From: legg@nospam.magma.ca
On Sat, 18 Aug 2012 16:43:09 -0700 (PDT), Patrick Turner
wrote:
>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.
Should have read 6AQ8/ECC85 and 6BK7A/ECC81
So no ECC83/12AX7's there.
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 6AU6, as
indicated in SAM's. and no major problems appear in this section.
The generic circuit in literature tends to use the components from the
SAM's documentation.
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. I guess 91R was a compromise.
The newer part suggests a later rev rather than an early one. If it's
an early one, perhaps they dumped the 6AQ8 because of higher cost or
poor availability.
The board art itself is copyright 1956 and is a complete re-spin for
the heater tracks and dual tube structure pin swap.
Anyways, it was up and running after a socket cleaning in the local
oscillator and an IF realignment.
--- SoupGate-Win32 v1.05
* Origin: you cannot sedate... all the things you hate (1:229/2)
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