Hello. I recapped a NAD 7020e. It worked after the recap. I went to set the bias when the meter leads got pulled and shorted out. The 2N3055 (Q415) and MJ2955 (Q417) got toasted and have been replaced. In looking for further damage, would there likely be more components beyond Q411, Q413, R453, R455, R457, C431, C433, C435, C437, and C439?
Thanks,
Thanks,
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I agree with mbz for the most part but I became even more cautious over time. Long long ago I use to repair literally 100s of these these professionally.
In the day they were called "direct coupled" and that of course meant that almost every semiconductor in the output stage of a channel was "directly" connected to the one before in the signal path. As the connection between the devices is low impedance when a output device fails large and unexpected currents flow throughout the output stage often right back into the input devices. Damage due to these fault currents does not always result in total destruction of the smaller devices near the input stages but often these devices are "hurt" by having been exposed to high current spikes during the destruction of the output devices.
So testing the the smaller devices often would show them functional only to see them fail later (taking the newly installed outputs with them).
I found with time and experience to prevent callbacks of repaired amplifiers it was fastest to simply replace all the semiconductors in the failed output channel and that included any small signal diodes. You can often get away leaving Q401,402 in place but there is a risk in doing so.
Also carefully check R452,451,439,437,427,425 to make sure they are still on value. Be especially paranoid about resistors R441,445,442 that run the bias circuit as if these are off value or fail later the output devices will likely die again and you are back to square one.
Another helpful trick is always have a 60w bulb in series with one side of the power cord before you power it up as this will limit the current in the output stage and may give you a second chance if something is still not right. As well using a varaic to increase voltage slowly is best if you can get access to one. If the 60W bulb starts to glow stop and look to where all the current is going. Best of luck.
In the day they were called "direct coupled" and that of course meant that almost every semiconductor in the output stage of a channel was "directly" connected to the one before in the signal path. As the connection between the devices is low impedance when a output device fails large and unexpected currents flow throughout the output stage often right back into the input devices. Damage due to these fault currents does not always result in total destruction of the smaller devices near the input stages but often these devices are "hurt" by having been exposed to high current spikes during the destruction of the output devices.
So testing the the smaller devices often would show them functional only to see them fail later (taking the newly installed outputs with them).
I found with time and experience to prevent callbacks of repaired amplifiers it was fastest to simply replace all the semiconductors in the failed output channel and that included any small signal diodes. You can often get away leaving Q401,402 in place but there is a risk in doing so.
Also carefully check R452,451,439,437,427,425 to make sure they are still on value. Be especially paranoid about resistors R441,445,442 that run the bias circuit as if these are off value or fail later the output devices will likely die again and you are back to square one.
Another helpful trick is always have a 60w bulb in series with one side of the power cord before you power it up as this will limit the current in the output stage and may give you a second chance if something is still not right. As well using a varaic to increase voltage slowly is best if you can get access to one. If the 60W bulb starts to glow stop and look to where all the current is going. Best of luck.
Thanks to mbz and Bluesystems for the great advice and tips. I have checked the items suggested by mbz and found the only D669A to faulty.
I have ordered more transistors and will likely replace all the upstream ones for this channel.
I do usually use a dim bulb but on that day, I was rushing and pushed my luck. Noob error!
I have ordered more transistors and will likely replace all the upstream ones for this channel.
I do usually use a dim bulb but on that day, I was rushing and pushed my luck. Noob error!