Harman Kardon FM100 - early version?

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

Reply to
legg
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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.

Reply to
spamtrap1888

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

Reply to
legg

Should read 6AQ8/ECC85 and 6BK7A/ECC81

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

Reply to
legg

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

RL

Reply to
legg

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

Reply to
legg

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Regards,

John Byrns
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Reply to
John Byrns

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).

Reply to
William Sommerwerck

What cleaning agent did you use?

Reply to
William Sommerwerck

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

Reply to
legg

Friction.

RL

Reply to
legg

reversed.

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

formatting link

gone!

RL

Reply to
legg

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