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K7AAY wrote:
Suggestions on priorities for troubleshooting, by tube replacement or otherwise, appreciated. I feel the detector is the place to start? Have a multimeter, is all. In Portland (OR) Metro area. Maybe, or something before the detector. You really, really want a scope. If you don't have a scope, you can make the assumption that what has gone bad is _probably_ a paper cap somewhere in the IF strip or detector, and just shotgun all the paper caps out. If it turns out not to be a paper cap, you haven't wasted your time because sooner or later one of them will fail. The second thing you can do is measure grid, plate, and cathode voltages on each of the tubes on the IF strip and detector stage, and see if any of them is way out of wack. If you see 200V on a grid, you have a DC blocking cap issue. If you see 10V on a plate, you have a plate resistor gone open. If you see 200V on a cathode, you have a cathode resistor gone open. It's a lot easier with a scope and a signal generator because you can run signal in and trace it step by step with the scope, so you can identify the offending stage before you start poking around with the meter. This saves a lot of time. It's almost certainly not a tube. Tubes fail sometimes, but resistors and capacitors fail more often. I would make sure all the filaments are lit first, of course. --scott -- "C'est un Nagra. C'est suisse, et tres, tres precis." |
#2
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I agree with the previous advice. Paper & electrolytic capacitors should be
replaced eventually, so why not start now? This article has basic advice about recapping. http://antiqueradio.org/recap.htm You can download the schematic & service data from the BAMA site: ftp://bama.sbc.edu/downloads/hallicra/ I don't know offhand if all of the SX-100 tubes are glass, in which case you can look for duds by simply looking to see whether tube lights up inside. I believe this set uses a 0A2 regulator tube, which will glow purple rather than the usual orange-ish. Don't freak out -- that color is normal. Knowing that a tube lights up doesn't tell the whole story, since the tube might still be too weak to perform well. On the other hand, if you find a dud or two, you've learned something useful. You can also check for duds by measuring the resistance of each tube's filament with an ohmmeter. You can look up each tube's basing diagram at Nostalgia Air: http://www.nostalgiaair.org/ Click on Tubes, then enter the type (for example, 6SN7) of each tube you want to test. The tube filament is shown in the diagram as little mountain-peak sort of symbol. In a 6SN7 tube, for example, the filament pins are 7 and 8. If the filament is good, you should measure very low resistance between those pins using an ohmmeter. If you measure infinite resistance, the filament has burned out and the tube is no good. When you say that the radio "hums," if that means a loud, low-pitched hum which doesn't change as you turn the volume up or down, then you have bad electrolytic capacitors in the power supply. Replace those before doing anything else. You should do that anyway, to avoid burning up your power transformer if one of them fails. If by "hum" you mean just the normal faint background hum from a tube radio, then don't worry about it. There is a very simple test to tell you whether your audio amplifier is working. Turn up the volume, then briefly touch a turned-on soldering iron to the input terminal of the volume control potentiometer. (That is, the side that is not connected to ground.) If your audio section is working, you'll hear a loud growl from the speaker. Voltage checks are tedious, but can be very informative. When making any measurements on a powered-up chassis, follow the old-timer's rule of keeping one hand in your pocket when doing the test. This prevents you from getting a shock by touching two hands to places they should not touch (Bzzzt!). Have fun. That should be a great receiver when you get it working properly. Regards, Phil Nelson Phil's Old Radios http://antiqueradio.org/index.html |
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