I have a Nakamichi PA5 that has developed an issue where it sounds as if there is no bass form the right channel. Left sound perfect.
Any ideas of what I could check?
Any ideas of what I could check?
I don't have to look at a schematic to guess it's the electrolytic capacitor to ground in the resistive divider feedback path from the output to the negative input of the long-tail pair. Class AB amplifiers have had essentially the same schematic since at least the 1970s, maybe the 1960s. High capacitance is needed here, thus it's electrolytic, but it has virtually no voltage across it, so it eventually "deforms." A google brings up the schematic in the first post of this thread:
https://www.diyaudio.com/community/threads/nakamichi-pa-5-and-pa-7-design-help-requested.177101/
The cap in the PA-5 schematic is C102, 10uF/200V. That's for the left channel, but while you're in there you should replace that capacitor in both channels with new ones, as there's no telling when the other one will go out. You could put in a lower voltage cap here, I don't suppose it matters in this location. Maybe a non-polar would last longer.
https://www.diyaudio.com/community/threads/nakamichi-pa-5-and-pa-7-design-help-requested.177101/
The cap in the PA-5 schematic is C102, 10uF/200V. That's for the left channel, but while you're in there you should replace that capacitor in both channels with new ones, as there's no telling when the other one will go out. You could put in a lower voltage cap here, I don't suppose it matters in this location. Maybe a non-polar would last longer.
We'll I took the right channel board out and removed C102, it measures 9.95uF so it is perfectly fine, so that does not look to be the issue.
I went ahead and ordered new caps for all the electrolytic caps so we will see. Also found another post regarding the same cap in a PA7 and it is indeed a Mylar Cap.
I went ahead and ordered new caps for all the electrolytic caps so we will see. Also found another post regarding the same cap in a PA7 and it is indeed a Mylar Cap.
That's remarkable that it's not that cap. Obviously they had good reason to use a Mylar cap there.
Looking at the schematic in post #4, there's relay contacts on the output, and (for some reason I can't think of) a 1 uF capacitor across it. I didn't trace closely through the circuitry, but this is apparently to give a turn-on delay at power up, and to protect the speakers if there's a fault in the amplifier. That 1uF is enough to drive a tweeter, but of course not pass bass. It seems if that relay isn't activated, or the contacts for that channel aren't making a connection, that's what you'd hear.
Looking at the schematic in post #4, there's relay contacts on the output, and (for some reason I can't think of) a 1 uF capacitor across it. I didn't trace closely through the circuitry, but this is apparently to give a turn-on delay at power up, and to protect the speakers if there's a fault in the amplifier. That 1uF is enough to drive a tweeter, but of course not pass bass. It seems if that relay isn't activated, or the contacts for that channel aren't making a connection, that's what you'd hear.
