DeWald M-1000 Tuner problem.

K

kgetchmaine

Active Member
Good afternoon all, I have had the above tuner for awhile and use it randomly in my den. Seems to work ok but I recently picked up an Eico 625 tube tester for cheap on marketplace.

Anyway, decided to see if it works so I opened up the Dewald to test each tube. The tester seems to work fine. I get a "Good" reading on each tube except 2. The tubes with multiple test settings pass one test but not the other. They are the 12AT7 and 6BE6 tubes. I am really not sure what this second test is for and what affect it has on the tuner since it has been working. Any thoughts for any tube experts would be helpful in my understanding. This is the one and only tube gear I have and not fully comfortable opening this thing up.
 
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6BE6 is a pentagrid converter, its one tube, but kinda-sorta used like two. G1 and G2 form a "triode" and it runs as an oscillator. G3 gets the signal in from the antenna or RF amp. The other two work as shields to keep the internal capacitence low. All that going on between cathode and plate gives you four outputs, the amplified RF signal, the oscillator frequency, the difference of the two, and the sum of the two. Run that through a transformer tuned to the difference of the two (455kc or 10.7mc usually) and you have IF output. 6BE6 is usually used in the AM section, though some tuners did try and make them work for FM. its not a great high frequency tube.

12AT7 is a twin triode like John said, that one is more common in an FM front end. Can be a cascode RF amp stage, or one side can be the oscillator and the other a mixer.

Generally if either aren't working properly the tuner won't work or it will work poorly. If the tuner is working well I vote for ignoring what the tube tester has to say about it. Not specifically familiar with the 625, I owned one for a short time but never did anything with it or bothered to learn how it really worked. I think its just an emissions tester though. I'd expect for a 12AT7 it has two sets of settings, one tests output on pin 3 and one on pin 8 since those are the two plates. Absolutely no idea what its doing with the 6BE6 unless its setting up G2 as the plate for the oscillator section and the other testing the whole thing from cathode to plate.
 
Good morning, thanks for the explanation although trying to take it all in a nd process all the details. Per the DeWald service manual, the 12AT7 is the FM oscillator - FM AFC tube. The 6BE6 is the AM Converter. Dont use AM so that seems like not a big issue for the tuner while using FM. Read some on the Eico 625 and generally its stated to be a "GO - NOGO" tester.

When I run the second test for the 12AT7 I dont see anything at all on the meter. Does not even wiggle. My expectation would be that the meter would at least show "Replace" even if very low.

Just some added info, I have not done anything to the Eico 625 except to clean it up a little bit, open it up and do some basic dusting out and looking for blantent failures. I then put it on my dim bulb to fire it up. The dim bulb did not even flicker or glow at all. Should I get in there and clean the pots and lever switches? I think I read to not use deoxit on the lever switches due to possible issue with overload lamp being tripped. I do have CRC electronic cleaner.
 
do you have any other dual section tubes to see if they test OK? any 12a*7 should serve for this test. Same pinout so the switches ought to be the same.

so oscillator and probably mixer. AFC is auto frequency control. There will be a connection from the detector back to the oscillator section. If the voltage is anything other than zero volts at the detector, it will shift the frequency on the oscillator up or down to make it be zero. The mixer is what will blend the signal from the RF amplifier with the signal from the amplifier to output the intermediate frequency.
 
12AT7 is oscillator and AFC. Tuner will work if AFC half is dead, though of course AFC will not. But yes, I'd check the 625 on another tube before I'd trust it.

PM if you need the schematic and/or Sams folder.
 
kgetchmaine said:
Good morning, thanks for the explanation although trying to take it all in a nd process all the details. Per the DeWald service manual, the 12AT7 is the FM oscillator - FM AFC tube. The 6BE6 is the AM Converter. Dont use AM so that seems like not a big issue for the tuner while using FM. Read some on the Eico 625 and generally its stated to be a "GO - NOGO" tester.

When I run the second test for the 12AT7 I dont see anything at all on the meter. Does not even wiggle. My expectation would be that the meter would at least show "Replace" even if very low.

Just some added info, I have not done anything to the Eico 625 except to clean it up a little bit, open it up and do some basic dusting out and looking for blantent failures. I then put it on my dim bulb to fire it up. The dim bulb did not even flicker or glow at all. Should I get in there and clean the pots and lever switches? I think I read to not use deoxit on the lever switches due to possible issue with overload lamp being tripped. I do have CRC electronic cleaner.
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My bet is on bad tube tester. Instead what you want is a tester that tells you if the tube is good or bad (if you see what I did there :) )

It could be a switch, it could be a bad connection on the tester's tube socket. If the tester got a lot of use the socket could be worn out.

And FWIW the usual admonishment: the filter capacitors in the power supply should be replaced unless that's already been done else the power transformer is at risk. You will not love smell of power transformers burning in the morning (or any other time).
 
Tom Bavis said:
12AT7 is oscillator and AFC. Tuner will work if AFC half is dead, though of course AFC will not. But yes, I'd check the 625 on another tube before I'd trust it.

PM if you need the schematic and/or Sams folder.
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The 625 seems to work fine on all the other tubes that are NOT dual section. The other dual section tube, 6BE6 , was in the "?" section on the second test. First test of that was "GOOD" at approx 66 on the meter scale.
 
6BE6 isn't really dual section though. I'd see if you can dig up some twin triode with the same pinout as a 12AT7 to see if that works.
 
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