80's Boombox blown amp - UK Vintage Radio Repair and Restoration Discussion Forum

Miloburnz · 4th Aug 2025, 3:48 pm

Hi all,
I have a 1980's Boombox, Pantronic radio cassette that i was using for radio and had the aux connected to bluetooth. It has a plug for AC and DC and batteries so i tried using a 12V DC convertor to see how it was on DC but it blew the amp, upon further investigation i found that when DC was plugged in for some reason the positive and negative were in reverse on the connector at the board. Only damage i could see was a blown capacitor that i replaced but it was totally dead still afterwards. I took it to a local guy who repairs old hifi gear but he couldn't do anything with it.
I'm looking for advice as to what are the options,
1.Could i retrofit an amp using the existing controls, volume, bass ect...
2. Would the amp be totally unrepairable.
3. The reverse polarity that blew the amp would that have likely damaged the radio board also.
I was considering a bluetooth amp connected to the speakers as a last resort.
Cheers................

paulsherwin · 4th Aug 2025, 3:57 pm

You have probably fried some of the transistors/ICs. You're going to have to pay somebody to fix it if you don't have the expertise to faultfind yourself, which won't be cheap for a competent job. It's not going to be possible to fit a modern amp module while retaining the existing controls.

It's worth checking any protective fuses though, just in case you've been (very) lucky.

It's bizarre that manufacturers don't include a 5p reverse polarity protection diode to prevent these sorts of disasters, but they generally didn't in the 80s and still don't today.

Miloburnz · 6th Aug 2025, 9:36 pm

I did check for a fuse but the only thing i found that was obviously blown was the capacitor so I thought maybe that had saved the rest of the board but after sourcing a new one and soldering it in it was still dead.

I not opposed to fault finding myself it's just that I'm not really sure what i would be looking for or how to test the components. I have done the odd repair on circuit boards over the years, I work in marine engineering so I do a lot of AC/DC electrical work but this is the first time I have looked at an audio amp so all looks quite alien to me at first glance.

Edward Huggins · 7th Aug 2025, 8:23 am

Your Pantronic looks like one of the most advanced of all of that generation of Boom-Boxes, but I don't recognise the name - a Panasonic derivative? It all depends how important this is to you. The circuit boards of this will be very densely packed and with a possible Rat's Nest of wiring. I suggest you put up a Post in our "Help Wanted" section. There are Members of this Forum near to you - and you will not be ripped off.

paulsherwin · 7th Aug 2025, 9:45 am

It's a Panasonic lookalike, though I doubt if there's any connection. It does seem surprisingly sophisticated for something using knock-off branding.

G6ONEDave · 7th Aug 2025, 5:01 pm

For what it's worth my guess is that the audio output chip will have died. It will probably have a hole blown out on it. This was very common with in car equipment that had been reverse connected. The chip is probably available but you would need to know what it actually is to source a replacement.

Other possibilities are short circuit electrolytic capacitors and or diodes. That is a case of tracing the power supplies inside the set. Remember to keep well clear of any mains voltages at the mains transformer input.

Dave

Miloburnz · 8th Aug 2025, 9:46 am

I just took the CB out again to have a second look but as per photo the only thing i found on my original inspection was the big capacitor that is first inline when DC power is applied on the two pin plug was blown as in the canister popped open. Looking at everything else on the board it all looks ok, after a reverse polarity error would there normally be visible damage ??
Would this be considered a complicated circuit board ??

60 oldjohn · 9th Aug 2025, 8:45 am

PM sent.

paulsherwin · 9th Aug 2025, 9:59 am

It's a pretty typical PCB for boomboxes of that era. There's very likely to be damage that you can't see - as said upthread, sometimes there will be a visible hole in an IC, but that certainly doesn't always happen.

G6ONEDave · 9th Aug 2025, 10:06 am

The audio output chip appears to be under the aluminium 'U' shaped heatsink at the bottom of the image.

Dave

60 oldjohn · 9th Aug 2025, 10:09 am

Anyone have a circuit? without having to join another site.

John.

SiriusHardware · 9th Aug 2025, 11:42 am

Agree with Dave that the next most likely thing to have been damaged is the audio power amplifier IC which will be under that big aluminium heatsink, the reason being that they are almost always placed directly across the supply rails.

I note that the OP already took this to a repairer who said he couldn't do anything with it but that may just have meant that he didn't consider it economically viable to fix, with the possibility of multiple reverse-polarity-inflicted problems waiting to be found one after the other and little prospect of recovering the cost of the time spent investigating.

A local forum member with no interest other than helping to get this working may be a better bet, or at least may be able to offer a second opinion.

