Realistic DX400 White noise only - UK Vintage Radio Repair and Restoration Discussion Forum

M6RDPfly · 10th Apr 2023, 10:39 am

Hi folks....

I have a Realistic DX400 which has worked wonderfully for many years but yesterday when I switched it on it suddenly stopped! The S meter display lights up and there are gobbledegook digits on the LCD display. White noise only is apparent on FM but on AM and SSB there is no noise at all.

I have some experience in radio repair and I can solder and desolder quite well and so I wondered whether anyone might be able to point to me in the direction of what might be at fault.

The last radio I repaired was a crystal controlled airband receiver.... it took me the best part of five years but I got there in the end! In that case it was a damaged transistor in the crystal oscillator but I only really got there by process of elimination and a lot of forum help!

This Realistic DX400 is a short wave receiver, and I shall try and upload a photo of it as it displays currently when I switch it on.

Anyway thanks for reading.
Adam
M6RDP
UK

Radio Wrangler · 10th Apr 2023, 12:17 pm

Hi Adam,

I'm not familiar with this set, but it is likely that a single fault causes all your symptoms.

'The direct frequency entry' shows that the set is tuned by a frequency synthesiser. You type in a number, spin the tuning knob or poke a memory button and a little microprocessor updates its register of what frequency you want. It encodes this number in a way to suit the LCD display and it sends it. It also has to think about the frequency synthesiser and calculate what frequency is needed for the set's first local oscillator. It then has to format this number to suit the PLL chip(s) in the synthesiser, and senf that data to them.

It looks like your microprocessor isn't working properly and is sending gaga data or none at all, to both the display and the synthesiser.

Sending wrong data to the synth is quite capable of making a set play dead. The white noise with FM selected is from the limiting amplifier needed ahead of an FM discriminator bringing the noise floor right up to maximum signal levels. This is quite normal and to be expected.

So I'd go looking around the controller board, looking for dirty connectors, bad solder joints. I'd use an oscilloscope to look for the microprocessor's clock oscillator (crystal?) running. I'd go looking to see the reset circuit working at turn on, and then I'd go looking at all the data lines looking to see that there was at least some activity. I'd take precautions against static charge damage while working in ths area.

David

Station X · 10th Apr 2023, 12:50 pm

If you don't already have it the manual contains a circuit diagram.

https://rigreference.com/storage/man...3.93465455.pdf

John KC0G · 10th Apr 2023, 4:52 pm

IIRC the DX400 was a version of the Uniden CR-2021 which was made by them for Radio Shack / Tandy. I think that it was Uniden's version of a Sony ICF-2001. The radio is now very close to 40 years old.

I think that there are several tantalum capacitors in this radio. I had one where it appeared that one or more had failed. I swapped the RF/IF board from a radio with a very poor case. I do recall that the circuit diagram was a beast to work through.

The two large electrolytic capacitors around the IC audio amp are prone to fail. I know of at least two cases where replacing those brought a set back to life.

A Google search also suggest that the front-end FET(s) were susceptible to failure, particularly in cases where FM still worked and AM/SW was dead.

I think that a thorough check of the power lines might be in order. Is part of the circuit covered by a screening can?

73 John KC0G/M0KCY

SiriusHardware · 11th Apr 2023, 12:05 am

With anything which has a microprocessor at the heart of it, it may be worth consulting the unit's user manual to see if there is a hard 'factory reset' which can be tried - if there is, it is often just a matter of holding down one or more buttons while turning the unit on. Always consider things like that, things which you can investigate and possibly try without even taking the unit apart, before getting out the hammer and chisel.

Edit: Reading the manual linked to earlier I can see no mention of a specific hard reset process but it does mention a possible situation where the micro might not initialise properly especially after installing new batteries. The suggestion in the manual is to remove the batteries (and any other source of power) for at least a minute, then try again.

Edit #2: Reading further I see that the unit has distinct and separate 'radio' batteries (6 x C cells) and 'microprocessor' or 'backup' batteries (2 x AA). It is the 'microprocessor' batteries which need to be removed for a minute / checked / changed in the event of any microprocessor strangeness, as seems to be the case here. When were the 'microprocessor' batteries last checked or changed?

