The National RX-C60 in the photo arrived earlier today, from Facebook Marketplace. I know in advance that it wasn't 100% .. the seller said it 'worked but the volume control was bad'. Well, it was a bit worse than that .. the tape deck works physically but no sound output, radio zilch and line in I managed to get a bit of horribly distorted sound out of the right speaker when pressing play on the tape (?) .. the only LED working is the power LED. I took it apart (not too difficult, thankfully, the whole mechanism slides out without having to pull off any knobs) and inside it seems to be in pristine condition .. poked around a bit with a multimeter and put it back together and it sprang to life! So .. situation now .. radio works (on the LED bargraph, left shows VU and right seems to show signal strength), bargraph works, line in works, tape works but with low volume and a lot of hum, recording level works as far as the LED meters are concerned - I didn't actually test recording. What doesn't work at all is the left speaker or even the left channel on a plugged in headphones, that and the humming tapedeck. If you look inside, there are 3 boards, one at the back that has the mains transformer and speaker outputs on it - there is an IC under a heatsink here that I think is the main amplifier. This is connected by two ribbon cables to a board at the top which seems to be the radio receiver and tapedeck circuitry (I can see the Dolby IC there and lots of radio-related stuff, together with the AM ferrite antenna). The last circuit board, at the front I think is just for the controls. There are some small daughterboards for the LED graph etc. and quite a few of what seem to be 'late additions' to the design. I have searched the internet repeatedly and failed to come up with a service manual - has anyone maybe worked on one of these and can guide me in checking the amplifier section .. I read somewhere that the poweramp is an LA4505 IC but I am not sure about this. I was thinking of first seeing that a stereo signal is arriving at the IC .. I'm not seeing any exploded or leaky capacitors but there are LOADS of small electrolytics. Now that the right speaker is working sound quality there is fine which I think would indicate that most of the capacitors are still OK. Thanks in advance and thank you for letting me join this great forum!
The most likely problem is oxidised switches and contacts that need cleaning. Internally most radio cassettes have a large multiway switch for switching between Play and Record. Get yourself some Servisol switch cleaner (I'm not sure where now Maplin has gone) spray a bit in each switch and socket the operate it at least a dozen times.
I'll be trying contact cleaner this morning.. It might explain the tape hum.. I'm more worried about not having sound on the left speaker, even with line input when the led vu meter shows correctly. I am thinking that the power amp is an LA5404, it's a 20 pin IC similar to what's in an RX-C20 which I have the service manual for. The intention is to test with the oscilloscope to see if input is arriving at the IC or not.. Wish me luck I really like this boombox, it's like something big that was shrunk..most of the buttons are about 2x4mm,it's really much smaller than it looks but very dense.
So .. today I took the RX-C60 apart again .. I am pretty sure that the board at the top is the radio + cassette pre-amp, front is just the potentiometers, back is the PSU + poweramp .. I found out what makes it go dead and half-come-to-life .. it's the plug on the power amp board. With an oscilloscope I worked out that the amplifier is either an LA4505 or pin compatible. One channel I could see input and output .. while testing the other the radio went dead .. just the power LED, tape deck motors working but no sound. I am absolutely sure I didn't cause the problem myself So .. some further poking around - there is a 5 way flat cable going from the power amp board to the daughterboard with the headphone jack and one of those came off. I suspect this switches the left speaker. I am reasonably sure that the main problem is on the power board - somehow power is not getting to some parts of the circuitry .. when it goes into blink not even the Dolby Nr LED lights up, just Power/Battery LED. Sound from the line output is OK and both channels work (when the stereo is not totally dead like it is now) .. tape deck works but as before has a hum and pegs the meters due to the hum. Radio has the right channel louder and more distorted than the left. I didn't have my soldering iron handy so was a bit limited in what I could try out today .. but I took photos of the top and bottom of the power board and intend to overlay them and reverse-engineer the circuitry there. Wish me luck and if anyone can come up with a schematic, service manual etc. it would be really welcome! Incidentally I think someone has been in this stereo before me - some of the board headers just have the cables soldered onto them and not plugged in. It makes it more difficult to take the stereo apart, though by taking out 7 screws the whole internals can slide out as one piece with the back case half (while still being operational) I'll upload my findings .. there is precious little documentation on this model online.
Friends..i have this model too , some of its circuits use hidden resistors, also noticed a flaw in it. You may need to skip its original circuit, Restructuring its circuit from BA328,and more circuits
The output chip on these blows very easily & it sounds like that's what's happened to yours! I replaced one for a forum member a few years back - Not too tricky & I had a donor machine supplied with it! Everything worked 1'st time on replacing the chip!
i have 2, when they are working they are as good as the jvc pc5 but not as loud, well worth saving if you can
I have this cable on my rx-c60 that seems to be a ground but I can not seem to find where it connects to on the radio/cassette amplifier board. Anyone have a picture of where it connects to?
