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Working on a Rickenbacker TR35B that is shorting out somewhere. Put an new volume pot in and it worked for awhile and then experienced low, distorted volume. Put the old one back in and it worked while the chassis was removed from the speaker cabinet. Installed it the cabinet, and then it blew a fuse. Replaced the fuse and powered it up using a lamp limiter which showed there was a short somewhere. The 2 power transistors (Q9 -TIP42B and Q11- TIP41B) where bad. Replaced them with new a TIP42C and aTIP41C.Powered it up with the light limiter and it did not even light up. Powered it up without the limiter and heard a loud hum and then a pop and Q9 blew up! Don't know where to go from hear. I don't think using a TIP42C instead of a TIP42B was the issue since these are very similar transistors. Luckily I purchased a batch of TIP42C and TIP41C so I have spares. I attached the schematic for reference. I usually work on tube amps and was trying to help out a friend on this SS amp.
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Firstly the TIP41B is an 80volt version of the TIP41C no other differences.
Output transistors blow eithe due to an excessive load or excessive current demand.
Are your TIP***s from a reputable dealer; OnSemi, Farnell, RS, Mouser etc or fromthe amuzun or fleabay.
If the latter, suspect the poor quality TIPs.
If propper transistors, the most likely cause is over current. Check the bias adjust pot has not been damaged and set to about 1.4v between the bases of Q8 & Q12.
These power amplifiers, (designed originally by RCA back when transistors were in their infancy as a quasi symetrical amplifier), are quite simple and reliable.Support for Fender, Laney, Marshall, Mesa, VOX and many more. https://jonsnell.co.uk
If you can't fix it, I probably can. -
Thanks for responding Jon. I usually work on tube amps and avoid SS amps for these type of issues. I did buy these on Amazon and the supplier seemed reliable and had good reviews. I tested both of these before installing. I did notice that the 100uF, 40V e-cap was leaking green ooze and did not get any reading on it. I have a replacement cap I can install and hope this solves the issue. Will re-post after I get a chance to change it out.Comment
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Also, were you careful with the insulators for the output transistors? They can get lost in the shuffle. If they are missing or damaged, the collectors (metal case) of output transistors get shorted to the chassis.Originally posted by Enzo
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Yes, that is the insulator. There should also be a plastic shoulder washer which prevents that screw or nut from contacting the metal tab (case) of the transistor.Originally posted by Enzo
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The plastic shoulder washer is in place. I will be careful in reinstalling new transistors to make sure the case is not grounded out. I also noticed the 100uF/40V e-cap was leaking green ooze from the positive side and will replace.Comment
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If Q9 went, Q7 may also be bad. Check Q7 and Q11 as well. And all associated resistors if you have not already checked them.
Continue with limiter lamp, do not connect any speaker or load until amp is stable with no DC volts at the output.Originally posted by Enzo
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Installed new power transistors, Q9 and Q13 and checked Q7, Q11, and Q8 and they all passed the diode test. Replaced the 100uF/40V cap that was leaking. All of the associated resistors checked out. Plugged the amp into the light limiter and it did not light up at all.
Also wondering what this component in the attached picture. Looks like a red coil of some sortComment
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