I have a relatively nice Panasonic who was originally missing antenna and had a bad tape deck. I didn't much like the factory drivers so I researched some to match the small enclosure and that it was sealed. The result was actually very nice. It now produces crisp and well articulated sound regardless of genre, although it favors classical. I added a BT module which for now is simply double side taped to the back of the chassis. I repaired the tape deck with donor parts and new belts. Simple stuff since the Panasonic design team did an outstanding job of making these serviceable. Turntables and Needles had the belts and Parts Express had the 3-way cross overs and drivers I matched to the impedance and enclosure. Everything came together very well. The one downside is that the system now weighs 8 more pounds than stock. Oh well they weren't exactly light to start. The National was purchased to be a donor but I've decided to restore. The deck was shot and the output stage is scratchy due to (I'm guessing) bad capacitors. It was missing the handle, the metal binding straps and the 13 pin ribbon cable. All super hard to find. I fabricated the ribbon, super easy to do, and have begun acquiring various boom box handles to modify. The straps, which tie the receiver/amp to the cassette/power supply are going to be super easy to fabricate also. Various models of Panasonics helped by donating slider knobs, auto reverse heads and etc. Not much cross over from the C300 to other models and the tape decks in particular seem to be purpose made for this unit. Its not the foot print or the soft touch controls, its the ribbon cable from deck to PCB. Completely different configuration from anything else I've seen. Wood putty and some Tamiya model paint in nearly the exact shade of very dark grey will cover the repairs to the cabinets. All in all it's been very fun but sadly not cheap! I have bill of material if anyone is interested in the speaker mods. It does require larger cut outs for the tweeter and 2" driver but the woofer is fine. Its all reversible by replacing the front baffle with MDF sheet later if you want. I try to make all mods reversible in case I part with something and new owner won't like it. PS, yes the fabricated ribbon is a prototype. I've got new material in to remake it a lot cleaner. It does function though!
The National is taking shape. When I bought it from South America, for parts only, it was missing: handle, metal straps, some slider knobs and ribbon cable and the speaker wire connections on the receiver were broken and the two inch drivers were perforated. Using the ribbon of my Panasonic RX-C300, I was able to power up and evaluate. Tuner indicator was broken, the tape deck was not working and horrible static above 1/4 volume. Headphones proved it was the output amplifier responsible for static. I replaced all the belts and used a donor RX-RW43 for the tuner station indicator and equalizer knobs; a RX-5100 for a handle (had to cut it down); the gears from a RX-CD70 (which almost has the exact same tape deck minus the proper electrical connections) and replaced the two inch drivers. I've got Tamiya TS-48 paint on order, which looks damn close to odd near black on the radio. All the broken corners on the speakers have been built up with wood filler epoxy and I built my own 13 pin IDC cable assembly. Right now......everything works! At low volumes it sounds awesome but still has the quarter volume issue. Next is to replace the electrolytic capacitors on the out put. If that fails, all the caps. If that fails the two stereo IC chips. When it's "perfect" I will put up for sale and loose my ass hahaha.
More progress to the National. Availability of docs for these is scarce. I have the Panasonic version and the manual and I can say, having disassembled both and swapped parts for troubleshooting, the amplifier may plug in and work but it is different with respect to amp chips and some circuitry. Handle is fabricated, ribbon cable fabricated and done and total repaint done. Many parts replaced. Still annoying distortion at anything above 1/4 volume. Replaced all transistors, amp chips and 90% of the capacitors and still distortion. Ugh. Cosmetically it looks better than I hoped and the fabricated handle permits speaker removal or retention like the original, it just doesn't come off as easy but that is not required due to the way I engineered it to slide up when spring pins retracted. Very fun project overall. Luckily I'm gainfully employed and can afford to have this hobby hahaha.
Big project with a high "hassle factor"... good job! That handle rebuild looks great. I wonder why the distortion persists?
It has to be simple. The only thing remotely complex are the two Sanyo STK4311 chips. I'm rusty and have no troubleshooting gear but a Fluke meter and soldering iron. A tone generator and O-scope could potentially point to clipping. Initially, I believed it had to be the output stage because the headphones sounded good but, saw another post about the impedance difference between the speakers 4Ohms and headphones 300-ish. Amp not driven hard for headphones so they sound clear. I don't do much in the way of this type of analysis and my degree is ancient so, I'm stumped, and local electronics places won't touch it lol. I see STK4311s on eBay and have ordered one set but that didn't fix. In fact one channel dropped out so I reverted back to the original ones. I'll keep studying it, can't quit now! I'm getting pretty knowledgeable on these radios at least.
