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hartke3500 blown transistor

Started by js1970, October 12, 2014, 08:00:21 PM

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J M Fahey


js1970

   Ok, so I plugged direct,no limiter. My offset voltage is around 20 mV and the supply is 87 V.
   I connected speakers and still have that hum.  I also noticed the offset increased with volume. I assume this is normal, as its driving the speakers?
  The hum is affected by ,and increased with, the master volume, and the contour knobs  ( low pass and high pass). To some degree the EQ will filter the hum by turning down the 64,125,250,500 Hz,and 1 KHz knobs.
   
    It would seem as this is a different issue unrelated to the blown transistor. If so let me know and I'll start another thread. Thanks to all who responded for your helpful guidance

phatt

You may have missed a ground connection somewhere while fixing the other problem,,,or unwittingly added an extra ground path, both can cause the dreaded hum.
Phil.

Roly

Quote from: js1970It would seem as this is a different issue unrelated to the blown transistor.

So it does.

Quote from: js1970The hum is affected by ,and increased with, the master volume, and the contour knobs  ( low pass and high pass). To some degree the EQ will filter the hum by turning down the 64,125,250,500 Hz,and 1 KHz knobs.

This suggests that it is originating before the graphic EQ in the signal chain.


Check all the inter-board connections, firmly plugged on or none broken if soldered.

Clean all four sockets, Send, Return, Active and Passive, with dry metho, isopropyl, or DeOxit, and work a plug in and out when you do (cleans the internal contacts).


Is the Compression switched OFF?

Is it in both Solid VR and Tube VR channels?

Does pulling the twin triode make any difference?
If you say theory and practice don't agree you haven't applied enough theory.

js1970

   Adjusting the solid or tube knobs makes no difference. Niether does the compression / EQ. All ground connections to the chassis showed good for continuity. I don't know what the twin triode is.
   I'll go ahead and give the board another good look for bad joints.
   Could a bad pre amp tube give me that hum?

Enzo

The twin triode IS the preamp tube.

You just now said the Compression/EQ makes no difference, but earlier you posted

QuoteTo some degree the EQ will filter the hum by turning down the 64,125,250,500 Hz,and 1 KHz knobs.

So either the EQ controls affect the hum or they do not.  Which is it?

This amp, like any other, is a stream through a circuit.  The hum comes into the stream at SOME point.  We are trying to find what is before and after that point.

Roly

Signal Flow

(circuit ref: http://www.ssguitar.com/index.php?action=dlattach;topic=3592.0;attach=5068

Start - top-left, "Passive" and "Active".

These go into two op-amp input stages with differing gain, IC101A and B.

These are then mixed together at IC102A, and the signal then split three different ways;

- 1. via the preamp twin-triode valve/tube, the "Tube VA" path,


- 2. via IC103A for the "Solid VA" path,


- 3. into the Compression control circuit (which boils down to a gain control voltage going to IC403 pin 11).


The first two are re-combined at IC109A, and fed into a gain control block IC403, for Compression control (pin 11).

From pin 14 the signal then goes to IC107A where it is lifted from -2dB to +4dB nominal, and sent (left) to the Fx Send/Return sub-assembly, IC501A and B.

IC107B and IC108A bandwidth limit before the "Contour" controls around IC201B and A.

If the EQ is engaged, SW101, then the signal goes via the rank of gyrators in the graphic EQ section IC205 and IC206.

IC202A is the final output buffer for the preamp section which drives the main power amp input.
If you say theory and practice don't agree you haven't applied enough theory.

Roly

If you say theory and practice don't agree you haven't applied enough theory.

js1970

  I pulled the tube with limiter engaged and no speaker load before. There was no change in the bulb brightness. I didn't know I could pull the tube and run it through the speakers. I will do that this weekend for sure.
   As for the ICs you mentioned, again I see the +/-15 off of pins 4and 8. Is that how I would check that they were functioning correctly?

    The hum is present, whether the EQ is on or off. It makes no difference. However, with the EQ on, I can filter the hum down to some degree.

g1

 The point about the eq was to try and narrow down the area.  If the eq can affect it, then it is happening before the eq.

J M Fahey

#25
It's funny how they cheat with the tube  :grr

After Ic102a , signal is sent to two parallel connected preamps; "Tube" and "SS" .

