Motorboating is a low frequency oscillation caused by feeback between stages of a radio. Usually caused by bypass caps or lack of tube shields. Does your 6D6 and 6Q7 have shields. If 6Q7 is metal is pin #1 grounded?
Don't use too large of coupling cap. If the original was .01 mfd and replacement .1 mfd a radio will have better low frequency response. In some cases too much low frequency gain causes motorboating.
Norm
:I am currently restoring an Emerson radio from about 1937. I can't find the model number but the tube line up is like the BL200 with a 6A7, 6D6, 6Q7, 41 & 80.
:I have so far replaced the two filter caps and the
:coupling cap from the detector to the audio output.
:I hooked up an antenna and the radio plays fine starting at the mid part of the AM band and above. However, when trying to tune in the lower part of the
:band, around 680 or 850 KC ( two local AM stations ) all I get is a motor-boating noise. What causes this?
:I know I will be checking the plate voltages and most
:likely repace other caps and resistors in the radio but
:are there other possible causes? Thanks, Randall
I think one tube has a shield. I will double check tonight. More importantly, I may have more than doubled the size of the cap, from .01 (as the schematic calls for) to .044. I'll try a .01 tonight and see what happens. Actually, I have a bunch of .022s, which I use often and now have two in parallel to the output tube. I'll just use one and look for the change.
As always, thanks for your logical explination and help
: Motorboating is a low frequency oscillation caused by feeback between stages of a radio. Usually caused by bypass caps or lack of tube shields. Does your 6D6 and 6Q7 have shields. If 6Q7 is metal is pin #1 grounded?
:
: Don't use too large of coupling cap. If the original was .01 mfd and replacement .1 mfd a radio will have better low frequency response. In some cases too much low frequency gain causes motorboating.
:
:Norm
:
::I am currently restoring an Emerson radio from about 1937. I can't find the model number but the tube line up is like the BL200 with a 6A7, 6D6, 6Q7, 41 & 80.
::I have so far replaced the two filter caps and the
::coupling cap from the detector to the audio output.
::I hooked up an antenna and the radio plays fine starting at the mid part of the AM band and above. However, when trying to tune in the lower part of the
::band, around 680 or 850 KC ( two local AM stations ) all I get is a motor-boating noise. What causes this?
::I know I will be checking the plate voltages and most
::likely repace other caps and resistors in the radio but
::are there other possible causes? Thanks, Randall
1) Most common cause of motorboating is an open output filter capacitor. You might double-check your connections there. Also, there is the remote possibility that your new output filter cap was defective. Try bridging it with another.
2) An open in a grid circuit can also cause motorboating. Check all grid circuits with an ohmmeter. Especially check the grid-load resistors in the AF stages.
3) You mention you are getting no station reception below mid-point on the dial. Check your tuning condenser--a bent plate, foreign object, corrosion, etc. might be causing plates to touch through part of the dial movement, thus cutting off reception at the low end of the dial. If this turns out to be the case, it is possible that the motorboating is happening all across the dial, but is being masked by strong station reception above dial mid-point.
Poston
::Hi Randall
:Thanks Norm:
:
:I think one tube has a shield. I will double check tonight. More importantly, I may have more than doubled the size of the cap, from .01 (as the schematic calls for) to .044. I'll try a .01 tonight and see what happens. Actually, I have a bunch of .022s, which I use often and now have two in parallel to the output tube. I'll just use one and look for the change.
:
:As always, thanks for your logical explination and help
:
:
:
:: Motorboating is a low frequency oscillation caused by feeback between stages of a radio. Usually caused by bypass caps or lack of tube shields. Does your 6D6 and 6Q7 have shields. If 6Q7 is metal is pin #1 grounded?
::
:: Don't use too large of coupling cap. If the original was .01 mfd and replacement .1 mfd a radio will have better low frequency response. In some cases too much low frequency gain causes motorboating.
::
::Norm
::
:::I am currently restoring an Emerson radio from about 1937. I can't find the model number but the tube line up is like the BL200 with a 6A7, 6D6, 6Q7, 41 & 80.
:::I have so far replaced the two filter caps and the
:::coupling cap from the detector to the audio output.
:::I hooked up an antenna and the radio plays fine starting at the mid part of the AM band and above. However, when trying to tune in the lower part of the
:::band, around 680 or 850 KC ( two local AM stations ) all I get is a motor-boating noise. What causes this?
:::I know I will be checking the plate voltages and most
:::likely repace other caps and resistors in the radio but
:::are there other possible causes? Thanks, Randall
Randall: The feedback between stages is probably wherein your problem lies. I recommend the following:
1. Set up your radio to receive some kind of signal from any station, signal generator, or even severe motorboating.
2. Use either a scope or an AC voltmeter and check for the presence of ANY ac whatsoever at all the filter caps AND all the screen grids. They should be operating at AC ground and have no appreciable AC potential unless there is an open cap.