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 Post subject: DX-440 FM & Broadcast MW good. No SW reception
PostPosted: Mar Wed 25, 2020 3:44 pm 
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Joined: Mar Wed 25, 2020 3:33 pm
Posts: 2
I volunteer on electronics for the thrift store that supports our local hospice and a DX-440 has got to the front of my queue.

with the rod antenna (lost its tip but still about 5ft long), I'm pulling in FM signals that I normally can't receive in my mountain valley location, very impressive. Also MW is good.

Nothing but broadband noise on AM outside MW.

I'm in Colorado, close to WWV, so I should be pulling that in loud and clear.

I'm competent with audio electronics and I am well equipped with signal generators up into the MHz range, scopes etc, but it's about 40 years since I investigated any radio electronics.

Any advice would be appreciated.


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 Post subject: Re: DX-440 FM & Broadcast MW good. No SW reception
PostPosted: Mar Wed 25, 2020 9:54 pm 
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Joined: Jan Tue 31, 2012 1:55 am
Posts: 12659
Location: Mountains of Mourne. Ireland.
Hello Dave.

RTM (read the manual)... :)


To reset the microprocessor, remove all batteries for ten+ minutes.
When troubleshooting a transistor radio, always use a fresh set of batteries.

User manual...
http://www.radiopics.com/-%20PHP%20-/1. ... g%20Manual).php

Service manual...
https://www.manualslib.com/manual/11387 ... e=1#manual

Weird stuff!
https://www.w8an.net/sangean/sangean_ats803a.pdf


:) Greg.


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 Post subject: Re: DX-440 FM & Broadcast MW good. No SW reception
PostPosted: Mar Thu 26, 2020 7:17 am 
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Joined: Mar Wed 25, 2020 3:33 pm
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egg,

Thank you for the prompt reply.

Well I had found the user manual and read the relevant bits but not the service manual.

However the weird stuff seems to be pointing in the right direction.

There is no doubt the FM is very good and SW is dead less some noise, so that lines up with some of the FET zapped reports.

I can talk radios from a systems point of view, but not at circuit design level, so I'm unclear why a front-end FET failing (failing open?) can leave anything working, even an FM receiver working as well as this one is. Can you help with that please?

In any case, I'll locate the FET and test it and replace if required.

Dave


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 Post subject: Re: DX-440 FM & Broadcast MW good. No SW reception
PostPosted: Mar Thu 26, 2020 2:00 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 21096
Location: Utica, NY 13502 (USA)
According to the schematic, the FM tuner uses a separate FET in its front end which is probably why the FM band still works.

Dave


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 Post subject: Re: DX-440 FM & Broadcast MW good. No SW reception
PostPosted: Mar Thu 26, 2020 5:50 pm 
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Joined: Jun Sun 15, 2014 5:37 pm
Posts: 390
Location: Montreal, Quebec
There's generally little overlap between the FM section and the am/shortwave section, until the audio circuitry. So one could work without the other.

That's changed in recent years, a single IC for all bands, converting to digital to process it.

I remember getting into an argument online with someone a quarter century ago, he had an expensive shortwave portable and insisted it had good FM reception. But generally for a shortwave receiver, the fm band reception is an add on, so it isn't automatically a great fm receiver.


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 Post subject: Re: DX-440 FM & Broadcast MW good. No SW reception
PostPosted: Mar Sat 28, 2020 2:33 pm 
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Joined: Mar Mon 16, 2020 4:18 am
Posts: 14
Hi Dave. I am not familiar with the DX-440 in particular and hopefully someone will chime in on its peculiarities. A few things I might suggest at first (after downloading a free schematic if possible,lol,) is to check for the local oscillator. If something is wrong hear you will have unturned band noise.Also, if it is REALLY deaf you probably might not hear any change in hand noise. If it takes 1000 uv to make a squeak WEB wouldn't come through. You should be able to check this with a sig gen cranked way up in level at both the tuned signal freq and maybe the i.F too if you know it's freq. If you can spot a peep of signal turned way up you should be able to compare its sensitivity roughly against another radio. If you can find out its I.F frequency you could listen for the radio's local oscillator in another shortwave radio. Listen for it, listen for it to tune around, move in freq. with the tuning dial. Some first things to check and look for. E.G= local oscillator freq. = dialfreq +/- the I.F. Some old popular I.F's = 455kc or .455 megacycle or 10.7 megacycle? I am interested if you find anything out. I hope I am understandable. Best Regards, Doug E.


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 Post subject: Re: DX-440 FM & Broadcast MW good. No SW reception
PostPosted: Mar Sat 28, 2020 6:14 pm 
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Joined: Jun Sun 15, 2014 5:37 pm
Posts: 390
Location: Montreal, Quebec
A lot of digitally tuned portables have a 450KHz IF. It's related to the synthesizer IC used.

And if the shortwave portable is double conversion, in recent decades the IF is above 30MHz, though I don't think there's real standardizatiin of the specific frequency.


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 Post subject: Re: DX-440 FM & Broadcast MW good. No SW reception
PostPosted: Mar Sun 29, 2020 9:23 pm 
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Joined: Mar Tue 27, 2012 1:24 pm
Posts: 1166
SW off the whip was never good on my DX-440, and with conditions being so poor, I doubt you'd hear much off the whip in much of the US.

