5150ii, recrimp OT primary pins?

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castlev
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Joined: Fri Jul 08, 2016 3:42 pm

5150ii, recrimp OT primary pins?

Post by castlev » Wed Dec 21, 2022 5:25 am

For a while, my 5150ii has been experiencing volume drops. I've tried everything, swapping tubes (power and pre) cleaning tube sockets, all TS jacks, reflowing cracked solder joints that I found (both on the power tube pcb, and preamp board). After, I still had the issue. I believed the problem was on the preamp side, as I recorded the output of both the preamp out, and speaker out, into my DAW, and both outputs drop in volume when the problem occurs. I also ran a different amps preamp out into the power amp of this amp, and it seemed to have no drops in volume, but maybe I didn't do that test long enough.

Last night with the chassis out of the shell, doing the chopstick test (again), I noticed a bit of a static, or intermittent mute, when I would push on the OT primary wires connected to the powertube PCB, the 4 pin molex with only 3 wires in it. Specifically the brown (ground?) wire would produce static or intermittent milisecond mutes when pressure was applied to it. I powered off the amp, drained the caps, and retentioned the female pins on the molex plug, noticing that the female pin for the brown wire looked visibly slightly streched, compared to the other two.

Since I originally suspected the preamp to be the issue, I threw in (yet another) spare set of preamp tubes I had for good measure, put the amp back together and so far so good. I'm going to put the previous preamp tubes back just to isolate if the issue, if the amp is still good, then the problem was the loose female OT pins on the connector that plugs into the power tube PCB.

Assuming it the loose pin (specifically on the brown wire), I'm assuming that with all of those heat and cool down cycles the amp has been through, it will eventually happen again.

I picked up a used USA made 6505+ for $500 to use reliably until this old 5150ii is proven to be reliable. I have to say I am jealous how much more resistance there is when turning the pots, or plugging in cables, everything feels new despite it even being an old amp. I never realized how worn my old 5150ii is. Almost seems like new pots and jacks will be in order in it's future. I'm temped to just sell it once I fix it and keep the 6505+.

TLDR, is here a specific name, gauge, nomenclature for the female pin that is used on the OT primary side? I would prefer to not solder this connection to keep the internal components modular.

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