Harman Kardon FM100 - early version? - Harman-Kardon Model FM-100 Centerpoint.pdf (0/1)
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 |
Harman Kardon FM100 - early version? - Harman-Kardon Model FM-100 Centerpoint.pdf (0/1)
On Mon, 30 Jul 2012 23:59:10 -0500, flipper wrote:
On Mon, 30 Jul 2012 13:59:54 -0500, 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. Well, I can really answer why it's broke but some general observations. 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. Can you find a manufacture date on it? The SAMS may be the 'original' version and they couldn't know what would be done 'later'. Or you may have a 'pre-production' unit. The board art itself is copyright 1956. 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, it's an input/mixer combo - same difference? Not sure what 6BK7A does that's special or not except for the biasing differences. 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. The two are listed as 'substitutes' for each other but I do think the 6BK7 is 'preferred'. Both should work I got this a little bassackwards in my description. 6AQ8/ECC85 (!!) is used in the documented 12AT7/ECC81 (!!) and 6BK7 positions, for the functions noted. The newer part suggests a later rev. If it's an early one, perhaps they dumped the 6AQ8 because of higher cost or poor availability. 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). Well, that's obviously different, and not pin compatible, but the two types are 'similar' and rather close in both mu and gm (6BK7 requires more heater current). I mean, you could use either when designing, but not swap once the traces are down. See below. 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. Sounds like a bias tweak, maybe due to the slightly different tube being used, but it probably doesn't make much difference one way or the other and might have been done, regardless of the tube, just to lower current drain. gm would likely change a bit, perhaps altering gain a bit, but neither should make it 'work' or 'not work'. For self-bias, the cathode resistor for 6AQ8 is supposed to be roughly 4X that of the 6BK7, so an increase here would be expected. It's in the AFC circuit 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 6AQ8 pulls significantly more current per plate volts than the 12AT7 so the higher plate resistor cuts plate volts to keep current down. I don't know if that value makes it 'the same' as the other tube but being a higher value makes sense. It's also not 'operational'. I mean, it's bypassed and, so, just setting B+ to the plate, which is probably lower than SAMS indicates because it doesn't need as much plate volts to get the same current draw. Gain might be a little different but it doesn't really matter for an OSC, as long as there's 'enough', and they're usually overdriven anyway. Easy enough to find out. Is it oscillating? Btw, note that if AFC is screwed then so is the OSC and everything else. That seems to be the issue. Oscillator is not oscillating. AFC control line seems to be functioning, in simple IF signal tests. 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. The 22k sounds pretty common. Why I don't know but you see it everywhere. It's bypass and, so, out of circuit at RF. I also see that in typical schematics of the time. 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 What is the 'malfunction'? RF front end went out, on the shelf, 2yrs after last power-on. (It is mono, after all.) This is the first time I've actually paid much attention to the innards - was happy to just leave it alone, as it warnt broke. I don't have an FM signal generator or a controllable source (like a wireless audio link). So I've been limited in what I can actually check. The SAM's IF and discriminator calibration procedure, using 10.7MHz cw after the mixer, showed it to be pretty out of tune, but functional in those sections. No 6AQ8's in my spares cabinet. RL |
Harman Kardon FM100 - early version? - Harman-Kardon ModelFM-100 Centerpoint.pdf (0/1)
On Mon, 30 Jul 2012 13:59:54 -0500, legg wrote:
6BK7A/ECC83 RL I assume this is a typo. An ECC83 is a 12AX7. It doesn't have the same pin connections as a 6BK7 and isn't particularly suited to VHF uses. -- Jim Mueller To get my real email address, replace wrongname with dadoheadman. Then replace nospam with fastmail. Lastly, replace com with us. |
Harman Kardon FM100 - early version? - Harman-Kardon Model FM-100 Centerpoint.pdf (0/1)
On 05 Aug 2012 23:10:03 GMT, Jim Mueller wrote:
On Mon, 30 Jul 2012 13:59:54 -0500, legg wrote: 6BK7A/ECC83 RL I assume this is a typo. An ECC83 is a 12AX7. It doesn't have the same pin connections as a 6BK7 and isn't particularly suited to VHF uses. A lot of typos that day, including the header, which should have read Counterpoint, as in the message body. Should have read 6AQ8/ECC85 and 6BK7A/ECC81 Also, again 6AQ8/ECC85 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. 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. The IF strip was functional, but seriously out of tune. Would accurate notes have indicated a different IF? Anyways, I swung it back to 10.7MHz. The main problem was the silent local oscillator. This turned out to be a persistently bad tube pin contact, which finally responded to vigorous polishing in the socket connector. The whole commercial band didn't really wake up until the base and cover were reattached. Luckily, most fine tuning can be performed with the case fully assembled, as the appropriate access points are present in the metalwork. All OK for now. Wish there was a real schematic floating around, or at least some explanation of the revision history. |
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