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28 volts pills

Motorola Engineering Bulletin EB-27A concerned 2x mrf422. It used 3T/22Ga Input and 3T/16Ga Output for the Impedance transformers. Down from typical 4T for the 2879. The article however, notes that the amp runs hot, extra cooling required.
 
Here is the modified 2SC2879 to MRF422 and a scrap 2SC2879 for comparison. I ended up with 4T input and output XFMR.

SL
 

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Here is the modified 2SC2879 to MRF422 and a scrap 2SC2879 for comparison. I ended up with 4T input and output XFMR.

SL
Alexis, if you have specific questions that I can answer, feel free. I will look at the amp and tell you what's going on there. This amp was built in the mid 90's and has not been used in decades. I am going to see how it works soon.
 
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Reactions: Alexis Mercado
use metal clad caps on the output xformer... start with 800-1000 pf.
This amp was built almost 30 years ago and it works. I used what I used. If you have a reason, for using metal/mica's, as opposed to silver mica's and HV trimmer, provide your reasoning or your statement means nothing.
 
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Reactions: Holydvr
10x better than motorola.. and you can drive them.. thanx.
Another baseless and useless post, that does not answer the question. By the way , Motorola wrote the book that the designs were based on. Google, "Helge Granberg".

If one was to build an amp using the BLW97, where would one procure them from? Please don't say eBay or Ali.
 
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Reactions: Alexis Mercado
Super Lid,thanks for posting the pics.
I have not seen the word "Lid" used since the early 70's lol.

I forgot to review the voltage rating on the electrolytics on the original Palomar 900 board. I had to order 35v replacements & I'm waiting on those now.

My first power-up Bias circuit will use MC78S15 (3A rating) for dropping 30 to 15 followed by LM1086-1.8 to voltage divider for some standing current.

I finding a discrepancy re BE current. I expected a Max of 800mA (200 each) but I'm reading reports of much higher.
What do you use for 28vdc Power Supply?
The Astron 28v psu is pricey but the 28vdc regulator board only $30. I'm using an old Pyramid chassis. Replaced Edgewise V meter with a 40 unit & higher V Transformer, wires, etc. Have outboard OVP unit (AliExpress) controlling a 100A relay to power amp.

This is my first experiment above 14vdc.
I'll post pic soon.
 
  • Like
Reactions: Alexis Mercado
Super Lid,thanks for posting the pics.
I have not seen the word "Lid" used since the early 70's lol.

I forgot to review the voltage rating on the electrolytics on the original Palomar 900 board. I had to order 35v replacements & I'm waiting on those now.

My first power-up Bias circuit will use MC78S15 (3A rating) for dropping 30 to 15 followed by LM1086-1.8 to voltage divider for some standing current.

I finding a discrepancy re BE current. I expected a Max of 800mA (200 each) but I'm reading reports of much higher.
What do you use for 28vdc Power Supply?
The Astron 28v psu is pricey but the 28vdc regulator board only $30. I'm using an old Pyramid chassis. Replaced Edgewise V meter with a 40 unit & higher V Transformer, wires, etc. Have outboard OVP unit (AliExpress) controlling a 100A relay to power amp.

This is my first experiment above 14vdc.
I'll post pic soon.
I use 3 Glenayre SMPS's in parallel. They are 14A each, removed from 250 and 500 watt paging transmitters. I have an empty Motorola CDM base station cabinet I am filling with 6 Mean Well 6.5A PSU's right now. It is better for the desktop, than the rack mount Glenayre's.
 
After reading about, I may be short on base current.
www.ifwtech.co.uk/g3sek/tr-bias/tr-bias1.htm used a Tip41 for an amp using 13..8v. Won't know until I try.

Hoped to avoid the diode to ground/Tip31 by increasing voltage to 28v. Backup plan is adding Tip41 series-pass fed by same regulator combo. Still waiting for capacitors. neil
Starting point.jpg
 
Last edited:
After reading about, I may be short on base current.
www.ifwtech.co.uk/g3sek/tr-bias/tr-bias1.htm used a Tip41 for an amp using 13..8v. Won't know until I try.

Hoped to avoid the diode to ground/Tip31 by increasing voltage to 28v. Backup plan is adding Tip41 series-pass fed by same regulator combo. Still waiting for capacitors. neilView attachment 59097
Whose PCB is that? I am just guessing, but is that an RM board? Why did you opt for HG devices? The Motorola part is still available as Ma/Com. I have not tested a single HG part that met the spec of the part it is supposed to replicate. I won't use them in anything. I am curious to see how this turns out. As I offered to the OP, if you have any questions about my 2879-MRF422 conversion, just ask and I will pick it apart. The scrapped 4 module 2879 KW was such, to donate parts for a 4 module MRF422 KW.
 
Super Lid,thanks for posting the pics.
I have not seen the word "Lid" used since the early 70's lol.

I forgot to review the voltage rating on the electrolytics on the original Palomar 900 board. I had to order 35v replacements & I'm waiting on those now.

My first power-up Bias circuit will use MC78S15 (3A rating) for dropping 30 to 15 followed by LM1086-1.8 to voltage divider for some standing current.

I finding a discrepancy re BE current. I expected a Max of 800mA (200 each) but I'm reading reports of much higher.
What do you use for 28vdc Power Supply?
The Astron 28v psu is pricey but the 28vdc regulator board only $30. I'm using an old Pyramid chassis. Replaced Edgewise V meter with a 40 unit & higher V Transformer, wires, etc. Have outboard OVP unit (AliExpress) controlling a 100A relay to power amp.

This is my first experiment above 14vdc.
I'll post pic soon.
It occurred to me later on, but which reference to "Lid from the 70's" are you referring to?
 

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