Hi everyone,
This might be a strange question, but does anyone know which NAD amplifier was used in the 705 receiver? Similar to how the 3020 amplifier section is in the 7020 receiver.
I'm asking because I want to troubleshoot a 705, but the schematic available on HiFi Engine is so blurry that I can't make out much of it. I was hoping to find the schematic for the specific amplifier model used in the 705 and use that as a reference instead.
Any ideas would be greatly appreciated!
Cheers
This might be a strange question, but does anyone know which NAD amplifier was used in the 705 receiver? Similar to how the 3020 amplifier section is in the 7020 receiver.
I'm asking because I want to troubleshoot a 705, but the schematic available on HiFi Engine is so blurry that I can't make out much of it. I was hoping to find the schematic for the specific amplifier model used in the 705 and use that as a reference instead.
Any ideas would be greatly appreciated!
Cheers
I hope this one is better than the one form hifiengine. At least it has schematics that are sort of readable.
Hi maudio,
Thanks for trying to help! Unfortunately it is the same version as HiFi Engine, even has the same hand written scribble on the scanned pages. But really appreciate you trying to help. 🙂
Cheers
Thanks for trying to help! Unfortunately it is the same version as HiFi Engine, even has the same hand written scribble on the scanned pages. But really appreciate you trying to help. 🙂
Cheers
Was worth a try. It's the best I could find when I had to repair mine. Anyway, what is the problem with your 705?
Thanks BSST! It's the same file as well. I've found this one on a number of sites now, all are the same file, down to the scribbles. 🙂
Yes absolutely! Q117 and Q119 in the left channel power amp circuit gets incredibly hot and causes the amp to go into thermal shutdown. This happens within about a minute of switching it on. Music plays fine before thermal shutdown so I think the transistors are not faulty.
I haven't done much troubleshooting yet and not sure where to start. Looking at the schematic (top of page 15 of the PDF) there is a capacitor connected between the base of Q117 and Q119. I think the parts label is C121 but I can't find that number in the parts list at the end...
Yes absolutely! Q117 and Q119 in the left channel power amp circuit gets incredibly hot and causes the amp to go into thermal shutdown. This happens within about a minute of switching it on. Music plays fine before thermal shutdown so I think the transistors are not faulty.
I haven't done much troubleshooting yet and not sure where to start. Looking at the schematic (top of page 15 of the PDF) there is a capacitor connected between the base of Q117 and Q119. I think the parts label is C121 but I can't find that number in the parts list at the end...
Check the bias by measuring voltage over R143. Should be 4.4mV.
And use a dim light bulb protection, place a 60W filament bulb in series with the mains supply to prevent things from blowing up while you're poking around.
And use a dim light bulb protection, place a 60W filament bulb in series with the mains supply to prevent things from blowing up while you're poking around.
4.4 mV, when the dim bulb is not connected. Or with the variac turned up high enough so you get full line voltage at the plug when using the bulb. It should be lower than 4.4 mV with the bulb engaged. If the problem is just mis-adjusted bias, turn it down to zero on the dim bulb, then bring up supply voltage and then bring bias back where it’s supposed to be. With no emitter resistors, overshooting the bias can do a lot worse than just making the amp run hot so one needs to be extra careful.
Thanks guys.
Turns out bias in the left channel is way off. I measured over 1V across R143; reading seems to stabilise around 1.006V. In the right channel R144 measures correctly at 3.3mV.
Looking more closely at the board I see that D107 is gone. Looks like someone had cut it off at the legs... Not sure if the dark residue is burn mark, but I see that in the right channel as well which seems to work fine, so I'm inclined to think it's just glue. Someone also installed extra heatsink on Q115 and Q113, but they don't heat up anywhere near as much as Q117 and Q119.
Should I start with replacing D107 and see what that does? Or would you recommend I test other parts of the circuit first?
Turns out bias in the left channel is way off. I measured over 1V across R143; reading seems to stabilise around 1.006V. In the right channel R144 measures correctly at 3.3mV.
Looking more closely at the board I see that D107 is gone. Looks like someone had cut it off at the legs... Not sure if the dark residue is burn mark, but I see that in the right channel as well which seems to work fine, so I'm inclined to think it's just glue. Someone also installed extra heatsink on Q115 and Q113, but they don't heat up anywhere near as much as Q117 and Q119.
Should I start with replacing D107 and see what that does? Or would you recommend I test other parts of the circuit first?
