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Motor Boating Caused by SS Preamp. Help!?
Motor Boating Caused by SS Preamp. Help!?
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Old 21st October 2019, 08:33 PM   #21
Robbie010 is offline Robbie010
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Some pics attached.

Anything jump out at anyone?
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Old 21st October 2019, 08:39 PM   #22
Robbie010 is offline Robbie010
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I’ve just had a quick poke around and found a loose solder joint on the positive banana connection. Thats not a good start!
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Old 21st October 2019, 09:13 PM   #23
DF96 is offline DF96  England
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As it appears to still have the old carbon composition resistors in place you should check the value of every one of them.
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Old 21st October 2019, 09:47 PM   #24
pcan is offline pcan  Italy
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Motor Boating Caused by SS Preamp. Help!?
Pictures aren't clear, but it seems to me that the ground terminal of the new filter capacitors has been relocated on the main ground bus wire, and this wire is ultimetely connected to the painted chassis with a screw. This remind me a issue were I got a intermittent motorboating on a freshly recapped amplifier. I traced the fault to a bad connection between the negative terminal of the new main filter capacitor and the chassis, due to the paint and oxide between the terminal lug I used as new anchoring point and the chassis. I now try to ground new capacitors on the same spot of the old ones (a bad ground choice may be a hum source) and I check the resistence between the chassis and the circuit ground. According to your picture, one of the old capacitors has been left connected to the circuit. I do the same on collector items that may be turned on shortly once in a while (if ever), to keep authenticity, but I usually do a full capacitor replacement on devices of this age that will be used daily.

EDIT: I just cheched the MA12 assembly and operation manual, and there are NO 33uF capacitors listed on the part list. The new capacitors I see in the picture seems to be 33uF capacitors (all of them). Component substitution could lead to issues.

Last edited by pcan; 21st October 2019 at 10:03 PM.
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Old 21st October 2019, 10:12 PM   #25
pcan is offline pcan  Italy
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Motor Boating Caused by SS Preamp. Help!?
Quote:
Originally Posted by DF96 View Post
As it appears to still have the old carbon composition resistors in place you should check the value of every one of them.
A resistor has been changed, so they have been checked, I presume. But I wonder what value the output tubes cathode capacitors may have now, since they are still the original ones.
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Old 21st October 2019, 11:13 PM   #26
directdriver is offline directdriver  United States
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Default full schematic

I would make sure to check if C1 and C2 decoupling caps are faulty.

And make sure to add a high value input grid resistor to ground in case the volume control (R1) wiper lose contact.

Click the image to open in full size.
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Old 22nd October 2019, 12:37 AM   #27
Alan4411 is offline Alan4411  Wales
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Quote:
Originally Posted by Robbie010 View Post
Anything jump out at anyone?
1: as pcan points out, the EL84 cathode bypass caps (C10 + C11) are the original Plessy ones. They have a poor reputation and should be replaced. They are likely open circuit...
2: pcan again spotted the electrolytic caps are low in value compared to the originals, they were 60+250uF (C1) and 50+50uF (C2). They are grounded near their original points though. I think the connection to the original cap is just a tag point now, the black ground is not connected and the can is insulated.
Then, C3, the 510pF cap, looks like it is a Dubilier 'domino' cap. These can fail randomly, it would be worth a careful check.
And strangely the wire to the red socket (3 ohms) is red. This is normally yellow. Has the output transformer been changed?
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Old 22nd October 2019, 09:21 AM   #28
Robbie010 is offline Robbie010
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Thanks all, plenty for me to go at there. I’ll pull out the multimeter and soldering iron!
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Old 22nd October 2019, 09:23 AM   #29
Robbie010 is offline Robbie010
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Quote:
Originally Posted by Alan4411 View Post
1: as pcan points out, the EL84 cathode bypass caps (C10 + C11) are the original Plessy ones. They have a poor reputation and should be replaced. They are likely open circuit...
2: pcan again spotted the electrolytic caps are low in value compared to the originals, they were 60+250uF (C1) and 50+50uF (C2). They are grounded near their original points though. I think the connection to the original cap is just a tag point now, the black ground is not connected and the can is insulated.
Then, C3, the 510pF cap, looks like it is a Dubilier 'domino' cap. These can fail randomly, it would be worth a careful check.
And strangely the wire to the red socket (3 ohms) is red. This is normally yellow. Has the output transformer been changed?
Yes, the output transformer on the faulty amp is not original.
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Old 22nd October 2019, 10:14 AM   #30
Robbie010 is offline Robbie010
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Something has just occured to me, I have assumed all along that the 15ohm tap was redundant, owing to the fact that the new banana plugs were fitted. When testing the amps I have been using an old set of Wharfedale W4 speakers that are 15ohms! I have just hooked the amps up to the speakers using the 15ohm tap and cannot hear the motorboating.......

Could it be something as simple as that, the 15ohm speakers over driving one of the amps?
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