Pin Diodes used in MB8719 chassis old and new

[loosecannon]

Hi all.

This thread is about two things having to do with the pin diodes used on the front end of this chassis.

first, in the old versions of this chassis, MC301 pin diodes were used, and now, if you look at the schematic for the current production 148's, they just use 1N60's there.

why?

this leads me to the second part of the thread.

where can i find current replacements for the MC301 pin diodes?
I know they cross reference to an NTE553, but i believe that is also an obsolete part number.

what is everyone else using to replace pin diodes these days?

I am working on two different radios right now, and they seem to have the exact opposite problems, both seeming to point to the AGC circuit.

one is under-amplifying weak signals, and the other is over-amplifying weak signals.

by the time the signal reaches 100uV, both radios act the same (S9 on the meters).

one of the places i am looking for problems is the pin diodes, and i realized i don't have any replacements on hand.

thanks for any advice.
LC

 

[Robb]

https://www.rfparts.com/1ss53.html

 

[nomadradio]

1SS53 works fine.

73

 

[loosecannon]

i was under the impression that the 1ss53 is just a silicon diode and not a PIN diode.
LC

 

[nomadradio]

Hmm. Checked for specs and it shows that it's a "switch" diode.

Oops. Need to go back and correct the inventory record that says to use this one when we're out of MC301.

Oops. Thanks for pointing that out.

73

 

[loosecannon]

thanks for the confirmation Nomad!

This is not the only place i've seen this diode recommended as a replacement for a PIN diode, and i was really hoping to get a ruling from a pro on this.

now i just have to go scour some datasheets and see if i can find something with similar specs.

as for the AGC problems im having, have you ever heard of a problem like this?
LC

 

[Robb]

Sooo....RF Parts is passing off the wrong kind of part?

Guess this proves once more that it pays to have old radios to scavenge parts out of.

 

[loosecannon]

Robb i'm not sure what Merit's thinking on this is, but the turn on voltage for those diodes he recommends is too high for them to do their job properly.

the new 148's use germanium diodes, which have a low enough turn on voltage, but without that "I" layer, my concern is that they are going to overload on strong RF signals.
LC

 

[loosecannon]

well after much searching, i found what i think will be a good candidate for a replacement PIN diode for the old Uniden chassis.

The BA479 looks to be an equivalent:
https://www.mouser.com/ds/2/427/ba479g-105032.pdf

however, i am neither an engineer nor a datasheet expert, so i am hoping that someone here can look over the datasheets and confirm whether or not this is a suitable replacement part for the MC301 diode.

here is a datasheet for the MC301:
http://www.datasheets360.com/pdf/5430447173458661123

thanks in advance to anyone who can decipher these better than I.
LC

 

[nomadradio]

Wow. Never tried to look up the MC301. Says nothing about "PIN", just says it's a switching diode.

Like the 1SS53 datasheet does.

Odd.

The BA479 data looks just like what I would expect for a front-end attenuator/AGC circuit.

73

 

[loosecannon]

thank you nomad.

I got some advice today that the BA479 might not be a suitable replacement, but haven't gotten a reason yet.

the tough thing for me is that i don't have a way to test all the characteristics that could be affected by using something different, so i have to rely on others' knowledge of the specs of the original and the replacement.

i just came across a surface mount part that might work:
https://www.mouser.com/ds/2/348/1ss356tw11-e-1017133.pdf

again, i am just trying to match up specs, and could be missing something.
LC

 

[loosecannon]

here is a short quote from the transmission1 forum on this subject:


"The stock AGC diodes are chosen as MC301 because they have a good range of dynamic resistance against forward bias current and reasonably high Q and lowish capacitance. These are desirable attributes here as they are placed across a highish impedance tuned circuit and must not load it (when AGC is not required) or add too much capacitance. They should be able to get as low as 1 Ohm with AGC applied. This should give the potential for lots of AGC range. I'm not sure of their linearity at the onset of AGC action however."

thought this might help someone.
LC

 

[OldTech03]

Hi all.

This thread is about two things having to do with the pin diodes used on the front end of this chassis.

first, in the old versions of this chassis, MC301 pin diodes were used, and now, if you look at the schematic for the current production 148's, they just use 1N60's there.

why?

this leads me to the second part of the thread.

where can i find current replacements for the MC301 pin diodes?
I know they cross reference to an NTE553, but i believe that is also an obsolete part number.

what is everyone else using to replace pin diodes these days?

I am working on two different radios right now, and they seem to have the exact opposite problems, both seeming to point to the AGC circuit.

one is under-amplifying weak signals, and the other is over-amplifying weak signals.

by the time the signal reaches 100uV, both radios act the same (S9 on the meters).

one of the places i am looking for problems is the pin diodes, and i realized i don't have any replacements on hand.

thanks for any advice.
LC

I took quick look at the data sheets but first the most significant difference between 1n60's and any pin diode would be the 1n60's will be a little bit more abrupt in putting the breaks on high level RF while the PIN Diodes will be more linear in the response and most likely from what I can see on the specs the BA479 will be a bit smoother in the effect it has on toning down the RF. One could argue PIN diodes are better suited for this job because their response more linear in the resistance sense and 1n60's are a sharper cutoff. Don't know if that makes any sense but as for trying the BA479 go for it, that is what experimenting is all about I will say the old PIN's used to have about the frequency bandwidth and that makes them a bit more suited to the 27 Mhz region.

 

[loosecannon]

thanks VT!

I appreciate you looking at the datasheets for me.

I'm getting better at it, but cross referencing semiconductors by comparing specs is not one of my strong points.

I am going to buy some BA479's and experiment a bit with them.
I don't really have a way of quantifying the results, but i will at least be able to tell if they work similarly to the MC301's.

LC

 

[Robb]

Wow...I bought a couple of the 1ss53's a couple of years ago from RF Parts just to have them in inventory. You know; to make the shipping cost more worthwhile when buying finals and drivers? Figured I'd need them eventually. But now I'm not sure if they will do the job. I'll stay tuned and see what you come up with loosecannon . . .