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Vintage Amateur and Military Radio Amateur/military receivers and transmitters, morse, and any other related vintage comms equipment.

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Old 19th Mar 2017, 3:11 pm   #41
Techman
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Default Re: Harvard H407 CB RX audio problem

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Originally Posted by Damo666 View Post
Can anyone suggest a decent entry level frequency counter to look out for, and that way I can create a "followed search" on eBay & be instantly notified if something is listed?
You may be able to pick one up at one of these junk ham radio auction type sales. I bought a couple a few months back for a quid each, untested! I may start a thread on them for the sake of interest and information. One of them worked and the other needed some 'attention' to get it working properly. Remember that any old counter that you buy may be out of calibration.
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Old 19th Mar 2017, 3:17 pm   #42
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Default Re: Harvard H407 CB RX audio problem

Will do Techman.

I'll have to fetch the rig downstairs I think, as the strong signals necessary to show the problem tend to only come on in the late evening, by which time I've had to vacate the bedroom it's in due to my toddler sleeping in there. He's a light sleeper.

The aforementioned, in addition to various other commitments is the reason this is taking so long to get to the bottom of.

I'll fetch it downstairs right now before I forget and keep it switched on permanently. There's no access to the outside vertical here, but I do have a very long length of wire which should bring in a decent signal to trigger things off.
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Old 19th Mar 2017, 3:20 pm   #43
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Default Re: Harvard H407 CB RX audio problem

My advice on the way forward would be to slow down and try and fix this radio in slow time. Even if that means fixing it later in the year. Otherwise you could end up throwing loads of time and money at something you don't really need once it finally gets fixed.

My advice on a frequency counter would be to be patient and buy an old used counter from one of the big names like Racal or Philips or maybe HP. Definitely buy something with an external reference option on the rear panel.

Otherwise there are new entry level counters available on ebay for similar money that you can buy straight away. Eg the VC3165 seems to be very popular. But I wouldn't want one. I'd rather take a chance and buy a top brand used counter. The prices seem to vary, I wouldn't want to pay more than 50 for a used counter if it was just going to be used for old school CB and ham stuff. If you are patient you will find one for much less money.
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Old 19th Mar 2017, 5:42 pm   #44
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Default Re: Harvard H407 CB RX audio problem

I did the pin 7 tests this afternoon and it showed 6.11V when cold although this wandered from 6.07 to 6.2V but once it was on for a while it was typically 6.25V with no signal. Note that I also set my PSU to give 12.67V at pin 11 of the LA1230 to match the homebase radio reading here.

I did try optimising the quad demod coil adjustment and this thing is very critical on the adjustment. It only has to be out by a tiny amount to wreck the audio. Note that I did this with a two tone test waveform to look at 2nd and 3rd order distortion on pin 6 (on an AF spectrum analyser) as I adjusted the coil.

I also put the tip of my little finger on the centre of the LA1230 chip and held it there and my thermal camera is correct. The centre of the chip runs quite hot. Much hotter than you would think! I assume that mine is OK because it seems to be working fine in FM receive. These things were never that great in terms of 'fidelity' (despite the name on the front panel ) but I think it is working as I remember back in the day. Not as harsh as a 134 radio but other radios will be nicer to listen to.
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Last edited by G0HZU_JMR; 19th Mar 2017 at 5:52 pm.
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Old 20th Mar 2017, 1:49 pm   #45
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Default Re: Harvard H407 CB RX audio problem

Jeremy,

When I was testing the voltage of pin 7, I found I was getting fluctuations in voltage readings like yours when I was using the chassis as the negative. However, I was getting different but solid readings when I was using transformer cans as the negative test point, if that makes sense? I found this odd, as the transformer cans are still connected to ground - unless my DMM is prone to slight RFI?

Anyway, in a nutshell, it seems that the dropping voltage of pin 7 on mine is presumably something to do with my problem - because your readings are constant as opposed to dropping.

I've ordered some 100uF Cap's, because they're handy & I have none, so I'll replace this one connected to pin 7 when I get them - as maybe the cap is leaky?

I'll hold off on the LA123O as you've all suggested & take things slow - besides, it's been pretty quiet on the CB these last couple of days, and no strong signals have been apparent which I really need.
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Old 21st Mar 2017, 12:38 am   #46
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Default Re: Harvard H407 CB RX audio problem

If you've got another room to try the radio in, you could do what I sometimes do and that is to connect another CB radio to your aerial, and with it set on low power on a blank channel, lock the mic on with a 'broadcast' radio set on a speech only type station placed within normal speaking distance of the mic. Then have the radio on test in another room, or even the same room for this particular test if you can put up with a bit of audio feed back, with no aerial connected and on the same channel as the other radio you'll get a good +30 signal for your test.
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Old 22nd Mar 2017, 1:32 am   #47
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Default Re: Harvard H407 CB RX audio problem

Techman,

That's a very good idea you suggested, so I'll do just that when I get back home later this week. In fact, I'll get my wife to modulate the mic' on my behalf.

Thank you for the idea.
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