The Fine Things are Always Hand Made
Could use some tech advice Scott 800B
Took my 800B out of mothballs but could not locate speaker so rigged up a jones plug with a pair of resistors for the field, the jumper to turn it on, and and some speaker wire to a bookshelf speaker.
After working the buttons with some deoxit spray I was able to turn it on, change bands and listen to some FM and a couple of shortwave stations. However, the Broadcast band was mostly absent except for one weak sounding local. So back to the deoxit on the SW BC switch with no improvement. I dug out the RF oscillator and st it to 10 mHz and got a strong signal, then set the radio to BC and could pick up the oscillator weakly at 1290 a local station. I then set the oscillator and radio for 630 and nothing. I reset both radio and oscillator back at 1290 and tuned them both towrds the bottom, the signal getting weaker until around 800 it disappeared. I had recapped the radio and amp except for the bathtub caps in the rf-if section before I retired it to the attic when my wife became ill. At that time the radio was quite sensitive on all bands.
The FM plays great. Any ideas on this problem? Thanks!
Replies to This Discussion
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Permalink Reply by David C. Poland on
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what are you using for an antenna? needs one.
Came with a loop antenna inside the cabinet.
Has provision for using a long wire - 50 feet to 75 ft including lead in should be sufficient.
Scott also offered an optional double di-pole antenna system.
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Permalink Reply by sloane freeman on
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I have the tubular loop contraption, but since I don't have the FM dipole or the multi wire AM one that are part of the combo, I have the switch set in the counterclockwise position and use a longwire antenna for AM and SW, and a twinlead dipole for FM that came with a stereo receiver I bought years ago.
As I said, I hooked a signal generator to the antenna input and a 10 mHz signal comes in loud and strong, but as I go down in frequency into the BC band the signal drops to the point it disappears below 800 kHz. I tune the radio down from the SW band into the BC band and I keep turning up the generator output as the signal gets fainter and fainter until the generator is at maximum output and no tone is heard. The lower the frequency I tune to, the less sensitive becomes the receiver until it won't pickup below 800 kHz, although there is plenty of background noise as if everything is working OK, just no more tone from the generator and no loud local on 630 kHz which I can pickup on a Rocket crystal set without hooking the ground lead to anything as the tower is only a few miles away. It has me puzzled. It worked fine before I put it in the attic three years ago. Thanks for responding.
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Do you have an oscillioscope? If so check the oscillator output in the converter to see if dropping.
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Hi,
Thanks for responding. This circuit uses a separate oscillator and mixer. Should I check the oscillator then? I have a digital meter with some sort of O-scope capability though I have not used it before. I will see if I can see something with it. I am going to recap the double and triple .1uf caps that I did not do before to see if that helps. There seems to be a disconnect from the antenna input to the mixer. If I couple the antenna through a .01 cap to the rf coil in the mixer stage I get several stations fairly loud. The tubes all check as new with no shorts or gas.
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Well since can you get stations by bypassing the rf stage I suggest you do checking there. It maybe something as simple as corrosion on the band switch. The oscillator can be checkin the voltage on the grid. If it drops to zero as you adjust the frequency you got a problem...
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I am going nuts! SW works reasonably well. I have a Buzit rf noise generator I put on the antenna input. With the band switch on SW the signal comes through loud the entire band. I change the switch to BC and starting from 550 I hear nothing as I tune up the band until I get to about 1200 then it comes in faintly and gets louder as I tune to the top but it is never gets very loud with the volume at max. I have two parts tuners if you can think of what has gone wonky. I rebuilt two of the tub caps that were way off and tested two others that were spot on. I used a heat gun to open them up. Very smelly and messy! LOL! Thank you very much for your advice
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It acts as if something was filtering out the signal below about 5mHz, the lower I go in frequency, the more it attenuates the signal.
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Permalink Reply by Kent King on
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Make sure that the ferrite tuning is still good and in place in coil L2. Also, make sure that trimmer C1 is grounded and in good shape. I chased a problem in the FM circuit on an 800B and finally found a corroded ground on a trimmer. While fixing it, the trimmer broke and I had to extract one from a parts chassis. L2 and C1 are the upper and lower trimmers for AM (respectively).
Kent
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Thanks a lot for your help Kent! I was losing it I tell ya! You put me right next to the problem, After checking out L2 and C1, I checked out the other trimmers and C15, an air variable had come apart and was shorted. I have never seen this happen to one of these before. It has a slotted shaft with a nut on it but it isn't a nut just a hex sleeve kind of crimped on the shaft. I cleaned it up and swedged the shaft a bit to hold the hex piece on and line the plates, then I soldered the shaft to the nut thing. I soldered the nut and shaft on the SW one next to it. BC is booming in now but the dial calibration is off especially on the lower end.
Could you give me any tips on aligning this tuner?
Kent King said:Make sure that the ferrite tuning is still good and in place in coil L2. Also, make sure that trimmer C1 is grounded and in good shape. I chased a problem in the FM circuit on an 800B and finally found a corroded ground on a trimmer. While fixing it, the trimmer broke and I had to extract one from a parts chassis. L2 and C1 are the upper and lower trimmers for AM (respectively).
Kent
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Did you pull all the technical manuals from the Scott Info Archive here? Set Folders >> 800B has all the manuals for the 800B, a well-documented set. See pages 42-43 in Technical Service Manual 2 for the AM alignment instructions. I'll give it more thought...Kent
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Just did and thanks! I'm wondering why the two 25 watt resistors I am using in place of the field get hot enough to burn your finger but when I have the field coil speaker hooked up the field coil only gets a little warm. Also I am using a Russian tube 6n3c instead of 6l6's. It isn't an N between the 6 and 3 but some Russian letter. They sound good and were dirt cheap a few years ago so I bought a dozen. How do I adjust the bias on these so they are balanced. I don't run it very loud but I was thinking of pushing a pair og Realistic Mach one speakers in series on the 16 ohm tap and figured I better have the outputs balanced.
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Permalink Reply by Norman S Braithwaite on
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Surface area of the heat dissipating element. The field coil has a much larger surface area over which heat may be dissipated hence less heat buildup within the field coil.
sloane freeman said:Just did and thanks! I'm wondering why the two 25 watt resistors I am using in place of the field get hot enough to burn your finger but when I have the field coil speaker hooked up the field coil only gets a little warm.
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