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Post by 238 on Aug 25, 2021 15:25:25 GMT -5
Started have trouble with a receiver i have been running. Meter started acting up, would not stay set. Have tested the tubes & cleaned all the pots. Been checked the voltages, seem to be ok. 2600 something you had said on a early post about checking the sq control. So i checked it an it stayed on 15k, no mater how you turn the knob. So i assume it is bad. Jim/238
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Post by 2600 on Aug 25, 2021 23:25:18 GMT -5
The test I recommended was across the two outside lugs. Should be 10k, plus or minus 10 or 20 percent. This reading will not typically change when turning the knob.
Only a measurement to the center lug will show a change as the knob is turned.
So just what kind of receiver trouble were you chasing down?
73
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Post by 238 on Aug 27, 2021 22:06:40 GMT -5
Sq control is ok. I was having trouble with the meter not staying set. An it seem to be having trouble with the receive. I found that the R41/220K resister was bad. It controls the voltage to pin 5 of the V8/ 6AL5 tube. Checked every thing in the "S" meter adj pot from R41/ 220K resister too Pin 7 of the V7/6BA6 tube. Everything checks ok. Ran it for 4 hours today seems to be working ok Not sure what the 6AL5 tube has to do with the meter. Thanks for the help. Jim/238
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mattbee
Mudduck
Tram & Browning Enthusiast :)
Posts: 38 
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Post by mattbee on Aug 30, 2021 6:17:58 GMT -5
AS YOU HAVE BEEN ASKING ON OTHER SITES, THE 6AL5 TUBE HAS NOTHING TO DO WITH THE METER  MATTBEE |
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Post by 2600 on Aug 30, 2021 23:50:45 GMT -5
AS YOU HAVE BEEN ASKING ON OTHER SITES, THE 6AL5 TUBE HAS NOTHING TO DO WITH THE METER MATTBEE Ah, not entirely true. One half of the 6AL5 is in the AGC circuit. Anything that affects the AGC will affect the meter reading. Unplugging the 6AL5 while powered up will definitely affect the S-meter. Cleaning the tube sockets is a good start. If gently rocking a tube in its socket produces any noise at all, this can cause intermittent trouble. You should be able to shift the tube slightly from side to side in the socket without seeing wacky changes in behavior. A non-residue solvent like CRC brand "QD" works well. Found at Auto Zone, Amazon and elsewhere. The tubes are suspect unless you have checked them in a decent-quality tube tester. Testers come in all levels of quality from useless to laboratory-grade and in between. Once the sockets are clean, a "thwack" of your fingernail against the side of the tube should also produce no noises. Holding the top of the tube firmly in place with one hand while tapping the side should not make any sound at all in the receiver speaker. A tube that goes "scratch" or 'click' when you do this is defective and prone to making the receiver's performance shift around randomly. This is bit like having a 1967 car that just won't idle smoothly. Never assume that you have 'only one' fault causing your symptom. 73 |
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mattbee
Mudduck
Tram & Browning Enthusiast :)
Posts: 38
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Post by mattbee on Aug 31, 2021 8:01:46 GMT -5
Chris, I disagree Unplug the 6AL5 tube, and pull the ANL out/off and the receiver works perfectly. On the Mark III according to the Sams schematic, page 30, the AGC is derived from diode tube V8a because C39 blocks the DC level FROM CR-3. Better than doing nothing with the wasted side of the diode tube. V8a and CR3 are mutually redundant. Doing the exact same thing. V9a is NOT a noise amp. It is a DC comparator. Mattbee |
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Post by 238 on Sept 5, 2021 19:36:08 GMT -5
Not being a real teck i cannot say 1 way or the other. All i know is that the R41/220K was bad. I changed it an 3 more on that tie strip, that put voltage back to pin #5 of V8/6AL5. Been running it off an on since 8/27. So for meter has been doing ok, have not had to reset it. As always thanks for the help. Jim/238
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