I've been a shortwave listener since the early 1970s when I was ten years old. Over that time I've built and bought a number of receivers. At a hamfest back in 2002 I came across an ICOM IC-R70 that was from the estate of an SK. The radio was more expensive than I really had planned to spend, but the more I looked at it the more convinced I became that it needed a good home.
I really didn't go looking for an R70, so I wasn't familiar with the radio. Getting it home and doing some research on the Internet convinced me that this was one great rig. The government used these in monitoring stations and it's descendant, the IC-R71A is considered by many to be among the best receivers around. There are even those who say the R70 is better because it has less digital hash than the later rigs.
My first impression was of a very solidly built receiver. Mine did have a problem when I bought it (see details below), but opening it up to work on it only further convinced me that this radio is built STOUT! There is no wide open space inside--it has a very sturdy frame and several PCBs.
The audio suffers a bit because of the small size of the built in speaker, but with a decent external speaker it sounds great! It has several crystal filters, so you get an audio response that makes sense whether you're listening to AM, SSB, or CW. So many other radios make compromises on the number of filters because they do cost money.
My R70 also came with the optional FM board.
I have since sold my R70, but have kept this page up in hopes that maybe some of the troubleshooting I describe below will help someone else.
A little playing around convinced me something was amiss. Even with the antenna jack shorted, I could only turn the volume control up about 1/3 of the way before the static reached ear splitting levels.
Since the noise was internally generated, I started at the antenna and worked my way back using a capacitor to temporarily 'short' the signal coming out of each section to ground. This is kind of a reverse signal tracing--instead of injecting a signal and following it, you have something generating the noise internally and you try to shunt that noise to ground.
I quickly isolated the problem to the noise blanker area. The 2nd mixer feeds an IF can (L2) which connects to another can (L3) which feeds the noise blanker diodes.
The input to the noise blanker is pulled from between L2 and L3 and fed into a JFET amplifer (Q1) which feeds an op-amp (IC1) which drives a transistor (Q3) that is used as a switch to turn off and on the noise blanker diodes (in the signal chain after L3).
I tended to suspect the op-amp and spent a while checking it and the rest of the noise blanker circuit, but everything seemed okay. After a bit of head scratching I finally figured out the the JFET isolation amp (Q1) was the problem. Instead of acting like a nice isolation stage, it had failed in a mode that turned it into a noise source.
After replacing Q1, the radio quieted down to the point where you could crank the volume all the way with no antenna and the static wasn't uncomfortably loud.
Now that the radio was quiet, I noticed a different problem...
In AM mode there was a faint 'beat note' in the background. For example, if I was listening to an AM station at 15.180 MHz, I'd hear a weak 100 Hz tone when I tuned to 15.180.1. When I tuned up another step to 15.180.2 the beat note changed to a weak 200 Hz tone. The further off the carrier frequency I tuned, the higher pitched the tone.
When tuning into a strong station, the note isn't detectable much more than 500 Hz above the signal. And it isn't detectable below the signal. Inside that window, the note was very weak and barely noticeable--but it was there.
My first thought was that the BFO was on (which it shouldn't be in AM mode), but when I check the output with a frequency counter I get good values when in SSB and CW mode, but no count when in AM mode.
Several people suggested checking all the screws holding the PCBs were tight. Others suggested checking that the cables running by the display module were properly routed. Neither of these yielded any change.
On a repair mailing list, one person I have a great deal of respect for (Bill, K0ZL) seemed certain the noise was related to the display module. I double checked the cable routing and all the screws in this area with no results. Finally in desperation I broke out the roll of aluminum foil and tried putting temporary 'shielding' in the rig the see if that made a difference. Nothing changed.
After thinking about it several days, I decided that the best way to remove suspicion from the display module was the remove power from the display and see if the problem went away. I tuned to a beat note and listened as I did this... And found out that when you pull the plug on the display board, the receive pretty much goes dead.
I powered the receiver down and reconnected the display board. I was disappointed not to be able to strike this possibility off the list. But then I noticed something. When I tried to find the beat note again, it was gone.
Somehow the connector had been the cause of the problem. Having worked as a computer tech during college, I know a lot of times you can 'fix' strange problems by reseating chips and wiggling some of the connectors. I'd never really though about doing this to the R70. Now I used a good contact cleaner and went through all the connectors.
The radio worked like a champ! While I had it open I ran through the alignment process and the rig works great!
P.S. I put this whole thing here because maybe someday someone will be troubleshooting similar problems and find it useful.
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