AR-88lf noise floor

[ve8xj]

As I mentioned in a post recently my favorite receiver is my AR-88lf . I also have 2 CR-88a's which are great as well. I have noticed that my AR-88LF has an extremely low noise floor compared to the CR-88A ,even though signal strength is comparable on sigs between the two when I do tests.. You can hear a pin drop ,and weak CW sigs that would be covered up by the noise on the CR-88A . And I have low noise here to start with.

 The only reason for this that I can think of is that the IF frequency is different in the AR-88LF model . Its IF freq is 735kc which is fairly unique .

Could the noise floor be a result of different IF freqs ?? Or perhaps something else?

Has anyone run into this before with LF versions of radios with strange IF frequencies ?

[ke7trp]

I do not know the answer but would like to learn so I am posting.  I have noticed very similiar situation when comparing receivers here.  The old HRO50T has a very very low noise floor. Maybe 1 SU of noise.  Yet, When someone keys up, its 20 over full clear audio.  The Hallicrafters rigs, i have can blow you out of the room with noise. Like 7 or 8 SU of just noise. When someone keys up its the same 20 over clear audio.

 AGC/AVC variances?


C

[KM1H]

The AGC isnt functional without a signal.

Some of the noise is in sets that use a converter tube (6K8, 6L7, 6SA7, 6BA7, 6BE6, etc) in the front end and a self excited HFO. Using a seperate HFO as in the HRO's makes a difference. Using a pentode or triode mixer is much better.

A noisy RF tube such as 6K7, 6SK7, 6SS7 are only good to about 20M in a bandswitched radio and quite a bit higher in a HRO and those with the sliding coil carriages such as the NC-100 thru NC-240D family

Other sources are tubes with H-K leakage, and lots of out of tolerance resistors. Im assuming here that all paper caps and electrolytics have been replaced first.

A high IF will let more noise thru to start with just as sets with only 1-2 455kc IF stages that are overcoupled...S-38/S-40 as an example.

There are a few very simple octal tube swaps that can help with the higher frequencies.

Carl

[ve8xj]

Thanks Carl

 That rules out my high IF freq theory then since the receiver with the high IF is the quietest ( AR-88LF ) . I will check the tube line up on the CR-88A versus the AR-88 LF . Been a while since i looked but I think the tube line ups are fairly close except for the audio tube. Much stronger audio on the CR-88a .

 I did go through and replace all capacitors on the AR-88LF but perhaps its time for a resistor change out . But like I said the noise is very low but the signals equal the other rec.

Thanks for the input ,I will let you know what I find one day when i get motivated to take it out of the rack . But first it will be the r-390a on the surgery table.