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Thread: Hartke HS1200 - what keeps the bias V in the middle ?

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    Senior Member chazpope's Avatar
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    Hartke HS1200 - what keeps the bias V in the middle ?

    I guess the 'other thread' is now so deep and old that nobody bothers to look anymore So I am cross posting this as a fresh problem, well it really is


    Hi Guys


    I am back onto trying to fix this HS1200, and I have started pulling out transistors to find what's wrong. It generally behaves as if one side is not working - I guess what I 'hear' is half the signal distorted at low and higher volume. The problem is apparent in a static state, but I can't yet figure out what causes it.

    Without load, the base of the Q210 driver sits at .9V and the base of Q211 is -1.2V. If I plug headphones the sound is OK.

    But when I plug the speaker the base of Q210 goes to -5V and the -side goes to -7V and so the whole thing is offset by about -6 volts!

    I have taken the heat sink off - and when I play the amp in that state the negative side outputs do not get warm (only the driver does). Conversely the + side does get warm while its driver doesn't.

    I have pulled out and checked Q210, 205, 206, 207, 208. I also removed the caps of the limiter just to eliminate them for now. The Mid point seems to be at DC 0 and the pot seems to operate predictably, also the biasing pot works and modifies the voltage difference between the two bases, but not where that is with regards to 0.

    So now I wonder what in this circuit makes the biasing voltage stay in the middle rather than go off like I get ?

    Thanks

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    Senior Member chazpope's Avatar
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    Well nobody answered so I carried on pulling stuff out and, after checking every other transistor I found that all the B688s have their B-Es blown, two are showing nothing, the other one .9V

    This amp has been played weekly but only at very low volume say at 15W max and has never heated up during play. I wonder why this has happened and how to prevent it from happening again.

    The drivers seem to be OK. Now I have ordered new outputs but they will take 3 weeks to come, in the meantime I am tempted to try a TIP35/36C pair only they are rated to 100V and the rails are 65V, so I would expect the B688s to be rated at above 130V but they are 120V , now isn't that too close for comfort.

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    Noodle of Reality Steve Conner's Avatar
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    What makes the bias voltage stay in the middle is the global negative feedback. It forces the output voltage to zero, so the bias stacks up evenly either side of zero.

    The MJL3281, MJL1302, MJW21194 etc. are all pretty good output transistors. They come in a few different package styles with slightly different part numbers.

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    Senior Member chazpope's Avatar
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    Quote Originally Posted by Steve Conner View Post
    What makes the bias voltage stay in the middle is the global negative feedback. It forces the output voltage to zero, so the bias stacks up evenly either side of zero.
    Well in my case the output DC was 0 but the voltage at the driver bases was way off the middle ? I know I should have pulled the output transistors first but the fact that there was no DC on the output made me think they are OK, I had not seen this kind of burn before, usually I see a EC short. I wonder what caused it, the drivers seem OK

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    Noodle of Reality Steve Conner's Avatar
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    For me the most likely cause of a B-E short would be the transistor losing the main supply to its collector, so that the driver stage tries to supply all of the speaker current through the output device's B-E junction. This would also explain the blown drivers as they are not up to driving the speaker by themselves.

    Check the supply rails to the output stage, solder joints etc.

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    "Enzo, I see that you replied parasitic oscillations. Is that a hypothesis? Or is that your amazing metal band I should check out?"

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    Senior Member chazpope's Avatar
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    well it is rather a BE 'open' than a short - one is showing open circuit the other .9V. The drivers seem OK, I thought I could try it just on the drivers but then I thought not cause their emitters do not go to the speaker. What is the point in this floating arrangement is it to save a resistor? Could it be that somehow the outputs got too much BE Voltage before burning, esp that all of them are burned the same way. Funny that the amp was played for an hour in this distorted state before anyone noticed

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    Supporting Member rf7's Avatar
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    What are you using for the output devices? I can't find 2SD718's or 2SB688's anywhere. Do you have a good source for them? Working on a KM-200 though, not a HS-1200.

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    Senior Member chazpope's Avatar
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    well yeah - ebay !

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    Supporting Member Jazz P Bass's Avatar
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    Quote Originally Posted by chazpope View Post
    The drivers seem OK, I thought I could try it just on the drivers but then I thought not cause their emitters do not go to the speaker.
    It is possible, for testing the circuit, to run a 1 watt, 1K resistor from the output transistor base pad to the emitter pad. Each side.
    This is for testing the preoutput stage.
    I would not run the amp into a load.

