50s Japanese am/fm tube dual speaker radio

[maxhifi]

Picked this up tonight for $20. Was filthy, took apart and cleaned. Am works well, fm was weak. Replaced 5uf capacitor across output of fm detector and fm came alive, by although stations are all strong it seems they are all slightly distorted, like it's always slightly out of tune. I like the looks of it, and AM sounds great, it will be my new morning listening radio once it works properly.

[maxhifi]

More pics - iPhone let's me post one per message for some reason

[maxhifi]

More

[maxhifi]

Inside

[maxhifi]

Better photo - daylight would help

[bob91343]

The front end seems to be mounted on its own chassis. And the schematic diagram doesn't show the audio output tube type, probably a 35C5.

[maxhifi]

Yes, it's a little tiny sub chassis. The voltage amp is a 12AV6 and the output is exactly a 35C5. The schematic is glued to the bottom of the radio. The dial light comes on strong when you first turn on the radio, and then extinguishes. I am quite sure it is a neon light, but it's the physical size of a normal 47 pilot light - something may be causing low B+, which may be responsible for the distortion on FM. The 17EW8 is a weird tube, I don't have any spares on hand. The little speakers actually sound pretty good on AM, I am looking forward to figuring out why FM isn't working properly, hopefully it is something simple like bad filter capacitors causing low B+.

[bob91343]

The lamp you have is probably NE-51 or more likely NE-51H (high brightness, high current version), will only glow on one electrode because it's got DC on it. It takes 90 Volts or so to get it to strike but will run at 67 Volts after that. The dropping resistor controls the current and brightness. If B+ drops below about 70V the lamp will go out. You should have about 110V on the filter capacitors; if not, one may be lacking in capacitance. They are supposed to be 60 microfarads and I am guessing they aren't close to that. When you get good capacitors there, the lamp will stay lit and the FM should work better. In fact, everything should work better.

[Sandy G]

Still, a NEAT little set.. Maybe it was from the very early stereo FM era, was advertised as a "Twin-Speaker" set to try to fool unwary consumers into thinking it was stereo...

[maxhifi]

Hmm that makes a lot of sense - I will order some new capacitors and go from there

[maxhifi]

Update: filter capacitors seem to be fine, no characteristic hum, and b+ on all three sections is over 110v. On AM the radio plays like new, on fm the volume is much quieter, and when you turn it up the sound is distorted, although it does pick up a lot of stations. The AGC voltage is only 0.6v with the strongest fm station. There are no paper or oil capacitors in the radio circuit only audio, and it works fine on am so I assume they are good.

All tubes original and if transformers still have undisturbed drops of white paint on alignment screws... Got to be something wrong in the fm section which isn't obvious to my amateur radio repair skills

All tubes test good on eico 667 including for h/k leakage

[bob91343]

The problem is either in the V4 circuit or the FM detector. Check those germanium diodes, the OA-70s. Replace with something like 1N270 or other germanium if they aren't good. Also the detector transformer might have a problem, like misalignment or even a bad internal capacitor.

[maxhifi]

Originally Posted by bob91343

The problem is either in the V4 circuit or the FM detector. Check those germanium diodes, the OA-70s. Replace with something like 1N270 or other germanium if they aren't good. Also the detector transformer might have a problem, like misalignment or even a bad internal capacitor.

Your advice is much appreciated, thank you for following the thread - will do exactly that tonight. Is it common for Ge diodes to fail? I will check them with a digital multi meter, diode test function, is that an adequate test for small signal diodes? I could also use the EICO 667 to check them, I suppose.

I acquired a B&K 415 sweep marker generator some time ago, but have never used it. Am thinking towards setting it up and checking the IF bandpass and ratio detector S curve (youtube has some awesome videos by current and former VK members), but having never done that type of work before and not knowing the condition of the B&K I want to approach it with caution - definitely don't want to make things worse!

Just so you know who you are talking to, I would say my skill level is moderate, I am an electrical engineer, but focus on power systems and not electronics professionally. From a hobby perspective I have done a lot of work with building and repairing tube and solid state audio and guitar amplifiers, and have been poking around with old televisions for a while. I've also aligned and restored antique AM radios, but IF and RF work at higher frequencies (such as an FM radio) is kind of new ground for me, so I really do appreciate the guidance from experienced people. I do plan to replace all the electrolytic capacitors in this radio, but would like to make it work first, and then do preventive maintenance, so I don't end up introducing a new problem which makes the existing one even harder to find.

As for the neon indicator, it must just be bad, it lights up and then goes off, but the voltage does not fall below 100 at the socket base in any case. The lamp is the size of a 47, but with a screw base rather than a bayonet.

[Robb]

nice radio

[maxhifi]

Originally Posted by Robb

nice radio

Thank you! I think your photography skills would do it a little more justice, especially the reverse painted plastic front panel. My iPhone photos are quite crappy - although I am impressed how well the schematic diagram photographed.