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Old 17th Jul 2021, 4:21 pm   #1
ChristianFletcher
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Default HRO M-Type AVC

National HRO early M Type

I have just bought an HRO and noticed that it overloads quite badly on the broadcast band actually on all bands. Careful adjustment of the RF gain helps but the audio still sounds clipped and distorted. The AVC switch doesn’t appear to have any effect and I suspect an AVC problem. Perhaps someone with a HRO could confirm the effectiveness of this control on a work set. The set is sensitive but one doesn’t have any experience of this set so not sure how well they should perform? The set has has a few of the capacitors changed out but it looks mainly original so probably needs some work. I haven’t investigated further as yet.

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Old 17th Jul 2021, 4:58 pm   #2
Bazz4CQJ
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Default Re: HRO M-Type AVC

Oh yes, the AVC switch is like night and day on the HRO.

B
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Old 17th Jul 2021, 4:59 pm   #3
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Default Re: HRO M-Type AVC

Measure the voltage on the AVC line and compare with post 10 in the thread at https://www.vintage-radio.net/forum/...d.php?t=111380
Also, turning on the BFO should markedly desensitise the receiver with the AVC switch in the 'On' position.
Finally, it doesn't take much leakage to effectively ground the AVC line (e.g. possibly C14).
- Peter
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Old 17th Jul 2021, 10:04 pm   #4
ChristianFletcher
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Default Re: HRO M-Type AVC

Ok thanks for the operation reports. Having played a bit more with the set the AVC is working but does not have a big effect. My next step is to have a look at the AVC line from what I can see the sets mainly original so will probably benefit from changing out some of the older capacitors.

Thanks regards Chris
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Old 18th Jul 2021, 2:34 am   #5
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Default Re: HRO M-Type AVC

Fitst suspects are leaky/shorted AGC time constant capacitors from the AGC rail to ground. Zero volts AGC corresponds to max IF and RF gain and overload on all but very small signals.

What is the S-meter doing? and what effect does the RF gain pot have?

David
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Old 18th Jul 2021, 2:06 pm   #6
ChristianFletcher
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Default Re: HRO M-Type AVC

The Radio is overloading on larger signals. The RF gain pot works normal. The signal strength meter kind of works but is a bit erratic. Spends a lot of time with the need pushing toward the zero stop but does work when the rf gain is increased.
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Old 18th Jul 2021, 2:38 pm   #7
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Default Re: HRO M-Type AVC

Yes I'd be checking all the AVC-line decoupling capacitors as a first step. You could also try measuring the resistance from the AVC line to ground [with the radio switched off of course] - it should be several Megohms. If it's lower you have a leak!

In any old radio you can find leakage in odd places - old paxolin/SRBP insulation, dirt/oil/grease/tobacco-film on ceramic valve-sockets, rotten cotton-covered cables, and I've even seen a toggle-switch which had internal green fluffy corrosion between the contacts and the body that made it look like a 50KOhm resistor to ground.
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Old 18th Jul 2021, 2:54 pm   #8
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Default Re: HRO M-Type AVC

Quote:
Originally Posted by G6Tanuki View Post
You could also try measuring the resistance from the AVC line to ground [with the radio switched off of course] - it should be several Megohms. If it's lower you have a leak!
AVC off....approx 250k.

AVC on approx 1meg.

Lawrence.
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