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

On Jul 30, 11:01*am, 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.

Just to clear the decks: I'm assuming there are no date codes on
chassis or PCB that would indicate early or late production, and that
the tube types present match the numbers silkscreened on the PCB, or
the tube diagram inside the unit, if present.

On Mon, 30 Jul 2012 11:17:58 -0700 (PDT), spamtrap1888
wrote:

On Jul 30, 11:01*am, legg wrote:
snip
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.

Just to clear the decks: I'm assuming there are no date codes on
chassis or PCB that would indicate early or late production, and that
the tube types present match the numbers silkscreened on the PCB, or
the tube diagram inside the unit, if present.

No room for tube diagram. Chassis cover is perforated, chassis base
has standoffs and adjustment ports. This is pre-silkscreen era -
copyright 1956 in foil pattern.

Just seeing a printed circuit in consumer audio surprised me.

Board is PC-A-5, DWG P1702265A

MFR 'Operation and Service Instructions' dated 1956 stops at page 4,
without a tube list or schematic. SAM's is dated 1957.

RL

On Mon, 30 Jul 2012 13:01:32 -0500, legg wrote:


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.

Should read 6AQ8/ECC85 and 6BK7A/ECC81

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.

Again 6AQ8/ECC85

As well, first IF is 6AU6, vs the 6BA6 in the documentation. Didn't
figure that this was major, because the other two IFs are 6BA6, as
indicated in SAM's and no problems appear in this section.

One oddity. Swapping the 2nd and 3rd IF tubes result in either large
DC values at the multiplex output (~3V), or normal values (100mV),
indicating a part-dependent instability. They both test with similar
u.

RL

On Mon, 30 Jul 2012 18:11:21 -0500, legg wrote:

On Mon, 30 Jul 2012 13:01:32 -0500, legg wrote:

As well, first IF is 6AU6, vs the 6BA6 in the documentation. Didn't
figure that this was major, because the other two IFs are 6BA6, as
indicated in SAM's and no problems appear in this section.

argh - sghould read
'the other two IFs are 6AU6, as indicated in SAM's.'

RL

On Mon, 30 Jul 2012 13:01:32 -0500, legg wrote:


Anecdotal info suggests that ECC85 (6AQ8) was developed and marketed
after ECC81/12AT7, and that ECC85 was developed specifically for VHF
input/oscillator combinations. An improved inter-device screen
supposedly reduced oscillator stage radiation. This isn't an
input/oscillator schematic application, in any event.Not sure what
6BK7A does that's special.

When converting from 6BK7 to 6AQ8 or 12AT7, the cathode bias resistor
in self-bias is supposed to increase from 56 to 200R, to maintain
similar plate currents.

Looks like this is actually a later revision.

Just wish they'd documented the changes.

RL

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/

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.

This falls into the "It's not a defect, it's a feature" category.

If the AFC time constant isn't long enough, the AFC will try to null out the
audio's bass.

I've never heard of a "rumble" filter for FM tuners, but it might have made
sense in the days when phonograph records were the principal program source,
and perhaps even during live concerts (when traffic and subway noise might
have intruded).

Bad contact in the socket of the local oscillator.
Had cleaned the tube pins, but sockets presented
a more serious challenge.

What cleaning agent did you use?

On Tue, 31 Jul 2012 16:48:00 -0700, "William Sommerwerck"
wrote:

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.

This falls into the "It's not a defect, it's a feature" category.

If the AFC time constant isn't long enough, the AFC will try to null out the
audio's bass.

I've never heard of a "rumble" filter for FM tuners, but it might have made
sense in the days when phonograph records were the principal program source,
and perhaps even during live concerts (when traffic and subway noise might
have intruded).


All done. Bad contact in the socket of the local oscillator. Had
cleaned the tube pins, but sockets presented a more serious challenge.

Output coupling cap had shorted as well. Don't expect some amps would
like that. Only noticed on the scope.

RL

On Sat, 4 Aug 2012 15:48:28 -0700, "William Sommerwerck"
wrote:

Bad contact in the socket of the local oscillator.
Had cleaned the tube pins, but sockets presented
a more serious challenge.

What cleaning agent did you use?

Friction.

RL

On Tue, 31 Jul 2012 16:30:47 -0500, John Byrns
wrote:

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.


Going......going.....

http://www.ebay.com/itm/300757745636...84.m1555.l2649

gone!

RL