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  10. #10
    Supporting Member Jazz P Bass's Avatar
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    Please let us know how you make out with the transistors.

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    Senior Member chazpope's Avatar
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    FIXED! So I got new B688/D718 transistors, tried on pair on and it worked. I had also bought new drivers but they haven't arrived so I left the old ones in. I then put in the remaining 2 output pairs and closed it shut. I set the bias pot at about 5% off its lowest point. Next I played it in the pub last night for about 3h and it worked ok.

    Again, in the hot pub and in the kickback position I found it had gotten really hot to the touch, despite the fact that it was never more than 9 o'clock turned up. With 3 sets of outputs it feels as if there is no heatsinking going on whatsoever. I will try it with a fan next time but this must be bad design - can't think of what will happen if I play it the nominal 120W, surely it will cook itself to death!

    The question remains about the voltage rating of the stock outputs - does that explain the way they had failed their BE junction. Also I wonder if I should have replaced the drivers - although they did check out ok and the amp works fine.

    Finally I should have checked the outputs first as I usually do to diag this fault, but I guess too much reading of forums and the odd way they had burned made me waste time looking for other faults

    Also this amp has always had a nasty DC pop about 2 secs after switching off, looks like as the voltage drops something suddenly gets out of balance and the speaker is pushed out wildly. Not a big deal but it makes me wonder if it will turn on next time ..

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    Last edited by chazpope; 09-02-2013 at 04:12 PM.

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    Lifetime Member Enzo's Avatar
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    You need to properly set the bias for the amp. Setting it by which way the control points is not right. If the design were bad, then ALL of them would be sitting there burning themselves up. AMps that get hot just idling are not properly adjusted.

    Drivers? I'd replace them. Sure the amp is working, and you checked the transistors. I bet you checked the parts with a hand meter. A very low current test at a very low voltage. That does not remotely tell you if the internal structure of the transistor is cracked. For a couple of dollars, is it really economy to take the chance?

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    Supporting Member Jazz P Bass's Avatar
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    The EBay transistor supplier was located where?
    All of the China, Thailand, Hong Cong parts are remakes.
    If you want to dive in again, I would measure the idle voltage across each 0.47 ballast resistor.
    That will give you an idea how much current each transistor is trying to pull.
    Another item you can check is the wattage that the amp is drawing from the mains, at idle.
    50 watts is a rough estimate.
    If it is drawing more than that, the bias is way hot.

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  14. #14
    Senior Member chazpope's Avatar
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    Transistors came from China - (somehow within 2 days).

    Now the amp is definitely not getting hot at idle - I checked that first. I did not measure it but it can't have been drawing anywhere near 50W -at idle the limiter lamp was barely showing a colour. Longevity was my only criteria when setting the bias - and not knowing what it is supposed to be emitter-current-wise - I set it straight to the lowest possible point. There was no audible distortion and this is good enough for pub bass, so I set it at just above that.

    I did try what happens when I turn the pot the other way and at almost full you can barely see a difference in the lamp (40W) limiter, I did not stay there for long though.

    Enzo this is exactly what I was not sure of - can you have this sort of half-damage on the drivers. I will change them as soon as they arrive (from china

    I guess I should test the thing in upright and non upright state and see what the T difference is - but I do not yet have a consistent test signal, I did buy a cheap scope but the chinese manuals were unbelievable so I returned it straight away and I will look for some other scope solution

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    Supporting Member Jazz P Bass's Avatar
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    How about measurements.
    The lamp limiter is 'limiting' the amp.
    That tells very little, as far as what the amp is actually doing.
    The measurements across the .047 resistors is a must do.
    If you have one 'hogging' current, that may be a problem.

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    Senior Member chazpope's Avatar
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    well I did measure the current at TP2 (forgot what it was) but not knowing what value to go for I proceeded to setting it 'by feel' - so that with no heatsink the outputs do not get hot, and the lamp does not shine - I guess it likely draws 5W at idle.

    I left the amp at the pub so it won't be until Sunday that I can measure and report it for science sake

    Like I said I idled it for some time and it did not get hot, but not sure if it does not go into some other state after some real play. I got this amp in a cooked state to start with - I replaced 1 shorted output pair and that fixed it but there was no apparent reason why it had died. Since that I have played it for about 200 hours at lowish volumes until it burnt again. The bias has always been set to the lowest setting.

