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Thread: Marshall TSL:100 bias drift problem

  1. #36
    Old Timer Leo_Gnardo's Avatar
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    Quote Originally Posted by lowell View Post
    Thanks guys...

    Leo.. I should have mentioned that mod has already been done but did not fix the bias drift. It truly seems as though removing C46 fixed it. Odd.
    C46 is undoubtedly a liability. I've seen 'em charred - a big clue even I can appreciate. The TSL100 is an amp with multiple failure modes, and you've addressed 2 of them. Board? Cap? Yes, both. And... once you get 'em working, they still sound terrific.

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  2. #37
    Don't forget the joker g1's Avatar
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    I think the terminology may be the problem here. You were actually measuring idle current at the cathode (90mV) rather than bias voltage at the grids, correct?
    In that case, it was 'idle current drift' rather than 'bias drift'. Bias drift will always cause change in idle current, but idle current drift is not always caused by bias changes. Hope that makes sense.

    Change in plate or screen voltage can cause change in idle current, this cap could have caused that. Hum/noise could also be seen as change in idle current by the meter, and this cap may have caused that too.

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  3. #38
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    Agreed.. BUT, now I am back to where I was before... months ago. I took a break from this amp cause I couldn't figure it out.

    I put it back in the cab and it is now running away really bad... I cannot get one of the pots below 100mv and it's still climbing after an hour. I then remembered that it went away when I pulled the PI. Pulled the PI and it absolutely goes WAY back down. Also, over an hour of leaving it on, there is a hum that gets louder. I did notice some bulging B+ caps, and figured I'd replace them when I fixed this problem... but NOW I'm thinking they ARE the problem. My theory at this point is that those 2 caps are failing, and failing more as they heat up, producing more hum, which seems to come thru the PI and then on to the PA. So my theory is that the hum is what's causing the HOT bias readings, not thermal runaway or drift. Gonna replace those caps and see what happens. It absolutely makes sense being that the problem goes away when the PI is pulled. I can't come up with any other hypothesis. The 2 caps are the main high V B+ caps.

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  4. #39
    Supporting Member The Dude's Avatar
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    I'm wondering if you have an oscillation problem. Did you replace C46 or just remove it? Do you have a scope? If so, scope the output and see if you spot any oscillation.

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  5. #40
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    This was happening before I removed C46... and after. I am just now recalling all the tests I ran on this amp months ago. Usually I have notes on these things... but have been out of the routine.

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  6. #41
    Lifetime Member Enzo's Avatar
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    Did you replace C46 or just remove it?

    Did you scope the output of the amp or at least around the power tubes to see if there was high level RF oscillation?

    You say pulling the PI drops the tube current. OK, monitor MAINS current draw, does that go up when the PI is installed? Does it look higher than average?

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    Unfortunately due to circumstances my scope is not available at the moment.

    I removed C46 altogether.

    Running a mains current test now... at power on it's sitting at 1.16A. Seems very high... in standby it's at 560ma. I'm way out of practice... but that seems high.

    If I recall... Filament secondary draw is about 3.6A.

    3.6A x 6.3V is about 23W. 23 / 120vac primary is only .19A - So in standby it should only be pulling .19A... save for some HT current charging the main filter caps.

    Thoughts?

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  8. #43
    Lifetime Member Enzo's Avatar
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    OK, make an RF detector in place of a scope. Your meter cannot measure high frequency AC, so take a diode and put in series with your meter probe, and add a smallish cap to ground from the probe tip as well. Not critical, maybe 0.01uf? And measure the output as it sits there. Normally an idle amp makes zero output. If it is sitting there making 50 watts of 50kHz, this diode thingie will rectify it and the cap will smooth it, and your meter will read it as a DC voltage.

    There are various ways, here is one:
    https://www.youtube.com/watch?v=QUmPBBnzkRE

    I fogure an amp like this at idle should usually draw ROUGHLY 40-50 watts from the wall. Half an amp sounds close enough. But your half an amp was in standby, in operate, your 1.16A is about 140 watts. So either it is biased way hot or it might be cranking HF oscillation.

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    As always Enzo thanks for the help. Wouldn't this just show ANY noise the amp produces? IE If there's hum anywhere in the amp that's being amplified to the output, wouldn't that show as DC voltage with the probe you describe? How do we know it's an oscillation?

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  10. #45
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    Lemme answer my own question. The smallish cap won't register lower frequencies. This will only yield rectified high freq.

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  11. #46
    Lifetime Member Enzo's Avatar
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    Well, that aside, we would already know we didn't have a bunch of hum in the speaker. SO we would find some several volts of DC and nothing audible to explain it. Implying RF. I suppose you would be right, if we had a bunch of hum, we would not be able to detect simultaneous RF with this. In other words the context answers your concern.

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    So at power on, had nothing. After about 40 minutes, the hum is loud, and there is about 60mvDC using the diode/cap. It's a bit odd, if I set my meter at 6v it reads 500mv... if I set it at 600mv it reads 60mvDC. Seems to be a weird meter symptom/issue. The hum is gone at power on and grows over the 40 minutes.

    Doesn't seem like enough DC voltage to be symptomatic of an oscillation does it? Even the hum itself I would think would read higher on the meter as it's a good amount of hum. (technical speak)

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  13. #48
    Lifetime Member Enzo's Avatar
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    No, I was expecting like several volts.


    You have a signal generator? Set it up to something like 50kHz or 100kHz. Something WAY above audio freq. Set the output level to something strong. What would be a couple volts at audio. Now measure that with your diode thingie. What sort of reading does it give? Just an experiment.

    By the way, that diode circuit is what we call a "detector" and is the basis for an AM radio.

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  14. #49
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    Don't have the signal generator either, but can probably make this happen in the coming days.

    I am replacing the bulging main B+ caps. 330uf 315v. They are 20mm diameter and 10mm lead spacing. Cannot find these anywhere. Does anyone know where to find em?

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  15. #50
    Lifetime Member Enzo's Avatar
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    315v? Oh please. Look for something standard like 330uf at 350v instead.

    COmpanies like Marshall order parts in huge amounts. They can order them custom for their application. Off the shelf items may not match up. But it is just a power supply filter.

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  16. #51
    Don't forget the joker g1's Avatar
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    I guess it's the spacing giving you the trouble?
    Are they snap in type or radial leads?

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  17. #52
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    Haha I know Enzo... I don't care about the 315v... it's more the SIZE. 330uf 350v with the 10mm lead spacing and 20mm diameter is what's difficult. Marshall did not allow for larger radial caps. I order some larger ones on Mouser... and will make em work using silicone glue. Just figured I'd see if anyone had a direct hit on them.

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  18. #53
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    got those caps in and managed to get the amp to behave for 4hrs. I replaced the 2 main filter caps. After 4 hrs the bias was at 85mv each side. Still not sure I fixed it, as it still rose over time to 85mv, but after 4 hrs it's not running away. And I just suggested to the owner that he turn it off or put in standby between playing it.

    Enzo, I did your test. It seems that anything 50khz and above doesn't register on my meter. However at 50khz into the diode/cap filter, I injected 1vAC and had about 9vDC. Does NOT make sense haha but that's what my meter told me. I'm guessing the meter is no good at higher vAC, but does just fine with vDC.

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