Your not hearing WWV seems problematic, though, so it's possible the SW RF FET was blown. A weak spot in those radios because they didn't have diode protection.


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 Post subject: Re: DX-440 FM & Broadcast MW good. No SW reception
PostPosted: Mar Sun 29, 2020 9:24 pm 
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Joined: Mar Tue 27, 2012 1:24 pm
Posts: 1166
Dave Doughty wrote:
According to the schematic, the FM tuner uses a separate FET in its front end which is probably why the FM band still works.

Dave


MW works off its own RF amp, too. Which would explain why it works.


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 Post subject: Re: DX-440 FM & Broadcast MW good. No SW reception
PostPosted: Apr Thu 30, 2020 10:57 am 
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Joined: Jun Sat 27, 2015 5:35 am
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try to replace Q115 (2SK152-3)


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 Post subject: Re: DX-440 FM & Broadcast MW good. No SW reception
PostPosted: May Wed 13, 2020 5:17 am 
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Joined: Apr Tue 28, 2020 7:58 pm
Posts: 33
I have the same, almost, problem with my dx-440. I just posted a new thread on it. I can however get pretty strong wwv signal when connected to external antenna, just the audio sounds a bit harsh and distorted, also I don't get any LEDs light up on sw even when connected to external antenna receiving strong sw signal. Audio sounds good and clean on all bands except sw.

If Q115 was bad, wouldn't it also cause mw band to be dead?

In short, my dx-440 works perfectly on FM and MW, but it's very quiet on SW except with external antenna and even then the signal lights stay dark and audio is distorted on SW.

Any expert Insight to this vexing situation? I have already "reset" the radio by pulling the aa batteries.


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 Post subject: Re: DX-440 FM & Broadcast MW good. No SW reception
PostPosted: May Thu 14, 2020 1:50 pm 
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Joined: Mar Tue 27, 2012 1:24 pm
Posts: 1166
jamesc760 wrote:
If Q115 was bad, wouldn't it also cause mw band to be dead?


MW operates off of a separate RF amp transistor (Q110), per the schematic (on the schematic the MW RF amp FET is next to the loopstick). Signal from the loopstick goes into Q110, and then the MW signal appears to bypass Q115, and it connects with the IF chain right after Q115 and in front of L110 and L111.


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 Post subject: Re: DX-440 FM & Broadcast MW good. No SW reception
PostPosted: May Thu 14, 2020 4:31 pm 
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Joined: Apr Tue 28, 2020 7:58 pm
Posts: 33
Renton481 wrote:
jamesc760 wrote:
If Q115 was bad, wouldn't it also cause mw band to be dead?


MW operates off of a separate RF amp transistor (Q110), per the schematic (on the schematic the MW RF amp FET is next to the loopstick). Signal from the loopstick goes into Q110, and then the MW signal appears to bypass Q115, and it connects with the IF chain right after Q115 and in front of L110 and L111.


I see, so you're telling me that if we replace Q115, SW band will come back. I'm going to replace Q115 today and will report back! Thank you!


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 Post subject: Re: DX-440 FM & Broadcast MW good. No SW reception
PostPosted: May Thu 14, 2020 7:53 pm 
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Joined: Apr Tue 28, 2020 7:58 pm
Posts: 33
I just replaced rhe Q115 with a J113 transistor, its a direct pin to pin replacement. Shortwave is coming in strong now, the signal meter LEDs light up from 0 to 5. Thank you to those who said to replace it. Kudos!

Tldr, if your dx-440 suddenly develops low sensitivity to shortwave, replace Q115.


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 Post subject: Re: DX-440 FM & Broadcast MW good. No SW reception
PostPosted: May Fri 15, 2020 8:43 am 
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Joined: Mar Tue 27, 2012 1:24 pm
Posts: 1166
Glad I could be of some help.

Now, if and when you have the time or inclination, wire some back-to-back diodes between the whip antenna terminal and the battery ground. I did this for my DX-440 in 2003 or so, as a protective measure. Some hookup wire, diodes, and a little tape around the soldered, back-to-back diodes was easy enough to do.

The back to back diodes would help prevent hand caused static electricity discharges from zapping Q115 (like if you touch the whip on a dry winter day or in similar conditions.

The DX-440, unlike other, later Sangean radios, doesn't have diode protection on the whip and external antenna terminal.


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 Post subject: Re: DX-440 FM & Broadcast MW good. No SW reception
PostPosted: May Fri 15, 2020 4:20 pm 
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Joined: Apr Tue 28, 2020 7:58 pm
Posts: 33
Renton481 wrote:
Glad I could be of some help.

Now, if and when you have the time or inclination, wire some back-to-back diodes between the whip antenna terminal and the battery ground. I did this for my DX-440 in 2003 or so, as a protective measure. Some hookup wire, diodes, and a little tape around the soldered, back-to-back diodes was easy enough to do.

The back to back diodes would help prevent hand caused static electricity discharges from zapping Q115 (like if you touch the whip on a dry winter day or in similar conditions.

The DX-440, unlike other, later Sangean radios, doesn't have diode protection on the whip and external antenna terminal.


Aye, sir! Thanks for the tip, I did the diodes protection for my sony icf 2010 (that was for the external antenna jack, If I remember correctly) so I'm familiar with the concept. Much appreciated!


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