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Hello Everyone,
My first post but have visited quite a few times. Hope someone can shed some light onto this discovery I made while repairing a Fender Princeton 112 Plus amp.
Received only the amp chassis to repair. It was on Craigslist. Owner had told me it just needed replacement of 2 of the potentiometers that had broken off - otherwise he said amp worked fine.
Received in mail and put on test bench... well everything looked good so I hooked it up to test and was dismayed to see it blew the fuse within 10 seconds or so. Obviously NOT working fine!
Anyway did the troubleshooting and found both outputs bad. The TIP 142 (Q1) was badly shorted as well as 2 diodes (CR9 and CR10) in that immediate part of the circuit. Also the other output TIP147 (Q2) tested bad also but associated diodes tested good. Just for good measure I replaced the emitter resistor (.47 5w) for Q2 even though it tested ok.
That’s the “back story” on this guy. When I got around to installing the outputs, I noticed they were different than the old bad outputs. They are same size but are different in the fact the old ones didn’t have the thick plastic casing over the metal tab that has the hole for the bolt/screw to connect it to the heat sink. That really didn’t matter to me until I noticed odd marks on the secondary heat sink that is mounted between the actual output heatsink and the chassis. I may have been in an alternate universe at the time but, when I removed the PCB from the chassis, I made sure I didn’t disturb the secondary heatsink.
Ok, I know this long but I’m getting to the observation that prompted my QUESTION. The secondary heatsink had bolt marks dug into them that were caused by the mounting bolts for the output transistors. Due to the length of the bolt, they were forcing themselves into the secondary heatsink. I thought that was a pretty poor design and could cause problems with the effectiveness of the entire heatsink system used on this amp.
With the new thicker output transistors, it wouldn’t be an issue because the bolts wouldn’t protrude that far. So I started applying thermal grease to the top and bottoms of the heat sinks. To my surprise, when I turned over the secondary heatsink, low and behold there were machined holes in the heatsink exactly where the protruding bolts would have gone! I’m pretty sure this amp had never been serviced so I had to assume it came from the factory this way. So I have to guess this was a mistake done at factory and wondered how many amps were sent out this way and what effect it really had on the longevity of the amp due to the impaired efficiency of the heatsink (IMHO).
Has anyone run across this issue? I imagine that this chassis design was pretty popular with all the different Princeton solid state amps Fender made. Could have been a large production run for all we know!
Anyways I haven’t completed the amp repair yet, still waiting on some new potentiometers and bushings. Keeping my fingers crossed that I’ve found all the defective parts and idiosyncrasies of this amp (there were some erroneous values on some of the resistors - values in parts list didn’t match what was installed).
BTW, if anyone has any device on finding or building a cabinet for this amp, I’d be very appreciative.
Thanks for “listening” and enduring the long drawn out commentary.
Voxman
My first post but have visited quite a few times. Hope someone can shed some light onto this discovery I made while repairing a Fender Princeton 112 Plus amp.
Received only the amp chassis to repair. It was on Craigslist. Owner had told me it just needed replacement of 2 of the potentiometers that had broken off - otherwise he said amp worked fine.
Received in mail and put on test bench... well everything looked good so I hooked it up to test and was dismayed to see it blew the fuse within 10 seconds or so. Obviously NOT working fine!
Anyway did the troubleshooting and found both outputs bad. The TIP 142 (Q1) was badly shorted as well as 2 diodes (CR9 and CR10) in that immediate part of the circuit. Also the other output TIP147 (Q2) tested bad also but associated diodes tested good. Just for good measure I replaced the emitter resistor (.47 5w) for Q2 even though it tested ok.
That’s the “back story” on this guy. When I got around to installing the outputs, I noticed they were different than the old bad outputs. They are same size but are different in the fact the old ones didn’t have the thick plastic casing over the metal tab that has the hole for the bolt/screw to connect it to the heat sink. That really didn’t matter to me until I noticed odd marks on the secondary heat sink that is mounted between the actual output heatsink and the chassis. I may have been in an alternate universe at the time but, when I removed the PCB from the chassis, I made sure I didn’t disturb the secondary heatsink.
Ok, I know this long but I’m getting to the observation that prompted my QUESTION. The secondary heatsink had bolt marks dug into them that were caused by the mounting bolts for the output transistors. Due to the length of the bolt, they were forcing themselves into the secondary heatsink. I thought that was a pretty poor design and could cause problems with the effectiveness of the entire heatsink system used on this amp.
With the new thicker output transistors, it wouldn’t be an issue because the bolts wouldn’t protrude that far. So I started applying thermal grease to the top and bottoms of the heat sinks. To my surprise, when I turned over the secondary heatsink, low and behold there were machined holes in the heatsink exactly where the protruding bolts would have gone! I’m pretty sure this amp had never been serviced so I had to assume it came from the factory this way. So I have to guess this was a mistake done at factory and wondered how many amps were sent out this way and what effect it really had on the longevity of the amp due to the impaired efficiency of the heatsink (IMHO).
Has anyone run across this issue? I imagine that this chassis design was pretty popular with all the different Princeton solid state amps Fender made. Could have been a large production run for all we know!
Anyways I haven’t completed the amp repair yet, still waiting on some new potentiometers and bushings. Keeping my fingers crossed that I’ve found all the defective parts and idiosyncrasies of this amp (there were some erroneous values on some of the resistors - values in parts list didn’t match what was installed).
BTW, if anyone has any device on finding or building a cabinet for this amp, I’d be very appreciative.
Thanks for “listening” and enduring the long drawn out commentary.
Voxman
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