fender hot rod deluxe low volume & distorted output

[Enzo]

The cap is in series, and blocks any DC from going into your listening amp. A resistor to ground goes on the listening amp end of the cap. Your linked example adds a pot for level control. It also serves the purpose of resistor to ground, since one of its legs is grounded. I just use a 1 meg resistor, but having some level control there is a good idea.

The sleeve connects to the ground of the unit under test. Your example link does the same thing with a ground clip to the chassis of the listening amp. Somehow, the listening amp ground must get connected to the ground of the unit under test.

[Enzo]

The cap is in series, and blocks any DC from going into your listening amp. A resistor to ground goes on the listening amp end of the cap. Your linked example adds a pot for level control. It also serves the purpose of resistor to ground, since one of its legs is grounded. I just use a 1 meg resistor, but having some level control there is a good idea.

The sleeve connects to the ground of the unit under test. Your example link does the same thing with a ground clip to the chassis of the listening amp. Somehow, the listening amp ground must get connected to the ground of the unit under test.

[Djeans]

Ok,

Some useful results, I hope.

I created the quick and dirty signal tracer and started testing at the test points.

Test point 1 - all good.
Test point 2 - distorted audio
Test point 3 - No audio at all (the cap that had a cut lead that I repaired)
Test point 4 - all good again
Test point 5 - distorted audio
Test point 6 - No audio at all
Test point 7 - all good
Test point 8 - distorted audio

I didn't keep going, because it seems like the problem is already happening at this point.

What do you guys make of this?

[Rick Erickson]

Regarding the 82K PI Plate resistor: Has anyone come to a conclusion why this resistor fails in these amps? I don't see this happen in any other Fender, at least not like in the HR series. I do not have a good explanation for this. Perhaps a bad run of 82K 1/2W resistors?

[Djeans]

Still trying . . .

Still stuck, but still trying . . .

I found a newer schematic for this amp that more closely matches my amp. (some of the resistors were horizontal on the schematic, but vertical on the board etc with the old schematic). The new schematic also has more test points.

I decided to re test all of the test points, and got some different readings this time.

The first test points out of range are 4 and 7 - interestingly, as soon as my probe touches those test points, there is a loud buzz in the speaker similar to what it sounds like when you touch the tip of a guitar cable plugged into an amp. - Not sure if that means anything, but just throwing it out there in case it does.

I know you guys must be getting tired of me, but I'm still hoping to get this amp working.

Thanks again for all the help.

Here is the newer schematic and my latest test point readings.

djeans

[Enzo]

yiyiyi

I am not sure what to do now. Your first post said you had a "DeVille" which could mean Hot Rod DeVille or Blues DeVille, and now you have a schematic for a Hot Rod DeVilleIII. You mentioned it was a 2002 model. This most recent schematic shows 2009 boards. What year is printed on your BOARDS? Does the board in this new schematic match your board exactly?

Blues DeVille reissue looks to be from 2005. SO I will assume it is indeed a Hot Rod. There are two versions of boards in the attached files. Look at the last one, 2001 board. Does that look like yours?

[Djeans]

Hmm,

Very strange.

Mine is a Hot Rod Deville, at least that's what the plate on the back says - 2002 stamped on the board. But, my board does not match that schematic exactly. For instance, here is a shot of the footswitch area - very different from that schematic, but does seem to match the 2009 schematic for the DevilleIII.

Weird.

[Enzo]

Geez. Well, if the footswitch part is different, but we are not working on the footswitch circuit, I guess we can live with it.


When faced with multiple iterations of a circuit, and you might find more than one drawing number and within each numkber several revision levels, like A, B, C, etc., I'd call Fender and ask them about it, "Hey, just what schematic matches this board. Give them your serial number and the board dates.

[Jazz P Bass]

The numbers on the output transformer test points look bad.
Fender wants to see a 12:1 ratio & you are measuring a 32:1 ratio.
I am not saying that it is a bad transformer.
But it sure appears likely.

[Djeans]

The numbers on the output transformer test points look bad.
Fender wants to see a 12:1 ratio & you are measuring a 32:1 ratio.
I am not saying that it is a bad transformer.
But it sure appears likely.

Nice catch.

Is there an easy way to test the transformer without pulling it out of the amp? Can I just test the resistance across the blue and brown wires on one side and the green and black on the other side? What level of resistance is normal?

[Jazz P Bass]

If you send a signal into the preamp & measure the signal directly at the output tube grids, it should be a fairly hefty Vac signal.
If this is verified true & the output tubes are known to be good, if you still have a weak signal at the output of the transformer, I would say it is bad.
The dc resistance from red to brown/ blue will only indicate a gross error inside the transformer primary.
You can 'send' a signal into the secondary of the OPT & measure the signal coming out of the primary.
Red to brown & Red to blue should be the same.
This will also indicate the transfer function of the transformer.
Be aware this is not a full blown high voltage test.
For that refer to RG's Geofex web site.