Hi All
I am having issues with my 800C which seems to be in the FM limiter tube? V6(6AU6)
It is a new tube and has been swapped with others with same results.
I am getting more than double the stated voltage in schematic of (44V)
99Vdc at pin 6 (G2).
Plate voltage(pin 5) is 10v higher at 166 and pin 1 grid is a measly-.42V when it says it should be -3.4V
I have checked most resistors that have measured pretty close to stated value around FM section.
The FM brings in a strong signal at meter, works in FM stereo filter but FM Mono is weak and distorted audio.
AM doesn't work at all.
Any thoughts?
I am having issues with my 800C which seems to be in the FM limiter tube? V6(6AU6)
It is a new tube and has been swapped with others with same results.
I am getting more than double the stated voltage in schematic of (44V)
99Vdc at pin 6 (G2).
Plate voltage(pin 5) is 10v higher at 166 and pin 1 grid is a measly-.42V when it says it should be -3.4V
I have checked most resistors that have measured pretty close to stated value around FM section.
The FM brings in a strong signal at meter, works in FM stereo filter but FM Mono is weak and distorted audio.
AM doesn't work at all.
Any thoughts?
Check the HT resistor leading to g2, and the coupling capacitor leading to g1. but FM mono has nothing to do with the limiter if FM stereo is working, and AM has nothing to do with the FM limiter at all.
DC voltages around limiters will tend to be very sensitive to signal level, so that stuff is probably just a red herring. The strange mono/stereo behavior certainly points to the multiplex circuit. AM issue is either completely unrelated, or the AM audio is run through some stage common to the multiplex. Just theories at this early point, but maybe a better place to start. Schematic would be nice.
All good fortune,
Chris
All good fortune,
Chris
Thanks for the replies guys.
The 800C is weird as V3 is a 6HR6 that is the FM-AM IF amp
and V4 is a 6AU6 that is "FM IF amp and AM detector"
That's why I thought the problem may be linked.
It's all a bit out of my league but I want to learn and know the basic theory.
A very knowledgable fellow on AudioKarma says the same about the voltages as Chris just did.
He said it may point to an IF transformer having "dreaded Mica disease" unfortunately I don't have the test equipment or knowledge
to start messing with those.
I am puzzled as to why it works on FM stereo filter and not very well on mono.
It also has a lot of signal strength on local stations so wouldn't that mean the front end is working as it should?
I wouldn't hesitate to take it to a good tech but I can't find anyone local who has the knowledge or equipment to work on it.
I am on Vancouver Island.
The 800C is weird as V3 is a 6HR6 that is the FM-AM IF amp
and V4 is a 6AU6 that is "FM IF amp and AM detector"
That's why I thought the problem may be linked.
It's all a bit out of my league but I want to learn and know the basic theory.
A very knowledgable fellow on AudioKarma says the same about the voltages as Chris just did.
He said it may point to an IF transformer having "dreaded Mica disease" unfortunately I don't have the test equipment or knowledge
to start messing with those.
I am puzzled as to why it works on FM stereo filter and not very well on mono.
It also has a lot of signal strength on local stations so wouldn't that mean the front end is working as it should?
I wouldn't hesitate to take it to a good tech but I can't find anyone local who has the knowledge or equipment to work on it.
I am on Vancouver Island.
What test equipment do you have available? Can you get a look at test point 3 (right off the discriminator) as a baseline?
All good fortune,
Chris
All good fortune,
Chris
As far as test equipment goes I have a Hitachi V-212 analog scope and a DMM
and an old Heathkit audio gen.
I measured voltage at TP3 and it was -12.3V whereas schematic says -33V
Thanks for tip EJP.
and an old Heathkit audio gen.
I measured voltage at TP3 and it was -12.3V whereas schematic says -33V
Thanks for tip EJP.