Fender Hot Rod Delxue channel switching issue

[jazztastic]

Here is a link to the latest Service Manual:

http://badraven.com/Fender_Manuals/H...d%20Deluxe.pdf

Also, this amp has not sounded right since it was left on overnite early last year. Rechecked Test Points TP1-30 this morning and found some definite voltage discrepancies.

[52 Bill]

The yellow led should be off when in clean mode and turn on in distortion mode.

Markus is right, change IC U3.

[MarkusBass]

The schematic contains detailed information on how the footswitch works. I didn't say that U3 had failed. I only said that the voltages of its output should be measured and analysed. The problem may be caused by U3 IC, or by Q3 transistor, or the channel switch, or any Zener diode on the input, or even by the footswitch jack. I suggest that you measure (and provide here) the voltages starting from TP31 up to TP36 and compare them with the data provided on the schematic. Please note that if you have incorrect input voltages (TP31, TP32, TP34, TP35), then output voltages may be also incorrect (TP33, TP36). The U3 IC works as a simple comparator. If it gets incorrect input voltages, the output voltages are also incorrect. But this does not mean that U3 has failed.
Please also note that the footswitch works due to 39V AC provided by R97 (at the top of the attached picture). If there is anything wrong with the voltage, or the resistor, the switch will not work. So I suggest that you check that this voltage is present on R97 and R97 is not open.

Mark

[jazztastic]

R97 is good at 1.2K ohms and 39vac.

TP31
+1.03
+0.84
+0.84

TP32
+0.63
+2.62
+0.627
+0.627

TP33
+4.75
+2.13
+2.13

TP34
-12.5
-12.5
-12.5

TP35
-3.5
-6.65
-0.55
-9.44

TP36
-16
-16
-16

[MarkusBass]

Nice table. And what is the conclusion? Remember I wrote ?

the voltages should be measured and analysed.

And you just measured them. Where is the second part of the task: analysis?
I can help you only with the first measurement (TP31). This is the input voltage for the comparator (U3B). You have 1.03V instead of 1.37V on non-inverting input of the comparator. Where does it come from? It comes from voltage drop on CR20 and CR27 diodes. The correct voltage drop is 2x0.65V=1.3V. You have a little bit less but this may be due to incorrect voltage on the output of U3B.
On the inverting input (TP32) you have 0.63V which seems almost good.
So the question is the following: you have a comparator supplied with +/-16V. On the non-inverting input you have 1.03V and on the inverting input you have 0.63V. What voltage should be on the output of the comparator? In my opinion it should be +16V (correct me if I'm wrong). And you have only 4.75V. So either U3 has failed, or the output is heavily loaded. To check the load you may just desolder R84 the output resistor (and see if this changes the voltage on the output - T33). Depending on the result you may decide to replace U3, or continue with the investigation. Of course, there is always a chance that the incorrect voltage in TP33 is caused by problems with the LED diode, or by Q3 - you decide.
Similar analysis should be performed (as an exercise) for all other cases.

Mark

[jazztastic]

Thank you Mark! I'm learning a little at time. I will investigate U3 & L1 further.

Changing gears, I think I've found why the amp doesn't sound quite right.

TP27 & TP28 are reading 23.85vac instead of 66.5vac. Does this point to the 050438 Output transformer being bad?

TP24 & TP25 are low as well, 8.5vac & 7.24vac instead of 11.5vac (power to V3)

[Jazz P Bass]

The Vac TP's are all related.
The input condition must be met first of all.
1K, 4 mv sine wave input.

And follow Note #4.
-Rvb full CCW
-All other controls at 50%.
-Pull Bright Out.
-Drive (yellow LED) Channel selected.

When seeing low voltages at the end of the circuit (ie: output section), it is best to keep working back to find where the signal does not agree with the TP's.
Divide to Conquer.
Jump right to the beginning. 4 mv's.
If that is true, jump to the middle somewhere.
This procedure can help narrow down the fault area rather quickly.

[jazztastic]

Hi Jazz P Bass!

I put a 1k sine wave to input and measured TP1 at close to 4 mv. But, TP1 (or any other TP) doesn't really change when I vary the input signal level.

All conditions are met for Note #4 except Drive yellow LED is always on and doesn't truely switch to Drive mode.

I'm a bit baffled...

[Jazz P Bass]

I think that you may be having test equipment issues.

Anyway, if you have 4 mv's at TP1 that fact is irrelevant.
Move down the TP's & tell us the first one where your amp does not agree.

[jazztastic]

TP2 is already off measuring 2.46vac instead of 180mv.

Below is Test Point measurements 1 thru 30:

[MarkusBass]

I'm affraid that you have serious problems with performing the measurements. TP3 is OK - it means that the current through V1A is correct, as well as the power supply for the tube. With the voltages you specified for TP1 and TP2 the tube would have gain of 600 which is rather impossible. My guess is that you are providing 40 mV input signal instead of 4 mV. On digital display 4 mV may be displayed as 0.004. Do you have 0.004, or 0.04 (one zero less)?

Mark

[g1]

Some meters can't properly measure AC if there is DC present. What kind of meter do you have? Sometimes you need to put a cap in series with the probe (like .047 or .1 600V) to block the DC.

[jazztastic]

It's a fairly cheap Craftsman 82139 Auto Ranging DMM. You may just be right, i'll try the cap in series. I sometimes get voltage off of the ground probe being connected to the amp without the positive probe.

[jazztastic]

I had to take a step away from this for a while.

I primarily use the effects loop in this amp to attenuate with a volume box.

Question: If there is an issue with one of the IC's (U1,U2 or U3 - TL072) does this affect the actually sound quality of the amp as well as channel switching, reverb etc. ?