Look for a connection to a capacitor that disconnected. Motorboating is usually cause by lack of bypass capacity or tube shielding.
Norm
:A while back I picked up a Crosley Super8 radio. Recapped the radio and replaced a number of resistors. The radio played fine at this point. A few days ago I turned the radio on and all I got was a loud "Motor Boat" sound from the speaker. Checked the tubes, especially the rectifier (5Y3), they seem OK. Does anyone have an idea of what else I could check.
Thanks for your reply, I have checked all the cap connections and they seem OK. Just finished working on the radio. Changed the two 6U7 tubes at the IF transformers and the radio played great, for about 5 minutes and then slowly started to motorboat. I do not have a setting on my old jackson tube tester for the 6U7 tube so cannot test these tubes. There are a couple small ceramic caps on the second IF stage, one by-pass and one coupling that I suspect might be bad.
:Hi George
:
: Look for a connection to a capacitor that disconnected. Motorboating is usually cause by lack of bypass capacity or tube shielding.
:
:Norm
:
:
::A while back I picked up a Crosley Super8 radio. Recapped the radio and replaced a number of resistors. The radio played fine at this point. A few days ago I turned the radio on and all I got was a loud "Motor Boat" sound from the speaker. Checked the tubes, especially the rectifier (5Y3), they seem OK. Does anyone have an idea of what else I could check.
Are your 6U7's shielded with shields grounded to chassis? This is a bad type tube for motorboating. It's tall and without a shield can easily go into oscillation. Motorboating problems won't be found with a tube tester.
Norm
:Hi Norm,
:
:Thanks for your reply, I have checked all the cap connections and they seem OK. Just finished working on the radio. Changed the two 6U7 tubes at the IF transformers and the radio played great, for about 5 minutes and then slowly started to motorboat. I do not have a setting on my old jackson tube tester for the 6U7 tube so cannot test these tubes. There are a couple small ceramic caps on the second IF stage, one by-pass and one coupling that I suspect might be bad.
:
::Hi George
::
:: Look for a connection to a capacitor that disconnected. Motorboating is usually cause by lack of bypass capacity or tube shielding.
::
::Norm
::
::
:::A while back I picked up a Crosley Super8 radio. Recapped the radio and replaced a number of resistors. The radio played fine at this point. A few days ago I turned the radio on and all I got was a loud "Motor Boat" sound from the speaker. Checked the tubes, especially the rectifier (5Y3), they seem OK. Does anyone have an idea of what else I could check.
The 6U7's do have a shield around them. I checked to make sure they were grounded to the chassis. You are correct in that this tube seems to be prone to unwanted oscillations. I have ordered two new 6U7's and I will post the results when they arrive.
Thanks for your input
:George
:
: Are your 6U7's shielded with shields grounded to chassis? This is a bad type tube for motorboating. It's tall and without a shield can easily go into oscillation. Motorboating problems won't be found with a tube tester.
:
:Norm
:
::Hi Norm,
::
::Thanks for your reply, I have checked all the cap connections and they seem OK. Just finished working on the radio. Changed the two 6U7 tubes at the IF transformers and the radio played great, for about 5 minutes and then slowly started to motorboat. I do not have a setting on my old jackson tube tester for the 6U7 tube so cannot test these tubes. There are a couple small ceramic caps on the second IF stage, one by-pass and one coupling that I suspect might be bad.
::
:::Hi George
:::
::: Look for a connection to a capacitor that disconnected. Motorboating is usually cause by lack of bypass capacity or tube shielding.
:::
:::Norm
:::
:::
::::A while back I picked up a Crosley Super8 radio. Recapped the radio and replaced a number of resistors. The radio played fine at this point. A few days ago I turned the radio on and all I got was a loud "Motor Boat" sound from the speaker. Checked the tubes, especially the rectifier (5Y3), they seem OK. Does anyone have an idea of what else I could check.
Make sure that speaker wires are nowhere near the 1st audio tube.
