R75 Help Needed

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SCPD

QRT
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Feb 24, 2001
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Hey guys,

So I dug out my R75 to repair a couple of gremlins. Before I started working on it, I checked its operation and there's 2 main problems that I could reproduce:

1. The DC power jack was coming loose - to the point where it could short. I later saw this was due to very poor soldered joints. I repaired this easily.

2. The radio seems to have a problem where it loses it's incoming RF input signal. For example, I'm monitoring WWV/H or something strong and suddenly the signal strength meter drops to zero - almost as if the antenna connection suddenly was pulled. If I very lightly tap on the top or side of the case, the signal would return. To me, this spelled a simple contact joint - bad soldering, etc.

So I examined the PLL board. The higher-grade crystal ref osc that I added years ago was a tad loose. The part itself seemed loose but the soldered side looked okay. I resoldered this part anyway. After the resoldering job, it seemed tighter and had a stronger fit.

After putting everything back together, I now have a new problem. I'm now not able to get any RF audio output.

Example: Tune to WWV/H @ 5 Mhz and the signal strength meter shows a full signal, but no RF audio. At first, I thought a speaker problem, but the keypad "beeps" come thru just fine. The headphone and speaker jack both have the same output - no RF audio but control beeps. I removed the DSP unit and also the optional 455khz filter just to eliminate any other variables. I also performed a full reset to see if that would help - but it did not.

I have the R75 service manual and have done the basic tests. I checked the PLL board first. My DC voltages were not exactly 4.0 V as per the service manual but seem reasonable (only a couple of tenths higher.) My P1 output (Ref Osc) was per spec at 60.00002 mhz. There is one test of P151 (the 3rd LO) where the value is suppose to be very close to the freq setting of the VFO. This test was about 500khz-1mhz off. The example shows 9.4615 and the value is suppose to be identical. I was getting 10.22-10.83. This seems a bit off? I don't have a RF voltmeter, so I cannot check the RF voltages.

I know the PREAMP and ATT functions work with respect to the meter. Tuning on to say a local AM station seems to function correctly. So the VFO is working fine. My best guess is something in the PLL unit is not working.

Any ideas on how to troubleshoot? Would measuring DC voltages on the PLL board help any? Any other ideas?


Thanks
-Nick
 

nanZor

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May 28, 2009
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Sounds like you covered your bases, but I'll throw out something that bit me a few times.

I know that when you do a CPU reset on the R75, the RF/SQ defaults to "SQ", and if you have the RF gain pot mostly clockwise, everything is squelched in that position and sounds like what you are describing even though you can tune around and see the s-meter moving. (p14)

Seems simple, but this has thrown me off before since you have 3 different setup options on how the rf-gain pot operates:

SQ
AT
RS

I normally run with the other options, either "AT", or for the most part in "RS" mode, where I have the rf-gain mostly clockwise. If I do a cpu reset (which I did just to test this out again), the rig defaults to SQ squelch mode, and with the pot mostly clockwise - bingo - no audio except for keypad beeps. Not even the "RX" icon in the upper right.

This drove me crazy at first and hope this might be the case here.
 

SCPD

QRT
Joined
Feb 24, 2001
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0
Location
Virginia
Good thought Hertzian, as I did have the SQL setting back to the default. But I was pretty sure I had checked that before. (I did several resets.)

Now it's set to RS and I rarely use the SQL control anyway. I always leave mine fully open (counterclock.)

When hooking the power cord back up again, I noticed that the power jack is acting up again. I think the inner conductor piece is not providing a solid connection to the plug. Everything else on the radio seems to operate okay despite that flakiness however.

I might order a replacement power jack to see if that helps any. It's a cheap part to replace and simple to solder.
 

nanZor

Active Member
Joined
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You know what? I tossed the OEM power supply after verifying that even under load, mine was also putting out 18v as described here:

ICOM IC-R75 Receiver (AD-55 modification) - N9EWO -

I just tossed mine - too risky so I run with a different small well-regulated 12V supply. I saved the plug from the oem supply and used that. Maybe the plug/wire on your power supply is bad and not the radio's jack itself??
 
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