Re: {Collins} R388 Progress

----- Original Message ----- From: <ka4koe@xxxxxxxxxxx>
To: "Richard Knoppow" <dickburk@xxxxxxxxxxxxx>
Cc: "Collins@xxxxxxxxxxxxx" <collins@xxxxxxxxxxxxx>
Sent: Thursday, July 18, 2013 8:09 AM
Subject: {Collins} R388 Progress

OK, adjusted trimmer marked XTAL as suggested offline. Only managed to get around 1 KC of shift, if that much. It wasn't alot.

Aligned BFO, Aligned calibrator with WWV on 15 MHz. The fiducial is dead center on 2 MHz.

Still out some bands. It is off on band 1.

Will make a record of which is which and likely just move the KHz dial to split the difference. I do not want to buy crystals if I can avoid it. If it is one or two, maybe, but not a whole batch.

Also replaced 100 PF cap for the AVC issue. Signal meter needle is not going full scale anymore, but rests at around 1/3 of full scale on signal meter. I assume the other 100 PF will need replacing as well.

Getting close. I am almost to the point of "is this good enough".

Philip KA4KOE

I have not read all this thread so maybe repeating something. The XTAL trimmer does very little. The instructions say to put it near minimum capacitance. You can just put it right at minimum. The capacitance across the crystals is not easy to measure. After trying other methods I used my ancient Boonton Q-meter. 30pf was very slightly more than the minimum setting. I experimented and found that the setting made very little difference in frequency or amplitude so left it at the measured point. When you are on the two bands with red calibrations no crystals are used. The dial should be set to read right on these bands with the fiducial at the center of its range. Set the both dials at center and run through the bands. There will be some deviation from band to band due to crystal ageing. I think the spec was something like 1 khz. If its more than a couple of Khz the crystals need to be replaced. This can get expensive if a lot of them are off frequency. However, you can live with the deviation if you are not a perfectionist and the fiducial will correct for quite a bit. I am more concerned with the linearity of the VFO than having exact crystal frequencies. I would leave the fiducial centered at 2Mhz since this is a band with no crystals. If its not centered on 3 Mhz check the BFO as below. When the BFO is exactly at 500 Khz both 2 and 3 Mhz should come in at the same fiducial setting. Note that you can set the BFO at exactly 500 Khz by setting the bandswitch on the lower red band. With no antenna connected set the dial to a frequency that is not a multiple of the calibrator. Turn on the calibrator and the BFO, you will hear a beat. Adjust the BFO for exact zero beat and it will be at 500 Khz. This is important for checking the calibration. FWIW, I replaced a couple of crystals in my R-388 with units from ICM, they were exactly on the nose. I don't have a suggestion for the signal strength meter but its certainly worth checking for bad caps and drifted resistors. I assume you have tested the tubes to find really bad ones. I checked my receiver for the minimum level that operates the AVC. This is the "AVC delay" level and is about 2.5 uV. That's the level where the AVC voltage begins to increase with the signal. I found the meter to be reasonably accurate in measuring signal level above this. My meter will pin on very strong signals and should pin when the RF gain is at minimum. You can test the meter by using the audio readings. You should be able to get full scale peaks. This is just to be sure the meter isn't sticking. There are some who criticize the 51J-3/R-388 for various reasons but IMO its an excellent receiver and fun to use.

Richard Knoppow
Los Angeles

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