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 Post subject: Hallicrafters SX101A S Meter Circuit Problem
PostPosted: Jan Sun 21, 2018 8:33 pm 
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Joined: Jan Sun 06, 2013 9:57 pm
Posts: 95
Location: Buffalo, NY
Meter shows unvarying reading when receiver on. Smeter CAL and ADJ move level but again unvarying with received signal.

Items checked:
1. V15 6BA6- tube and surrounding components all test OK. Voltages on spec.

2. V7 6BJ7- tube and surrounding components all test OK. Voltages out of spec: PIN1 spec. 30v; 4 v measured. PIN2 spec. 30V; 0 v measured.

3. All rectifier and regulator tube voltages are within spec.

4. S Meter- getting full deflection with 5MA. Meter mechanically sound.

5. All tubes test OK.

Any suggestions on what else needs testing would be appreciated.


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 Post subject: Re: Hallicrafters SX101A S Meter Circuit Problem
PostPosted: Jan Sun 21, 2018 9:34 pm 
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Does receiver output change when switching AVC on/off with a strong signal? Does it overload with AVC off (but not on) with a very strong signal?

Rodger WQ9E


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 Post subject: Re: Hallicrafters SX101A S Meter Circuit Problem
PostPosted: Jan Sun 21, 2018 11:34 pm 
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Is your RF GAIN control at max?
On many receivers, reducing its setting introduces a bias on the S-meter.

- Leigh

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 Post subject: Re: Hallicrafters SX101A S Meter Circuit Problem
PostPosted: Jan Mon 22, 2018 12:56 am 
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If you look at the schematic, there is a potentiometer in the grid circuit of the S meter amp tube, 6BA6, V15,--R88. This controls the sensitivity of the S meter. It is not the zero control--R86--which is in series with the meter in its half of the cathode path to ground. On page 18 of the manual I have, after the RF alignment, BFO alignment, and Notch alignment, the manual tells how this is set for an S-9 reading with a specific set of signal requirements.

Norm

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 Post subject: Re: Hallicrafters SX101A S Meter Circuit Problem
PostPosted: Jan Mon 22, 2018 1:52 am 
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Joined: Jan Sun 06, 2013 9:57 pm
Posts: 95
Location: Buffalo, NY
Thanks for the input guys!

The receiver output does not change with a strong signal while switching the AVC - ON or OFF.

The RF gain control was at maximum.

I have the manual but have not performed the S meter setting because I could not get the meter to move with varying signals.

When I initially tested R88- the sensitivity control- I was seeing 0-200 ohms on my DMM. I deoxid it before putting it back in the circuit. I now notice with further turning of the control that there is a spot where it momentarily gets the meter to move with incoming signals- BUT it does not remain that way and goes back to unresponsiveness. Perhaps I would have noticed this "glitch" if I had used my analog ohmmeter.When I say unresponsive, I mean it stays constant at about 2 units. I'm now wondering if R88 is the culprit?


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 Post subject: Re: Hallicrafters SX101A S Meter Circuit Problem
PostPosted: Jan Mon 22, 2018 2:11 am 
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Location: Costa Mesa, California
R88 should be 0-2 meg. It would be at the 2 meg setting of the control that the meter would be most sensitive. At the 0 end, R88 is shorting the grid to ground and there would be no change with the detector voltage. R33 is a resistor going from the grid and one end of R88 to the plate of the detector, V7 pin 8. R33 is 1 meg and if it has gone open or high, then that would affect the change at the grid of the S meter amp tube and therefore the change in the current through the meter. The AVC voltage also comes off V7 pin 8 so something may be going on there. Have you replaced C77, a .047 capacitor that is connected after R44, a 1 meg resistor also connected to V7, pin 8? Is R45 still reading 2.2 meg? Is R44 good? Those are the questions I would be asking myself at this point.

Norm

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 Post subject: Re: Hallicrafters SX101A S Meter Circuit Problem
PostPosted: Jan Mon 22, 2018 2:27 am 
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Joined: Mar Thu 05, 2015 2:31 am
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Look at the AVC line going out of the detector with a high z meter. Should be seeing neg values proportional with strong signals. There is a 270K resistor in series with the detector. Also I have seen a number of bad Halli AVC switches that leak and ground the AVC line.

