57 champ reissue output problems

[delray]

Bought a 57 champ reissue from a local store that was selling it as a customer return. It makes a little distored sound sometimes when a guitar is plugged in but sometimes no output.

I have done the following: checked 12ax7 and 6v6 and they test good. I have also swapped them for known good tubes with the same result.


Signal traced it to the output of the 6v6 and it is going through all stages as best as i can tell with my scope.


Replaced output transformer with a replacement (same result, put original back in)

Replaced volume pot, no result, put original back in.

Checked the outpit jack, verified solder joints, etc.

One odd thing is i am seeing -420 volts when the schematic says -340. I believe the schematic is at 100v input and my wall voltage is  125 or so.


Here is a brief YouTube video that shows the issue with a signal generator . 


Any thoughts?

i=2ojXfXfHE6Jo9HaQ

[/youtube]

[tubeswell]

Some suggestions


1) use your V-meter to check DC voltages on the High Tension Rail (including all filter cap nodes) - you should see a progressive drop at each filter cap node - if you don't, it may mean that the gain stages aren't decoupled (which would create unwanted signal feedback through the PS rail leading to distorted signal and reduced power). Look for a dead filter cap or a problem with DC connectivity in 2) below.


2) related to 1) above, each filter cap ground-lead should have DC continuity/zero resistance with the ground - and each positive filter cap lead should have DC continuity (zero resistance) with the relevant PS supply resistor node. If one (or more) of the filter caps leads has lost connection, it will cause the filter cap node to become uncoupled, leading to the sort of distortion you appear to be describing. (The cause of any loss of DC-connectivity could be a broken/cracked PCB trace cold solder joint, faulty jumper or ribbon cable - so use your R-meter to check for continuity if the HT supply voltages look dodgy)


3) check the coupling caps between gain stages aren't leaking DC - this would let DC onto the subsequent gain stage's grid - affecting bias, leading to reduced signal (but going by your description of distortion - not as high on the suspect list as 1) or 2)

[shooter]

that was selling it as a customer return.

this a PCB amp?
did you ask why the customer returned it?

[delray]

Some suggestions


1) use your V-meter to check DC voltages on the High Tension Rail (including all filter cap nodes) - you should see a progressive drop at each filter cap node - if you don't, it may mean that the gain stages aren't decoupled (which would create unwanted signal feedback through the PS rail leading to distorted signal and reduced power). Look for a dead filter cap or a problem with DC connectivity in 2) below.


2) related to 1) above, each filter cap ground-lead should have DC continuity/zero resistance with the ground - and each positive filter cap lead should have DC continuity (zero resistance) with the relevant PS supply resistor node. If one (or more) of the filter caps leads has lost connection, it will cause the filter cap node to become uncoupled, leading to the sort of distortion you appear to be describing. (The cause of any loss of DC-connectivity could be a broken/cracked PCB trace cold solder joint, faulty jumper or ribbon cable - so use your R-meter to check for continuity if the HT supply voltages look dodgy)


3) check the coupling caps between gain stages aren't leaking DC - this would let DC onto the subsequent gain stage's grid - affecting bias, leading to reduced signal (but going by your description of distortion - not as high on the suspect list as 1) or 2)

Thank you. I did check #1,2 and 3 and everything seems to be checking out ok. No DC present, caps to ground, voltage dropping over the stages, etc.

I haven't pulled the board out of it but I think that may be my next step to see if there is something on the back side of the board.

[delray]

that was selling it as a customer return.

this a PCB amp?
did you ask why the customer returned it?

This is the reissued 57 which is "hand wired". This was from my local guitar center and was listed as no sound output, tubes not lighting up. The tubes are fine and pilot works as expected.

[jukelemon]

Did you sub in a known good speaker?

[delray]

Did you sub in a known good speaker?

Yes using a known good external at this point.

[AlNewman]

I doubt the schematic would be rated at 100V wall voltage, but I may be wrong.  Share the schematic you're using.

Draw up a voltage chart and post it here.
Maybe 420 isn't high voltage when dealing with 25% over schematic, OR, maybe something isn't drawing current. 

[delray]

Here is the schematic I am using. I have it all apart at the moment but will try to update with my values vs schematic values.

[SEL49]

One odd thing is i am seeing -420 volts when the schematic says -340. I believe the schematic is at 100v input and my wall voltage is  125 or so.

That amp is available with three different PTs, depending on the desired line voltage. Look at the POWER TRANSFORMER AND LINE FUSE table on the schematic. Verify that your PT number matches the 120V PT number.

