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 Post subject: Early HRO avc issue
PostPosted: Feb Thu 01, 2018 12:56 am 
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Location: Roslyn,Pa 19001
I have a 1936 HRO that seems to have a distortion problem related to the avc circuit. It goes like this With the avc on on the rf gain at max a strong broadcast station will have some distortion. I can back off the rf gain control and it clears up the distortion.

This is what I've done, good set of tubes, align if and rf sections, replace paper/foil caps, check resistors for tolerance (all seem to be pretty good there are a couple in the avc circuit that are o/t but added parallel resistors to bring then into spec. Cleaned coils, switches, and sockets. HV supply is about 240vdc

Measured the avc voltage and with no signal it about -1 1/2 volts, good undistorted signal is around -5v and very strong signal is about -12. It seems to like voltages around from -1 1/2v to -7v above that distortion creeps in.

I tried loading the avc line down a little w/a high value resistor. This did drop the voltage down a little but made the distortion worst. Backing down the rf gain clears it up but it pegs the S meter unless the gain is up.
http://www.one-electron.com/doc_archive ... al_HRO.pdf

So is this the nature of the beast or am I missing something?

Tnx
Terry N3GTE


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 Post subject: Re: Early HRO avc issue
PostPosted: Feb Thu 01, 2018 1:15 am 
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Radioroslyn wrote:
I tried loading the avc line down a little w/a high value resistor. This did drop the voltage down a little but made the distortion worst.
I was about to say, "That's going to make it worse," before I even finished reading the sentence.

The symptoms you describe are indicative of some type of leakage to ground on the AVC line and the resistor just made that worse.

The leakage could be the AVC filter capacitor(s), or it could be grid leakage of one of the AVC controlled tubes. Either of these can pull the AVC line closer to ground than it should be resulting in excessive gain and distortion in the RF/IF stages.

Tube grid leakage may tend to get progressively worse as the tube warms up (maybe OK for the first 30 seconds to a minute or a few minutes). So if it starts off not being too bad and gets worse, that might easily be a tube problem.

Curtis Eickerman

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 Post subject: Re: Early HRO avc issue
PostPosted: Feb Thu 01, 2018 2:30 am 
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Measure carefully on each side of the 500K individual tube grid filters coming from the AGC line and this can help you locate a tube that is creating the problem because the AGC voltage will be less negative on the tube side of the filter resistor.

But sometimes the tube fault isn't one that tries to place a positive bias on the AGC line but is one that has changed its gain control characteristics and the most practical way to catch that tube is via substitution. As an alternative you can use a signal generator and scope to see where the distortion starts in the receiver but you are dealing with very low signal levels which makes it more difficult.

Rodger WQ9E


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 Post subject: Re: Early HRO avc issue
PostPosted: Feb Thu 01, 2018 3:02 am 
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Location: Liberty, Missorui
The modulation content of the received signal, and the amount of capacitance present on the AGC line, can have an impact on distortion levels, and is most noticeable at lower audio frequencies. This is a fairly common occurrence on older receivers.

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 Post subject: Re: Early HRO avc issue
PostPosted: Feb Thu 01, 2018 3:11 am 
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I think the distortion is 'created' in the last IF due to clipping, but that is due to the gain of a previous tube(s) being too high. This overdrives the innocent last IF to cause the distortion. Make sure that you have 0 leakage in any of the caps on the agc line. Weak caps on the agc line can also cause distortion. You could scope the agc looking for audio.

Regards, Larry


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 Post subject: Re: Early HRO avc issue
PostPosted: Feb Thu 01, 2018 10:20 pm 
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Joined: May Fri 01, 2015 2:33 am
Posts: 1026
Location: Roslyn,Pa 19001
Tnx guys for the sage advise.
Measured the voltage on each side the the avc filter resistors, all had abt a .6v drop. Was hoping to see one with a volt or two drop but no such luck. Tried swapping the tubes making sure I didn't switch the 6D6 for a 6C6. No improvement. Then I thought maybe I should look at the avc voltage at the grid caps of the tubes, maybe a problem with the connection from the avc resistors the tuned circuit. Using a Heathkit V-6 vtvom I found that the voltage at the rf amps were much higher the the if amp tubes. The odd thing is that when I measured the 1st if amp grid it was abt -4v along with all the others. Touching it with the high Z meter probe must have shocked it in to submission. So I'm thinking that it's a tube issue either socket or grid cap connection.
Now that the avc is working the S meter won't zero. Back to the ohm meter to solve this issue.

Terry


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 Post subject: Re: Early HRO avc issue
PostPosted: Feb Thu 01, 2018 11:40 pm 
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Radioroslyn wrote:
Now that the avc is working the S meter won't zero. Back to the ohm meter to solve this issue.
S-meter zero is something that should be checked with the antenna disconnected and perhaps even shorting the antenna terminals of the receiver.

In many cases plain old RF noise coming in through the antenna will keep a an S-meter well above zero (atmospherics, lighting, switching power supplies, blenders, etc.).

Curtis Eickerman

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 Post subject: Re: Early HRO avc issue
PostPosted: Feb Fri 02, 2018 2:06 am 
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Joined: May Fri 01, 2015 2:33 am
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Location: Roslyn,Pa 19001
I grounded the ant and I can't get the meter below 1/2 scale.
Checked the pair of 2K resistors and pot those are ok. The circuit is a little difficult to untangle as the meter measures the difference between the SG current of the rf and if tubes vs the plate current of the if amps.
I'm trying to scare up a voltage chart for this little beastie to give me some clues as to whether it's the sg side or the plate side at fault.

Terry N3GTE


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