RCA CR-88 Receiver Test Results

[W1VD]

Receiver: RCA CR-88

Band	MDS	Blocking DR	Two-tone DR
		(20 kHz)	(20 kHz)
80 meters	-140 dBm	102 dB	77 dB
40 meters	-139 dBm	105 dB	78 dB
20 meters	-137 dBm	102 dB	78 dB

AM audio S/N: 48 dB

AM Audio Frequency Response: in dB, referenced to 1 kHz measurement   

100 Hz	200 Hz	400 Hz	600 Hz	800 Hz	1 kHz	2 kHz	3 kHz	4 kHz	5 kHz	6 kHz
-4	-2	0	0	0	0	0	0	0	-2	-8

AM Audio Distortion:

Mod	100 Hz	200 Hz	400 Hz	600 Hz	800 Hz	1 kHz	2 kHz
30%	6.3%	5.0%	4.0%	3.5%	3.2%	3.2%	4.0%
50%	5.6%	3.5%	2.8%	3.2%	3.2%	3.2%	5.6%
70%	5.0%	2.5%	2.5%	3.2%	3.5%	3.5%	7.1%
90%	6.3%	3.2%	3.2%	3.5%	4.0%	4.5%	10%
100%	8.9%	4.0%	4.0%	4.0%	4.0%	5.0%	11%

Notes: MDS, blocking and two-tone DR measurements performed using the narrowest crystal filter setting. AM audio recovery tests performed using the widest bandwidth setting - about 12 kHz @ 6 dB down. AM distortion numbers show the somewhat typical increase at lower audio frequencies - most often caused by the low audio frequencies riding on the AGC line. Also there is the usual increase in audio distortion as the modulation percentage increases. This is due to the detector's inability to cleanly demodulate the higher amplitude levels.

Thanks to W1MPY for use of this receiver.

Info on the measurement setups and test results on other receivers can be found at:

http://www.w1vd.com/BAreceivertest.html

[WA1GFZ]

wow, my guess was off for 2 tone. I wonder if you tried reducing the RF gain a bit? I suppose the first mixer got trashed easily with all that rf gain.

[W1VD]

Frank

I expected better dynamic range, too.

Yes...the gain was varied over a range with little improvement. As with most receivers without a product detector it was run manual gain with the 'rf gain' (includes 2 stages of i-f) backed off and af gain cranked up. Before making measurements I use the receiver (on an antenna) and note the range of appropriate settings for these controls.

The lineup is two rf amplifiers and only one mixer...this is a single conversion receiver with a 455 kHz IF. The 6SA7 mixer is operated not exactly in line with the RCA data sheet. Screen voltage is about half that recommended and LO injection seems to be on the low side. All this results in a cathode current about 1/3 of that recommended. Not my receiver so can only go so far. 

On the plus side, this is one nice sounding hifi AM receiver. The widest BW is only slightly narrower than the SX-28A which is my unofficial 'benchmark' for tube hifi AM reception. Low end could be extended with one or two coupling cap changes.

Lots of time spent on this one...especially the swept i-f alignment (13 adjustments!).   

       

[WA1GFZ]

I bet the mixer is hitting cut off. What a beast of a radio. Looks like something that belonged on a battle ship.
Hey how about feeding the Perseus after the second RF. Now that would be performance.

[WD5JKO]

Low end could be extended with one or two coupling cap changes.
Lots of time spent on this one...especially the swept i-f alignment (13 adjustments!). 
   

Jay, I wonder if you'd be willing to share how you swept the IF alignment specific to the CR88? I ask because I have one here holding down a table. This one is an AR-88 that I got from Ozona Bob. Bob bought it as a 'Mint" receiver, but it came in minus two front panel controls, and NO wiring harness. The crystal phasing IF can was also missing. I got the beast running again, but I never got the IF alignment right since I just peaked for maximum gain.

You say that the low end can be extended "with one or two coupling cap changes". I did this, added NFB, and it just wasn't doing as I had hoped. What I found was that the audio output tube B+ was being modulated by the tube current draw pulsing (male voice being played) such that the power supply filter resonance was being excited. I saw a wavering B+ that was going from about 150V to 350V at a rate of about 80 Hz. This was kind of like driving a car with no shock absorbers! Adding more bass to the audio of a AR/CR88 means addressing power supply issues first.

Regards,
Jim
WD5JKO

[W1VD]

Jim

Haven't noticed anything unusual with the audio ... although I haven't looked that close at it either. Will put a scope on it tonight.

For the swept i-f alignment I used an HP-3325A sig gen/Tek 2465 scope. The procedure I followed is basically that in the CR-88 manual available from bama

http://bama.edebris.com/manuals/rca/cr88/

...but with a few changes:

* Preset C75 the crystal phasing variable capacitor to half mesh.

* Determine the crystal filter center frequency and use this as the center of the i-f alignment.

* In steps 2 and 3 make the adjustments in bandwidth position 1 to get the double humps + and - 6 kc off center. Check for proper bandwidth 'shrinkage' in position 2. This is backwards from the book but seemed to work out better.

Don't be concerned with the noticeable double hump response during steps 2 and 3. With step 4 the first i-f can is added in (a single response) and the top becomes amazingly flat. The first and last i-f cans are single response and the two middle sets of i-f cans are double humped.   

If you need more detailed info feel free to PM me.