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 > > _______________________________________________ Drakelist mailing list Drakelist@zerobeat.net http://mailman.zerobeat.net/mailman/listinfo/drakelist -- This message has been scanned for viruses and dangerous content by MailScanner, and is believed to be clean. _______________________________________________ Drakelist mailing list Drakelist@zerobeat.net http://mailman.zerobeat.net/mailman/listinfo/drakelist