    So it seems to me that the heatsink does not work when in kickback state - there is absolutely no air movement in that hole and even 20W of power makes it really hot, especially in a hot pub. The only proper test for this is by generator which I don't have right now.

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    Senior Member chazpope's Avatar
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    Quote Originally Posted by Jazz P Bass View Post
    How about measurements.
    So I took the amp back from the pub and I have measurements:

    Voltage at TP1 and TP2 is 0.5 mV, idle consumption is 25W

    Turning the bias pot full up results in TP to 13mv and power to 40W.

    I measured AC current and I am not sure how valid this is to measure power. I can't see how 20W is dissipated so it might be less in fact.

    There is no heat build up at idle.

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    Senior Member chazpope's Avatar
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    so have you any comments guys? The drivers have arrived but I am wondering now whether I should break it up again, remove the heat sink etc just to replace them.

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    Supporting Member Jazz P Bass's Avatar
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    I forget what you are trying to fix.

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    Senior Member chazpope's Avatar
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    Quote Originally Posted by Jazz P Bass View Post
    I forget what you are trying to fix.
    well you asked for measurements - I gave measurements. Question was about how to set the bias, and the amp works only I do not have a reason why it blew up after only light use and why it gets so hot when played

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    Supporting Member Jazz P Bass's Avatar
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    O/k.
    That 5-13 mv reading sounds really low.
    Like it's pointing to something is wrong yet.
    As it is, that will result in very low mains current draw & low idle temperature.(which is good in a way)
    Do you have a digital thermometer.
    It would be nice to know what temp you are calling 'really hot'.
    I would try to get a reading directly across each & every ballast resistor.
    That will tell what each transistor is doing.
    They should be relatively the same.
    Is the TH1-TH2 (R3) connection good. (do you read 500 ohms)
    Is this part firmly afixed to the heatsink.
    This part is supposed to help track the heatsink temperature.
    Voltage readings at the base of the output & driver transistors would help. (static Vdc, no signal)

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    Reading some of your earlier posts, sound like you were setting the bias with the amp still plugged into the light bulb limiter. If that is true, remove the limiter and set the idle bias again.

    And also be sure to allow the amp to run until it has warmed up and become stable before biasing.

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    Senior Member chazpope's Avatar
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    Why do you think i13mV is too low ? It produces 10W of idle outputs power.

    Nobody has yet said what the bias should be. Again, I get no heat at idle, no audible distortion at low volumes. No I was not doing bias with the lamp in.

    I have a thermometer but how do I get it hot - it ain't easy playing two hours bass to the neighbors, I don't have a suitable dummy load nor a generator. Do you mean that I should work up the amp to a temperature and then set bias ? I guess I can do that when I take off the heatsink , might be dodgy.

    I am opening it again to replace drivers, hope it's not for science only...

    An hour later I have new drivers in and measuring voltages (with lamp)- rails 50V, output bases .55V driver bases 1.13v, wildly variant emitter currents : Q217 ans Q214 want to run away while the others stay cool. I guess this is normal with no heatsink still did not expect such a wide variation. so emitters are between 3mv and 15-20mv depending on temperature.

    now I am not sure what happens with heat sink on - it will be harder to measure/touch.
    also the pcb has just about had it but does it make sense to swap and put the HOT transistors next to the Thermal probe - I really don't want to pull them again

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    Last edited by chazpope; 09-14-2013 at 12:39 AM.

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    Lifetime Member Enzo's Avatar
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    It should run forever at the coldest bias. The only negative is that the crossover distortion would be the most. If you are not hearing any crossover distortion, then that is moot. If the amp sounds good, I see zero reason to set it hotter.

    DO NOT POWER UP THE AMP WITH THE HEATSINK REMOVED. JUST DO NOT DO IT.

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    Supporting Member Jazz P Bass's Avatar
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    If I am reading this correctly, the amp functions fine.
    It simply gets really hot.
    I asked for a heatsink reading when it's too hot.
    The outside of the heatsink is accessable to probe.
    It may be that what the OP feels is too hot, is not.
    One of the original questions was' if the amp was only used at moderate volume, why did it initially fail?'