M6RDPfly · 12th Apr 2023, 4:06 pm

Hi everyone,

All this is really helpful and I'm extremely grateful for your pointers. I will have a look at the micro processor batteries as they haven't been changed for a couple of years as that would be a really easy place to start and it didn't even occur to me. To be honest I had forgotten they were even there!

John thank you for the information regarding the capacitors and FETs. That might well be my first point of call if the above doesn't resolve things.

I will also go on to check the leads and connections around the micro processor.

Thank you also for the circuit diagram.

I should get a few hours to work on it over the next few days so promise to come back with an update.

Yes the reason I bought it was because it was so similar to the Uniden CR2021 I had when I was a boy - my first shortwave receiver!

I'll be back......!

M6RDPfly · 15th Apr 2023, 10:24 am

Sadly replacing the memory batteries didn't make any difference. That would have been nice! Never mind. I had been using the radio on a homemade HF active loop. I wonder now if that was not such a good idea, and my guess is the front-end FETs that John KC0G referred to might be a logical starting place. There are an awful lot of them: five in a row I think. And I will have a look at changing the 2200uF electrolytic capacitors c195/196 as I have a few of those spare.

I found a fault-finding flow chart for the DX390 and will look for one for the DX400.

Yes, part of the circuit is covered by a large screening can.

Thanks again and I'll report back when I have more news.......

73 Adam M6RDP

M6RDPfly · 16th Apr 2023, 1:46 pm

OK......I have taken the main board off and around the large 1000uF electrolytic capacitor adjacent to the power transformer (that gets hot, I have read) there are clear sign of damage to the nearby diode and resistor, possibly also the 2SC2458Y transistor. The resistor even wobbles and has come away from the mounting hole, so this is quite likely the source of the trouble.

I will replace these and report the results.

There are several electrolytics in this area and the two 2200uF capacitors appear to my untrained eye that they might be leaking.

I will upload a photo from my phone shortly. I'd be interested to know if you agree and if so I'll add these to my component order.

Thanks. 73 Adam

M6RDPfly · 16th Apr 2023, 1:59 pm

Here is the photo of what I think is a leaking capacitor

Lloyd 1985 · 16th Apr 2023, 7:25 pm

It might not be the capacitor leaking, that looks like the glue that Sony used to like sticking components down with, it’s awful stuff, when it goes dark brown like it has in your photo, it becomes conductive, and it also corrodes components while it’s breaking down, hence that bit of green on the ceramic capacitor leg. No chemicals seem to touch it, so you end up having to scrape it off the board. The electrolytics may be ok if their legs haven’t corroded off!

Regards
Lloyd

M6RDPfly · 17th Apr 2023, 7:15 am

Hi Lloyd

Thanks for the information about that sticky stuff around the capacitors. I'll get my Jewelers loupe out and have a good look at the capacitor legs. Learning so much from each radio repair thanks to the forum. Cheers
Adam

M6RDPfly · 21st Apr 2023, 3:22 pm

I just wanted to thank everyone who has helped me with the radio. The wobbly diode and resistor were upon closer inspection connected to a corroded piece of PCB pad. I can only presume it was as Lloyd described and caused by the broken down manufacturing glue as it is actually some distance from the transformer and adjacent to the speaker magnet although it does look like a burn mark. I replaced D47, R153 and TR45, althougj again, as you said Lloyd, the components turned out to be perfectly okay once I checked them after removal. This is the second time I have come across a minute PCB pad that has come away from the board. I improvised but must now learn how to repair for future projects. I cleaned the damaged area also and with hindsight next time it would have been easier just to clean up the area and reheat the joint connecting the three components on the damaged pad. But it's been fun and what joy when I switched it on and the radio burst back into life again.

