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Messages - Enzo

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Amplifier Discussion / Re: National GA920P Help
« on: November 06, 2019, 11:10:51 PM »
There is a thread about this model right on this forum.

The power supply is a simple single voltage.  Look at your power transformer.  Are there any unused wires coming from it?  Or are the wires that connect with the mains cord doubled?   If so, you MIGHT have a dual voltage transformer.  I tend to doubt it, but looking is free.

Looking at the thread, further down is a real schematic, it appears you need a transformer that produces  26-0-26 volts AC.  SLightly odd values, so 24-0-24 would work just fine and be easier to find.  That is same as 48vCT  (Center Tapped)

Amplifier Discussion / Re: 15A5K pot question
« on: November 06, 2019, 05:38:05 PM »
Seems to me, and I could be wrong, that it is a percentage.  Perhaps at mid point, the percentage of the full resistance.

Tubes and Hybrids / Re: JTM60 Combo rectified diodes fried
« on: November 05, 2019, 04:54:39 PM »
Possibly nothing.  One diode shorts from random component failure, and causes the others to fail.  Otherwise, a shorted power tube is the first source of damage in a repair until shown otherwise.  Beyond that and all farther down the list would be anything to short the B+ to ground, like bad filter, bad flyback diode, shorted OT, and so on.

Sump pumps and furnaces were just examples, we had a drop forge a half mile from our shop, and the heavy motors and such in that place put noise on the power mains.  At another location, we had a traffic light with a noisy triac a half block away, and so whenever ONE direction had a yellow light, we could hear power line noise, so every minute or so, we got 10 seconds of noise.  Point being, we need to eliminate the possibility of external sources.  We cannot just reason them away.   One trick is to set an AM portable radio between stations so all you hear is hiss, set it near the amp.  DO you ever hear noises from the radio at the same time as the amp?

I realize this could be something totally unrelated, but it is just a question to answer:  inside the amp or out.

Fender has little to say about it.  Fender may not RADIATE interference, but how sensitive it  might be is a mater of how well do they shield it.

The whole essence of troubleshooting is to isolate the problem, so the first thing to answer is whether the problem is even inside the unit or out.

fender or anyone else takes standard steps to reduce sensitivity to external noise, but there is a limit.  Common sources are cell phones.  Many have found that if they sit their phone on top of the amp, the amp picks up ticking sounds.  Ever been in or lived in a home where every time the furnace kicked on or the sump pump kicked on, you heard a quick pop or noise on your TV or AM radio? 

Something external to the amp can do this.  Does the amp act this way if you take it to a different place, like a friend's house?

By the way, not only power amp, but power supply is included in your narrowing down.

Watch with a scope. 

The Newcomer's Forum / Re: I want to ask a question
« on: October 22, 2019, 08:05:39 PM »
Gs65D is a legit model name for KMD, I have the schematic in my files  in my storage unit.  I will dig them out next time I go there.

I agree, the output devices is most likely failure.

The Newcomer's Forum / Re: Peavey Audition Plus
« on: October 17, 2019, 01:18:22 PM »
I provided the right schematic, it is the link at the bottom of my previous post.

The Newcomer's Forum / Re: Peavey Audition Plus
« on: October 16, 2019, 10:44:21 PM »
The Audition Plus is its own model.  You can get this or any other Peavey schematic from the factory:

This amp, like many other small amps, uses a power amp IC to drive the speaker.  This  one is a TDA2040.  A TDA2050 would also work.  Those TDAs are no longer common, but an LM1875 should be a good substitute.

I point that out because it is almost certain your amplifier IC is bad.

One possibility to check first is a failed filter cap on one of the power supplies.  The two large-ish capacitors over near the fuse.  Any of them cracked free of the solder?

Amplifier Discussion / Re: Fender 85 tone help
« on: September 30, 2019, 08:12:11 PM »
Speakers are still the single most effective way to change the tone of an amp.   Try connecting the amp to a variety of speakers to see if you like any better.

Amplifier Discussion / Re: Gallien Kruger GT200
« on: September 09, 2019, 12:53:36 AM »
I see no reason to suspect the OT yet, but I do have some shop wisdom:

never think up reasons not to check something.

SO go ahead.

But remember, the signal for Q2 comes directly FROM the speaker side of the OT.  Seems like it works then.

Before you test the OT, apply signal to the amp, and see if signal is on the hot pin of the extension speaker jack.

Look at the speaker jacks.  Note the main jack requires the signal to come THROUGH the extension jack contacts, so if they are bent or dirty... no sound.  And if you plug the speaker into the extension jack, there is nothing in the main jack to complete the circuit to ground, and... no sound.

A quick test?  Two speakers, one into each jack.  If that works then we can be fairly certain the extension jack contacts are bad.

Amplifier Discussion / Re: Gallien Kruger GT200
« on: September 08, 2019, 03:16:56 PM »
Bias is the difference between emitter and base, not just the base voltage.  SO with the base esentially at ground for DC, look at the schematic.  Your emitter then must be negative to turn on the Q1.

Note the top and bottom halves of this amp are the SAME, not opposite polarities as usual.  The upper side acts like a plain single ended amp, then the OUTPUT from the speaker winding becomes the signal for the bottom side.  That signal is inverted from the input signal, so the NFB acts more or less as a phase inverter.

ANyway, over left, R4, R5 are the emitter resistors for Q1,2.  The join, but not at ground, they are wired up and over to the right with the current control and H supply of -8.9V.   H comes from Q26 and ultimately from the -30v rail.

So... do you have voltage H at that control?  At the joint of R4,5?   My first suspect is an open R4, but surely a missing supply there would do it too.

Note by the voltages on the schematic the top and bottom halves mirror one another, so the voltages are all the same.  Use Q2 side to compare.

Amplifier Discussion / Re: Gallien Kruger GT200
« on: September 07, 2019, 06:54:59 PM »
SO is there signal between the cap and resistor?

Amplifier Discussion / Re: Zener vs IC regulator for the preamp section
« on: August 31, 2019, 01:30:47 PM »
And IC regulators have to be properly mounted and heat sunk.  I have drawers full of 7800/7900 Vreg parts, mainly 15v but also 12v, 18v, 5v and others.   I surely do have to replace them.  And I have found some without heat sinks that failed in commercial products.  I replace zeners too of course.  I used to buy 1N4744 by the hundred.  Like anything else, it isn't the zener's fault if the part is underspec'd or not mounted properly.  You are correct, the engineers who did the design needed to put sufficient thought into it.

Amplifier Discussion / Re: Zener vs IC regulator for the preamp section
« on: August 30, 2019, 04:00:45 PM »
Who told you zeners are unstable?   A zillion guitar amps use zener regulated supplies for the op amps.

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