Hartke LH1000

[lowell]

Came in with a blown fuse. I found one NPN and one PNP output shorted. Replaced the 2. Drivers test good. Powered up with limiter and have bad base voltages. Supplies are +/-60 w limiter.

Q34 Vc is +5v on the working module... and output Vbe's are 550mv.

Here's a pic of my schematic with voltages. I'm stuck unless I start replacing things to see if it fixes it. What would be your next step?

[The Dude]

Did you check the associated emitter resistors?

[lowell]

Yes they're all good

[lowell]

It's stumping me cause all transistors seem to be conducting and Vbe are looking good.

[nosaj]

It's stumping me cause all transistors seem to be conducting and Vbe are looking good.

You have 0 on one side of R317 and -57 on the other side Is it open?
You are talking about Q304 right? I don't see a Q34.
Thanks,
nosaj

[lowell]

It reads fine, and there would not be 850mv across R313 (Q306) Re, if it was open. Q306 is conducting, it would not be if R317 was open. On the working power amp, (there's a left and right PA module in this amp) there is +5v on the other side of R317.

My guess, and it'd require the shotgun replacement method, is that Q305 is conducting too hard, then again its Re voltage drop doesn't seem to be too high. The voltage would increase as current thru Re increases. So again, stumped.

[lowell]

Ok I used the ESGM (educated shotgun method)

It was odd to me that driver bases were skewed positive AND that there was 0v at Q304 collector.

If the output emitter junctions had +2v offset, this should then cause the Vb of diff pair transistor Q067 to rise. This would turn Q304 ON more thus making its Vc higher. And this should be higher than the correct reading from the other module. But alas it was lower. It seemed Q067 wasn't working right, replaced it - fixed.

[nosaj]

Ok I used the ESGM (educated shotgun method)

It was odd to me that driver bases were skewed positive AND that there was 0v at Q304 collector.

If the output emitter junctions had +2v offset, this should then cause the Vb of diff pair transistor Q067 to rise. This would turn Q304 ON more thus making its Vc higher. And this should be higher than the correct reading from the other module. But alas it was lower. It seemed Q067 wasn't working right, replaced it - fixed.

i like the way you explain things. I don't recall exactly what the amp was doing could you tell us?

Thanks,
nosaj

[lowell]

Had a blown fuse. I powered it down and found 2 shorted BJT power transistors on CH1 module. CH2 module was working fine. Replaced them, but then found that the drivers base voltage were both wrong. That led me to testing all the voltages throughout. My last post explains it...hopefully. Hope that helps.

[nosaj]

Had a blown fuse. I powered it down and found 2 shorted BJT power transistors on CH1 module. CH2 module was working fine. Replaced them, but then found that the drivers base voltage were both wrong. That led me to testing all the voltages throughout. My last post explains it...hopefully. Hope that helps.

Yea a little I'm still learning though.
nosaj

[lowell]

If you have some specific questions I'd be happy to answer em. I've learned so much over the years from everyone on this forum I'd be honored to do the same for anyone I may be of help to.

[Enzo]

Nothing teaches you about something as well as having to explain it to someone else.

[lowell]

Absolutely!

[nosaj]

If you have some specific questions I'd be happy to answer em. I've learned so much over the years from everyone on this forum I'd be honored to do the same for anyone I may be of help to.

So after replacing the blown transistors. Your voltage checks let you know the health or how stressed any other transistors were?

Did the amp pass sound at this point Before the last one you replaced(Brain says probably not if one was not amplifying.)

Thanks,
\nosaj

[lowell]

Immediately after replacing the power transistors I plugged into my bulb limiter and fired it up to test the bias. This is the static or quiescent Vb (base voltage) on the drivers. Probably checked this on the outputs bases too.

Anywho the top driver had about 3v and the bottom driver had about 1.5v. These voltages SHOULD be equal and OPPOSITE. And the voltage should be 500mv-1v approx. Or, just enough forward voltage to barely turn the drivers on so they're sitting there ready to amplify AC.

So looking at it, the upper driver was too high, and the lower driver was too low (it should be about -1v not +1.5v). Considering polarity...it was very "low" voltage. Skewed positive.

The Vc (collector voltage) of Q304 was what pointed me to the differential pair transistor as likely cuplrit.

The more ON that transistor, the lower its Vc would be. Its Vc is also Q304 Vb. The lower (higher neg voltage) Q304 Vbe, being PNP and drawn upside down, the more "pulled up" its collector should be. Hope that helps. The collector of Q304 wasnt as pulled up as it was on the working module.

I look at transistors as see saws. Base goes up, collector goes down. This image, being very basic, helps to have an estimate of where things should be voltage-wise. Direct coupled transistor amplifiers are tricky because everything is so codependent. If one transistor isn't behaving, it skews everything. IE, no coupling caps to separate the stages.