[Patrick Turner]

On Tuesday, 31 July 2012 04:01:32 UTC+10, 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.

6AQ8 is identical to 12AT7/ECC81 except for heaters which are 6.6V only for 6AQ8I hardly think ECC83 would be used in RF input stage because its 12AX7, and I don't recall ever seeing 12AX7 in any cascode or grounded grid FM input stages.


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.

12AT7 IS a ECC81. Check the heater arrangement. If there is 6.3V applied to across pin 4 to pin 5, its 6AQ8, but if its 6.3V from pins 4&5 to pin 9, its 12AT7. Sometimes 12.6V is across pins 4 to 5 with pin 9 as CT and usually grounded.

Consult all the well known schematics circuits for tube input stages which so often did use a pair of 12AT7 OR 6AQ8, one triode for RF input gain, second for F converter, 3rd for oscillator, 4th for AFC control. Usually Harmon Kardon and fisher and most others just copied whatever generic RCA schematic had been developed by RCA at that time. Learn how they work, to better understand R values.
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).

Well there you are then.

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.

Not a huge difference between 68 and 91 ohms in such circuits.

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.

My guess is the 6k8 goes from B+ to anode and there's a cap to a coil. if the triode oscillator has Ia at 6mA while oscillating, you'd get about 40Vdc measurable across 6k8, as opposed to only 6Vdc across 1k0. If B+ is only 100Vdc, then Ea would be a bit low with 6k8. But you need to be able to measure the Ea without the VM probe capacitance stopping oscillations, so maybe use 100k in series with probe tip. Again, you need to know what to expect BEFORE you probe around the circuit.


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.

Rk at 22k is typical value because when the circuit oscillates, considerable grid current flows and its this current which limits the oscillator output voltage level which needs to be kept stable. at much lower F on broadcast band typical triode oscillator grid R is 47k. Usually all R values useed at around 100MHz are all lower than those used for lower F, lest stray C values have too much unwanted effects. 10pF at 100MHz is a reactance 100 times smaller than at 1MHz.

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.

Seems like you have a generic type of circuit used in so many tubed FM sets.. Sometimes they also used 6DJ8.

Allow me to say it seems like Google is slowly trying to squeeze everyone out out from here and its no use complaining about it because they recently changed the way the Usenet site displays yet again and I find myself typing in t a tiny little box without being able to see your post properly.

Patrick Turner.