Hi All,
I picked up a NAD 7400 that goes into Red protection mode. No blown fuses.
Tuner seems to be working fine.
Preamp and Amp are not connected.
I measure 74VDC (L to G and R to G) at the speaker terminal A.
No voltage on Speaker B terminals when Speaker B is On.
All transistors passed diode test ( no shorts found ) in situ.
Unfortunately I mis-diagnosed (measured an open circuit across CE - this is expected for BJT!) Q017 and Q020, Q023 and Q026 and removed them. I plan to resolder these.
Idle Current in the Right channel was high (around 27mV and rising) I managed to adjust it to 14mV per service manuals. Felt a lot of heat early from that side of the heatsink before lowering the idle current.
Idle Current in the Left Channel was adjusted to 14mV. It was close to the expected value.
Faults I’ve found so far
Speaker Relay RY001 (NO type Relay) is shorted( to be replaced by Omron Electronics G2R-2A-DC24 )
I’ve read 74 VDC (rail voltage?) at speaker terminals points to a short in the power transistors. I haven’t been able to find a short in the power transistors Q821-Q824, if I’m correct. Note the right channel in the schematics is a boxed abbreviated formed.
Can the RY001 short cause rail voltage at speaker terminal A?
Service manual attached. Thank you.
I picked up a NAD 7400 that goes into Red protection mode. No blown fuses.
Tuner seems to be working fine.
Preamp and Amp are not connected.
I measure 74VDC (L to G and R to G) at the speaker terminal A.
No voltage on Speaker B terminals when Speaker B is On.
All transistors passed diode test ( no shorts found ) in situ.
Unfortunately I mis-diagnosed (measured an open circuit across CE - this is expected for BJT!) Q017 and Q020, Q023 and Q026 and removed them. I plan to resolder these.
Idle Current in the Right channel was high (around 27mV and rising) I managed to adjust it to 14mV per service manuals. Felt a lot of heat early from that side of the heatsink before lowering the idle current.
Idle Current in the Left Channel was adjusted to 14mV. It was close to the expected value.
Faults I’ve found so far
Speaker Relay RY001 (NO type Relay) is shorted( to be replaced by Omron Electronics G2R-2A-DC24 )
I’ve read 74 VDC (rail voltage?) at speaker terminals points to a short in the power transistors. I haven’t been able to find a short in the power transistors Q821-Q824, if I’m correct. Note the right channel in the schematics is a boxed abbreviated formed.
Can the RY001 short cause rail voltage at speaker terminal A?
Service manual attached. Thank you.
A shorted relay (contacts welded together) would allow any DC offset to pass straight through to the speaker terminal. Any offset such as you have should trip the DC protection and prevent DC voltage appearing at the terminals.
Being able to adjust the bias current and having a high DC offset on both channels seems an odd scenario tbh.
I would begin again by measuring voltages on one channel only and measure the offset from before the relay.
Make sure all supplies are correct... first rule of fault finding.
Being able to adjust the bias current and having a high DC offset on both channels seems an odd scenario tbh.
I would begin again by measuring voltages on one channel only and measure the offset from before the relay.
Make sure all supplies are correct... first rule of fault finding.
I thought it was odd too. I'll measure the offset on one channel before the relay and check power supplies. Awaiting solder in the mail to re-install those transistors.
Thank you.
Thank you.
You've got parallel transistors in the output but are measuring idle current only from one. That could account for why you can set idle current.
On the left channel, if the output offset voltage is positive, check Q817, Q819, Q821, and Q823 for collector/emitter short.
If negative, check Q818, Q820, Q822, and Q824.
Do the same on the right channel with equivalent Q's.
Be sure to measure that output offset voltage before the relays.
On the left channel, if the output offset voltage is positive, check Q817, Q819, Q821, and Q823 for collector/emitter short.
If negative, check Q818, Q820, Q822, and Q824.
Do the same on the right channel with equivalent Q's.
Be sure to measure that output offset voltage before the relays.
Thank you. no luck finding a collector/emitter short in the power transistors. I’ll check the output offset voltage before the relays.
It’s entirely possible that a defect in early stages can drive the output stages into a supply rail and that output stages are innocent.
If you suspect this might be the case, tabulate voltages at output before relay, feedback at base of Q801 (right side), and input base of Q801 (left side). Then look at inter-stage transistor voltages. Often you can find the defect by probing with scope or meter to find a device that doesn’t behave as it should.
Your’re in good hand with Mooly. Good luck!
