skal1

federal am-864/u problem
« on: June 21, 2009, 06:39:58 PM »
Need some help with this.


1}Ok their seems to be a resistor missing from the leg r11 witch is a 47k to pin 1 of v2  does any know what it is .


2}A wire is hanging of the 13 pin turret board , i have traced it back to pin 3 of the attenuator pot where does this go on the turret  board


3}in this pic i can see a green where connected to the pin 2 of the turret board where does the other end connect.   http://twin-x.com/groupdiy/displayimage.php?pos=-771


i do not know if this is a federal 864 thing but there is a lot of hum when the device is idling what would cause this also why when it releases to i here a hum.




any advice will be invaluable


thanks


skal1


« Last Edit: June 21, 2009, 09:07:42 PM by skal1 »


skal1

Re: federal am-864/u problem
« Reply #1 on: June 22, 2009, 02:29:16 PM »
i have taken some measurements   of all  the values  with the attenuator counter clockwise


v1
pin1,2,3 and 5 0volts


pin 4. - 6.92

pin7 .7.15 vac

pin 8. 187 vdc

v2
pin1,2,3 and 5 0volts


pin 4. - 6.92

pin7 .7.15 vac

pin 8. 190 vdc

v3
pin1,0.01vdc

pin2,196vdc

pin3and6.4.38vdc

pin4 0.01

pin 5 ,196vdc

pin7 ,7.15vac

pin 8. 0vdc

v4

pin1,0.01vdc

pin2,13.30vdc

pin3,14.16vdc

pin4 ,-6.25

pin 5 ,-6.25vdc

pin 6,157vdc

pin7 ,7.15vac

pin 8. 0vdc



208vdc after rectifier

current and threshold both clockwise pos..

do these values look ok


can i get some help on this


usual problems hum ,hum on release and motor boarding with no signal

any advice will be invaluable  :-\


thanks


skal1



skal1

Re: federal am-864/u problem
« Reply #2 on: June 23, 2009, 08:15:26 AM »
anyone Know the voltages of c2 ,c3, c5 and c6 in the federal amp


thanks

skal1

EmRR

Re: federal am-864/u problem
« Reply #3 on: June 23, 2009, 02:20:43 PM »

1}Ok their seems to be a resistor missing from the leg r11 witch is a 47k to pin 1 of v2  does any know what it is .

pin 1 v2 is grounded; r11 isn't.   not understanding.  unless r1 or c1 is missing. 


Quote
2}A wire is hanging of the 13 pin turret board , i have traced it back to pin 3 of the attenuator pot where does this go on the turret  board

attenuator pot has 6 pins, based on schemo.   no idea what you're calling pin 3. 

Quote
i do not know if this is a federal 864 thing but there is a lot of hum when the device is idling what would cause this also why when it releases to i here a hum.

better front end tube match will fix a lot.  good balance of R's 3/4/13/14.   convert the 6.3 to normal grounded CT filament wiring, while adding a hum adjust pot.  I don't know how practical that actually is with these.    the stock filament wiring is a recipe for hum. 

values posted look reasonable, but no specific info here.

I believe there's a long  thread with every possible blow by blow on voltages and mods somewhere here.  find the mod for current balance of v1/v2; I think rafafred (sp?) posted a drawing.   Your voltage balance looks reasonable. 
Best,

Doug Williams
Electromagnetic Radiation Recorders

"I think this can be better. Some kind of control that's intuitive, not complicated like a single knob" - Crusty

"Back when everything sounde

skal1

Re: federal am-864/u problem
« Reply #4 on: June 23, 2009, 03:25:57 PM »
Hey thanks emrr i will look into to the converting  the heater circuit  to  a normal grounded CT filament wiring, while adding a hum adjust pot mod.

although i have read that thread about 10 times ,i am sure their must be some new info i did not pick up on


thanks

skal1


skal1

Re: federal am-864/u problems
« Reply #5 on: July 03, 2009, 07:13:35 AM »
OK  , i think have narrow the noise down too the  transformer .


Steps have have taken


1}discounted the heater supply from the rectifier


result= still mains noise on output


2}discounted the b+ supply from the rectifier

result= still mains noise on output


3}discounted all leads from secondary of transformer


result= still mains noise on output


For so reason the noise from the mains tx is transferring  to the output transformer which has know direct coupling to the output tx how is this possible .

can someone with  a little more experience help me with my problem


thanks

skal1 


 

Curtis

Re: federal am-864/u problem
« Reply #6 on: July 03, 2009, 08:20:45 PM »
Try unbolting the mains transformer from the chassis and move it around to see if the noise gets better or worse.  I'm not familiar with the layout of the Federal so this may or may not be practical.  The mains transformer can easily induce hum in adjacent transformers if the layout isn't up to scratch.

However, my Federal clone (and others too IIRC) always suffered from hum problems.  I think it just tends to be a noisy beast at the best of times.  It was never really designed for low-noise full-range studio work in the first place.  The best results I had were to add an extra filter capacitor at pin 3 of V4 to ground (say 47uF @ 350V) and sticking a 20dB pad on the output transformer.  The Federal has way too much gain by itself to be usable on line level stuff.  Padding the output brings it down to a more usable level, and knocks the residual hum down by the same amount.
--------------------
http://www.thethirdending.com
--------------------

skal1

Re: federal am-864/u problem
« Reply #7 on: July 04, 2009, 03:17:10 AM »
Thanks Curtis ,


I was wondering if anyone had the same issues with these beasts , it looks like the hum thing is is a family treat.

I will try that extra cap mod .


thanks again

skal1

drpat

Re: federal am-864/u problem New
« Reply #8 on: July 15, 2009, 10:05:03 AM »
...
« Last Edit: September 18, 2010, 07:36:16 AM by drpat »
I use the LA2a on very hot signals like lead vocal.

skal1

Re: federal am-864/u problem
« Reply #9 on: July 15, 2009, 02:01:03 PM »
Are you connected balanced or unbalanced?



balanced yes.


If balanced, is the shield/ground connected on both the output and input. If both are connected, remove the output ground to see if that helps.

no connection at pin 1 xlr from the ct of the i/o transformers.



what do you mean by interface problem ?

skal1



drpat

Re: federal am-864/u problem New
« Reply #10 on: July 15, 2009, 07:03:24 PM »
...
« Last Edit: September 18, 2010, 07:36:51 AM by drpat »
I use the LA2a on very hot signals like lead vocal.


 

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