Hot Rod DeLuxe I switching problem

[fuzzman72]

Hello.

I'm troubleshooting the switching circuit of a Fender Hot Rod Deluxe.
The problem is that after a while it's not possible to switch from drive to more drive, the led stay yellow and doesn't turn red.
I've checked the test point as suggested on the schematic by Fender; here what I've found.


TP31

Normal 1,4V

Drive 1,13V

More Drive 1,11V


TP32

Channel Select Out 0,52V

Channel Select In 9,89V

TP33

Normal 16,6V

Drive -10,53

More Drive -13V

TP34

Normal -11,8v

Drive -11,75v (Slowly Increase)

More Drive -4,5v

TP35

More Drive Out -0,535v

More Drive In -9,96v

TP36

Normal -16,18v

Drive -16,18v

More Drive -16,18v


I've replaced all the trannies, mosfets and zeners.
I got a steady +16/-16 from the power supply.
Now I'm running out of ideas.

Greetings From Italy.

[52 Bill]

The only voltage that I see that are wrong are the Drive voltages at TP33 and TP34.

Have you tested the LED indicator and made certain that it has not come loose from the board?

[Enzo]

Agree. That dual LED is not just a pretty indicator, it is part of the circuit.

[nap_alfa55]

Fender FM212r no channel switching

[fuzzman72]

Thank you for your replies.

The Led tested good outside the board on a Atlas Peak DCA75.
Once resoldered, all the connections tested good as the components, but the problem still remains.

When you switch on the amp already in drive mode you see the right voltage at TP33 and TP34, but after half a minute or so the voltage at TP33 begin to go more positive while at TP33 more negative.

[52 Bill]

I've rechecked the first post and am wondering about the voltages at TP36. Does the voltage stay at -16.18 volts in all modes?

[Jazz P Bass]

Quote:"TP33 begin to go more positive while at TP33 more negative."

Can you clarify the TP's?

I am thinking that Q4 is leaky.

[fuzzman72]

I've made a mistake...

At TP33 the voltage go more positive, while at TP34the voltage go more negative.

At TP36 I have -16 volts in all positions.

[52 Bill]

At TP36 I have -16 volts in all positions.

This voltage should change from negative to positive in the more drive mode. So recheck your reading.

If it does stay negative in all modes, the last thing to change is the dual opamp U3.

[fuzzman72]

...another mistake.

The actual voltages at TP 36 are:

Normal Mode - 16

Drive - 16

More Drive + 16

So they looks correct according to the schematic

I've already changed U3 with an MC4558 IC but the problem still remains

[52 Bill]

...another mistake.

Sorry, but I'm lost here. Let's start from the beginning.

The amp when cold will work and all three modes are available. When the amp gets hot the more drive (red) stops working. You have replaced a bunch of parts somewhere in the amp, but the problem remains.

Have you inspected and checked the low voltage power supply circuit? Mainly R78 and R79 and Zener diodes CR13 and CR14. They have been known to cause problems when they warm up.

What transistors and diodes and chips have you replaced already? Part numbers would be helpful.

[fuzzman72]

Eheh...I'm lost too!
I'll try to make it more clear.

Monitoring TP33 from the turn up in “Drive Mode”


Drive -13,5 after about one minute and a half the voltage suddenly rise to -12V and goes up
till -11,27V when the “More Drive “ stop working; the voltage rise up till -10,85V

Normal +16V


Monitoring TP34 from the turn up in “Drive Mode”

Drive -4,98V after 3 seconds begin to go down till -11,76V when the more drive stop working

Normal -11,6v


Monitoring TP36 from the turn up in “Drive Mode”


Drive -16,23V

Normal -16,18V

More Drive + 16V after one minute and a half the more drive stop working and there's no more
change from -16v to +16v

[52 Bill]

The problem voltages are at TP33 and TP34. The voltage at TP34 changes first and then TP33 changes. You have changed U3, Q4, CR17 and CR22, and maybe other parts as well.

You will need to start monitoring the voltages at TP31 and TP32 at start up, when the circuit is working and when it fails. What you are looking for is a reason for the voltage at TP33 to change.

You also need to start monitoring the voltages at TP35 and TP36 at start up, when the circuit is working and when it fails. What you are looking for is a reason for the voltage at TP34 to change.

If there is no change at the input of the opamp that correlates to the change in its output, then you can assume that the problem is in another section of the circuit. These circuits are basic comparators, so the output should be changing in response to a change in the input.

You are not using a footswitch when testing this amp, correct? Have you cleaned the contacts of the footswitch jack and checked for cold solder joints as well? And you are certain that both 16 volt power supplies are steady and stable?