Also, the plate voltage for the 6V6 will be much higher than 340V if the tube is not plugged into the socket.
 

[tubeswell]

Thank you. I did check #1,2 and 3 and everything seems to be checking out ok. No DC present, caps to ground, voltage dropping over the stages, etc.

What voltages?

[mresistor]

Hopefully he will verify what the part # is on the power transformer. If it's the 100v then the amp came from Japan or S. Korea. 
It should be this one for US 120VAC   P/N   0078046000


If the pilot lamp is working then there is filament voltage to the tubes,, but what is the voltage at TP13 and 14 ?

[Willabe]

If the pilot lamp is working then there is filament voltage to the tubes, ....

Maybe, maybe not.

If the heater wires go to the pilot light 1st, of if the heater wire string were run ||, 1. to the light/2. to the tubes, either way the pilot light could work but the wires going to the tubes could be broken somewhere along the heater wire string.

[delray]

Hey thanks for the feedback everyone. Here are the measured voltages as of a few minutes ago. All tubes were plugged, output speaker plugged in, signal tracer plugged in and volume was at zero.

[delray]

Hopefully he will verify what the part # is on the power transformer. If it's the 100v then the amp came from Japan or S. Korea. 
It should be this one for US 120VAC   P/N   0078046000


If the pilot lamp is working then there is filament voltage to the tubes,, but what is the voltage at TP13 and 14 ?

Good call out. I did check this and the part number is correct.

[AlNewman]

There's some voltages missing.

Like the plates and cathodes of the tubes, etc.

The easiest way, now that we have the voltages at the dropping resistors, is to measure and record all voltages at all pins on the tubes.  AC at the heater pins, across pins 2 and 7 on the power tubes, and across pins 4 - 9 and 5 - 9 on the little tubes. And measure DC at all the other pins.

[AlNewman]

Also, with the power off, and amp discharged, measure resistance between pin 8 of the rectifier and pin 3 of the 6v6.

[delray]

Also, with the power off, and amp discharged, measure resistance between pin 8 of the rectifier and pin 3 of the 6v6.

All of these are measured based on socket numbers:

6V6 (sorry edited, made a mistake)
---
1 - 430
2 -
3 - 460
4 - 430
5 -
6 -
7 - 3.36
8 -146


5Y3
---
1 -
2 - 457
3 - 338
4 - 312
5 -
6 - 280
7 - 338
7 - 456

12AX7
---
1 - 251
2
3 - 1.8
4 -
5 -
6 - 237
7 -
8 - 1.8
9 - 3.3 ac


Measurement from pin 8 to pin 3 is ~260ohm

[Willabe]

The 5Y3, are the 1N4007 diodes mounted on the rectifier tube socket from pin 3 to pin 4 and from pin 7 to pin 6?

Pins 4 and 6 should not be 32dcv difference. 

[AlNewman]

You shouldn't be measuring 146V at pin 8 of your 6v6.

Check again, and if it's a good number, turn off the amp and check resistance of R10.

[delray]

Yeah i need to do some more measurements… got in a hurry earlier.

Is the 3.6v heater value acceptable?

[Willabe]

Yeah i need to do some more measurements… got in a hurry earlier.

Is the 3.6v heater value acceptable?

How did you measure it?

And you only have 1 heater # for each tube, should be 2 the way you measured.

For heaters, you measure for acv across pins 2 and 7 for the 6V6 and across pins 4/5 and 9 for the 12AX7. (Pins 4 and 5 are tied together.) Not hot prob to 1 pin and the other prob to ground.

[delray]

Can't attach these for some reason but here are my after work measurements. The voltage still seems a little high but I dont know if that's the issue.

[labb]

How do you get 58 vdc at pin 8 of the 6v6. That would be give over 50 watt for a tube rated for 14 watt.

[Willabe]

How do you get 58 vdc at pin 8 of the 6v6. That would be give over 50 watt for a tube rated for 14 watt.

Because he has 463dcv on the 6V6 plate. 

[Willabe]

That Fender schematic shows 250acv for each B+ lead. Your showing your getting 345 and 346acv.

We've seen guys get strange voltage readings from their meter because the batteries were low. Change the batteries just to make sure.

You need to take out all the tubes, and measure for acv at pin 2 and then at pin 8.

What do you get? 

[SEL49]

58V across a 470Ω/2W resistor means 7 watts dissipation. That resistor is going to burn crispy in short order.

Maybe your meter is lying to you.

[Willabe]

Maybe your meter is lying to you.

I told him to change the batteries in his meter.