G6ONEDave · 9th Aug 2025, 3:20 pm

Sorry I don't have a circuit, not even heard of Pantronic as a brand name. That does not mean that it can't be fixed though. My next move would be to remove the audio output chip and find out what it is, taking a note of which end pin 1 is. The blow hole that I mentioned is probably going to be under the chip, hopefully the type number will be intact on the top surface of the chip. Once the number is known, my guess is an LA something, it's just a matter of tracking one down. Who knows, a forum member might have one.

Dave

paulsherwin · 9th Aug 2025, 6:00 pm

But, even if the output chip is obviously damaged, there are likely to be other faults. Buying and fitting a replacement chip may well not fix things, even if the OP has the skills to do it properly.

G6ONEDave · 10th Aug 2025, 9:43 am

Maybe I've been lucky in the past but every time I had a reversed supply in an audio stage the dead devices have been the electrolytic capacitor across the supply, likewise protection diode if fitted and the audio output chip. Just occasionally there has also been a blown PCB track.

That doesn't mean to say that another fault won't develop at a later date but I've found that to be rare. Just my own experience.

Dave

60 oldjohn · 10th Aug 2025, 10:05 am

I have offered to look the set, but so far no response. If necessary I have all the gear to remove chips and LSICs without damaging tracks, but fear it may be too late.

John.

Miloburnz · 10th Aug 2025, 9:01 pm

Thanks for the input from everyone, so going back to the audio chip which I guess would be the difficult part to track down, I took a photo of it and there are two labelled Power amps L & R, both are the same PN. LA4440 3G5 14 Pin.

paulsherwin · 11th Aug 2025, 9:04 am

That's an odd choice for a boombox as the LA4440 is actually dual channel, though they can be bridged for higher powered mono. There are some cheap Chinese ones on eBay at the moment, but there's a significant risk they are fakes: https://www.ebay.co.uk/itm/388419103261

https://www.datasheetcafe.com/LA4440-pdf-datasheet-11407/

Please observe the forum's rules regarding eBay discussions.

Miloburnz · 11th Aug 2025, 9:06 am

Just found this info on wikiboombox.com
"Nippon CX-473F
The Nippon CX-473F is a large 15 volt ghettoblaster that was made in 1986 or 1987. It's about 69cm wide. It has 3-way speakers. It's very loud, but the bass isn't as good as you would expect from such a big model. It is a re-styled version of the Silverstone CX-473L, Trident CX-473, and TiSonic PR-7000. There is also a twin-deck version called the Tecsonic MX-900E."
So it seems it is the same Boombox sold under various labels, mine is a Pantronic CX-473F so not actually sure who made them all but a few of the components have 'Japan' on them.

SiriusHardware · 11th Aug 2025, 9:16 am

A few more questions:

How 'Dead' is the unit? Completely lifeless, nothing lights up, no sound, no motion from the cassette mechanism when 'play' is pressed?

Or -

Does it do everything you expect except make sounds?

If the latter, does the unit have a tape-out or AUX-out output which you could try connecting to a stereo amplifier or to active PC speakers? If you get sounds out of the AUX-out or tape-out output, that reinforces the possibility that the audio power amplifer ICs have failed.

4th Aug 2025, 3:48 pm	#1
Miloburnz Triode Join Date: Feb 2025 Location: Hull, Humberside, East Yorkshire, UK. Posts: 11	80's Boombox blown amp Hi all, I have a 1980's Boombox, Pantronic radio cassette that i was using for radio and had the aux connected to bluetooth. It has a plug for AC and DC and batteries so i tried using a 12V DC convertor to see how it was on DC but it blew the amp, upon further investigation i found that when DC was plugged in for some reason the positive and negative were in reverse on the connector at the board. Only damage i could see was a blown capacitor that i replaced but it was totally dead still afterwards. I took it to a local guy who repairs old hifi gear but he couldn't do anything with it. I'm looking for advice as to what are the options, 1.Could i retrofit an amp using the existing controls, volume, bass ect... 2. Would the amp be totally unrepairable. 3. The reverse polarity that blew the amp would that have likely damaged the radio board also. I was considering a bluetooth amp connected to the speakers as a last resort. Cheers................ Attached Thumbnails

4th Aug 2025, 3:57 pm	#2
paulsherwin Moderator Join Date: Jun 2003 Location: Oxford, UK Posts: 30,437	Re: 80's Boombox blown amp You have probably fried some of the transistors/ICs. You're going to have to pay somebody to fix it if you don't have the expertise to faultfind yourself, which won't be cheap for a competent job. It's not going to be possible to fit a modern amp module while retaining the existing controls. It's worth checking any protective fuses though, just in case you've been (very) lucky. It's bizarre that manufacturers don't include a 5p reverse polarity protection diode to prevent these sorts of disasters, but they generally didn't in the 80s and still don't today.

