Nad 2200 Recap and Repair

Status
Not open for further replies.
Jump to Latest
#41
Hi MBZ

Just changed 303 and 304 for KSC3503's. the voltage on the connector JP202 pin 2/3 is slowly increasing but is below the 92V it has been previously. I shut off the power at about -80V as it was still climbing. I could put the dim bulb in series witht he supply but then I'm not sure what the voltages should be...
 
#43
JP202 pins 2/3 climbing to -80Vdc indicates you still have a problem.
Your voltage measurements suggest -94V on Q327 b/c/e and about the same on Q331.
Also ZD307 looks suspicious with -92V on both anode/cathode. Can you re-measure
all these.



need to do some reading on how the PE is supposed to work.
 
#44
Looks like I've found the problem. The schematic is different to what I have in that the amp boards are earthed through the wiring. Once I started having problems I removed the wires to try and remove issues and narrow in on the faults. While I was checking voltages around Q216 I thought I would check the collector connection to earth as one of the previous posts had suggested there could be a problem with earthing at the transformer. It turned out that this was not earthed so I searched for an earth but couldn't find one. I then connected all the wires back to the JP's and it's working correctly. the 92 and 62 volt rails are correct in both boards.

What a plonker.... Now to put it back together and to the next step....
 
#46
I don't know if it's been mentioned yet but these amps had a real bad problem with cold solider joints. Also the speaker relays would stop making a solid connection and would cause intermittent problems. I have one of these amps. I was given the amp by a neighbor who was going to throw it out because of the sound always dropping out. I took it apart and re-solidered the cold joints and replaced the relays and it worked fine.

I don't know if this has anything to do with your situation but since you are in there you might want to replace the relays and re-solider all of the places on the board that connect to the rear panel of the enclosure. Low level inputs and speaker terminals.

It is a pretty decent amp. I've been using mine for 12 years now.
 
#47
Thanks Iamjack. The problem now is that I have 2.4V on the output of the right channel board. I can short this out with a light bulb as a resistor and the relays close, ie protection is happy.

TP102 I have 2.41V. R370 & 372 are good.
 
#48
Base of Q204 = 0.6V,Between ZD304 and ZD 302 I have 2.4V which I believe should be 0V. This is the same line as the output.

Q314 B = 0.36V
Q320 and Q316 I have B-E 2.4V. Is this okay or does that suggest they are fried? Q314 and Q318 B-E are 0.2V
 
#50
Tried adjusting with idler adjustment but makes no difference. Checked svr's, all okay. On the left channel where things are working adjusting the svr makes a big difference to the output and fine adjustment brings the output back to 0V however doing the same on the right channel makes no difference to the 2.4v on the output. Checked the obvious R 348 and svr 302 and both seem okay. Base of Q334 does not change from 1.7V.
 
#51
Have you tried to adjust the dc offset SVR202, though 2.4V seems like a big adj, this
would not explain the inconsistent voltages, perhaps dry solder joints, google for pix
if needed. Or maybe mm ground issue, try using any black speaker post as GND.
 
#52
Hi MBX

Tried adjusting it but no change whatsoever. On the left channel I get big changes with very small adjustments. I believe the effect of this is being overridden by something else. I have had a suggestion that Q202 and the Q302/304 could be the cause. Doing a bit more testing now.
 
#53
Juljon said:
Thanks Iamjack. The problem now is that I have 2.4V on the output of the right channel board. I can short this out with a light bulb as a resistor and the relays close, ie protection is happy.

TP102 I have 2.41V. R370 & 372 are good.
Click to expand...

You might still want to pop the tops off the relays and look inside of them as the copper likes to turn green. Corrosion = resistance. Just saying.
 
#57
Juljon said:
Checked the relays, look good and clean, no corrosion. I am now looking for the reason the ZD306 and 308 being disconnected allow the relays to close.
Click to expand...

That's good. How much of the board have you re-soldered? These boards had a real problem with cold solder joints and most of them looked fine but weren't. I re-soldered all of the joints that where connected to the back panel and all of the joints at the transformer. The joints looked good until you stressed the board a little bit and you could see the fractures in the joints.

I know that there where features on this amp that where unique to this amp like inverting the polarity of the low frequency input signal to draw from both rails at the same time when the signal was base heavy on right and left channels.

I have the original literature and sales brochures for this amp and the matching pre amp that I also own. They explain all of the amps features and technologies in good detail unlike today's ad campaigns that wow you with flashy images but don't tell you much. If you think it might help you I can photocopy it and post it here.

I know it was a power envelope design with loads of headroom on demand and would switch from a high current amp to a voltage amp on the fly depending on the source material. I understand that there isn't a whole lot of info available on the internet regarding the PE design so if you think it might help I will be happy to post what I have.
 
#58
Hi Iamjackalope, Thanks for the info. I have replaced all the caps, diodes and transistors on the right channel board. I have also redone quite a few of the resistors. I'll have to go and look a bit more closely at the joints to the back panel and I'll do them. I have the Service Manual and the Owners Manual for the amp. This is my second 2200 I already have one but I want to try two in mono. I am a bit of a NAD fanboy, my pre amp is a 3020 which I bought with my first salary back in 1981. It has been recapped and re powered a couple of times but the case is one and the same! I also have a few other NADs in the shed , TV room etc. Also just bought a 716 for 0.99c but it needs fixing....my next project.
 
#59
Fitted new ZD 306 & 308, no difference.

Found Q328 to be fried (collector was -94V). Put in a new one so now get -60V on the collector as it should be. No change to output however, still unable to adjust the 2.4V.

Looking for an imbalance between the ZD’s. Checked R336 and 338, all good at 12k.

Q334 bce 1.8/1.8/1.8. beginning to suspect this….. BUT Just shorted C 324 and everything goes right. 2.4V goes to 0V, D308 and 306 go to + & - 7.5V. Removed C324 to test and tests as good.

I’m now thinking I need to find the earth connections.. which are different to that on the schematic.

Tried shorting JP202 pins 4 & 5 to earth. No good, nothing changed.
 
#60
I don't think "shorting" C324 is the answer. It just sets the voltage at that point to GND
so you now have a low impedance path to GND for the faulty voltages to be dissipated.


Q334 1.8/1.8/1.8 looks suspect. Would expect c to be +ve, e to be -ve
 
Status
Not open for further replies.
Top Bottom