Peter


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 Post subject: Re: Hallicrafters SX101A S Meter Circuit Problem
PostPosted: Jan Mon 22, 2018 3:00 am 
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Sounds like the trouble is in the AVC circuit or there isn't enough signal to activate it and if the receiver is working normally the calibrator definitely would be strong enough. The receiver output should show a major increase in audio output when the AVC is turned off and on strong signals it should distort.

So you could have either a gain issue or an AVC issue but until the AVC is operational on a strong signal there is nothing to troubleshoot in the S meter circuit since that is just a simple amplifier controlled by the application of AGC to the control grid.

Rodger WQ9E


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 Post subject: Re: Hallicrafters SX101A S Meter Circuit Problem
PostPosted: Jan Mon 22, 2018 12:37 pm 
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Joined: Jan Sun 06, 2013 9:57 pm
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Location: Buffalo, NY
I'm wondering if we all are using the same schematic for the SX-101A.

Mine shows the SMeter ADJ pot as R27- 200 ohms.

The SMeter CAL pot as R26- 2 MEG.


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 Post subject: Re: Hallicrafters SX101A S Meter Circuit Problem
PostPosted: Jan Mon 22, 2018 12:59 pm 
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That is the schematic I have. The S meter and its 200 ohm zero adjust pot R27, configured as a variable resistor, are in parallel with the 820 ohm cathode resistor and the S meter is configured to respond to changes in cathode current. S meter sensitivity adjust R26 acts as a voltage divider determining the level of AGC voltage reaching the tube.

If you don't see changes in AGC voltage with varying signal strength then there will be no change in reading on the S meter because the S meter amplifier is running with cathode bias only until the AGC system develops voltage which provides control grid bias further reducing S meter amp gain. You should see a negative going voltage at pin 1 of V15 which goes increasingly negative with increases in signal strength.

Rodger WQ9E


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 Post subject: Re: Hallicrafters SX101A S Meter Circuit Problem
PostPosted: Jan Mon 22, 2018 3:20 pm 
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Joined: Jan Sun 06, 2013 9:57 pm
Posts: 95
Location: Buffalo, NY
I don't see any variation on PIN1 of V15 with negative voltage with increasing signal strength.


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 Post subject: Re: Hallicrafters SX101A S Meter Circuit Problem
PostPosted: Jan Mon 22, 2018 3:29 pm 
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sbvoigt wrote:
I don't see any variation on PIN1 of V15 with negative voltage with increasing signal strength.


That indicates the issue is with the AGC circuit or lack of enough gain in the front end or IF strip to create enough signal to develop AGC bias. If you have a signal generator hit the front end with around a 50 mv signal and there should either be significant AGC voltage developed or severe overload and distortion. If neither occurs there is a gain problem.

Rodger WQ9E


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 Post subject: Re: Hallicrafters SX101A S Meter Circuit Problem
PostPosted: Jan Mon 22, 2018 10:46 pm 
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Joined: Jan Sun 06, 2013 9:57 pm
Posts: 95
Location: Buffalo, NY
A second cleaning of the RF gain control did the trick! Who would have guessed- I guess I didn't get it clean on the first round.

I'm now getting movement with incoming signals and much better gain.

I'll save the S meter alignment/tuning for a later time.

Appreciate all the help from the forum.


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 Post subject: Re: Hallicrafters SX101A S Meter Circuit Problem
PostPosted: Jan Tue 23, 2018 2:05 am 
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Joined: Mar Thu 05, 2015 2:31 am
Posts: 36
Congrats! While at the gain pot make sure it zeros out completely with an ohmmeter. I have seen many that did not quite zero affecting senslesstivity.

The Halli S meters were composed of reversed meters, meaning when the meter went to zero, it was at full deflection. So when AVC is applied, the plate current of the S meter and front end/IF amps deflect to the right. I decided to put a meter on an older Halli receiver a few years ago that didnt have one. I didnt have a spare Halli meter and ended up kludging in a regular meter and zeroed it at half scale :D :D . I still use it.

I have owned a few 101s in the past. A nice receiver. Big chassis!

Peter


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