[Hammarlund] HQ-110 stability, or, making a silk purse out of a pig's ear,

[Carl]

Or get a HQ-170 and not have to rebuild it so it works well.

The HQ-145 is also a nice GC and Ham set, sort of a scaled down HQ-180 and 
mostly off the radar..

Carl
KM1H


----- Original Message ----- 
From: "Kenneth G. Gordon" <kgordon2006 at verizon.net>
To: <Hammarlund at mailman.qth.net>
Sent: Tuesday, February 09, 2010 5:54 PM
Subject: [Hammarlund] HQ-110 stability, or,making a silk purse out of a 
pig's ear,


>I spent some time last night studying the schematic and the manual
> for the subject receiver and discovered a few interesting things,
> most of which were NOT covered in the manual.
>
> First of all, the 0B2 regulator is connected not simply to the HF
> oscillator, but also to the screen of the 1st IF amplifier and to both
> "screens" of the 1st mixer, the 6BE6 V-2, AND is fed to the RF
> amp's AGC rectifier, providing "delayed" AGC voltage for the RF
> stage.
>
> Thus, any problem with the regulator would also effect the mixer, RF
> amp, and 1st IF amp. Conversely, any problem with the mixer, RF
> amp, the RF amp's AGC rectifier, and/or 1st IF amp tubes, could
> also cause a problem at the HF oscillator.
>
> Secondly, the BFO is NOT regulated at all, but receives its plate
> voltage directly from the output of the second HV filter cap. I
> suppose that due to the NEXT issue, the BFO output must have to
> be at a fairly high level, and couldn't run effectively from 108VDC
> regulated. It also probably draws too much current for the 0B2 to
> handle.
>
> Thirdly, and to me most interesting, is that the 2nd IF amp, the
> pentode section of a 6AZ8, is also used as a "linear detector" (since
> when is a detector or, in this case, a mixer "linear"?) for CW/SSB,
> the BFO output being fed to this tube's grid.
>
> Audio from the plate of this stage, when in the CW or SSB condition,
> bypasses the last IF transformer's two tuned circuits, and is fed
> directly to the grid of the first audio amp.  This reduces CW/SSB
> selectivity, and makes it impossible to use the AGC and "S" meter
> on CW or SSB.
>
> This also reduces the available audio level, a fact mentioned in the
> manual. The manual says to "turn up the audio gain a bit..." on
> CW/SSB.
>
> The 2nd IF stage obviously functions as a sort of product detector
> for CW or SSB.
>
> Lastly, the AGC is tuned for AM, being a fast-attack, fast-decay
> type, which is NOT suitable for CW or SSB.
>
> It is very interesting to me what Hammarlund engineers did with this
> receiver, accomplishng a lot with a little.
>
> However, I feel that there can be considerable improvement made
> to the operation of this receiver with a little effort.
>
> I am considering doing the following:
>
> Install a REAL dual-triode product detector which will take its IF
> signal from the output of the last IF transformer, thus preserving as
> much selectivity as possible, and enabling the use of AGC and "S"
> meter in CW/SSB.
>
> I would use a 7963 subminiature very hi-mu dual-triode since it can
> be mounted on a terminal strip below deck.
>
> I would remove the BFO signal being fed to the 2nd IF stage and
> connect the BFO to the new product detector, adjusting the BFO
> output for the proper level.
>
> Then I would disconnect the audio signal to the 1st audio amp from
> the 2nd IF, connecting the audio signal from the separate product
> detector in its place at the mode switch.
>
> I would leave the AM detector and the two AGC rectifiers in place.
>
> I would change the MAN-AGC switch to one with at least one more
> position so that I could enable fast and slow AGC, adding a
> capacitor to the AGC circuit where appropriate.
>
> The regulator circuit must be looked at carefully.
>
> I may use some strategically placed zeners of the proper values. For
> one thing, I think the BFO voltages should be regulated, and I hope
> to be able to separate the regulated voltage sources from one
> another to preclude interaction.
>
> Well, so much for that.
>
> Ken Gordon W7EKB
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