I’m currently stuck with this contraption. I am not sure if I can safely power the receiver with the majority of the tuner unpatched. Otherwise, I’ll have to insert about 14 jumpers and extend other wires to gain easy access to the main PCB. Any tricks 🙂?
The back side of the main PCB is not printed. I thought of printing a mirrored scan of the PCB, but the resolution is poor and I am not good and reading backwards.
Still planning to measure power supplies. It’s taking a lot of time figuring out what’s what on the back of the main board.
Input set to CD w/ volume at minimum
Q801 and Q802 (I think I got my left and right correct).
Left Base to Ground : 25mVDC
RB to G 7.4VDC !?
Is 25mV at the base of Q801 and Q802 one fault?
Verified Left +/-77VDC and Right +/-80VDC Rails voltages.
Main In
R-G 5mVDV
L-G 2mVDC
Pre Out
RG 58mVDC
LG 420mVDC
Speaker A (installed new Relay) (off) looks like Protection mode is blocking me from turning on speaker relay.
L 1.4VAC, 0.437VDC
R 1.04VAC, 0.1782VDC
Speaker B off
L 1.4VAC, 0.003VDC up and down 100mV… unstable
R 1VAC, ? Unstable
Why would speaker A and B have different DC measurements?
1VAC is huge at the speaker terminals.
Idle current L and R 4mV (lowered intentionally).
The back side of the main PCB is not printed. I thought of printing a mirrored scan of the PCB, but the resolution is poor and I am not good and reading backwards.
Still planning to measure power supplies. It’s taking a lot of time figuring out what’s what on the back of the main board.
Input set to CD w/ volume at minimum
Q801 and Q802 (I think I got my left and right correct).
Left Base to Ground : 25mVDC
RB to G 7.4VDC !?
Is 25mV at the base of Q801 and Q802 one fault?
Verified Left +/-77VDC and Right +/-80VDC Rails voltages.
Main In
R-G 5mVDV
L-G 2mVDC
Pre Out
RG 58mVDC
LG 420mVDC
Speaker A (installed new Relay) (off) looks like Protection mode is blocking me from turning on speaker relay.
L 1.4VAC, 0.437VDC
R 1.04VAC, 0.1782VDC
Speaker B off
L 1.4VAC, 0.003VDC up and down 100mV… unstable
R 1VAC, ? Unstable
Why would speaker A and B have different DC measurements?
1VAC is huge at the speaker terminals.
Idle current L and R 4mV (lowered intentionally).
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Verified 26VAC
Verified 34VDC
+/- 18 VDC is faulty
Q011 C-G: -0.7VDC
Q010 C-G : 1.0VDC
Verified 34VDC
+/- 18 VDC is faulty
Q011 C-G: -0.7VDC
Q010 C-G : 1.0VDC
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If the speaker relay is not engaged then the speaker terminals are just floating and so its normal to see all sorts or odd voltages across the terminals. It is just stray pick up and noise. If you put a 1k or similar resistor across the terminals you should read zero volts.
You must be methodical. Pick one channel and with your meter on chassis ground measure the DC voltage to this point. What do you see?
Now measure the voltage across each of these four 0.22 ohm resistors. So four measurements. What do you see?
It doesn't sound normal for the bias to be correct with a major DC fault so we need to do these tests to be absolutely sure.
You must be methodical. Pick one channel and with your meter on chassis ground measure the DC voltage to this point. What do you see?
Now measure the voltage across each of these four 0.22 ohm resistors. So four measurements. What do you see?
It doesn't sound normal for the bias to be correct with a major DC fault so we need to do these tests to be absolutely sure.
That you see +74V on both outputs is suggestive of a fault common to both channels, but not conclusive. Suggest you confirm low voltage supplies on both PA sections. If 18V supply is failed, that’s a possible reason.
So a true and massive offset. I find it strange the bias current seems to adjust though... however 🙂 Having zero volts across one of the four isn't necessarily an issue and could be just down to differences in matching of the output transistors.
This has to be the next step. Remember there is a - and + 18 volt supply so two to check.
I’ll update with voltage reading of the +- 18 and 24 VDC power supply circuit later. All of the transistors in that circuit passed the diode test. Thanks again for the guidance!
What measurements would you recommend? I can see input and output voltages. I don’t understand how this circuit works or what I’m looking for. Is the first step measuring all leads WRT ground in the blue square area, and looking for discrepancies between positive and negative voltages from right to left?
Also can 0mV across one of those four resistors (R885) indicate a short?
Also can 0mV across one of those four resistors (R885) indicate a short?
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