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Author Topic: HQ-170A circuit oddity ?  (Read 2620 times)
K7PEH
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Posts: 1144




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« on: October 23, 2017, 12:50:19 PM »

I have a working HQ-170A but I was just about to embark on replacing the Electrolytics and maybe the paper capacitors in the rig.  However, in comparing the actual wiring to the circuit diagram I find that there is an extra 82 ohm 10-watt power resistor in the HV power supply circuit.  The resistor lies in series between the HV output of the rectifier diodes and the Electrolytic capacitor on the input to the choke.  The circuit diagram does not have this resistor.

As a result, it seems that the HV voltage values are about 9 volts less, that is, about 272 volts rather than 281 volts.  That is, the output of the rectifiers is 281 volts and the other side of the resistor is 272 volts.

In all other cases that I have found, my circuit diagram matches the HQ-170A wiring.  It is a diode rectifier rather than the older tube rectifiers used in the earlier version of the HQ-170.  Is this some customization for some reason or is this resistor just missing from the published circuit diagrams, that is, a mistake?

73, phil K7PEH
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K1ZJH
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« Reply #1 on: October 23, 2017, 04:02:54 PM »

I can't answer whether that is factory or not, but it is common practice to add a resistor between the first filter cap the rectifier cathode when modifying a set to use silicon diodes instead of a vacuum tube rectifier.  The sold state diodes have much less voltage drop, and the resistor serves to keep the B plus voltage where it should be.

The resistor probably adds a bit the filtering action, and reduces peak currents in the transformer.

Pete
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KM1H
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« Reply #2 on: October 23, 2017, 05:25:17 PM »

That radio would work better if the B+ was at 175-200V.

Hammy never did learn that but National, Collins, Hallicrafters, and others did.

Carl
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K7PEH
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« Reply #3 on: October 23, 2017, 06:25:59 PM »

@Pete:

All the tube voltage measurements reflect the ~3 percent drop in the main B+ supply due to the resistor I mentioned.  That is, if the resistor were not in place then all voltages I measure would correctly match the documented tube voltage measurements.

So, it seems like the resistor itself was a modification made by someone and not by Hammarlund.  With the HQ-170A, one of the circuit changes was the solid state diode rectifiers replacing the previous (HQ-170) rectifier tube.  I would expect that Hammarlund would design the circuitry to eliminate the need for the resistor voltage drop with the diodes instead of the tubes -- that is, probably a different power transformer.  Of course, as I write this I am thinking it is also conceivable that Hammarlund had extra inventory of power transformers from the lineup of HQ-170 so they merely used the voltage drop resistor but apparently forgot to update the documentation in a like manner (i.e. tube voltage measurement table and circuit diagrams).  Or, maybe they did update that documentation and I merely do not have that particular version.

Of course, this is more of a mystery rather than any sort of real problem -- the radio does work.  Back in 1965 I had a brand new HQ-170AC in my shack.  Sold it while in college as I had drifted away from Ham Radio.  But, I always loved that receiver and sorry I got rid of it. 

73, phil, K7PEH
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W9ALD
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Posts: 38




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« Reply #4 on: October 24, 2017, 02:00:16 PM »

As I recall the Hammarlund HQ-170A and perhaps others were notoriously hard on the high frequency oscillator tube, a 6C4.  The tube was run at it's limits or beyond and didn't like it all that well.  I would guess someone was trying at add some longevity to the active devices in the circuit.
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K7PEH
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« Reply #5 on: October 24, 2017, 03:56:06 PM »

Some of my own research (googling mostly) comes up with the following:

  • --The 6C4 tubes would wear out because the original HQ-170A (and maybe the 170) has these filaments always on instead of switched with the power on.  This was to stabilize them to eliminate drift as the temperature settled.  Sometimes a fix to this problem is either to change the circuitry so that the filaments are switched with power on.  The problem was (as someone said) replacing the 6C4 tube once a year as a result of filaments being always on.
  • --The power resistor that is inserted between the rectifying diodes and the input to the capacitor/choke filter (my posted question) is usually due to the need to protect the diodes when higher value electrolytic capacitors are used such as switching the 40 mfd for 100 mfd.  This was to fix some hum detectable on the higher bands with the volume turned down low.  In my case, that input capacitor is still only 40 mfd.
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WB6BYU
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« Reply #6 on: October 24, 2017, 04:26:34 PM »

Power-up surges were not as severe with the old tube rectifiers. 
It may be that the resistor is to reduce the peak current when the
set is switched on - this might have been excessive on the original
design when they switched to the solid state diodes.
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KM1H
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« Reply #7 on: October 24, 2017, 06:33:55 PM »

Quote
As I recall the Hammarlund HQ-170A and perhaps others were notoriously hard on the high frequency oscillator tube, a 6C4.  The tube was run at it's limits or beyond and didn't like it all that well.  I would guess someone was trying at add some longevity to the active devices in the circuit.


