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Author Topic: MIRAGE 2530G 144-148Mhz (25 in/300 out) help  (Read 184 times)

KO4ENU

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MIRAGE 2530G 144-148Mhz (25 in/300 out) help
« on: May 05, 2022, 11:24:44 AM »

I have an older MIRAGE 2530G 144-148Mhz (25 in/300 out) where the T pad input attenuator legs have broken off. I have called MFJ/Ameritron and numerous technicians that are not willing to work on the amplifier because they do not have a schematic. I am looking for a schematic or anyone that may know the value of the T pad input attenuator. THE pad is labelled with KDI in red on it.

Speculation
I looked on the KDI data sheet/website and it looks like a KDI PPA20 but what is the db value.
I have seen schematics online and they all omit the value. At this at this point I will also take best logical guess answers or any suggestions.
Thank you in advance
73
Steve
KO4ENU
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WA3SKN

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Re: MIRAGE 2530G 144-148Mhz (25 in/300 out) help
« Reply #1 on: May 05, 2022, 01:58:58 PM »

This might help...

2dB 20w attenuator.

https://launch3telecom.com/kdi/ppa202.html

-Mike.
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WB8VLC

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Re: MIRAGE 2530G 144-148Mhz (25 in/300 out) help
« Reply #2 on: May 05, 2022, 02:42:10 PM »

I think the attenuator power rating is correct for  the info that WA3SKN  gave  but the attenuation value should be greater  than 2 dB as that amp was made in either a 5 watt or a 25 watt drive option as I remember.

If that is the amp that I am thinking of then it was made in a 5 watt or a 25 watt drive option and as I remember mirage used the power amp stage/stages in both amps then the attenuator was added in for the 25 watt drive model and it should be at least 7 db at a 20 watt power rating or more for a 2 to 1 resistor safety margin. 

The attenuation value required is at least 25/5 =  10log5 = 6.9897 dB ~  or a 7 dB then the following calculations should apply.

Also If the attenuator is a single ceramic part and you can't find the actual part then You can replace the attenuator with 3 series discrete resistors for the Tee pad as follows.

25 watts/5 watts puts the attenuator at ~ 7 dB and for the T pad the two series equivalent resistors should be 19.124 ohms (20 ohms is closest value) and the middle shunt resistors is 55.802 ohms (56 ohms is probably OK here).


 I used 20 ohms series and 56 ohms  shunt for my following calculations of the input attenuator and a times 2 power rating safety factor minimum for resistor power ratings, hence the 20 watt rating of the attenuator in my calculations especially  if that attenuator used is a 3 legged ceramic part then the input resistor would set the maximum power rating of the attenuator used with a 2 to 1 safety factor.

The input Tee pad 20 ohm attenuators first series resistor will see ~.707 amps thru it so it needs the greatest  power rating and  the first series resistor should dissipate a power of   (.707^2) amps X 20 ohms ~ or around 10 watts so I would use 20 watts for the first series attenuator component which sets the max 2 to 1 safety factor rating of the attenuator.

The 56 ohm shunt resistor should see ~ .39137 amps so it needs to dissipate (.391375^2) X 56 around ~ 8.577 watts, use 16 watts or greater for this shunt and the last output series R should dissipate around (.3131 amps squared) X 20 ohms so it should be rated ~ 1.959 watts, I'd use at least 4 watts here.

Additionally; I have the high power version of that amp but mine is for 220 MHz and it's at home but I can double check my attenuator value and make some measurements and get back later next week with what I find.
« Last Edit: May 05, 2022, 02:48:19 PM by WB8VLC »
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