I sold my old Technic SA-200 receiver that I had been using as my bench receiver. I brought it along to the NEARC Westford show mostly in case someone wanted to audition the restored Voice of Music speakers and tuner I was trying to sell, so I didn't really want to sell it. I priced it higher than I thought it was worth, but someone liked the looks of it and bought it.
So, I dug into the pile and brought out a Nikko NR-719. It mostly worked as-is on FM. Deoxit on the controls along with vigorous exercising of them cleaned up the noisy contacts and potentiometers. I put a little Deoxit on the tuning capacitor contact points, too. The AM section was dead and the signal strength and tuning meters didn't move. And, of course, the pilot lights were soldered in and every one was burnt out.
I couldn't find a service manual for the NR-719, but HiFi Engine had a service manual for the NR-819. I suppose the power amplifier and power supply sections are different, but the RF and preamp sections of the NR-819 manual seem to match my NR-719.
The unit had a little rust on the bottom cover and a little less corrosion on the top cover and some of the internal structure. It also came with a few dead insects inside. It may have been stored outside in a covered location.
I cleaned up the rust on the bottom cover with a wire wheel in a hand-drill and coated it with Krylon clear spray to protect it. The rest of the corrosion was minor enough that I just let it go; it's only a workbench receiver, after all.
I next tackled the meters. The service manual says to release the spring wire clamps from the read of the meter and just pull it out. Mine were held in by hard-to-see clear tape in addition to the wire clamps. I had to pry them out a little with a screwdriver to stretch the tape a little, then get in to cut it with an X-Acto knife. When I put them back in, they seem to be secure enough with just the wire spring, but if I were to ship it, I'd want to replace the tape to prevent the meters from being jarred loose.
Here the meters are removed from the front panel. The meter bezel is molded into the clear plastic front cover for the tuning scale, so the needle and movement are exposed when they are removed. So be careful, and only do this in a clean environment so crud doesn't get into the movement.
I unsoldered the meters, put my DMM on a high resistance range, and put the probes on the meter lugs. The meter moved but stuck returning to zero.
I played with the needles on both meters; they were both sticky.
I examined the movements carefully but I couldn't see any crud that would rub against the armature.
I guessed that the moist storage environment corroded the bearings. So, I started backing out the pivot screws. It took about three-quarters of a turn before the meters would reliably return to the zero point. The signal meter zero was a little off, but the zero adjustment had been sealed with black lacquer and I couldn't get it to budge with the limited amount of force that I felt comfortable applying to it.
I reassembled the meters and resoldered them and they are now working nicely on FM.
I started investigating the AM section. I could get a signal if I cranked my signal generator output level all the way up, but below about 50 millivolts the receiver abruptly cut off. I checked the ceramic filter with a scope; there was a strong signal coming in, but nothing coming out. I at first suspected that the filter was bad, but then I measured the IF frequency by plugging my counter into the vertical output of my scope - the IF was set at 530 kHz! So nothing was going to get through the 455 kHz ceramic filter. Someone had golden screwdrivered the alignment.
I started following the alignment instructions in the service manual (with a little extra bit of interpretation, as the manual would have you use a sweeper to do the alignment). I got a big clue there: the adjustment on the second IF did very little. After I got the local oscillator on the right frequency so that I was getting a signal through the ceramic filter, I started looking at the second IF.
The AM receiver uses a Hitachi HA1197 chip, so the second IF is a little strange - it's a series-resonant LC circuit. Examining the signal with a scope, I saw a strong signal coming in but only a small signal after the LC circuit, and diddling the core didn't make much difference. The IF can had a few extra pins; the Nikko schematic didn't show them, but the data sheet for the HA1197 did. Assuming the can matched the Hitachi data sheet, I measured the coil resistance as 6 ohms, so probably good, and the capacitor measured about 13 kOhms. Aha! The cap appears to have shorted, so the circuit can't be tuned to resonance, and the series resistance of the short is high enough to greatly attenuate the signal.
I still have to test this theory and see if I can repair the second IF can by replacing the capacitor. I also still have to test out the phono preamp.
Update:
Yes, replacing the capacitor in the second IF can fixed the problem.