Thank you again and I will upload a picture of the area. 73 Adam

M6RDPfly · 21st Apr 2023, 3:26 pm

Here is the photo

Lloyd 1985 · 21st Apr 2023, 3:42 pm

Great news! It’s always nice when you get something working again.

That PCB does look rather black, I wonder if there is something that generates a bit of heat in that area too, that glue breaks down even quicker if heat is involved. That area of the circuit looks like a voltage regulator, so I wouldn’t be surprised if there was a little heat there when the radio is operating.

Regards,
Lloyd

M6RDPfly · 21st Apr 2023, 4:12 pm

Hi Lloyd, thanks for that. It worried me reassembling it thinking about what might be making it black. I'll do some checks. Family enough I was going to ask somebody what that area of the circuit was but didn't want to be asking too many questions. The radio was bought in the USA by the former owner whilst studying out there. Could it be that it wasn't designed to be used by the 240 volts in the UK? The user manual says power source ac/120 (240v for units purchased in the UK and Australia).....

10th Apr 2023, 10:39 am	#1
M6RDPfly Triode Join Date: Oct 2019 Location: Dawlish, Devon, UK. Posts: 34	Realistic DX400 White noise only Hi folks.... I have a Realistic DX400 which has worked wonderfully for many years but yesterday when I switched it on it suddenly stopped! The S meter display lights up and there are gobbledegook digits on the LCD display. White noise only is apparent on FM but on AM and SSB there is no noise at all. I have some experience in radio repair and I can solder and desolder quite well and so I wondered whether anyone might be able to point to me in the direction of what might be at fault. The last radio I repaired was a crystal controlled airband receiver.... it took me the best part of five years but I got there in the end! In that case it was a damaged transistor in the crystal oscillator but I only really got there by process of elimination and a lot of forum help! This Realistic DX400 is a short wave receiver, and I shall try and upload a photo of it as it displays currently when I switch it on. Anyway thanks for reading. Adam M6RDP UK Attached Thumbnails

10th Apr 2023, 12:50 pm	#3
Station X Moderator Join Date: Jan 2003 Location: Ipswich, Suffolk, IP4, UK. Posts: 21,288	Re: Realistic DX400 White noise only If you don't already have it the manual contains a circuit diagram. https://rigreference.com/storage/man...3.93465455.pdf __________________ Graham. Forum Moderator Reach for your meter before you reach for your soldering iron.

10th Apr 2023, 4:52 pm	#4
John KC0G Hexode Join Date: Mar 2009 Location: Minneapolis, Minnesota, USA. Posts: 278	Re: Realistic DX400 White noise only IIRC the DX400 was a version of the Uniden CR-2021 which was made by them for Radio Shack / Tandy. I think that it was Uniden's version of a Sony ICF-2001. The radio is now very close to 40 years old. I think that there are several tantalum capacitors in this radio. I had one where it appeared that one or more had failed. I swapped the RF/IF board from a radio with a very poor case. I do recall that the circuit diagram was a beast to work through. The two large electrolytic capacitors around the IC audio amp are prone to fail. I know of at least two cases where replacing those brought a set back to life. A Google search also suggest that the front-end FET(s) were susceptible to failure, particularly in cases where FM still worked and AM/SW was dead. I think that a thorough check of the power lines might be in order. Is part of the circuit covered by a screening can? 73 John KC0G/M0KCY

11th Apr 2023, 12:05 am	#5
SiriusHardware Dekatron Join Date: Aug 2011 Location: Newcastle, Tyne and Wear, UK. Posts: 11,560	Re: Realistic DX400 White noise only With anything which has a microprocessor at the heart of it, it may be worth consulting the unit's user manual to see if there is a hard 'factory reset' which can be tried - if there is, it is often just a matter of holding down one or more buttons while turning the unit on. Always consider things like that, things which you can investigate and possibly try without even taking the unit apart, before getting out the hammer and chisel. Edit: Reading the manual linked to earlier I can see no mention of a specific hard reset process but it does mention a possible situation where the micro might not initialise properly especially after installing new batteries. The suggestion in the manual is to remove the batteries (and any other source of power) for at least a minute, then try again. Edit #2: Reading further I see that the unit has distinct and separate 'radio' batteries (6 x C cells) and 'microprocessor' or 'backup' batteries (2 x AA). It is the 'microprocessor' batteries which need to be removed for a minute / checked / changed in the event of any microprocessor strangeness, as seems to be the case here. When were the 'microprocessor' batteries last checked or changed? Last edited by SiriusHardware; 11th Apr 2023 at 12:26 am.

