Fender 75 amp issues again and again.....

Dreadneck

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Hi!

So I chose to start a new thread rather than reviving the old ones.

I just replaced the PT that was smoked. The old PT was the stock model for EU market with 230VAC from the wall. Model number 013917. The replacement transformer I got is the drop in from Hammond, model 290YEX.

The difference right away is the orange wire (Electrostatic shield) and a third heater wire (assuming it is a CT for the 6.3V winding).
Should I use that CT for the 6.3 or go with the dual resistors at the pilot lamp i used before? The humdinger pot was shot when I got the amp, so rather than replacing it I installed a faux CT by two resistors.
The orange ES wire I just grounded. I suppose it was made to reduce noise.

Now the amp can be switched on without blowing a fuse, which is a MAJOR improvement!

However the voltages does not seem correct.
I took measurements with and without tubes all in the high power setting.

I have drawn the measurement points on the schematic (re-drawn by a user here, I attach the original schematic as well )

Without tubes:
1, 473VDC
2, 236VDC
3, 432VDC
4, -21VDC
5, -53VDC
6, 434VDC
7, 433VDC

With tubes:
1, 300VDC
2, 151VDC
3, 255VDC
4, -15VDC
5, -35VDC
6, 256VDC
7, 225VDC

So, the voltages goes down dramatically with tubes installed. Why? Seems odd.

Another thing is heater voltages from the PT goes to 5,2VAC and not to 6,3.... As a matter of fact all the PT voltages was some odd volts lower than what was spec. Not much, so I assume Hammond thinks it is within tollerance.

I use a light bulb between the amp and wall socket. Without tubes it does not light up at all. With tubes the wire in the bulb glows a bit, not enough to call it lighting up though. It is a 100W bulb.

Any help would be greatly appreciated!!


Fender 75 Version 3 voltages.jpg
 

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hamerfan

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5.2 Volt for the heaters is way to low. Did you still use faux CT when measuring? Using the real CT could be different.
What was mains voltage? In real life it's mostly lower than 230.
 

Dacious

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Ok. Don't use the faux centretap and real centretap. Use one or the other. Yes there can be a big variation between tubes in and out.

It sounds like youau have the primary wired for 240 volts, not 230. Your heater voltages might be being pulled down by having the faux and real centretap connected.

You are aware this amp has a low power 15 watt switch. This works by dropping B+ voltage via an alternate tap on the secondary. Perhaps this is engaged, or secondaries aren't wired properly.

Also look out for a resistor getting cooked tubes in.
 

Dreadneck

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I'm only using the PT CT for the heaters. I'll take measurements with the faux CT instead and see what happens.

Yes I know about the low power switch. It is in high power, I checked it before and again now =)

Thing is, the PT only has three primary windings. And it do say 240V @ primary side. However I'm using a variac so I know exactly what voltages I get. If I pull it up to 240V it doesn't change the voltages in the amp with more than 0.x volts.
 

Dreadneck

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Turned out that the lightbulb limiter ate a whole lot of volts. I checked right at the powercable lugs in the amp. About 180VAC. Without the lightbulb it went up to what came out of the variac. 240volts in this case.
Which made the heaters go up to correct voltages =)

However the original issue appeared again. The two 0.5AT fuses blew when I powered the amp up again without the bulb...
Back to square one......


At the times I got it up running now it does make sound. But it is at the volume of a very small pratice amp, even though I have the volume at max and ugly distorted.
 

Dreadneck

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Rectifier is dead?

It is SS rectification consisting of four diodes and four .002 caps. All of the diodes and caps are new, but I'll check to make sure.
Any way you would test it in particular? Besides testing the diodes.
 

Dreadneck

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Strange thing happened. I put my variac directly to the HV leads going to the rectifier circuit, effectively bypassing the PT.
The fuse in my variac blew!
I replaced it and turned to power up to about 17VAC slowly. The variac is humming! And after just a short while I saw a little stream of smoke from it.
Naturally I turned it off imidiately.

