fender hot rod deluxe low volume & distorted output

[Jazz P Bass]

It sounds like the output tubes are not conducting.(at least not fully)
What is Test Point 30? (see the attached pdf for the location)
And how about a Vdc reading on each output tube pin #5 (grid)
Are the screen resistors good? (R61 & 62 _470 ohm/ 1 watt)
Are they getting voltage?

[52 Bill]

It sounds like the output tubes are not conducting.(at least not fully)
What is Test Point 30? (see the attached pdf for the location)
And how about a Vdc reading on each output tube pin #5 (grid)
Are the screen resistors good? (R61 & 62 _470 ohm/ 1 watt)
Are they getting voltage?

Earlier on he found that one of the screen resistors had been replaced with a 470K. It was replaced and both were checked.

[Djeans]

It sounds like the output tubes are not conducting.(at least not fully)
What is Test Point 30? (see the attached pdf for the location)
And how about a Vdc reading on each output tube pin #5 (grid)
Are the screen resistors good? (R61 & 62 _470 ohm/ 1 watt)
Are they getting voltage?

Pin 5 read 51.2Vdc on both power tubes

Both resistors good (1reads 471, the other 468)

Both are getting 480Vdc

TP 30 reading 60.3mVdc

I should probably also mention that whoever had this amp before that replaced R61 with the wrong value resistor, also buggered the Connection on the board. When I removed it. The trace was bad. I soldered a jumper wire from that side of the resistor to the ribbon (pw4b) that the trace leads to. The connection seems good, but I figured I should mention it.

[J M Fahey]

Hi djeans.
You are doing *very* well, it'll be easier to help you this way.
Some considerations:
1) is yours a Hot Rod Deville or a HR Deluxe?
The schematics are basically the same, but the TPvalues stated in both differ somewhat, which can be confusing for you.
So far I just woke up, and compared only as far as TP7, go figure.
And so far, *parts*,values match in both. Voltages show slight differences.
Anyway I would be happier by following the "matching" schematic.
2) Let me tell you something, because maybe you think you are receiving conflicting suggestions, from one side saying varying values are of no importance, from other side the exact opposite.
Fact is, tubes vary *a lot* , so plugging another tube in the exact same socket will furnish different results.
Then, how much deviation is normal and which one is troublesome?
I'd say (as a general rule but not an *exact* LAW) that DC voltage variations of +/- 20 or 25% are normal, and *gain* variations up to +50%/-30% too.
That's referring to a single stage; if stacking many, errors do not add up but multiply.
So if you have a tube which has 50% more gain in both halves (reasonable because they come from the same batch and were presumably made by the same operator) and they are cascaded, the first one will receive normal signal, will provide 50% more than expected (still within what's reasonable) but the second one will supply 1.5x1.5=whopping 225% higher signal level than expected from the TP chart.
Yet it's not out of spec!!
It's still within 50% ... of what it received !!!
That's what Enzo means by not following *rigidly* what the TP chart states.
Or in a nutshell: if fed higher voltage, any given stage *will* provide higher output.
That's why he says to try to guess "normalcy" stage by stage.
What we are looking for here is for a stage which provides *catastrophically* less gain or output or which shows *very* weird DCvoltages.
I only checked up to TP7 so far, because I thought it would be important to give you an insight of "the Troubleshooter's approach" rather than a dry list of voltages.
Back to sleep, later will continue.
PS: and don't worry, you are not receiving conflicting information from "the experts", (whatever that means ) , simply most of us are "thinking aloud" and our mumblings may *sound* garbled ... be certain they are not.
Back to sleep, I want to get to the TPs where there was *wild* data ... but let's go step by step.
And be *certain* you'll repair your amp.

[Djeans]

I really appreciate the help. You guys are awesome.

The amp is a 2002 Hot Rod Deville 4x10. I need to get some sleep myself, just got off work, 3:00 am here

[Jazz P Bass]

O/k, the static Vdc looks good for the output tubes.
Do you have a nice Vac AT the grids. (the test chart stops before the coupling caps).
And you have not supplied how much current is going through the tubes. (test point 30)
Pin 8 of the tubes should be 1 ohm from ground.
By measuring the Vdc at pin 8, to ground, you will get a millivolt reading which converts to milliamps by the one ohm resistor.

[Djeans]

O/k, the static Vdc looks good for the output tubes.
Do you have a nice Vac AT the grids. (the test chart stops before the coupling caps).
And you have not supplied how much current is going through the tubes. (test point 30)
Pin 8 of the tubes should be 1 ohm from ground.
By measuring the Vdc at pin 8, to ground, you will get a millivolt reading which converts to milliamps by the one ohm resistor.

For AC at the grids, just to make sure, pin 4 is screen grid, 5 is control grid, and 8 is cathode suppressor grid, correct?

Pin 4: it is erratic- high as .112V and low as .030V constantly changing
Pin5: also erratic - high 2.0V low .865V
Pin8: .001V

Pin 8 to ground dc
Pin 8 reads 58 mV and 59 mVdc


Test point 30 is reading 60.1 millivolts - this is the bias test point, correct?

[Jazz P Bass]

Yup.

[Djeans]

Updated Test Point data

Didn't see the explanation of TP 31 and up initially (I stopped reading after it said footswitch operation because I figured it was all info related to the footswitch - I'm a rookie) so now, I have tested all the test point values.

Here is the spread sheet I have them all listed out on in case it helps clarify anything.

Fender Deville Test Points.xls

Someone mentioned tracing the signal through the amp. I will need an Oscilloscope for that, yes? Or is there another way?

[Enzo]

Yes, a "signal tracer."

[Djeans]

Yes, a "signal tracer."

in my google search for signal tracer, I came across another forum with a post from you on a similar subject I will try to build a simple one and use one of my other amps for that. I may have some rookie questions for you on how to connect the cap and resistor tho.

[Djeans]

Googled for signal tracer and found this

Amp Tools

About three quarters down the page, "listening amplifier"

Would the resistor go in place of the pot shown here? After he cap, right?

[pontiacpete]

here's one:

Signal Tracer parade 2

[Djeans]

so, if I understand this correctly, to build the probe, I connect the tip of a guitar cable to a metal probe with a cap and a resistor in between them with the other end plugged into another amp. Which comes first in line, the cap or the resistor, or does it matter? Probe to cap, cap to resistor, resistor to tip of guitar cable? What does the sleeve of the guitar cable connect to, if anything?

Thanks again for answering my noobie questions.

[J M Fahey]

Build the Hoffman as is.
And then plug the free plug on the right to any small amp you have available.
Start from zero and rise volume slowly to desired level.
On the left you have a shielded wire which ends in an old multimeter probe, so you can touch the TP you are interested in, and a wire with a crocodile clip which gets clipped to amp grpund or chassis at a convenient point.