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Old 12th February 2013, 11:14 PM   #211
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Alright Nigel thanks man, this is information I can use.

BR
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Old 12th February 2013, 11:27 PM   #212
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Quote:
Originally Posted by nigelwright7557 View Post
Look under DC current gain or Hfe.

269 looks good to me as the upper limit is 300.
So Current at the Collector of 150mA should produce 100 to 300 hfe, correct?
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Old 13th February 2013, 12:10 AM   #213
Enzo is offline Enzo  United States
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BR, I was not picking on you, I think you understand that. Just changing parts as a strategy is what I was talking about. I am not sure I can always articulate my approach, but it rarely involves shotgunning the parts (replacing all of them). Parts are not the only source of problems, cracked or broken copper traces are just as deadly to a circuit, and are not rare. Also, little beads of solder can get free or short across places they don't belong. Replacing a lot of parts doesn't find those.

This is hard, and this amp in particular has stumped people before. It is an unecessarily complicated disaster. Give me a good old reliable peavey amp any day. It is a rare Peavey 400BH that takes me more than 5 or 10 minutes to diagnose. This here circuit is just cruel.


Imagine you needed to fix the light in your refrigerator - it won't go off when the door closes, so you need to get inside the switch, but the switch is not accessible when the door is closed. We then need to develop a strategy to overcome this obstacle.
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Old 13th February 2013, 03:33 AM   #214
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Enzo I know you're not picking on me. Not to worry man...
I have been looking at schematics for other amp and you are correct dude, this amp is a cluster F___! In all honesty, this amp has left such a bad taste in my mouth I doubt I will ever buy another Fender. Bad thing is, I really enjoyed playing the Deluxe Tweed Reissue. I was like OMG this is the sound I have been looking for!!! I mean it sounded like I was finally home with my Carvin plugged into it.

I have my eye on a Bugera V22 at the moment. Its relatively small, low powered so I can crank it into the sweet spot without killing the neighborhood and it sounds pretty darn good to me.

As for my amp, I honestly do not think a broken track is the problem. I have been meticulous in searching for a break. I have also struck a chord and flexed the PCB seeing if I could get any crackles, pop, hums etc. Nothing...

Now as for a cold joint, I have had several I suspected and I desoldered using solder wick and resoldered. Of course I could very well be missing one and there is always the chance its that one piece causing all this grief (wink).

I am gonna just take a couple days off to get a break till my order arrives. Then I will pull each transistor and test it and if I find anything abnormal I will change it out. Unless you have a better idea.

If I am going to check the power at the transistors I have to take the 42v and use ohms law to see what I should get after the resistor at a given base and the bias voltage from there?

There is so much math involved in this, its like too much to ever hope to solve.
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Old 15th February 2013, 03:10 AM   #215
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I pulled Q18 and 19 the other night and installed new ones just because.
TIP31c calls for hfe max of 50 my meter is showing 136 / new one shows 159 What's up with that?
TIP32c calls for 50 as well, meter shows 114

Is this normal? If these are showing this high, wouldn't this be causing Q20 and 21 to saturate and overheat?
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Old 15th February 2013, 05:05 AM   #216
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I am not surprised that this simple amp is not yet repaired and I doubt ever will it be, the way it goes.

U have been learning theory in the practical class.

Gajanan Phadte
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Old 15th February 2013, 04:52 PM   #217
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Simple amp? I dont think this is a simple amp. I think those that designed it should be publicly horse whipped.
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Old 15th February 2013, 05:07 PM   #218
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Quote:
Originally Posted by gmphadte View Post
I am not surprised that this simple amp is not yet repaired and I doubt ever will it be, the way it goes.

U have been learning theory in the practical class.

Gajanan Phadte
It isn't that simple. FYI, using only one pair of outputs, and expecting 100W, it looks like they tried every trick in the book to hopefully get 100W out of the design, but I doubt that 100W can be obtained without clipping.
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Old 16th February 2013, 09:47 AM   #219
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Originally Posted by ouimetnick View Post
It isn't that simple. FYI, using only one pair of outputs, and expecting 100W, it looks like they tried every trick in the book to hopefully get 100W out of the design, but I doubt that 100W can be obtained without clipping.
Not really - power output is a function of power supply voltage, and it's capability of providing enough current. The main function of the amplifier is to 'survive' providing that level of output.

The supply rails are 84V, which assuming zero losses (which won't even get anywhere close to zero) could provide 200W to 4 ohms, and 100W to 8 ohms.

The actual spec on the amp (printed on the circuit diagram) states 90W at 1KHz and 5% THD - which is pretty poor for such a high voltage rail. I suspect the mains transformer and reservoir capacitors are pretty small from that value, but it's only a guitar amp, so it doesn't need anything great.

The circuit seems over-complicated, and for no good reason - it looks to have more in common with a HiFi amp than a guitar amp?.
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Old 18th February 2013, 12:13 AM   #220
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Default Developements

My order arrived yesterday. I have been working on my amp today re-checking values etc.

I ordered zener diodes 5.1v and 10v.
I assumed that when a zener is rated at 10v, it would show 10v after the zener. D20 showed 42.1v before and 41.9v after. I pulled it and checked it with the diode setting and it showed 7.231 and open the other direction. The new one showed almost the same. I installed it and rechecked and got 42.1v and 42v after. This doesn't make sense to me. I guess I don't understand why a zener is rated at 10v and lets more voltage pass.

Enzo was talking about how one of the smaller transistors could be turning the output transistors on too hard. So I ordered replacements just in case.
I pulled Q15 and replaced it with a KSA1013. I had planned to replace Q14 as well because both are mounted in a small pocked cut into the heatsink next to Q20 and 21 respectively. I figured the heat could have effected them. But, I wasn't watching what I was doing and ordered the replacement with the wrong pin configuration. Anyway, changing Q15 did not change the over-heating problem.

Also;

I had previously found R88 to be out of spec. Schematic calls for 47ohms, I got 80k in circuit, removed I got 79ohms. I replaced it with what I thought was a 47ohm but when I checked it today it showed 470k. I removed it and checked and it was close to the same. Apparently I had my head in the posterior orifice last time. I installed a real 47ohm and plugged in the amp and no more fuzz.
The amp plays as good or pretty close to as good as it ever did. But it still overheats after about ten minutes of play. It seems like it heats up faster on the drive channel.

So, I went about checking all the resistors I could find that are obviously part of a power divider in any given area. The idea being that improper feedback would obviously cause the respective transistor to put out too much or too little. I also pulled one leg of every resistor that shows a value even slightly over or under, in circuit. Each one, however, showed to be within 5% with the exception of one that was about 6% out.

It occurred to me that while there is a heatsink on the PCB its normally mounted the the aluminum chassis, which obviously adds to the capacity of the heatsink. So I took a pair of locking pliers and clamped them to the heatsink next to Q21 and turned the amp on and played. As usual, Q21 got hotter than the heatsink. In fact it gets about 8 or so degrees hotter than Q20. In fact the screw that secures it to the heatsink is the actual hotspot. The transistor runs about 10 degrees cooler than the head of the screw. And even with the locking pliers in place, it still overheated. So... I am wondering if maybe the issue is that the connection between the transistor and the heatsink may be an issue. I used the grease and the insulating sheet that came with the transistors. I'm not sure just how snug that screw is supposed to be. I snugged it but I was careful because it didn't want to crush the end of the transistor. Is there a rule of thumb about this?

Thanks for your time.

BR
Attached Files
File Type: pdf KSA1013 PNP TO-92L Transistor.pdf (55.5 KB, 2 views)
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