What the hell!? Am I missing something that I really should have known? I'm ready to slap my forehead =)
 

fidopunk

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It is SS rectification consisting of four diodes and four .002 caps. All of the diodes and caps are new, but I'll check to make sure.
Any way you would test it in particular? Besides testing the diodes.
I have to think about this some more-- I had two old tube amps with solid state rectifiers that were displaying the same behavior. I didn't even bother testing them, I just swapped all four out and it solved the problem. The way you troubleshot it still sounds like the diodes in the rectifier are either orientated incorrectly or something downstream of it is shorting out.
 

W.L.Weller

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Clearly something is drawing a ton more current than it's supposed to. Why did you bypass the new power transformer with the Variac? The tubes won't work without heater current. Is your Variac able to "stand in" for the HT side of the power transformer at the correct voltage and current?
 

Dreadneck

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I'll change the diodes and see if that helps.

But if the diodes were backwards wouldn't all the voltages simply go to ground? Unless some of them were backwards and some right. I'll double check that again!

The reason I bypassed the PT with the Variac was so that I could apply different voltages across the rectifier circuit and minimize steps. I wanted to test the rectifier specifically.
The Variac handles 0-265VAC and 3.15A so it should be fine. At least that is what I thought. Since there was no fuses in the two 0.5A fuse sockets at the moment it couldn't send voltages back through the PT. In my mind there is only one way the voltages could go and that is through the rectifier.
What happened to the Variac though is beyond me. Something ate it alive.
 

Dreadneck

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I'll change the diodes and see if that helps.

But if the diodes were backwards wouldn't all the voltages simply go to ground? Unless some of them were backwards and some right. I'll double check that again!

The reason I bypassed the PT with the Variac was so that I could apply different voltages across the rectifier circuit and minimize steps. I wanted to test the rectifier specifically.
The Variac handles 0-265VAC and 3.15A so it should be fine. At least that is what I thought. Since there was no fuses in the two 0.5A fuse sockets at the moment it couldn't send voltages back through the PT. In my mind there is only one way the voltages could go and that is through the rectifier.
What happened to the Variac though is beyond me. Something ate it alive.
 

NTC

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But if the diodes were backwards wouldn't all the voltages simply go to ground? Unless some of them were backwards and some right.
If all of them were backward, your ground would be positive wrt the B+, so it probably isn't that - but it is worth checking.
 

Dreadneck

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They were all in the correct orientation and all of them checked out good. I've taken them out of circuit to test them as well. So I will put in new ones eighter way. Looking at the large filter caps, they all look good as well. Good connections and good ESR values. Anything else to look for here?

Could it be the OT that draws current? All the tubes are known good tubes borrowed from my main amp that works like clockwork.

FWIW I do not have the reverb tank hooked up to the amp now that it is on the bench, but the pedal is hooked up as well as the speaker.
 

Dreadneck

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Ok.
So.

New diodes installed in rectifier bridge. 0.5AT fuses blow

Installed standby switch (non existing in EU model). 0.5AT fuses blow...

I'm thinking about unhooking the OT primary and take it out of circuit. If it still blows fuses the OT is out of the question at least.
 

Dreadneck

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New update!

Progress!!

Turns out that the speaker jack was short to ground. I fixed it and BAM! fuses intact and sound+volume!

Now another weird issue shows up instead. Together with the normal clean guitar sound is a smaller distorted sound. Almost like there was a smaller speaker next to the big one that was distorting. This is new to me. The amp itself is not humming excesively or noisy. The distorted sound only shows up when playing.
It is not affected by the volume, tone or master pots. Not the bright switch eighter.
It doesn't matter if i run the amp in low or high power mode or if i use the clean or lead channel.

Any ideas?
 

Wally

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This is a complex amp on which to learn, imho. Cross your fingers that the OT Has not suffered damage during all of this. Yes, an OT can put sound out and be rendered useless due to certain damage.
 
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NTC

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Gently tighten the speaker and anything else that can vibrate. It can also be a tube issue - try substituting other tubes if you have them.
 

W.L.Weller

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Kind of a difficult and expensive OT to use the substitution method of troubleshooting on, but output transformers don't love dead shorts across the secondaries. (At least it wasn't open)

It looks like Hammond sells an OT that would work (https://www.hammfg.com/part/1650P ), more or less (4-8-16 secondaries instead of 2-4-8). Not sure if there are Swedish manufacturers of transformers that might save you a few bucks on shipping/customs.
 




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