Ive been working on a federal AM864 all day today and have hit the wall. check it:
http://www.montagar.com/~patj/am-864u-sch.gif
This damn thing is so simple Im pissed I cant track this down.
I got:
1. 60 hz ish Hum.
2. Lotsa hiss.
3. No compression.
4. waaaaaay too much output level.
So far Ive:
replaced the V4 with a known working tube. Unit behaves the same with the tube installed and with it pulled entirely- no compression
put it a known working 5y3, get the same hum
Replaced c2, c3, c5, c6
replaced threshold pot.
It appears the previous owner bypassed C4 with a 47uf/450 electro on either side of L1 with the negative side of the cap to ground.
Seems that as soon as I open the input attenuator the littlest bit, the unit goes to full output gain. Im assuming this is where the hiss is coming from.
get the following voltages on V4:
7- 6.8 vac
6- 183 v (manual shows 140 normal)
5- -.54v
4- -.54 (manual shows -1.42 normal)
3- 2.5v
Any thoughts? Is there something obvious to check? Is that 1 volt discrepency on pins 4/5 ruining my day there? Could R7 and R6 be throwing off the voltages on pin 6 and 5/4? I have a feeling the voltage differences on those pins are a symptom, not the problem, but I really dont understand enough to draw a conclusion either way.
dave
http://www.montagar.com/~patj/am-864u-sch.gif
This damn thing is so simple Im pissed I cant track this down.
I got:
1. 60 hz ish Hum.
2. Lotsa hiss.
3. No compression.
4. waaaaaay too much output level.
So far Ive:
replaced the V4 with a known working tube. Unit behaves the same with the tube installed and with it pulled entirely- no compression
put it a known working 5y3, get the same hum
Replaced c2, c3, c5, c6
replaced threshold pot.
It appears the previous owner bypassed C4 with a 47uf/450 electro on either side of L1 with the negative side of the cap to ground.
Seems that as soon as I open the input attenuator the littlest bit, the unit goes to full output gain. Im assuming this is where the hiss is coming from.
get the following voltages on V4:
7- 6.8 vac
6- 183 v (manual shows 140 normal)
5- -.54v
4- -.54 (manual shows -1.42 normal)
3- 2.5v
Any thoughts? Is there something obvious to check? Is that 1 volt discrepency on pins 4/5 ruining my day there? Could R7 and R6 be throwing off the voltages on pin 6 and 5/4? I have a feeling the voltage differences on those pins are a symptom, not the problem, but I really dont understand enough to draw a conclusion either way.
dave