Pops when switching bands is a good indication. Clean the band switch and check voltage on your 6SB7Y to be sure it's oscillating. Pin #5 should be a few volts negative. Connect a short antenna to pin #8 on this tube.
http://www.nostalgiaair.org/PagesByModel/713/M0024713.pdf
Norm
:Have recapped the chassis, replaced brittle wiring, fixed glaring errors and refurbed the cobra matic (a popular after-addition, not orig.). Not getting any sound though... I do not have a signal generator or o-scope handy. Does anyone have some good suggestions as to how to trace and troubleshoot my lack of audio? I appear to get final stage amplification (pops from switching bands get louder, VERY minor fuzz not to be confused w/static gets louder), but it's got nothing to amplify apparently. I had to do my best guess with the 7F7 arrangement refurb, it's not in the original Zenith schematics. Thanks for any suggestions. -Alex
Information on the 7F7 phono tube can be found in Riders 19, Zenith Record Changer page 14. On Nostalgia Air Resources for Zenith, look for model S-14006. Schematic and wiring diagrams are there, along with mechanical adjustments, etc.
Meade G.
:Hi Alex
:
: Pops when switching bands is a good indication. Clean the band switch and check voltage on your 6SB7Y to be sure it's oscillating. Pin #5 should be a few volts negative. Connect a short antenna to pin #8 on this tube.
:
:http://www.nostalgiaair.org/PagesByModel/713/M0024713.pdf
:
:Norm
:
::Have recapped the chassis, replaced brittle wiring, fixed glaring errors and refurbed the cobra matic (a popular after-addition, not orig.). Not getting any sound though... I do not have a signal generator or o-scope handy. Does anyone have some good suggestions as to how to trace and troubleshoot my lack of audio? I appear to get final stage amplification (pops from switching bands get louder, VERY minor fuzz not to be confused w/static gets louder), but it's got nothing to amplify apparently. I had to do my best guess with the 7F7 arrangement refurb, it's not in the original Zenith schematics. Thanks for any suggestions. -Alex
:Hi Alex
:
: Pops when switching bands is a good indication. Clean the band switch and check voltage on your 6SB7Y to be sure it's oscillating. Pin #5 should be a few volts negative. Connect a short antenna to pin #8 on this tube.
:
:http://www.nostalgiaair.org/PagesByModel/713/M0024713.pdf
:
:Norm
:
::Have recapped the chassis, replaced brittle wiring, fixed glaring errors and refurbed the cobra matic (a popular after-addition, not orig.). Not getting any sound though... I do not have a signal generator or o-scope handy. Does anyone have some good suggestions as to how to trace and troubleshoot my lack of audio? I appear to get final stage amplification (pops from switching bands get louder, VERY minor fuzz not to be confused w/static gets louder), but it's got nothing to amplify apparently. I had to do my best guess with the 7F7 arrangement refurb, it's not in the original Zenith schematics. Thanks for any suggestions. -Alex
:Thanks for the suggestion, voltage at pin #5 is a steady -0.5V. It changed to -0.6V when I switched to a known good backup tube.
: More guidance? Thanks so far, Alex
:
::Hi Alex
::
:: Pops when switching bands is a good indication. Clean the band switch and check voltage on your 6SB7Y to be sure it's oscillating. Pin #5 should be a few volts negative. Connect a short antenna to pin #8 on this tube.
::
::http://www.nostalgiaair.org/PagesByModel/713/M0024713.pdf
::
::Norm
::
:::Have recapped the chassis, replaced brittle wiring, fixed glaring errors and refurbed the cobra matic (a popular after-addition, not orig.). Not getting any sound though... I do not have a signal generator or o-scope handy. Does anyone have some good suggestions as to how to trace and troubleshoot my lack of audio? I appear to get final stage amplification (pops from switching bands get louder, VERY minor fuzz not to be confused w/static gets louder), but it's got nothing to amplify apparently. I had to do my best guess with the 7F7 arrangement refurb, it's not in the original Zenith schematics. Thanks for any suggestions. -Alex
Not negative enough. The oscillator most likely isn't operating. Grid #1 voltage should be -5 volts or so. The oscillator is made up of pins #4, 5 and 6. Pin #4 must be positive. Check coils for continuity and resistors in this area for value. Coils open and resistors can be high in value.
You should also be able to hear an oscillator in a second radio. A second radio tuned the IF frequency above you Zenith should pick up the signal. With an IF of 455 and your Zenith tuned to 1000 a second radio should pick up a signal around 1455.
I will be away for a week but others will be reading posts.
Norm
:Bump...
:
::Thanks for the suggestion, voltage at pin #5 is a steady -0.5V. It changed to -0.6V when I switched to a known good backup tube.
:: More guidance? Thanks so far, Alex
::
:::Hi Alex
:::
::: Pops when switching bands is a good indication. Clean the band switch and check voltage on your 6SB7Y to be sure it's oscillating. Pin #5 should be a few volts negative. Connect a short antenna to pin #8 on this tube.
