09-16-2017, 10:06 PM
Well, I spent the day today on the 48-482, starting with your suggestion of injecting a signal at the front end and looking for AVC action. I had plenty of negative voltage, but no change with changes in the strength of the input signal which made me think the AVC wasn't working for some reason.
But, I went ahead and tweeked the FM IFs, gaining in speaker output along the way. With that done I still got no reception other than weak, garbled signal. I proceeded with the FM alignment procedure in the literature all going well up to the the point of tuning for the high and low ends of the FM spectrum. Since my signal generator only goes to 30 Mhz I had to use broadcast stations for this, but still no reception.
I then had a revelation. I didn't know that the FM antenna was a different connection than the interior loop antenna. It's pin 2 on the small four prong jack at the back of the chassis.
I wondered what that was for. The schematic says "to Z100", which I couldn't find anywhere. Guess it's the external antenna.
When I connected a long-wire antenna to that - ta da - I started receiving numerous stations. So, I went back, did another AM alignment, FM alignment and it works amazingly well. I guess it was just massively out of alignment to begin with. I don't think there was a single screw that didn't need turning.
The only anomaly I came up with was the adjustment on C503 for maximum at 580 Khz. It had no effect whatsoever on the output.
It's now playing strongly, sounding good. I'm working on cleaning it up and putting it back into the cabinet.
I'll post photos tomorrow.
But, I went ahead and tweeked the FM IFs, gaining in speaker output along the way. With that done I still got no reception other than weak, garbled signal. I proceeded with the FM alignment procedure in the literature all going well up to the the point of tuning for the high and low ends of the FM spectrum. Since my signal generator only goes to 30 Mhz I had to use broadcast stations for this, but still no reception.
I then had a revelation. I didn't know that the FM antenna was a different connection than the interior loop antenna. It's pin 2 on the small four prong jack at the back of the chassis.
I wondered what that was for. The schematic says "to Z100", which I couldn't find anywhere. Guess it's the external antenna.When I connected a long-wire antenna to that - ta da - I started receiving numerous stations. So, I went back, did another AM alignment, FM alignment and it works amazingly well. I guess it was just massively out of alignment to begin with. I don't think there was a single screw that didn't need turning.
The only anomaly I came up with was the adjustment on C503 for maximum at 580 Khz. It had no effect whatsoever on the output.
It's now playing strongly, sounding good. I'm working on cleaning it up and putting it back into the cabinet.
I'll post photos tomorrow.
