is adjusted. Then at Q611,613 there should be that 0.6Vbe drop then another 0.6Vbe drop for Q615
and Q617. All these voltages should respond slightly to the bias trimmer. Start at Q609.

 

[madtech]

Apologies for the basic question but assume you had speaker A selected when attempting to set bias/idle?
Bit strange that both channels are affected. Protection relay clicks after 3-8 seconds of power ON?
Amp plays audio?

0mv bias/bias not responding to trimmer normally means something has failed (transistor/resistor/...)
Start at the bias transistor Q609, both the emitter and collector voltages should vary when the trimmer
is adjusted. Then at Q611,613 there should be that 0.6Vbe drop then another 0.6Vbe drop for Q615
and Q617. All these voltages should respond slightly to the bias trimmer. Start at Q609.

No worries. Sometimes the basics are missed. Speaker A was selected, protection kicks in after 4 or 5 seconds after power on and the amp plays perfectly, as it did before the recap.
DC offset is 0 mv both sides. Output transistors are running cool under load. Idle current alignment pots VR 603 and 604 both have an affect on the measured voltage, until a certain point.
The last several turns on the Bourns pots has no effect. I can run right up to 0 mv but cannot get to +4.4 mv. Both channels are behaving identically.

I know the 3150 (integrated, same power amp) had some undocumented modifications from the factory done around R676 and D165 (relay), but I'm not sure what issue they were trying to correct.

I'm tempted to leave things alone but I would like to have idle current alignment somewhat close........ tks

 

[mbz]

Q609, 610 collector should be at about +1.1Vdc while the emitters at -1.1Vdc
Vaguely recall that those tweaks to the 3150 were to bump up the relay current
possibly to counter dropouts.

 

[madtech]

Could this be a multimeter lead orientation issue? Looking at the idle current adjustment procedure above, it just says 4.4mv - 11mv. I'm assuming this is +4.4mv? I'm connecting my red multimeter lead to the positive red speaker output for both L and R channels and the negative lead to the corresponding Q615, Q616 emitters (TP1 and TP2).
I can easily get to -4.4 mv with the trimmers but nothing in the positive range. I can read +1.1 vdc (collector) and -1.1 vdc (emitter) on Q609 and Q610, when first turned on, but they slowly decrease as the amp warms up. I thought is was my Fluke multimeter but I put a new battery in and nothing changed. Meanwhile the amp works fine. This one has me puzzled......

 

[mbz]

Could this be a multimeter lead orientation issue?

Yes, I think so, I would expect Q615, Q616 emitters to be at a slightly higher voltage than the red speaker post.
I'd reverse polarity of probes and adjust for 7-9mV (bias will drop as the amp warms up)

 

[madtech]

Yes, I think so, I would expect Q615, Q616 emitters to be at a slightly higher voltage than the red speaker post.
I'd reverse polarity of probes and adjust for 7-9mV (bias will drop as the amp warms up)

Adjusted for 7mv and we're off to the races! Thanks for your help.

 

[Binkman]

Super!.. that's a nice pic, shows Nad's styling very well. Honestly, I'm a little green.
(You're not going to stack for use.. ? )

 

[madtech]

Thanks. No, they won't be stacked for daily use. Stereo stand with proper air space for the power amp.

 

[NAD80]

Nice NAD stack. One question is how do they sound after the recap? 1020a Preamp, 2150 Amp,

 

[madtech]

I initially drove them for about 5 hours, at moderate volume, into a pair of 80's Energy ESM 1S speakers and I can say I was impressed with the combo.
For a 50 wpc amp it punches above it's weight. Detailed highs with the Vifa tweets and impressive low end response. Well worth the time and effort to recap the pre-amp and power amp.

 

[NAD80]

Always liked those NAD watts. Have a 3140.