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Thread: Battle-scarred, floridian GK 400RB-III, some questions

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    Battle-scarred, floridian GK 400RB-III, some questions

    Gambled on an "as is, taken from working environment (yeah right)" GK 400RB-III bass head. It was cheap and I'm patient.

    So far I haven't hooked up a speaker or instrument, just to be safe.

    Previous cowboys who've been inside it seem to have had something against zobel networks, or against putting small-value capacitors in parallel against signals anyway. C25, C26, and C27 all had one leg lifted. Can't imagine why.

    They also had something against cleaning their flux off of boards when they are done.

    It's all fillthy and lightly corroded, of course, as though it spent its life at a beach-front bar.

    Right now the major symptom i am chasing is that on power-on it stays in protected mode until it is lightly tapped. And by lightly tapped i mean you can get slightly aggressive using a bamboo toothpick like a drum stick against any part of the power board and it will come out of protection.

    I've reflowed a bunch of sketchy solder joints and spent a fair bit of time cleaning gunk off of the power board.

    I guess i will need to start checking individual components more closely. Are there any known suspects on this thing?

    I'm also curious if the fan is expected to spin all the time? it does a little twitch at power on and so far that's it, though i have verified that the fan is good.

    Is the fan supposed to blow out the top or down onto the heatsink? As i received it, it blows out the top.

    I have the "service manual" that is out on the internet for these but it doesn't go into much detail past setting the bias and checking wave forms.

    Thanks for any insight you may have.

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    Senior Member nevetslab's Avatar
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    Any chance the 85 deg OOR Thermal Switch J1 is stuck open, and has to be 'throttled' to close (normally closed). If open, it will be in protect, and the front end is turned off, since it's not turned on, so the constant current source on the front end isn't running.

    I assume you've re-connected C25, C26 & C27. I see no reason NOT to have the Zobel network. On the 1001RB, those output caps across the output jacks are 4.7nF instead of the 10nF you have on this.

    The Fan normally doesn't run until the heat sink gets above 45-50 deg C, then begins running, speed being temp-controlled. I forget air direction. If this is installed in the combo case, it seems there's no gap above the top cover, so if it were exhausting out the top, it would put back pressure on the fan.

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    Lifetime Member Enzo's Avatar
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    Perhaps the idea was not to eliminate a zobel network, but maybe someone was lifting parts in hopes of tracking a symptom.

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    Click image for larger version. 

Name:	400rb III pre (3).jpg 
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ID:	52416 and just in case Click image for larger version. 

Name:	gk 400rb III (2).jpg 
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    Last edited by shortcircuit; 02-08-2019 at 01:15 AM.

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    Quote Originally Posted by Enzo View Post
    Perhaps the idea was not to eliminate a zobel network, but maybe someone was lifting parts in hopes of tracking a symptom.
    Well, not if you see where they are in the circuit. It's as though they wanted to make sure that ultrasonic noise doesn't have a path to ground.

    Click image for larger version. 

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    Quote Originally Posted by nevetslab View Post
    Any chance the 85 deg OOR Thermal Switch J1 is stuck open, and has to be 'throttled' to close (normally closed). If open, it will be in protect, and the front end is turned off, since it's not turned on, so the constant current source on the front end isn't running.

    I assume you've re-connected C25, C26 & C27. I see no reason NOT to have the Zobel network. On the 1001RB, those output caps across the output jacks are 4.7nF instead of the 10nF you have on this.

    The Fan normally doesn't run until the heat sink gets above 45-50 deg C, then begins running, speed being temp-controlled. I forget air direction. If this is installed in the combo case, it seems there's no gap above the top cover, so if it were exhausting out the top, it would put back pressure on the fan.
    I'll check that thermal switch. Thanks.

    Of course i reconnected those caps. I could see making them smaller to increase the high end sparkle. Might do it.

    It's the stand-alone head, but I'm considering shoving it into a headless (modern) fender bassman 200 combo cabinet on some rack ears ($10 thrift store purchase and the 15" celestion and misc tweeter are both fine, and the gap turns out to be exactly 2 rack units).

    In that situation, i might cut a matching hole in the top of the cabinet and mount the fan to the top of the case as an exhaust. I'm pretty sure there are good reasons why CPU heatsink fans always blow down onto the heatsink, though.

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    Old Timer J M Fahey's Avatar
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    I could see making them smaller to increase the high end sparkle. Might do it.
    Donīt.
    They have ZERO effect on equalization and *all* the effect on stability.

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    Juan Manuel Fahey

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