12th Apr 2023, 4:06 pm	#6
M6RDPfly Triode Join Date: Oct 2019 Location: Dawlish, Devon, UK. Posts: 34	Re: Realistic DX400 White noise only Hi everyone, All this is really helpful and I'm extremely grateful for your pointers. I will have a look at the micro processor batteries as they haven't been changed for a couple of years as that would be a really easy place to start and it didn't even occur to me. To be honest I had forgotten they were even there! John thank you for the information regarding the capacitors and FETs. That might well be my first point of call if the above doesn't resolve things. I will also go on to check the leads and connections around the micro processor. Thank you also for the circuit diagram. I should get a few hours to work on it over the next few days so promise to come back with an update. Yes the reason I bought it was because it was so similar to the Uniden CR2021 I had when I was a boy - my first shortwave receiver! I'll be back......!

10th Apr 2023, 12:17 pm	#2
Radio Wrangler Moderator Join Date: Mar 2012 Location: Fife, Scotland, UK. Posts: 22,876	Re: Realistic DX400 White noise only Hi Adam, I'm not familiar with this set, but it is likely that a single fault causes all your symptoms. 'The direct frequency entry' shows that the set is tuned by a frequency synthesiser. You type in a number, spin the tuning knob or poke a memory button and a little microprocessor updates its register of what frequency you want. It encodes this number in a way to suit the LCD display and it sends it. It also has to think about the frequency synthesiser and calculate what frequency is needed for the set's first local oscillator. It then has to format this number to suit the PLL chip(s) in the synthesiser, and senf that data to them. It looks like your microprocessor isn't working properly and is sending gaga data or none at all, to both the display and the synthesiser. Sending wrong data to the synth is quite capable of making a set play dead. The white noise with FM selected is from the limiting amplifier needed ahead of an FM discriminator bringing the noise floor right up to maximum signal levels. This is quite normal and to be expected. So I'd go looking around the controller board, looking for dirty connectors, bad solder joints. I'd use an oscilloscope to look for the microprocessor's clock oscillator (crystal?) running. I'd go looking to see the reset circuit working at turn on, and then I'd go looking at all the data lines looking to see that there was at least some activity. I'd take precautions against static charge damage while working in ths area. David __________________ Can't afford the volcanic island yet, but the plans for my monorail and the goons' uniforms are done

15th Apr 2023, 10:24 am	#7
M6RDPfly Triode Join Date: Oct 2019 Location: Dawlish, Devon, UK. Posts: 34	Re: Realistic DX400 White noise only Sadly replacing the memory batteries didn't make any difference. That would have been nice! Never mind. I had been using the radio on a homemade HF active loop. I wonder now if that was not such a good idea, and my guess is the front-end FETs that John KC0G referred to might be a logical starting place. There are an awful lot of them: five in a row I think. And I will have a look at changing the 2200uF electrolytic capacitors c195/196 as I have a few of those spare. I found a fault-finding flow chart for the DX390 and will look for one for the DX400. Yes, part of the circuit is covered by a large screening can. Thanks again and I'll report back when I have more news....... 73 Adam M6RDP

16th Apr 2023, 1:46 pm	#8
M6RDPfly Triode Join Date: Oct 2019 Location: Dawlish, Devon, UK. Posts: 34	Re: Realistic DX400 White noise only OK......I have taken the main board off and around the large 1000uF electrolytic capacitor adjacent to the power transformer (that gets hot, I have read) there are clear sign of damage to the nearby diode and resistor, possibly also the 2SC2458Y transistor. The resistor even wobbles and has come away from the mounting hole, so this is quite likely the source of the trouble. I will replace these and report the results. There are several electrolytics in this area and the two 2200uF capacitors appear to my untrained eye that they might be leaking. I will upload a photo from my phone shortly. I'd be interested to know if you agree and if so I'll add these to my component order. Thanks. 73 Adam