6th Aug 2025, 9:36 pm	#3
Miloburnz Triode Join Date: Feb 2025 Location: Hull, Humberside, East Yorkshire, UK. Posts: 11	Re: 80's Boombox blown amp I did check for a fuse but the only thing i found that was obviously blown was the capacitor so I thought maybe that had saved the rest of the board but after sourcing a new one and soldering it in it was still dead. I not opposed to fault finding myself it's just that I'm not really sure what i would be looking for or how to test the components. I have done the odd repair on circuit boards over the years, I work in marine engineering so I do a lot of AC/DC electrical work but this is the first time I have looked at an audio amp so all looks quite alien to me at first glance.

7th Aug 2025, 8:23 am	#4
Edward Huggins Dekatron Join Date: Sep 2015 Location: Southwold, Suffolk, UK. Posts: 9,137	Re: 80's Boombox blown amp Your Pantronic looks like one of the most advanced of all of that generation of Boom-Boxes, but I don't recognise the name - a Panasonic derivative? It all depends how important this is to you. The circuit boards of this will be very densely packed and with a possible Rat's Nest of wiring. I suggest you put up a Post in our "Help Wanted" section. There are Members of this Forum near to you - and you will not be ripped off. __________________ Edward.

7th Aug 2025, 9:45 am	#5
paulsherwin Moderator Join Date: Jun 2003 Location: Oxford, UK Posts: 30,437	Re: 80's Boombox blown amp It's a Panasonic lookalike, though I doubt if there's any connection. It does seem surprisingly sophisticated for something using knock-off branding.

7th Aug 2025, 5:01 pm	#6
G6ONEDave Nonode Join Date: Dec 2019 Location: Owston Ferry, North Lincolnshire, UK. Posts: 2,088	Re: 80's Boombox blown amp For what it's worth my guess is that the audio output chip will have died. It will probably have a hole blown out on it. This was very common with in car equipment that had been reverse connected. The chip is probably available but you would need to know what it actually is to source a replacement. Other possibilities are short circuit electrolytic capacitors and or diodes. That is a case of tracing the power supplies inside the set. Remember to keep well clear of any mains voltages at the mains transformer input. Dave __________________ Quote "All is hyperthetical, until it isn't!" (President Laura Roslin, Battlestar Galactica)

8th Aug 2025, 9:46 am	#7
Miloburnz Triode Join Date: Feb 2025 Location: Hull, Humberside, East Yorkshire, UK. Posts: 11	Re: 80's Boombox blown amp I just took the CB out again to have a second look but as per photo the only thing i found on my original inspection was the big capacitor that is first inline when DC power is applied on the two pin plug was blown as in the canister popped open. Looking at everything else on the board it all looks ok, after a reverse polarity error would there normally be visible damage ?? Would this be considered a complicated circuit board ?? Attached Thumbnails

9th Aug 2025, 8:45 am	#8
60 oldjohn Dekatron Join Date: Dec 2004 Location: East Yorkshire, UK. Posts: 4,317	Re: 80's Boombox blown amp PM sent. __________________ My favourite text message "I'll be there in five minutes, if not read again"

9th Aug 2025, 9:59 am	#9
paulsherwin Moderator Join Date: Jun 2003 Location: Oxford, UK Posts: 30,437	Re: 80's Boombox blown amp It's a pretty typical PCB for boomboxes of that era. There's very likely to be damage that you can't see - as said upthread, sometimes there will be a visible hole in an IC, but that certainly doesn't always happen.

9th Aug 2025, 10:06 am	#10
G6ONEDave Nonode Join Date: Dec 2019 Location: Owston Ferry, North Lincolnshire, UK. Posts: 2,088	Re: 80's Boombox blown amp The audio output chip appears to be under the aluminium 'U' shaped heatsink at the bottom of the image. Dave __________________ Quote "All is hyperthetical, until it isn't!" (President Laura Roslin, Battlestar Galactica)

9th Aug 2025, 10:09 am	#11
60 oldjohn Dekatron Join Date: Dec 2004 Location: East Yorkshire, UK. Posts: 4,317	Re: 80's Boombox blown amp Anyone have a circuit? without having to join another site. John. __________________ My favourite text message "I'll be there in five minutes, if not read again"

9th Aug 2025, 11:42 am	#12
SiriusHardware Dekatron Join Date: Aug 2011 Location: Newcastle, Tyne and Wear, UK. Posts: 13,785	Re: 80's Boombox blown amp Agree with Dave that the next most likely thing to have been damaged is the audio power amplifier IC which will be under that big aluminium heatsink, the reason being that they are almost always placed directly across the supply rails. I note that the OP already took this to a repairer who said he couldn't do anything with it but that may just have meant that he didn't consider it economically viable to fix, with the possibility of multiple reverse-polarity-inflicted problems waiting to be found one after the other and little prospect of recovering the cost of the time spent investigating. A local forum member with no interest other than helping to get this working may be a better bet, or at least may be able to offer a second opinion.

