The "rumble filter" circuit in this tuner is sort of interesting. Rather than
inserting a low pass filter in the audio section, it changes the time constant
of the AFC network, when the time constant is lowered more of the "rumble"
frequencies are feedback cutting the low frequency audio response. This means
that the "rumble filter" won't work when the AFC is off. Also the labeling for
the "ON" and "OFF" positions of the "rumble filter" switch appear to be reversed.
In article ,
legg wrote:
I'm working on a Harman Kardon FM100 "Counterpoint II" that shows
signifigant variations, internally, from the documentation provided by
SAM's HF3 repair and servicing manual, or other references searchable
on the web.
For a start, the valve pins are reversed, by section, in V1 and V2,
from the SAM's schematic. The tube sections are swapped - confusing,
but not a biggy. Second, the tube types differ from SAM's and all
other references.
1) V1 dual triode in the RF amp and mixer stage are the older
6AQ8/ECC81, rather than the later 6BK7A/ECC83 expected in the SAM's
schematic and listed in other web references.
2) V2 dual triode in the AFC and oscillator sections is 6AQ8/ECC81
rather than the 12AT7 expected in the SAM's schematic and web
references.
3) The printed circuit board has the correct heater connections for
the single 6V heater of 6AQ8, without any rework in evidence (pins 4/5
receive 6V normally - same as the other 6v heaters, pin9 is grounded).
4) Cathode resistor of the grounded grid 6AQ8 RF input amp is 91R, vs
the 68R expected. No note in SAM's about variations here.
5) Plate resistor in V2 oscillator is 6800R, vs the 1000R expected in
schematic. No SAM's note on variations here. This seems to be an
extreme circuit change.
The non-functioning mixer self-biases at half the schematic grid
voltage value and 2/3 the plate current - but expect this when input
signal is missing Grid resistor is 20% low with age/dirt. Oscillator
grid resistor seems low in schematic at 22K, but what do I know.
As this is the section that seems to be malfunctioning, I'm wondering
if there's any advice related to the earliest versions of this tuner
that might ease in reviving it. Tubes test functional for heater,
emissions and transconductance, if a little slow to warm up.
RL
--
Regards,
John Byrns
Surf my web pages at,
http://fmamradios.com/