:::
:::http://www.nostalgiaair.org/PagesByModel/713/M0024713.pdf
:::
:::Norm
:::
::::Have recapped the chassis, replaced brittle wiring, fixed glaring errors and refurbed the cobra matic (a popular after-addition, not orig.). Not getting any sound though... I do not have a signal generator or o-scope handy. Does anyone have some good suggestions as to how to trace and troubleshoot my lack of audio? I appear to get final stage amplification (pops from switching bands get louder, VERY minor fuzz not to be confused w/static gets louder), but it's got nothing to amplify apparently. I had to do my best guess with the 7F7 arrangement refurb, it's not in the original Zenith schematics. Thanks for any suggestions. -Alex
If the majority of the RF section is in good operation, check the AVC resistor for being open, and check the AVC condenser for being leaky or shorted. Usually a leaky condenser will just cause the radio to blast. An open resistor can cause the set to cease all operation. Either will cause the voltage to not be negative enough. If the RF section has some other fault, AVC may not be going much negative due to lack of a signal. Now, if the AVC is not connected to the oscillator coil, the grid will not be biased from the AVC, and you can disregard all of the above.
Also, just a "heads up" for everyone: the tube everyone calls the "pre-amp" tube in the Cobra-Matic circuit is not a pre-amp at all. The tube after this tube is a pre-amp, but the very first tube in the Cobra circuit is an actual honest-to-goodness radio frequency oscillator, just like the advertisement says. There is a coil in the Cobra cartridge, and also a stainless steel diaphram that is attached to the needle. The true magic isn't really in the radio frequencies, as they'll be modulated by record scratch just like anything else. The magic is in the fact that the needle isn't driving anything to generate electricity. The old magnetic cartridges of the time were somewhat hard on records, though by the time the Cobra came out, huge improvements had been made. The crystal cartridge, as we all know, was a real blunderbuss at the time. Since the Cobra doesn't require the needle to twist a crystal or move an armature that is clamped tightly in a magnetic field, it can move quite freely, and not ruin records. Furthermore, since the needle is mounted perpendicular to the diaphram's movement, the majority of surface noise, which the majority of is vertically modulated, does not horizontally modulate the diaphram, and so it seems as though all hiss and scratch have been magically erased! If the cartridge is not mounted correctly, or the rubber dampers inside the cartridge are in poor condition, the cartridge will do no better than any other. During the 1950s, Zenith also put a special acoustical cream in the cartridge to dampen mid-range harmonics. An extremely small film of Vaseline will work on cartridges that don't have this cream, but you must be careful or you'll mute all the high frequencies. Vaseline is not a direct replacement for the original cream.
If your Zenith keeps the radio oscillator in operation (AM) while you play records, if you tune in a station and then play records on the phono setting, sometimes you can hear the Cobra heterodyne with the radio oscillator. If you wiggle the needle, you'll hear the heterodyne change pitch. On very rare occasion, while I was playing records, I would even get morse code pick-up. Actually there was a time when I'd get this several times a day, though I havn't in over a year now.
Thomas
:Hi Alex
:
: Not negative enough. The oscillator most likely isn't operating. Grid #1 voltage should be -5 volts or so. The oscillator is made up of pins #4, 5 and 6. Pin #4 must be positive. Check coils for continuity and resistors in this area for value. Coils open and resistors can be high in value.
:
: You should also be able to hear an oscillator in a second radio. A second radio tuned the IF frequency above you Zenith should pick up the signal. With an IF of 455 and your Zenith tuned to 1000 a second radio should pick up a signal around 1455.
:
: I will be away for a week but others will be reading posts.
:
:Norm
:
::Bump...
::
:::Thanks for the suggestion, voltage at pin #5 is a steady -0.5V. It changed to -0.6V when I switched to a known good backup tube.
::: More guidance? Thanks so far, Alex
:::
::::Hi Alex
::::
:::: Pops when switching bands is a good indication. Clean the band switch and check voltage on your 6SB7Y to be sure it's oscillating. Pin #5 should be a few volts negative. Connect a short antenna to pin #8 on this tube.
::::
::::http://www.nostalgiaair.org/PagesByModel/713/M0024713.pdf
::::
::::Norm
::::
:::::Have recapped the chassis, replaced brittle wiring, fixed glaring errors and refurbed the cobra matic (a popular after-addition, not orig.). Not getting any sound though... I do not have a signal generator or o-scope handy. Does anyone have some good suggestions as to how to trace and troubleshoot my lack of audio? I appear to get final stage amplification (pops from switching bands get louder, VERY minor fuzz not to be confused w/static gets louder), but it's got nothing to amplify apparently. I had to do my best guess with the 7F7 arrangement refurb, it's not in the original Zenith schematics. Thanks for any suggestions. -Alex