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    Senior Member chazpope's Avatar
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    ok it will be hard to measure that temperature, let's say it was too hot based on my experience of how hot amps get during play. This is not exact of course but let's say hot so that you cannot hold it indefinitely, I expect this was some limit T that it should go to after being on full (120W?) power for a while.

    I wonder if anyone ran this at the rated 120W and for how long.

    Enzo I see your point now - I had not seen this before (runaway at idle) but I have quick T sensing fingers so I spotted it in time Changed the drivers too almost killed - as I put the heat sink back on one of the tired track on a driver base had torn off and it looked dead for a while until I took it off again and fixed - it is still alive thank god

    I will test this tomorrow and at the sunday gig, might try the temp gauge or my hands again and one day if I have a gen I will test what effect the kickback position has on heat dissipation.

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    Lifetime Member Enzo's Avatar
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    Sure, but the why did it fail question is usually one that never finds an answer. That question usually resolves into the asker assuming there is a weakness in the design, or that some particular action or use caused the failure. Both those things are usually not the case.

    Many failures are due to random component failure. In the world of physics, there may be some direct trigger for the failure of a part that had a minor flaw. In other words a part is already compromised at birth, and some peak just happened to push it over the edge. Similarly, parts can be damaged or stressed by something and ultimately cause the part to fail days, weeks, months later. Like an oversize truck crossing a small bridge can crack its frame, then 3 months later it collapses when a Honda Civic crosses it. Not the Honda's fault. A solid state amp is one big feedback loop, so it is very hard many times to determine what part started a cascade failure. But even when we identify that first part to die, then we still don;t know why. For all was know, the pick and place machine put a tiny crack in a resistor when it stuffed the board. Over time with enough thermal cycles and vibration eventually the part gave way and burnt up. No one would ever know.


    The amp may well still have a problem, but if the amp sounds OK with the bias all the way cold, I still see no reason to run it hotter. It SHOULD be happy as a clam to run that way. If it isn;t we need to find why not. Biasing it hotter would only mask other causes of overheating. Just my opinion, I am open to contrasting views.

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    Lifetime Member Enzo's Avatar
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    Nobody runs an amp at its rated power for a length of time. Not unless their music is a steady sine wave at max volume. A sine wave is great for testing, but a totally unrealistic signal for the amp. Play at that power level is a bunch of pulses with space between them. Watch a VU meter. It is only making 120w when it is making the loudest sound it can make.

    remember too that your volume controls are not power limiters, they are merely sensitivity controls. If you play with the volume on 6, then turn the amp down to 3, you can play the guitar louder and come out the speaker just as loud as before. So the knob setting is not a good indication of how many watts the amp is producing.

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  29. #29
    Supporting Member Jazz P Bass's Avatar
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    As an aside, a number of enclosed amps rely on the speaker movement to push air around in the enclosure.
    Maybe at a lower output power setting you have found the 'doesn't do enough' spot.

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  30. #30
    Senior Member chazpope's Avatar
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    No of course the knob does not show the power, I do it all by ear - to start with the speaker (was it 70W) is totally inadequate for the power of this amp an it starts to distort fairly early. Say fine you plug a proper cab if you are going to play loud. Now say the amp plays audibly well up to power level X , I mean I use it at audibly half that level. It sounds to me like a 15W bass amp on full.

    Re the power rating when an amp says 120W`I take it this means it can run a sinusoidal power of 120 reasonably indefinitely - of course not every manuf follows this I guess. So there is got to be a comparative measure and isn't that quoted what rms power stands for.

    So tonight we did the blues jam with this amp in the same pub conditions. In the kick back state, 2 hours of play at moderate volume, ambient T 25C - and it got hot so that I could put my hand on the heat sing and hold it for no more than 5 secs. I say this is the hottest it should ever get on full power, I think I will try it just upright next time and see if there is a difference.

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    Last edited by chazpope; 09-16-2013 at 12:33 AM.

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    I'm going to assume the bias is set by VR202 since it has the word "idle" stamped next to it, and that turning it all the way to left is the "coldest" setting. Is this correct?

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  32. #32
    don't forget the joker g1's Avatar
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    If it says idle, it should be the bias pot. But left is not always coldest, there is no standard. Some are opposite. Try with your limiter lamp till you find out if counter-clockwise reduces bias.

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