9th Aug 2025, 3:20 pm	#13
G6ONEDave Nonode Join Date: Dec 2019 Location: Owston Ferry, North Lincolnshire, UK. Posts: 2,088	Re: 80's Boombox blown amp Sorry I don't have a circuit, not even heard of Pantronic as a brand name. That does not mean that it can't be fixed though. My next move would be to remove the audio output chip and find out what it is, taking a note of which end pin 1 is. The blow hole that I mentioned is probably going to be under the chip, hopefully the type number will be intact on the top surface of the chip. Once the number is known, my guess is an LA something, it's just a matter of tracking one down. Who knows, a forum member might have one. Dave __________________ Quote "All is hyperthetical, until it isn't!" (President Laura Roslin, Battlestar Galactica)

9th Aug 2025, 6:00 pm	#14
paulsherwin Moderator Join Date: Jun 2003 Location: Oxford, UK Posts: 30,437	Re: 80's Boombox blown amp But, even if the output chip is obviously damaged, there are likely to be other faults. Buying and fitting a replacement chip may well not fix things, even if the OP has the skills to do it properly.

10th Aug 2025, 9:43 am	#15
G6ONEDave Nonode Join Date: Dec 2019 Location: Owston Ferry, North Lincolnshire, UK. Posts: 2,088	Re: 80's Boombox blown amp Maybe I've been lucky in the past but every time I had a reversed supply in an audio stage the dead devices have been the electrolytic capacitor across the supply, likewise protection diode if fitted and the audio output chip. Just occasionally there has also been a blown PCB track. That doesn't mean to say that another fault won't develop at a later date but I've found that to be rare. Just my own experience. Dave __________________ Quote "All is hyperthetical, until it isn't!" (President Laura Roslin, Battlestar Galactica)

10th Aug 2025, 10:05 am	#16
60 oldjohn Dekatron Join Date: Dec 2004 Location: East Yorkshire, UK. Posts: 4,317	Re: 80's Boombox blown amp I have offered to look the set, but so far no response. If necessary I have all the gear to remove chips and LSICs without damaging tracks, but fear it may be too late. John. __________________ My favourite text message "I'll be there in five minutes, if not read again"

10th Aug 2025, 9:01 pm	#17
Miloburnz Triode Join Date: Feb 2025 Location: Hull, Humberside, East Yorkshire, UK. Posts: 11	Re: 80's Boombox blown amp Thanks for the input from everyone, so going back to the audio chip which I guess would be the difficult part to track down, I took a photo of it and there are two labelled Power amps L & R, both are the same PN. LA4440 3G5 14 Pin. Attached Thumbnails

11th Aug 2025, 9:04 am	#18
paulsherwin Moderator Join Date: Jun 2003 Location: Oxford, UK Posts: 30,437	Re: 80's Boombox blown amp That's an odd choice for a boombox as the LA4440 is actually dual channel, though they can be bridged for higher powered mono. There are some cheap Chinese ones on eBay at the moment, but there's a significant risk they are fakes: https://www.ebay.co.uk/itm/388419103261 https://www.datasheetcafe.com/LA4440-pdf-datasheet-11407/ Please observe the forum's rules regarding eBay discussions.

11th Aug 2025, 9:06 am	#19
Miloburnz Triode Join Date: Feb 2025 Location: Hull, Humberside, East Yorkshire, UK. Posts: 11	Re: 80's Boombox blown amp Just found this info on wikiboombox.com "Nippon CX-473F The Nippon CX-473F is a large 15 volt ghettoblaster that was made in 1986 or 1987. It's about 69cm wide. It has 3-way speakers. It's very loud, but the bass isn't as good as you would expect from such a big model. It is a re-styled version of the Silverstone CX-473L, Trident CX-473, and TiSonic PR-7000. There is also a twin-deck version called the Tecsonic MX-900E." So it seems it is the same Boombox sold under various labels, mine is a Pantronic CX-473F so not actually sure who made them all but a few of the components have 'Japan' on them.

11th Aug 2025, 9:16 am	#20
SiriusHardware Dekatron Join Date: Aug 2011 Location: Newcastle, Tyne and Wear, UK. Posts: 13,785	Re: 80's Boombox blown amp A few more questions: How 'Dead' is the unit? Completely lifeless, nothing lights up, no sound, no motion from the cassette mechanism when 'play' is pressed? Or - Does it do everything you expect except make sounds? If the latter, does the unit have a tape-out or AUX-out output which you could try connecting to a stereo amplifier or to active PC speakers? If you get sounds out of the AUX-out or tape-out output, that reinforces the possibility that the audio power amplifer ICs have failed.