SS is unity gain and flat .

Tube is 50X gain and passes through a fixed tone control, roughly Fender style , set to, hear this:
*  Treble on 10
*  Bass on 10
*  Mid on 5


and even considering tone control attenuation, signal is still hotter by some 5 to 10 dB :o

Talk about cheating  :trouble

This perpetuates the myth that "Tubes are always better than SS" yadda yadda yadda , no surprise users praise the`"hot punchy Tube sound" over the "clean but unintreresting SS one".

To check whether that tube adds some magic on its own, I have bypassed the added EQ network and padded the tube output so both channels have exact same gain and level.

Guess what? ... nobody could tell both channels apart ;)

The Fendery fixed tone control is:
R112/113/115/116 plus C109/110/111/112 .

Somebody might simulate it and post the curve here ;)
Thanks in advance.

Roly

Quote from: js1970There 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.






Quote from: J M FaheySomebody might simulate it and post the curve here ;)


8|
If you say theory and practice don't agree you haven't applied enough theory.

phatt

Now now Jaun!  Keep calm. Don't go spinning the wheels on your Falcon you might blow a tyre.  :lmao:

But yes I do see your point, So the valve gets tone while the SS does not.

I took the time to simulate the valve section.
The SS preamp is just the compressor so I'm not going to try to do that compressor and anyway the response will likely be flat.
(Roly Beat me to it but no matter)

In effect the Valve section uses one extra tone shaping system and as I discovered a long time back great things can be done with an extra tone stage. ;)

FWIW, Don't worry R118 is 150k which pulls ~5dB off the signal so they stuffed it anyway. ::)
I'm stunned that many big name gear fails to grasp the simple fact that those old tone circuits only work right when followed by a high Z input. I've added a second pic to show the loss.
Phil.

J M Fahey

THANKS guys, what would I do without you  <3)

Roly

Quote from: J M FaheyTHANKS guys, what would I do without you  <3)

Have a life?   :lmao:


Quote from: phattI'm stunned that many big name gear fails to grasp the simple fact that those old tone circuits only work right when followed by a high Z input.

Early in my career I had to work under the direction of a young engineer who was, by parts, truly obnoxious, arrogant, stupid, and accident-prone.  He had a really serious case of "I'm a ginger-beer and you can't tell me ANYTHING" (even that I'm leaning on my soldering iron and burning the crap out of my jumper, shirt, and arm - true; "no sense, no feeling" as my mother would say).

The sight of a pool of his blood on the floor was a frequent, if not weekly occurrence.  Warmly loved by one and all, we had a sweep going to guess when he would be flattened by a falling piano, and planned a right booze-up when it finally happened.

The major product prototype he had been working on was finally an installed prototype on the clients' site, and instead of coming good in a week or two, months were ticking by with the clients' machine still sitting idle (and on the phone demanding to know what we were playing at).

Then he simply stopped turning up at all and it fell to me to sort out his mess.

In a nutshell he had managed to get the logic levels between the machine feedback and the new control board inverted, High should have been Low, then tried to compensate by tacking resistors on each end as pull-ups and pull-downs until the logic was dithering in the error band.  A bit of Vero with three logic inverting stages did the trick.

So I reported back to the boss what I'd found and what I'd done to move things along, but I had a big question; how could a qualified engineer, supposedly an "expert" in micrologic, make such a basic error, then go right up the beach rather than finding and fixing it as I (a lowly tech at that time) had done in a day or so?

{It's also a great object lesson in knowing when to stop and ask for help.}

Good question.

He had presented all his qualification at interview when he was employed, but of course nobody bothered to actually check any of them, and it wasn't until somebody rang checking his employment history after he had walked out that the whole story started to unravel - he had failed out of first semester Electronic Engineering, so he had simply forged his qualifications, so by the time he made 30 he had red lines through his engineering qualifications, and a conviction for professional fraud on his CV.  I know he persisted with is own electronics company for a couple of years after that, pirating clients and designs of his previous employers, but then vanished from the Melbourne electronics scene.  Being into electricity and scuba diving, and being accident-prone, I expect he selected himself out of the gene pool.

That's the sort of thing that goes on in the background.

It's not often that it actually surfaces.
If you say theory and practice don't agree you haven't applied enough theory.