Gwyn -
Very interesting. I've been fortunate to avoid this sort of problem in
almost 50 years of working on Drake gear! :-)
I would suspect T5 at this point as well because it's about the only
other thing around! I assume you have subbed a 12BE6, a 'shaky' tube at
best. I HAVE seen transformers like T5 'partially' fail due to a
Screen-Suppressor short in V2, sometimes burning R9 in the process. The
excess current damages the insulation on the winding, causing shorted
turn(s). The 'repairman' replaces the tube, and fried resistor, and
when that doesn't fix it, sells it as 'working perfectly'.... :-)
It's possible your low sensitivity is related to damage in T5 as well.
The only information I have that might be useful is to expect ~1.5 to 2V
p-p of BAND, PTO and PRESELECTOR signal.
I'll be interested to hear of your progress.
73, Garey - K4OAH
Glen Allen, VA
Drake 2-B, 4-B, C-Line& TR-4/C Service Supplement CDs
<www.k4oah.com>
Griffiths G. wrote:
Garey -
Many thanks.
Linking across the (disconnected) filter brings the bandwidth to what one would
expect with just the IFTs at first IF. The selectivity is then properly
controlled at the bandpass LC filter as it should be.
I removed T6, the xtal filter can. There are only two xtals in a simple half
lattice configuration (marked 5645 and 5642kHz), no internal capacitors, and
the matching transformer at the output is not quite as in the circuit in the
manual I have. There it is shown as an autotransformer, in reality it has two
windings, the common output of the two xtals being to the hot end of the low
impedance primary winding at the earthy end of the secondary. Hence,
electrically, virtually no different to the circuit.
Out of the receiver, the filter tests OK with a sig gen, the two xtals working,
no sign of a problem, with series resonance at 5644.7 and 5641.0kHz.
So the next test when I have a workshop session will be on T5 secondary,
perhaps the secondary connected to the 5642kHz xtal has a problem.
By the way, the receiver is about 40dB low on sensitivity as well (I bought it
that way earlier this year - in need of a challenge!).
So far, I have the RF amplifier documented, but ideally would be good to
compare to a fully working set.
T1 has a measured Q of 60 at 3.6MHz, and a step up of about 2.5.
gm of the 6BZ6 at the bias point is about 2mA/V, and with the calculated
dynamic impedance of T2 at 3.6MHz of 4500 ohms, the stage gain is theoretically
9, though I measure 7, so not far off. I intend to go through like this, stage
by stage, as I've yet to find whether this level of detail is already available.
73
Gwyn
G3ZIL
________________________________________
From: drakelist-boun...@zerobeat.net [drakelist-boun...@zerobeat.net] On Behalf
Of Garey Barrell [k4...@mindspring.com]
Sent: 18 July 2010 14:46
To: drakelist
Subject: Re: [Drakelist] R4B xtal filter bandwidth problem
Gwyn -
Certainly sounds like a bad filter. Can't say I've ever seen or heard
of one failing that way. Seems highly unlikely.
I'd start by just connecting a wire between the leads going to the input
and output of the filter, after disconnecting them from the filter itself.
The filter is most likely just a series of four (it's a four pole
filter) crystals connected in series with small caps to ground between
them. It's a little hard to imagine THREE crystals all failing or
coming 'unglued' at the same time. I assume this was a catastrophic
failure. Does the receiver still seem about the same sensitivity, just
restricted?
Let us know what you find!
73, Garey - K4OAH
Glen Allen, VA
Drake 2-B, 4-B, C-Line& TR-4/C Service Supplement CDs
<www.k4oah.com>
Griffiths G. wrote:
Folks
Does anyone have experience of the R4B as-fitted xtal fliter showing a -3dB
bandwidth of just 250Hz, from 5644.910 to 5644.660kHz
This suggests to me that only one crystal is in circuit, but I do not have the
circuit of what is in the can so I can be sure.
Is there a circuit available?
Before I dismantle, suggestions welcome as to any other diagnostics I can do.
many thanks
73s, Gwyn Griffiths
G3ZIL
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