hartke3500 blown transistor

[js1970]

There was no change in the bulb brightness.

Now that the output half-rail is balancing to (near) zero volts correctly, have you tried powering the amp directly from the mains?  It could be that you hum is due to the voltage reduction of the lamp.

    I did take the plunge and fire it up off of mains power. Everything functions as it should, but with that incessant him. I removed the tube, with no change. When you gave me the run down of signal flow I started to wonder. Hear me out on this. The flow was from the instrument inputs, through the compression,through the EQ, then out the master volume.
What got me thinking, was that tbe amp hums with no signal in. So I followed the power input. What I found was a resistor ,R406 inline with the EQ switch, that appeared discolored. I will check its value when I get a chance.
    Also, when using the light bulb limiter, the bulb glows steadily even after power up. It is my impression that that means there is a short somewhere. I ve run other amps through the limiter to see how it worked. They were SS guitar amps and upon initial start up the bulb lights and fades out. Is it because of the preamp tube the bulb stays lit or is it signifying a problem? It is my theory that the limiter bulb and the hum are connected. If I find the problem, the bulb will go out and the hum will cease.
    Does this have any merit,or am I looking at it the wrong way?

[phatt]

Then measure R value of R411 which should read 10 Ohms it's likely the ground lift from the chassis. (I think)
If it reads very low then somewhere on that board is a second grounding path which will likely be the culprit.

I was working on a board once and after refitting with only a couple of screws to test I noticed that if I pressed on a corner the hum came back.

As it turned out the circuit had 2 ground points and that corner standoff hole had a ground copper trace across the hole. (design flaw,, Never assume the factory got it right, Hint)
I simply cut the track around the hole and not only had I fixed the initial problem I also fixed the bad hum.

Getting ones head around grounding can be harder than designing a circuit.
After dozens of builds I've learned to respect ground paths a lot more.
Phil.

Ed; R406 will run hot as it's the heater supply,,, says 10 Volts,,
Yikes should be 6.3V +/- 10% that Valve will have a short life :duh

[Roly]

I have a rack compressor/limiter somebody else took apart, a "bucket" job (comes in a bucket), with just this sort of grounding problem I have never been able to totally overcome - very tight case and missing fiber washers. 

Also, when using the light bulb limiter, the bulb glows steadily even after power up. It is my impression that that means there is a short somewhere. I've run other amps through the limiter to see how it worked.

That depends.

Every amp draws some residual power when it is idle, and this may result in a residual glow depending on how much idle power the amp pulls, and the wattage of the globe used.  With valve amps the residual power is normally higher due to the tube heaters.

If it's running okay, half-rail balancing to very low output offset, and it ran for more than a second direct-on-mains with no smoke or distress from the power amplifier, you can put the limiting lamp to one side.  I'd call that power amp a "goer".


At one point however you got distracted by the preamp +15V supply regulator, and I can't help but wonder if your problem now may be related to that.

{as a sidebar, this is why we try to tackle one fault at a time.  Normally amps come in with only one fault, such as your blown output stage (which may be due to a faulty speaker lead/connectors - it may well have a proximate cause you should look for.  You don't always find a cause, but you should always look for one.}


If turning the Master volume right down to zero kills the hum then it must be getting into the preamp somehow, and now that the power amp seems to be going okay we can concentrate on the preamp alone as a sub-system.  (and since the Master Vol is an op-amp stage it suggests that the supply rails are clean)

Try injecting a clean signal at Fx Return.  If this is hum-free then we need to look back towards the front end, however we really should confirm somehow that the preamp supplies are the correct voltage (easy) and hum-free (somewhat more difficult without an oscilloscope, but possible).

The Rx Return is low hanging fruit, and depending on what you report of this test we can work forward or back, and there are several places where you can harmlessly shunt the signal path to ground with a clip lead o.n.o. to isolate the problem to before/after.  (if this test comes up clean it very strongly suggests that the supplies are okay)

We normally come at problems like this by first applying binary splits, is it before or after the Master volume? (because the Master volume normally is between the preamp and power amp.  In you case we now think the power amp is okay, so we use the Fx Send and Return to split the preamp.

We keep doing this until we have a bite sized chunk where we think the fault is, then we work methodically, node by node, either backwards or forwards through the suspect circuit block.