Quote
-The 6C4 tubes would wear out because the original HQ-170A (and maybe the 170) has these filaments always on instead of switched with the power on.  This was to stabilize them to eliminate drift as the temperature settled.  Sometimes a fix to this problem is either to change the circuitry so that the filaments are switched with power on.  The problem was (as someone said) replacing the 6C4 tube once a year as a result of filaments being always on.

Nope.
The 6C4 was used in a lot of military and commercial equipment and run 24/7 for several years without problems.
Ive had many in here, plus the 180A and all had about 6.8 to 7V on the filament. Drop it down to 6.0V and reduce the B+ resulted in long life and much much less drift. My 180 with the always on factory mod hasnt needed a tube change in around 8 years and is no longer fussy about brand of 6C4 and the low end of band 6 crapping out. I made a few other changes to that circuit but those notes are not up here with the radio in the LR and Im off to bed, been up since 5:30.

Carl
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K1ZJH
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« Reply #8 on: October 24, 2017, 06:54:54 PM »

The resistor will also reduce the DC voltage; the voltage drop across a diode is negligible compared to a vacuum tube.  The impedance of the transformer windings is usually enough to limit the initial turn-on surge.
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G3RZP
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« Reply #9 on: October 25, 2017, 03:44:40 AM »

My father's HQ170A tended to get through 6C4s. The problem I believe was not that the tube was on 24/7 operating, but that it was on 24/7 with just the heater operating: a tube run 'cut off' or with no plate voltage will have an gradually increasing cathode interface resistance. See Millman and Taub 'Pulse, Digital and Switching Waveforms' on the subject.   
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KM1H
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« Reply #10 on: October 25, 2017, 06:33:44 AM »

That was what happened with the early computer tubes and I thought it was long corrected in regular production. Quite a lot of equipment has tubes biased well into cutoff for extended periods without damage.
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AC5UP
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« Reply #11 on: October 25, 2017, 08:54:55 AM »

Not familiar with the HQ-170A in particular, but the NJ7P tube database tells me the 6C4 is a medium-mu triode capable of 5.5 watts of dissipation at moderate frequencies.  2.5 watts @ 150 MC's.  For additional curves refer to the 12AU7A.

In a typical consumer application it was my experience the 12AU7 was long lived and not one of the usual suspects when troubleshooting...  (?)

( and it's understood that a HQ-170A is not the same as a Fisher 500 stereo receiver.  The Fisher sounds better )
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KD0REQ
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Posts: 2017




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« Reply #12 on: October 25, 2017, 10:36:00 AM »

instead of speculating, check the voltages at the socket against any reference values in the manual.  replacing a tube rectifier with diodes can cause a rise up to 50 volts, depending on the drop within the tube. this can slip the bias.

it was a common recommendation to add the appropriate 10-watt dropping resistor back in the 60s/70s when pulling the rectifier tube for diodes in just about every printed article. it could be the resistor lobby, or the protecting-engineers lobby, but more likely if a circuit is designed for a range of voltages, it was engineered to hit the middle. standard line voltage is up 10% now from the 50s and 60s. spike it perhaps 55 volts, it's easy to slide off the curve in a circuit just by wiring two diodes across the tube socket in the power supply.
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N2EY
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« Reply #13 on: October 25, 2017, 11:56:20 AM »

As I recall the Hammarlund HQ-170A and perhaps others were notoriously hard on the high frequency oscillator tube, a 6C4.  The tube was run at it's limits or beyond and didn't like it all that well.  I would guess someone was trying at add some longevity to the active devices in the circuit.

I don't think it was that the receiver was "hard" on 6C4s.

Rather, IIRC, the oscillator design was such that it was very dependent on the 6C4 characteristics being spot-on the book values (particularly gM). And of course this varied from receiver to receiver.

A similar situation existed with 5U4 rectifiers back in the tube-TV days. More than a few TV sets would not work well if the 5U4 low-voltage-B+ rectifier was just a little low on emission. Shrunken picture, loss of vertical hold, loss of brightness were common symptoms. Replace the tube and all would be well. The removed tube would work fine in other, less-critical applications. (This situation was most common back when "110 volt power" really was 110 volts in most homes.)

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K1ZJH
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« Reply #14 on: October 25, 2017, 12:43:42 PM »

http://www.w8ji.com/power_transformer_stress.htm
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