[Hammarlund] HQ-170A drift reduction progress

[Roger D Johnson]

On 4/15/2011 2:32 PM, Mitch wrote:
> All,
>      Again, thanks for all the replies and help. This morning, I am trying something different. I left the receiver turned on all night long (>8hrs) with the RF and AF gains turned down to the lowest setting. This morning, I am repeating the test of last evening. The receiver is definitely much more stable. Infact, I am quite happy with it this morning.
>
> Input to receiver coax antenna connection from signal generator for all tests -- 3 microvolts
>
> 1.9mhz             No discernable drift
> 3.8mhz             No discernable drift
> 7.150mhz          No discernable drift
> 14.2mhz            No discernable drift until the house heating system kicks on. Then it shifts in frequency about 100-200 hz.
>
> When the house heating system turns on, the air circulation in the room increases and I get the small amount of drift. Knowing that the bottom of the receiver is mostly sealed, I took a small plastic tube, and used it to blow air over various components in the top portion of the receiver. I discovered that the main tuning capacitor was affected by blowing air across the first section of the cap. the one closest to the front of the receiver. I took a tissue paper and covered the variable cap with it, then re-zero beat the receiver. It appears that it has greatly reduced the drift of the receiver. I did do a de-oxit treatment to the cap rotor contacts at the same time. No discernable improvement was detected. I think a small plastic or metal enclosure would benefit the stability by reducing the air movement across the tuning capacitor. Of course, anything in close proximity to the tuning cap affets the tuning and would probably require a re-alignment.
>
> All of the following tests are done with the tissue paper shield in place.
>
> 14.2mhz            No discernable drift! (with tissue paper air shield)
> 21.2mhz            No discernable drift! (with tissue paper air shield) (stable for over 30 minutes!)
> 29.0mhz            very slight drift (<50hz possible voltage shift)
> The antenna control also has an affect on the drift of the 10m band. Adjusting the antenna control while the receiver is zero beat to 29.0mhz, will shift the receive frequency out of zero beat. I set the control for the most tone then zero beat it again. I also discovered that on the 10m band, just touching the case will affect the frequency a small amount
>
> 52.0 mhz            No discernable drift!  I did have to turn up the RF gain and the output of the signal generator to 10 microvolts so that I could hear the tone.
>
> I removed the tissue paper air shield, re-zero beat, and the receiver immediately started drifting (still on 52mhz). I also blew some air across the main variable cap and was able to change the oscillator frequency. So it appears, the simplest solution to greatly reduce if not eliminate the drift is to give the receiver an extended warm up and add a enclosure around the main tuning Variable cap. I will construct a box in the next few days and see how it works.
>
> Other possibilities to stabilize the main tuning cap.
> 1. Install a small high wattage resistor under the main tuning cap inside the enclosure so that it heats the air inside the box and maintains a more stable temp (like a crystal oven). Since the VFO and first mixer are run off a seperate filament supply and the main transformer was not changed when they designed the 170A series, power for the resistor could come from the filament winding of the main transformer.
>
> 2. Possibly adjust the end bearing of the variable cap to see if it would reduce the drift. I do not think this would make much difference as the amount of air flow across the various section of the capacitor wasn't much and should not have affected the expansion/contraction of the capacitor rotor shaft.
>
> I also discovered something else cool about this receiver. When I tune the receiver for zero beat, the meter needle pulses when it appears I am exactly zero beat. The closer to zero beat, the more it pulses. It is a very fine adjustment from stable to pulsing needle.
>
> I haven't performed the alignment yet. I am still waiting for the VTVM I ordered to arrive. I have one, but it is an AC only VTVM. I could modify it, but decided to just but a Heathkit IM-28( ebay-$15.00). It should be here today.
>
> All in all, some very interesting results. I am very happy with the receiver and have been enjoying figuring out how to make it more stable. The air current affect on the variable cap is most interesting!
>
> Mitch KJ7JA
My HQ-180A has plastic covers over the tuning caps...my HQ-170 does not. The 
earlier receivers HQ-140/150 had the caps
in metal enclosures. The caps appear to be much better quality also. I would 
think caps with aluminum plates would be especially
bad due to the temp coefficient of Al.

73, Roger