We've seen that cause problems with false reading here many times.

[delray]

Maybe your meter is lying to you.

Will check again after work tomorrow with new batteries

I told him to change the batteries in his meter.

We've seen that cause problems with false reading here many times.

[delray]

I checked the voltages with all the tubes unplugged and with just the rectifier and it was still way too high.

Based on the comments about a crispy bypass resistor I checked it again and yes it was bad. 

So here is where we are at. I used a 5 watt 1k resistor on the center tap of the transformer to get the voltage down and that helped get it closer to 340. I also replaced the bypass resistor and hey it's back to a "working" amp. Using my external speaker it distorts at around 1/2 volume and you dont really get much more volume the more you increase it. That seems bad from a tone perspective but I need to check it with the factory speaker as well.

The resistor on the center tap seems like a hack and it gets too hot to touch. I could maybe play with the values and move it around but i dont feel really good about it. I think the root cause is my transformer is bad? I couldn't find the fender part number but there are several options for this amp.

Thoughts?

[SEL49]

So here is where we are at. I used a 5 watt 1k resistor on the center tap of the transformer to get the voltage down and that helped get it closer to 340.

Which transformer???

I also replaced the bypass resistor and hey it's back to a "working" amp.

Good. Now remove that 1K/5W hack and recheck voltages on pins 3, 4, and 8.

[Willabe]

Did you change the batteries in your meter?

Based on the comments about a crispy bypass resistor I checked it again and yes it was bad. ....

I also replaced the bypass resistor and hey it's back to a "working" amp.

.... I think the root cause is my transformer is bad?

What bypass resistor? What does it bypass?

Yes it seems the PT is either the wrong PT or it's gone bad.

[mresistor]

You have used two different power transformers, same part #   the odds of two bad pts is pretty slim but not unheard of. 


 

[uki]

I checked the voltages with all the tubes unplugged and with just the rectifier and it was still way too high.

Without tubes plugged voltages are usually way higher.

[delray]

Wanted to say thanks  to everyone that took time to read this thread and provide your input. I'm still a novice at this but learning a bit more with every repair attempt. Ultimately it looks like the cathode resistor was the fault. Everything is back to stock (removed power transformer resistor, original output transformer, etc.)

The final measurements are below and they seem pretty close to spec so I am going to play it as is for now.






So now on to the four silver tone amps that I have been delaying working on :)

[AlNewman]

That looks more better.
So did the factory install the wrong resistor, or did the resistor open up?

[delray]

That looks more better.
So did the factory install the wrong resistor, or did the resistor open up?

Looks like it opened up. I was not able to get a measurement on it.

[AlNewman]

There's still something not quite right with your voltages. 
Your plates are operating at 23mA, based on a transformer reading of 260 ohms, (which you measured earlier.)

Your cathode is dropping 21V over 470 ohms @ 370 minus 21V for cathode elevation, or 349 V.   So the tube is drawing 44mA.

That means either your screens are using 21mA, or you have a short somewhere, or you didn't get a proper reading of your transformer, or the voltages you're getting aren't correct.

Measure all the resistors around your power tube.

[Willabe]

Looks like it opened up. I was not able to get a measurement on it.

Wait a second, you gave us 2 different K R measurements before, now you say that K R was open. 

[SEL49]

Looks like it opened up. I was not able to get a measurement on it.

Wait a second, you gave us 2 different K R measurements before, now you say that K R was open. 

Look again. He never gave any cathode resistor measurement.

[Merlin]

He also claims to measure 335V on one side of the BYD33 diodes, and 242V on the other side. I don't know what Delray is doing, but his measurements are all over the place and cannot be trusted.

[Willabe]

Looks like it opened up. I was not able to get a measurement on it.

Wait a second, you gave us 2 different K R measurements before, now you say that K R was open. 

Look again. He never gave any cathode resistor measurement.

Yes he did.

Reply #17, 146dcv and reply #22, 58dcv. Fender schematic shows 18.2 dcv and he now says he has 20.9 dcv in reply #34.

[SEL49]

I thought you meant resistance (ohm) measurements.

[Willabe]

I didn't care what the value of the K R was. I was going by that if there's a dcv at the 6V6 K pin 8, then that tube is drawing current. And that would bring the B+ dcv down to where it should be with that PT. So it seemed something was wrong with the PT. 

We were thrown off track of what was wrong because he posted a dcv for the 6V6 K.      

Turns out it was a blown K R, so no current draw on 6V6, so PT B+ acv was way high, and so it follows that the B+ filter cap string was high too.