You should also try shorting both inputs, again with a couple of plugs and clipleads o.n.o.  If this shuts your hum up then one of the input shorting contacts in the socket isn't.  This should be an early test.


So I particularly want to know;

a) input a signal at Fx Return, clean or hummy?

b) both inputs shorted, silent or hummy?

[js1970]

   Roly, sorry but,I don't know what o.n.o means

[phatt]

Most likely means; Or Nearest Object.
In this case one which will conduct and bridge the switch.
, often a screwdriver.
Then again he could mean the nearest Observer.
Phil.

[J M Fahey]

(design flaw,, Never assume the factory got it right, Hint)

Getting ones head around grounding can be harder than designing a circuit.

After dozens of builds I've learned to respect ground paths a lot more.

Phil.

Print these 3 maxima with big bold letters and attach that paper to the shop wall, they will help you often.

[Roly]

   Roly, sorry but,I don't know what o.n.o means

o.n.o. "or near offer" (commercial), "roughly", "about", "something like".

[js1970]

   OK, so when I send a signal through the FX return, I get a fuzzy/ distorted sound,and the hum is still present. When I short the inputs it is still hummy.
   I also put the ohm meter on R411,like phatt suggested,and got 0.00.
With power off, I touched the leads to either side of the resistor while still in circuit

[Roly]

when I send a signal through the FX return, I get a fuzzy/ distorted sound,and the hum is still present

...and you eliminate the preamp.

This is now without the limiting lamp, right?  Then you still have a problem in your power amp or power supplies - somewhere.

So we return to basics...
Check that the voltage on the output half-rail with the speaker disconnected is zero +/-0.1V.
Main +ve voltage.
Main -ve voltage.
Preamp +/-ve supply voltages.

Please post five (exact) measurements.


Now you really need an oscilloscope, or at least that's what I would be reaching for at this point.

[js1970]

   Here are current supply voltages:
Half rail     23.1mV
Main +    IN  87.2V  / OUT  33.44V
Main --  IN  87.2V  / OUT  33.29V
Pre +   23.07V
Pre --   15.34V

  The B+ value on the pre amp is pretty high I see. I took that value from IC401( the voltage reg 7815). Is this the cause of my hum?
Earlier on I saw it was really loose. Could it have sustained damage to the board?
  I had tested it for output,and it checked out. When I resoldered it, I made sure to not run puddles together,so nothing is touching. As loose as it was though, could it have cracked,or otherwise stressed out, the trace in the board? Then, by me applying heat and solder, a bridge was created across the stressed areas.

[Roly]

uh huh.

The 7815 is a +15 volt regulator, and if you are getting more than that on the preamp +ve supply rail it certainly seems like the primary feed voltage (24V?) is somehow making it past the regulator; and yes, this could easily be the cause of excessive hum.

If you have a new one to hand (or it's easy to get) then I'd be inclined to just replace it, however while they are not totally indestructible they are pretty damn robust, so I suspect that the middle (ground) pin is not actually making ground for some reason (e.g. dry solder joint, cracked trace), so I would first check continuity from the middle leg (or tab) to a known ground point, and I think you'll find it's not zero ohms as it should be.


An alternative faultfinding pathway is that with the amp on, if the middle (ground) leg is open I'd expect about 23V on the output (right leg), a bit more, say 24V on the input (left leg), and perhaps 24-15=9V on the middle, supposedly grounded, leg, (but it could be just about any voltage, and it should be nothing).

If you have 24V in, 23V out and the middle pin is grounded, then the regulator is either stuffed or shorted somehow.

Can you post above and below macro pix of the 7815 area please?

[g1]

And in case you are unaware, the 7815 and 7915 regulators have their inputs and outputs in different positions.

[js1970]

    Yup. I checked continuity to ground and got OL.

[Roly]

Excellent.  :dbtu:   Now all you've got to do is find the open and fix it.

The first place I would look is the middle leg solder joint, and if this looks okay, measure continuity between it and some other ground point.  What frequently happens is that the track fractures just where it meets the solder donut, and it can be finer than a human hair.  If so, just scratch a bit of the green overprint off the ground track just back from the solder joint and bridge with a short bit of wire.

[js1970]

  Cool, I'll get after it and let you know the results.