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Thread: Hartke 3500-transistors

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    Hartke 3500-transistors

    I am looking at this amp to find noise problems (Crackling,hissing),I replaced the 12AX tube and it seems noise free.I have been using an IR temp sensor lately and probed around and found two transistors,The heatsink of one of them looks discolored due to heat.The discolored one is numbered(GS-GL7818-K30ROK) the other is(M-7918CT-QLL119). They are side by side and next to 2-50/50v caps.The GS showed a temp of 133 degrees farenheit,the other is 88.I am not good with SS but this seems wrong.

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    From the base numbers embedded in the part numbers (7818, 7918) I would guess you are looking at the positive (7818) and negative (7918) voltage regulators for the chip stuff in the amp.

    Find a handy dual opamp (4558, TL072, LF353, etc) and check to see if you get 18VDC at pins 4 (negative) and 8 (positive) referenced to ground.

    Sometimes the positive regulator gets heavier use than the negative one, as it may be used to drive LED's and stuff so would run a bit hotter. I think that 133F (53C) is still well within operational limits.

    Of course cooler is usually better for electronics. Manufacturers have a nasty habit of using the minimum heatsinking they can get away with. Make sure it is making good contact with the heatsink, and if it is insulated with a "sil-pad" rubbery insulator consider replacing that with mica & thermal compound.

    Good luck!

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    Lifetime Member Enzo's Avatar
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    I agree, those are three leg voltage regulator ICs and as part of their job, they get hot. If you think they are at fault, scope their output pins and look for glitches.

    Crackling and hisses sounds more like a bad op amp to me anyway.

    Isolate the problem, ANY control that affects the noise is after its source. so if the Master VOlume control tirns the hiss off, then it is coming from the preamp for example. Turn off any functions you can, such as graphic and compressor. Any circuits you can eliminate are ones you needn't troubleshoot through.

    The tubes in those are rarely the trouble, but there are a number of mylar caps on the board all around the tube, and they stick up in the air on their own leads. Vibration tends to crack them, so tug on each cap to see if any are loose.

    Scope the output pins of the op amps in the circuitry looking for noise.

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    Thanks for the replies,sorry for the delay in responding I have been away for the weekend.I tested the components as suggested and did find the 18 volts + and -.I checked the caps and they are secure.I replaced the tube and the noise went away,reinstalled the original and it was noisey.For peace of mind I am going to pull the board and check solder joints.I did notice noise when poking the small electrolytics on the exposed metal tops and measured a small voltage.Is this normal?

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    Good job narrowing the noise to the tube and then verifying it.

    Yes - I would characterize finding voltage and hearing resulting noise at the exposed metal tops of electrolytics as normal, depending on their function in the circuit, as the case is typically tied to the negative terminal.

    I also applaud your decision to pull the PCB and check for bad solders. Even when I can't see bad ones I usually end up retouching all the components which are large, heat-producing, or extend to the "outside world" as well (pots, switches, jacks etc.). Just seems like good sense and almost certainly will lengthen the time to the next failure.

    Best -O -luck!

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    I seem to get a lot of stuff that other local guys reject.I figure if someone will give me a shot and I can learn something why not try? I tend to be a bit obsessive,I hate to think I missed something because I got lazy.Its a side gig for me but I still take pride in my work.Thanks for the help again

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