Schottkey diode/transistor opinions

[tramd]

The swap to the Sanyo 2SC2999E never much amounted to anything. Someone who didn't know how to read data sheets and didn't understand that there is no test standardization amongst transistor manufactures saw that the noise level was slightly lower than the 2SC1675 and 2SC1674 transistors and thought that would make a difference and everyone fell for it. A dog couldn't tell the difference, let alone a person. The 2SC2999E has practically the same specs as a 2SC1674. The claims were all over the place on results. Some claimed better receive, some worse, and some said they were quieter. The biggest problem was that the 2SC299E had been obsolete for years. Most of the ones people bought were fakes, maybe restamped 2SC2999Ds. A transistor tester would easily reveal that the HFE ratings were usually too low for them to be a real 2SC2999E. I bought some at one time that were guaranteed to be original from a respected seller. They had a lower HFE reading that the original 2SC2999Ds that I had from years ago. Most probably never realigned the receiver after installing them so that is why it seemed quieter. For those that said it gave them better receive, it was most likely because the transistor they replaced was weak. The RF transistor is the first one after the antenna and takes a beating from extremely strong signals and electrical static in the air. On one of the forums years ago, a few guys with high-end audio equipment did some testing and concluded that they made no difference that a person could actually hear if a real 2SC2999E was used and the radio aligned. As far as using Schottky diodes, search for Handy Andy's posts about that here on this forum. They seem to help in noise blanker circuits, but germaniums are better for audio. I have personally tried Schotkeys at different times, and they cause a clicking noise in the audio of some radios, especially in the 858 chassis but do help in the noise blanker. A good set of germaniums such as original Motorola's sound much smoother in the audio circuits.

 

[Hawkeye351]

Cable,
If I unhook coax, I still get some hash noise (not as loud though) but the level drops to zero on the signal meter. I have read also that a leaky or weak transistor in the first stage could cause a lot of noise, this one must be very leaky then, lol...

The meter on AM stays around 5 to 7 s-units, and the meter on SSB floats around the same, no one talking, just the needle floating (I adjust L14 to drop it to just barely lift off zero). Both meter VR's adjust smoothly up and down, at the moment they're set around the halfway point as most of these Saturns land on receive alignment. I could turn the meter VR's all the way down, but then the audio won't correspond with the signals.

It's just an annoyance for me, lol...

 

[SuperLid]

I heard somewhere that germanium diodes were better than Schottkey but I'm no expert by any means.

The major difference between germanium and schottky, is the forward voltage drop. The schottky tends to be slightly lower, than the germanium. When increasing the voltages to the AGC circuit, that is when things get nasty. Overshoot, overload and has absolutely nothing to contribute, in reducing MDS or IP3. The only way to reduce inherent noise, is to reduce inherent noise. This would be without signal applied to the receiver front end, not when connecting your A-99. The schottky illusion, is more gain from the AGC (higher S Meter) , with the same signal, but the noise has not changed.

Lower noise figure RF amps could be employed in the front end, which I have done to some degree of success. The greatest faults I have seen in the RX's, is the AGC circuit, which can be tamed to a degree. I have done this in Ranger radios, which make them tolerable. But at the end of the day, the RX is shit, regardless of what fluff you put on it to make it taste sweeter. Shit, will always be shit... Interestingly, Ranger copied the Uniden AGC circuit verbatim and it still sucks. Component selection in the front end and around the AGC circuit has everything to do with this.

73,

SL

 

[Cable Guy]

Hawk, after alignment if I come across a radio that has any real static or hash with nothing plugged in, aside from a cobra 29 (misadjusted if amp) I would proactively replace the first RF amp and the mixer following. As a post above said, that first amp can take a beating. I would use whatever it calls out if I have it. If all the radios across your bench have a high hash level when connected to your antenna, probably something generating it locally. Just tune with the generator and if it's quiet on a dummy load or no antenna connected, it's good to go. Now, if all the hash is coming in from your antenna, that's another issue. Time to go hunting, may the odds be ever in your favor, or something like that.

 

[Hawkeye351]

I did try the NTE583 diodes last night, not really any improvement on the hash noise level. If there was any improvement it was minut, but I did notice a slight improvement on weaker signals coming through (not by much though).

I'm gonna try replacing TR17 this afternoon with one with a higher HFe and see what that does. It would be nice to eliminate completely that hash level, but it's no biggie because it's my personal radio.

I appreciate all the input so far. Y'all are answering a lot of others questions also, those that just read and not post are learning from y'all's input.

 

[Hawkeye351]

I may be chasing ghosts, lol...

My mobile, after I aligned it, exhibited the high hash level also. When I hooked it up in the truck to my whip and get away from my house the level drops to what I feel is a normal level. Maybe that's why I haven't had anyone complain of low or too high receive is because I align theirs to the generator and let it go, regardless of how high that hash level is on my desk, although it gets under my skin on my desk with that high a level, lol...

 

[SuperLid]

Hawk, after alignment if I come across a radio that has any real static or hash with nothing plugged in, aside from a cobra 29 (misadjusted if amp) I would proactively replace the first RF amp and the mixer following. As a post above said, that first amp can take a beating. I would use whatever it calls out if I have it. If all the radios across your bench have a high hash level when connected to your antenna, probably something generating it locally. Just tune with the generator and if it's quiet on a dummy load or no antenna connected, it's good to go. Now, if all the hash is coming in from your antenna, that's another issue. Time to go hunting, may the odds be ever in your favor, or something like that.

The 1st RF amp, should have the lowest noise figure in the chain. Everything after the 1st, amplifies the noise of that stage. Try to get your hands on 2SC1730 or MPSH10.

 

[SuperLid]

I did try the NTE583 diodes last night, not really any improvement on the hash noise level. If there was any improvement it was minut, but I did notice a slight improvement on weaker signals coming through (not by much though).

I'm gonna try replacing TR17 this afternoon with one with a higher HFe and see what that does. It would be nice to eliminate completely that hash level, but it's no biggie because it's my personal radio.

I appreciate all the input so far. Y'all are answering a lot of others questions also, those that just read and not post are learning from y'all's input.

Gain is not the answer. NF is....

 

[sp5it]

Changing silicon diodes to Schottky or germanium benefit in AM and NB detector. Because of lower Vf both detectors are triggered faster than silicon diodes.
Replacing input or mixer transistor won't help as long as you set correct Q-point.
Mike