National HRO avc.

[G3PIJpeter]

The S-meter on my HRO (R106-type) has never worked well, despite twiddling the pot and playing with the values of the bridge resistors. The other day, I measured the AVC line with the AVC switched on and the RF/AF controls set for AM reception. With no aerial input, the AVC line read around 500 mV positive and did not go negative until an input of 80 microvolts of RF. It took an input of several millivolts to the aerial connector for the AVC voltage to read greater than -3 volts.

Is this effect simply down to a soft valve somewhere along the AVC line? Or due to the AVC/det/1st audio valve (thumbnail attached) being soft? Is this an HRO problem or does it crop up in other sets?

- Peter

[Silicon]

The S meter makes use of the relative difference in the HT and screen grid voltages to deflect the needle.
There is no direct connection to the AVC line.

When the AVC is switched off the voltage on the AVC line could easily be positive because it connects to the cathode of V7 through the 250k resistor.

Faults in V7 or the components near it could keep the AVC line positive when the AVC is switched on.

Do you have the Octal or UX6 version?

[G3PIJpeter]

It's a complete rebuild following the R106 circuit with metal octal valves. There has been a previous thread about the OFF position of the HRO AVC switch - as I remember, it makes use of the contact potential of the right-hand diode to maintain a steady negative AVC voltage. You're dead right about the connection to the cathode resistor - in both the ON and OFF positions. However, are C12 and C15 present to maintain the negative potentials across R12 and R14 that result from the diode action on the applied IF voltage? I included the information about the S-meter because I am aasuming that lack of correct AVC swing will result in the RF/IF valves not operating where they should be for anode and screen currents to move in the required direction on increasing input to the aerial.
I'm just guessing - that's why I made the original post!
- Peter

[Silicon]

What type of voltmeter are you using to measure the AVC voltage?
What is the input resistance?

What are the AVC line voltages in the ON and OFF state?
What is the V7 cathode voltage? Have you tried a different valve?

Could there be leakage from the high voltage pins i.e. pin 2 & 3, and the diode pins 4 & 5 on the valve socket?

[ms660]

From the book of words if it's any use:

“The detector and A.V.C circuits employ the first diode of V7. A.V.C is fed via R19 to the filter circuits of the controlled valves when the A.V.C. is switched on.
When the A.V.C. is switched off the mean grid bias of the controlled valves is maintained by the second diode, at approximately the same potential as that existing under no signal conditions with A.V.C. on. The by-pass capacitor, C15, in the second diode circuit, removes any stray signal pickup or feedback which could produce partial A.V.C. with the switch in the off position”

Lawrence.

[G6Tanuki]

I'd be looking more for leakage in the decoupling caps associated with the screen circuit of the AVC-controlled stage which forms the 'bridge' that drives the S-meter. And checking that the resistors in the bridge have not drifted off to the resistor-equivalent of never-never-land.

[ms660]

I have both the R106 MK11 and HRO Senior, the slightly +ve AGC with the RF Gain adjusted to set the S Meter zero under no signal conditions with the AGC switched on rings a feint bell in my head with my receivers, can't say for sure.
Unfortunately I'm not in a position to fire them up and take some voltage measurements.

Lawrence.

[G3PIJpeter]

Originally Posted by pwdrive

I have both the R106 MK11 and HRO Senior, the slightly +ve AGC with the RF Gain adjusted to set the S Meter zero under no signal conditions with the AGC switched on rings a feint bell ....

I also have an HRO-MX with UX valves (purchased 1959) whose S-meter and AVC work fine on AM. When tomorrow's day jobs finish, I shall make some measurements on both of my HROs and let you know. I use a Heathkit valve voltmeter with a probe that contains a 2-megohm resistance - sufficiently high-Z not to load the circuit.
- Peter

[G3PIJpeter]

Update: measured values for my R106 (metal octal valves) are as follows - are they typical for AVC behaviour or is the trend a bit sluggish?

AVC switch OFF:
AVC line = +0.1V

AVC switch ON:
Cathode of det/AVC/1st audio valve (6SQ7) = +0.6V
Ant RF = 0V, RF gain = max: AVC = +0.36V
Ant RF = 0V, RF gain = min: AVC = +0.45V

With AVC switch ON and RF gain = max:
Aerial microvolts (@10 MHz): AVC volts
0: +0.36
10: +0.36
20: +0.36
40: +0.32
60: +0.24
80: -0.4
100: -0.52
200: -1.00
400: -1.5
800: -1.8
2mV: -2.4
8mV: -3.5
20mV: -4.5

I'll do the same for my HRO-MX (UX valves) later. Any thoughts on the above?
- Peter

[G3PIJpeter]

Comparative figures from my HRO-MX (UX valves)

AVC switch OFF
RF gain = max: AVC = +0.80V
RF gain = min: AVC = +0.86V

AVC switch ON; RF gain = max:
Aerial microvolts (@3.5 MHz): AVC volts
0: +0.6
4: 0.0
10: -0.9
20: -1.8
40: -2.8
80: -3.6
160: -4.4
400: -5.0
2mV: -8.8
4mV: -9.8
10mV: -11.0

Compared to the R106 figures, this lot seem to be a lot more responsive to increases at lower levels of aerial voltage. I have scoured all my radio textbooks and cannot find any suggestions for the typical range of AVC voltages to be expected in the typical sort of valve radio. However, there's clearly something up with the components around the AVC detector in the R106.

- Peter

[ms660]

The Hallicrafters SX28 book of words (available as an online download) shows an AGC graph/curve, it shows receiver output relative to RF input, I don't know if that would be a typical graph for comms receivers of that era, it might be worth plotting similar on your HRO MX and see if you get a similar graph/curve.

Lawrence.

[Silicon]

The AR-88 manual shows an AGC curve graph.
The X axis goes from 1uV to 1V. The most interesting bit of the curve is from 1uV to 10mV.
The Y axis goes from 0.1W to 10W measured across a 2.5 Ohm load.

Factors such as the level of RF and IF gain, conversion efficiency and whether 'delayed' AGC circuitry is used will have an effect on the AGC voltage and curve shape.

The 2 Megohm input resistance of the voltmeter might be a significant load on an AGC line.

[G3PIJpeter]

Interesting references - many thanks. However, the AR88 AVC curve shows aerial RF input against audio power out and the SX28 AVC curve shows aerial RF input against dB - which makes me think that both curves are showing how steady the receiver output remains for a change in input. In my last post, I was more wondering about actual AVC rail voltage for a given RF input to the aerial.
The input resistance to the Heathkit VTVM is given as 11 megohms, which should not load the voltage measured on the AVC line.
The Forum thread at https://www.vintage-radio.net/forum/...ad.php?t=84069 is about AVC in the R1155 and one post includes the observation that: "...the voltage starts at about -2.6V (no signal) and increases to -8.1V with a very strong signal applied". So that gives the upper and lower AVC voltage performance for the 1155 - fairly typical for a valve receiver of that era perhaps, and in line with the voltages I have measured on the HRO-MX. I think I shall replace all the capacitors around the 6SQ7 det/AVC valve in the R106 and see if that improves things . . . . .
- Peter