Garey Barrell <[EMAIL PROTECTED]> made an utterance to the drakelist gang
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Tom -
OK. Well, as I said, I've not seen many 2-Cs, but I agree that 10V
seems too low. The schematic says 12, and I would expect to see
12-13VDC there. R34, 0.82 @ 2W, is a wirewound (note double-wide first
color band), so unlikely to have changed value like a carbon comp.
They're usually either right or open! Check the rectifier diodes with
an ohmmeter. Perhaps one is open, giving you a half-wave rather than
full-wave supply. Also, in the later schematic, Drake added a second
1000 uF cap across C48, marked C86. I"m not sure where it is
physically. Does Q7 seem unusually warm? It should be warm, not HOT.
Measure the voltage across R66. The 12V bus drives ONLY the last two
audio stages, so either they are dragging the supply voltage down, or
the supply is faulty.
The rest of the solid state stages of the receiver are powered via R54
to what is marked as a +10 VDC bus, which is probably closer to 8V now.
I'm guessing an open D2 or D4! :-)
73, Garey - K4OAH
Atlanta
Drake 2-B, 4-B & C-Line Service CDs
<www.k4oah.com>
Tom Taylor wrote:
Just got some time to look at the 2-C again and go through all the
suggestions.
I swapped V4 with a NOS 12BE6. It made no difference to the s-meter.
Thanks for the clarifications on the resistances around V3. My 2-C is
#2804, well past the changes made at #1000.
I measured the DC voltage at C48: 10.1v. Seems very low.
Tom
On 10/12/06, *Garey Barrell* < [EMAIL PROTECTED]
<mailto:[EMAIL PROTECTED]>> wrote:
Tom -
OK. Next try V4. It, and V1 are controlled by the AGC #2 and are
the
"other side" of the S-Meter bridge.
Your readings on V3 (I'm assuming you're referring to Pin 2, not
Pin 3)
reflect a circuit change. There are at least two versions of the 2-C
that I'm familiar with, and the voltage/resistance charts weren't
updated to reflect the later version. Version 2 started at s/n 1000.
Your readings are correct for Version 2 units.
The AF collector voltage seems too low. A leaky Q6 can cause this.
What is the voltage at the top of C48? (The +12V power supply
filter cap)
73, Garey - K4OAH
Atlanta
Drake 2-B, 4-B & C-Line Service CDs
<www.k4oah.com <http://www.k4oah.com>>
Tom Taylor wrote:
> I tried swapping several 12BA6's (V5) at Garey's suggestion. All
> caused me to recalibrate the s-meter zero and I was able to get -13v
> at tp-1, but the s-meter is nearly unreponsive. It'll register S4 on
> only the very strongest of signals. I replaced the 12BZ6 with a NOS
> tube and that had no positive effect on the s-meter either.
>
> I did a voltage and resistance check against the table in the manual
> just to see if there was anything way off. Surprisingly, the
case of
> the output transistor was at 10.3v instead of 12.5v. Perhaps that's
> why audio isn't ear blasting. I'll have to investigate further.
As far
> as the resistance checks, everything was within reason for a 35
year
> old (perhaps older) radio, except for the resistances around V3, the
> mixer and VFO. Pin 3 is supposed to be 330 ohms and I measure .6
ohms.
> Pin 7 is supposed to be 7k and I measure 1m. Should I believe the
> chart in the manual? The VFO certainly seems to work fine. With
those
> measurements so far off, I wouldn't expect the receiver to be so
> operational, except for the S-meter.
>
> I guess I'll dive deeper and try and understand why those couple
> resistance measurements are so far from what's listed in the chart.
>
> Tom n7tm
>
>
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