These problems are difficult without testing equipment to systematically test down the signal chain. Cases and examples of tough issues to solve like this are what make me stop and think... real hard... about buying any more boom boxes. I am a perfectionist and have to have everything working again. I'm not a collector and can't understand having dozens of broken non-working boom boxes just stacked up to look cool and not worrying about fixing them! Anyway, just the messing around with that CF-550 I have was a nuisance enough for me. Luckily I like the old boxes with no integrated circuits at all so that makes it a bit easier. So if the new chip didn't solve anything I would be looking at the volume pot as somehow suspect or thinking about temporarily hooking up some new 4ohm speakers or new 8ohm speakers to drive them. Trying to check if the amp output really is putting out distorted audio or if something else is imposing the distortion further down. And Googling for hours to find other similar cases on those other boom box forums over the years. There certainly are qualified techs with the test gear who could fix just about anything. But one would have to fork over the big bucks I suppose. If one could even find them. I am only musing aloud, not talking about your specific case here, but, theoretically, if someone had a very nostalgic stereo, one that just had to work again, throwing a bunch of cash at it to get it fixed is a possibility. That or brute force component replacement where every suspect item is systematically replaced.
flood the volume pot with contact spray working it 50 times and let it dry for a few hours if needed do it again that should solve the issue i am also a perfectionist i believe in hitler lol also you say it works fine on cans so it could be something as simple the speakers you are using are not the correct match if you turn down the bass does it distort then if not then i would simply use less bass these things are not designed to be used at high volumes without turning down the bass test the unit on other speakers it could be the speakers you have can't handle the system it doesn't really matter what the specs state for example the airflow could be all out of whack with the new speakers try some hifi speakers that is what i would do. failing all this change the volume pot it seems you have changed everything else sir
The great thing about this situation is that I own two of these and one is perfect. The way these systems are built is really nice; everything is modular - speakers, tape deck, tuner, amp, spectrum analyzer and volume/equalizer are all plugged into one another directly or via a ribbon cable and not soldered. So I was able to swap out modules to see what's what. Found out the amplifier is exclusively at fault. I was an electronics tech in the Navy so I figured what the heck, just fix it. Ha that was over confident of me! I bought it for parts only since it wasn't working, had been abused and was missing a ton of stuff. Just couldn't "kill" it like that though and so decided to get working again. Fixed everything but the stubborn amplifier module. Worse comes to worse I can get a 45W per channel amp and stick it in the chassis, using it to bypass the power amp part. Parts Express has some really small amps that put out a ton of power per cubic inch. Assuming the pre-amp signal is clean. All the suggestions I'm getting are reassuring because they are pretty much where my mind went at first. Not getting discouraged cause I have time to spare and the best part is the chats on here with other enthusiasts! I love this group.
Hi there, I too am working on restoring this glorious machine and have nearly exactly the same amplifier fault. The key piece of information here is that the headphone jack sounds perfect at all volumes so the source signal is OK. To me this indicates that at light loads the amplification is OK but at heavier loads the modules distort. I replaced the two STK4311 modules and like you the fault still persisted. So I started looking at the supply voltage for the 4311s. Looking at the datasheet for the 4311 it lists the minimum Vcc is 35 volts. This is interesting as the supply voltage to these chips on the amplifier board is only 21 volts. I don't know if this was by design as the service manual I have is for the panasonic version with a different amplifier chip and output stage. You mention you have a working RX-C300, could you please measure the input voltages to the 4311s? Vcc is on pin 7 of the 4311. Pin 1 starts from the left looking at the front (black) chip. Also pin 11 and 12 of the ribbon cable as this is the AC voltage feeding the power supply for the amplifier. Thank you!
I was wondering about rail voltage too. I see what appears to be a full wave rectifier on the amp board and was getting ready to see what it's output looks like. Unfortunately I have decided to drop the unit off about 60 miles away to a specialist so, I won't be able to do this check. If he's fast and any good I will pass on fix. If you need cassette parts I can help with belt numbers and what other units have similar mechanisms.
After about eight months the highly recommended electronic repair specialist gave up and said troubleshooting this unit was too complicated. Of course they broke the power button to the spectrum analyzer and got sticky residue on the chassis first then gave up. At least they only charged me 45 dollars for the privilege. Just got radio home and will go through it again, so I should be able to answer the VCC potential question posed so many months ago. Honestly I am starting to really loathe "professional" repair persons across the spectrum. I should be able to answer the last questions this weekend. Assuming anyone is still interested. Sorry for the delay!
My claim to fame is that I had two of these models. The Panasonic RX300 with the auto reverse and spectrum analyzer. I had one new in the box that I sold for. I think $1,700 or something like that and it had another one that was somewhat broke that I sold for way less and the only reason why I sold them is because I just have too many radios still present day. Too many radios. I made videos about this model. I'm sure you viewed. Good day.
I just checked the output of the bridge rectifier on both Panasonic and National and getting 24.5VDC with a quarter volt AC ripple.
I'm still invested in this thread. Mine has been sitting in pieces until I have some time to really look into it. I want to build the amplifier circuit for the 4311 as designed on the 4311's datasheet. I'll use the original 4311 that came with the stereo and run some test and see if I can duplicate the fault.
