The AM Forum

I pulled the mod deck out to fix the high voltage meter dropping resistors, which had drifted up.
I built this about 30 (or more) years ago, back when I was into the black crinkle finish look.
It does 600 watts of audio in AB1.
I put in about eight 2 watt resistors to get the meter to read right, tested it with 2500 volts, and put it back in.
After 10 minutes it reads 400 volts high again, so I have to do it over again and add another resistor.


 

The top...

It looks great!

It seems you have the modulation transformer mounted outboard? No problems with the long plate leads?

You should see what Brett is using!  The only place for it on the bottom of the rack

Al

Its actualy on a shelf at the top of the rack, on plastic as insulation.
No problem with the long wires.
I swept it today, 40Hz to 10khz flat, it rolls of fast at the high end, slower on the low end.

I am getting another of the mod transformers soon, new in the original crate.
I might pick up another matching power transformer as well.

I use the same mod trans in the 3x4d32 rig, and one in the 2x813 rig, sold one with the push pull 812a rig.
I sold the crappy cvm5 a long time ago, it sounded poor.

Brett

Here is the power supply...

Brett

sweet Rig.. Are those 250s in the final?  Cant really see on the laptop.

C

VERY nice!

Hi Brett,

Nice, clean layout and square component placement. Not bad for 30 years ago.  My construction wasn't quite that good 30 years ago... ;D

I was looking at the underside with all the 1w and 2w resistors.  It occured to me that after years of looking for resistors in junk boxes that I can actually identify most of your resistor values just by a quick glance...  That may be a sign of over-exposure, dunno... :-)

Have fun.

T

I was given a pair of 4x150's as a child and have never been the same.

That mod xfmr looks like old RCA BC iron.

The 4 tubes in the pix are another modulator, ppp 100th's, but I dont use that deck.
Its all hooked up, to the voltage and mod trans, but with the power off, it does not do anything.
It does not sound quite as clean as the 4cx250b mod deck.
I dont know why I built it, just something to build I guess.

I should turn it on and use it once and a while I suppose...

The rf part of the trans is in the next cabinet over.

I am always amazed at broadcast rigs where the modulator stuff is next to the rf stuff....
I always kept them seperate as much as possible.

Brett

Oh.. Ok..  Really cool man..  100th's  Nice.  Really like your handy work!


C

I fired up the ppp 100th mod deck last nite, and found it works better at 2500 volts, it needs less drive power to get the power out at 2500 volts (instead of 2000 volts) and seems cleaner.
Its still at the limit of the 20 watt drive power though.
Maybe this weekend I will use it.

It DOES warm the shack....

Brett