Thomas
This is a tough one to isolate. I just replaced the two IF tubes (6U7) with two NOS and there does NOT seem to be any improvement. When I puchased this radio I put new filter caps and all bypass & coupling caps were changed. When I started having this problem I did change the filter caps again to make sure they were OK. Thanks for your suggestions, I will check out all of these areas
:IF you get motorboating with the volume down, the
trouble is elsewhere unless the volume control is in the antenna circuit. If you get motorboating only with the volume up, then the trouble is most likely in the RF circuit. Definitely check your main electrolytics, and any condensers that filter screen grids. The main electrolytics must be new. If not, make sure that they still respond well to your meter. If they are old stock, they may have dried up. You should definitely shield your tubes as Norm suggested. The 6U7 has an internal shield that works fairly well, but the tube is not perfectly shielded until a shield is placed over it and then is coupled to the chassis.
:
:Make sure that speaker wires are nowhere near the 1st audio tube.
:
:Thomas
Pull one of the RF tubes at a time. Does the motorboating disappear? If so, then it is related to the tube that you pulled (or its circuit...not necessarily the tube itself is at fault) and/or surrounding circuits.
If you can't get the motorboating to go away by removing RF tubes, then pull the 1st AF tube. If it goes away, then the trouble is in the AF section. The trouble is very likely to be due to wires being too close to eachother or lack of shielding, though it could be for other reasons. Also, perhaps you misconnected a new component.
Thomas
Found the problem, but let me answer your question first. The motorboating was most noticable when the volume was low but it was still there with the vol turned up. Spent some hours today going over each connection, and resoldering a number of them. Seperated wires that were close together, basically all the things you guys have suggested (and they were all good suggestions). Turned the radio back over, pluged in the speaker and hooked up the antenna, powered it up, and the same motorboat sound was still there. As the radio was playing I was pushing down the tube shields on the two 6U7's when I noticed a change in the motorboat sound. So I started moving my hand around that area and it seemed to have the most effect at the 6Q7G (1st AF). Changed this tube (new 6Q7GT) and the problem disappeared. Now this tube did check "good" on my tube tester, which I suppose shows that a tube can still be a problem even though it checks good.
Thanks Thomas for your help
:Does the motorboating occur with the volume up or
down or both??????????????
:
:Pull one of the RF tubes at a time. Does the motorboating disappear? If so, then it is related to the tube that you pulled (or its circuit...not necessarily the tube itself is at fault) and/or surrounding circuits.
:
:If you can't get the motorboating to go away by removing RF tubes, then pull the 1st AF tube. If it goes away, then the trouble is in the AF section. The trouble is very likely to be due to wires being too close to eachother or lack of shielding, though it could be for other reasons. Also, perhaps you misconnected a new component.
:
:Thomas
Save the 6Q7 and see if it doesn't work well in another radio. If it does, then it might have simply been sensitive in its location in this radio.
Thomas
As Thomas said save the tube. Did the 6Q7G have a shield? A shorter GT tube may not require shielding to operate? Higher gain in this tube might have caused motorboating but doubt the tube is bad.
Norm
:Could be that the tube was gassy and was oscillating internally somehow. Still, it is wise to shield around this tube well. Keep speaker wires and the speaker transformer far away from this tube. Keep its shield in place, especially if it's near the output tube. Placing a close spaced coil spring over the grid wire helps, too. Connect the spring to the chassis.
:
:Save the 6Q7 and see if it doesn't work well in another radio. If it does, then it might have simply been sensitive in its location in this radio.
:
:Thomas
I will try this tube (6Q7) in another Crosley Super 8 that I have. No, this tube does not have a shield, or even an attachment to hook the shield to the chassis, though it would interesting to put a shield on it just to see if that fixes the problem.
:Geroge
:
: As Thomas said save the tube. Did the 6Q7G have a shield? A shorter GT tube may not require shielding to operate? Higher gain in this tube might have caused motorboating but doubt the tube is bad.
:
:Norm
:
::Could be that the tube was gassy and was oscillating internally somehow. Still, it is wise to shield around this tube well. Keep speaker wires and the speaker transformer far away from this tube. Keep its shield in place, especially if it's near the output tube. Placing a close spaced coil spring over the grid wire helps, too. Connect the spring to the chassis.
::
::Save the 6Q7 and see if it doesn't work well in another radio. If it does, then it might have simply been sensitive in its location in this radio.
::
::Thomas