Clean is good, but it doesn’t do much for bad capacitors.
After spending the last few days in the Black Forest, I’m back at my workbench working on the Granada.
I took it apart today, and finished cleaning it up. I’m not going for spotless, just clean enough to see the parts.
It comes apart easier than expected, if you leave the speakers in the housing. I didn’t want to unscrew them - they are clean and they work. The speaker wiring is kind of complicated - Blaupunkt has some kind of free form, point to point wired crossover network going on that I didn’t want to fool with. I clipped the two wires from the chassis going to the speaker. I’ll solder them back together when I get done.
The chassis is held to the housing by five screws. They go in from the bottom. The sheet metal chassis has tapped holes for the screws. Strictly speaking, there’s four screws holding the chassis, and the fifth is to make sure the main transformer stays put - that sucker is heavy.
I parked the housing on the edge of the workbench and undid the screws from below because I didn’t want the chassis to fall on the glass front panel when it came undone. That worked well.
I thought I was going to have to take the knobs off the front to get the chassis out of the housing. As it turns out, I didn’t have to but I did make some pictures while figuring it out.
There are four knobs that protrude through the glass panel, but there’s only 2 axis. There’s a volume control, a tuner for the AM bands, a tuner for the FM band, and a rotation knob for the built in ferrite loop antenna.
The volume control and the FM tuner are the small knobs in the center of the two axis. They come off as shown in the photos above. They have set screws with straight slot heads - and there’s two of them on each knob. A modern knob would use a D-axle and one set screw.
I made fun of the Granada for being cheaply made last time around, but this time I found a spot where the engineers spent some money to do it right.
The set screws would be in the plastic in a modern radio. In the Granada, the set screws are in a drilled and tapped metal ring around the hole in the plastic for the shaft. They hold better, and the threads won’t strip out as can easily happen with modern knobs.
The large knobs are held in place with E-clips. I don’t recommend removing them unless you absolutely have to. If you remove the clips, the tuning mechanism (all the pulleys and strings) will come undone and you’ll have a tough time getting it back together properly.
As long as I had the two small knobs off, I used a cloth and some ethanol to clean out the grunge of 60 years. I found a price sticker rolled up and stuck inside one of them. It wasn’t doing anything (it wasn’t used as a shim or something,) it seems to have just fallen in and gotten stuck. Whatever is, it wasn’t the price tag of the radio. The price marked was 0.85DM - call it $0.40 US and the radio cost like 500 DM new.
Ethanol. I don’t use IPA to clean my PCBs like most folks do. A one liter bottle of IPA would cost about 20 Euros when ordered from Conrad Electronics - and they’ll only sell it to companies, not for private use. I can buy 99.9% pure ethanol in the hardware store. It’s called “bio starter fluid,” used to ignite charcoal or wood. It costs 1.99 Euros a liter. It smells better, and works better, than IPA. Just don’t dilute it and try to drink it. That 0.1% non-pure is some kind of icky stuff that’ll make you puke.
There was a lot more dust inside once I got the chassis out. Notice how the glass scale is attached to the chassis - I didn’t need to remove the knobs.
I used the vacuum cleaner on the housing, then had a look at the radio chassis.
The underside of the radio chassis was surprisingly clean - I pretty much left it alone because a large portion of it consists of black magic radio frequency (RF) stuff.
“Magic” includes things like this:
|Do not touch - magic|
There’s a red painted wire there in the middle, with a silver colored wire wound around it. That is a combined capacitor and inductor. That’s some picofarads of capacitance and some few microhenries of inductance - hand made, and by the appearance tuned in circuit to operate properly. Tube circuits are high impedance, which makes tuned circuits have to use very small capacitances and inductances. There’s a capacitor in my old D43 oscilloscope made of two pieces of wire (each about 4 centimeters long) that are wound around each other in three loose twists. Twiddling it influences the “tail” on the blanking for the horizontal sweep.
If you take an old radio apart, resist the urge to neaten up the RF areas. Just how a wire runs in relation to other parts can change the function. Usually just in small ways, but don’t take chances.
I used a small paint brush and brushed all the dusty surfaces. I used the vacuum cleaner to suck the dust out of the air rather than trying to get the nozzle in between the parts. I also took out each tube to brush and clean around them - and clean the (millimeter thick) dust off of the tubes themselves.
It isn’t perfectly clean, but it is a great improvement. I got all the dust out of the tuning capacitors and off of the ferrite loop antenna.
If you look over to the left, you’ll see an oscilloscope probe stuck in there.
I wanted to see how bad the ripple is on the power supply.
|Power supply ripple|
That’s about 14 volts peak to peak on the 250VDC B+ line. As far as I can tell, the 20300 uses a class A amplifier for the audio. Not a push-pull, just a single tube. That makes it more sensitive to junk on the B+ than other amplifiers would be. I’ve ordered some UF4007 diodes to build a replacement for the B250C100 selenium bridge rectifier, and some new electrolytic capacitors to replace the old ones in there now. I’ve also ordered a new bulb for the dial illumination - I would have sworn the old one worked, but it was dead this afternoon when I got things hooked up again.
Somewhere in all the clean up and wiggling buttons and knobs to get at things, the AM receiver started working again. Could have been a dirty spot on a switch or a bit of corrosion on a pin of one of the tubes. I don’t know. I hooked the radio up to measure the ripple, and found the AM receiver making noise when I turned the radio on. The last time I had it on, the AM section was deader than a door nail - nothing but hiss from the power stage. I spent some time this evening listening to a British station on about 1MHz - football, and not my thing, but I was fascinated that the old radio was working again.
That’s about it, except for replacing the power cord. The 20300 doesn’t have a safety ground, but somebody somewhen replaced the power plug with a Schuko plug with safety ground pins. That’s a no-no. I bought a new cord with molded on Euro plug and replaced the old one. I’m using the old cable as an extension on the speaker wires to run the radio with the chassis outside the housing.
I’ll replace the high voltage supply when the parts get here, and see how (or if) the hum improves.
Anyway, that’s all for today.
Computers are noisy on AM radio. I used to use an AM radio to provide “sound effects” while playing games on the TRS-80 Model I computer I had when I was a kid. The 20300 is full of all kinds of racket from the two computers here by my workbench. I expect the computers are better shielded than my old Model I was, but they produce so much more crap that it still gets through - and the 20300 is really sensitive, anyway.
Blaupunkt put a notice on the back of the 20300: “Störstrahlungssicher nach Empfehlungen der Bundespost.” “Interference shielded following the recommendations of the Bundepost.”
The Bundespost is responsible for the mail and radio communications in Germany - kind a combination of the US Snail mail and FCC.
Their recommendations back in 1960 must have been pretty good. The speakers on my desktop computer hum and buzz when my cell phone gets too close. The phone doesn’t bother the 20300 in the slightest.