16th Apr 2023, 1:59 pm	#9
M6RDPfly Triode Join Date: Oct 2019 Location: Dawlish, Devon, UK. Posts: 34	Re: Realistic DX400 White noise only Here is the photo of what I think is a leaking capacitor Attached Thumbnails

16th Apr 2023, 7:25 pm	#10
Lloyd 1985 Nonode Join Date: Jun 2008 Location: Coningsby, Lincolnshire, UK. Posts: 2,819	Re: Realistic DX400 White noise only It might not be the capacitor leaking, that looks like the glue that Sony used to like sticking components down with, it’s awful stuff, when it goes dark brown like it has in your photo, it becomes conductive, and it also corrodes components while it’s breaking down, hence that bit of green on the ceramic capacitor leg. No chemicals seem to touch it, so you end up having to scrape it off the board. The electrolytics may be ok if their legs haven’t corroded off! Regards Lloyd

17th Apr 2023, 7:15 am	#11
M6RDPfly Triode Join Date: Oct 2019 Location: Dawlish, Devon, UK. Posts: 34	Re: Realistic DX400 White noise only Hi Lloyd Thanks for the information about that sticky stuff around the capacitors. I'll get my Jewelers loupe out and have a good look at the capacitor legs. Learning so much from each radio repair thanks to the forum. Cheers Adam

21st Apr 2023, 3:22 pm	#12
M6RDPfly Triode Join Date: Oct 2019 Location: Dawlish, Devon, UK. Posts: 34	Re: Realistic DX400 White noise only I just wanted to thank everyone who has helped me with the radio. The wobbly diode and resistor were upon closer inspection connected to a corroded piece of PCB pad. I can only presume it was as Lloyd described and caused by the broken down manufacturing glue as it is actually some distance from the transformer and adjacent to the speaker magnet although it does look like a burn mark. I replaced D47, R153 and TR45, althougj again, as you said Lloyd, the components turned out to be perfectly okay once I checked them after removal. This is the second time I have come across a minute PCB pad that has come away from the board. I improvised but must now learn how to repair for future projects. I cleaned the damaged area also and with hindsight next time it would have been easier just to clean up the area and reheat the joint connecting the three components on the damaged pad. But it's been fun and what joy when I switched it on and the radio burst back into life again. Thank you again and I will upload a picture of the area. 73 Adam

21st Apr 2023, 3:26 pm	#13
M6RDPfly Triode Join Date: Oct 2019 Location: Dawlish, Devon, UK. Posts: 34	Re: Realistic DX400 White noise only Here is the photo Attached Thumbnails

21st Apr 2023, 3:42 pm	#14
Lloyd 1985 Nonode Join Date: Jun 2008 Location: Coningsby, Lincolnshire, UK. Posts: 2,819	Re: Realistic DX400 White noise only Great news! It’s always nice when you get something working again. That PCB does look rather black, I wonder if there is something that generates a bit of heat in that area too, that glue breaks down even quicker if heat is involved. That area of the circuit looks like a voltage regulator, so I wouldn’t be surprised if there was a little heat there when the radio is operating. Regards, Lloyd

21st Apr 2023, 4:12 pm	#15
M6RDPfly Triode Join Date: Oct 2019 Location: Dawlish, Devon, UK. Posts: 34	Re: Realistic DX400 White noise only Hi Lloyd, thanks for that. It worried me reassembling it thinking about what might be making it black. I'll do some checks. Family enough I was going to ask somebody what that area of the circuit was but didn't want to be asking too many questions. The radio was bought in the USA by the former owner whilst studying out there. Could it be that it wasn't designed to be used by the 240 volts in the UK? The user manual says power source ac/120 (240v for units purchased in the UK and Australia).....