Blues Deluxe Reissue PCB Conformal Coating Resistance?

[pcb777]

I have a 2004 edition of the Fender Blues Deluxe Reissue. After removing all of the tubes in order to do some touch-up soldering to try and fix an unrelated problem I powered it up and checked the voltages on V3 (phase inverter) and surrounding area. To my surprise I measured 6.5V across R23 (9k) which is connected to V3 pin 1 and nothing else, indicating about 0.7mA flowing through it! A check of R57 and R58 indicated the same thing. After visually inspecting the PCB board for solder bridges and components/traces not on the schematic, I decided to remove the entire tube socket, cleaned the board and rechecked. Still ~ 1mA through R23. leakage through coupling caps C26 and C27 could explain the drop across R57 and R58 but not R23. So my question is, can conformal coating, meant to insulate and protect, present a resistance? Voltage was +455DC on R57 (400V with tubes in place), +383V on pins 1 and 6 of V3.

[MWJB]

The 9.1K resistor at V3 pin 1 also connects to a 91K (R51) resistor that connects to the B+ rail, it's a split load plate resistor set up. The coupling cap comes off the junction of the 9.1K 7 the 91K.

Your R no's are confusing me a little, unless there's a new schem the 9.1K @ V3 pin 1 is R50 on mine, R57 & R58 are 470ohm sreen grid resistors?

[pcb777]

Schematic for 2004+ different than for 1993+ versions

The 9.1K resistor at V3 pin 1 also connects to a 91K (R51) resistor that connects to the B+ rail, it's a split load plate resistor set up. The coupling cap comes off the junction of the 9.1K 7 the 91K.

Your R no's are confusing me a little, unless there's a new schem the 9.1K @ V3 pin 1 is R50 on mine, R57 & R58 are 470ohm sreen grid resistors?

You are looking at the 1993 version. Here is the schematic for the newer models (go to end of pdf to view):
http://www.blueguitar.org/new/schem/...le_reissue.pdf
Besides adding trim pot R82 for biasing the power/output tubes, there were some minor changes to fix problems with the original.

I know it is a split resistor setup. Which is why current through R23 is so puzzling.

[MWJB]

What are the loaded voltages at V3 pins 1 & 6, when the B+ supply reads 400vdc?

[pcb777]

I have removed V3- tube and socket. One end of R23 is connected to R57. The other end of R23 is not connected to anything (just the PC board). I even removed C26. Why is current flowing through R23?

[MWJB]

What's the impedance of your voltmeter?

Do you measure any resistance to ground from V3 pin 1 or 6? Whatever is happening at R23/R57 is also happening at R58.

[Jazz P Bass]

What is the issue wth the amplifier.
And what is your real concern with the PI plate voltages.
A few ma,s current could very well be the flow through your meter.
To be more precise, the coating is called a mask.
Conformal coatings do indeed protect a circuit board in harsh enviroments.
Like a mig welder.
That is not the case here.
It is mask.
It helps to keep the solder flowing only to the bare copper pads.

[pcb777]

Of course- the DVM! I disconnected one end of R57 and there was still a drop across it! On further check the specs show 560kohms input resistance which correlates with the current and voltage. Georg Ohm is probably rolling over in his grave.
MWJB, Jazz P Bass thanks for your help.
Regarding the real problem I am having I'll post that separately if I need help.

[Enzo]

They beat me to it - the meter.

I have to ask, when you say there is a voltage across the resistor, did you actually measure from one end of the resistor to the other, or did you take a reading to ground from each end and then subtract?

Same with the mysterious R57. With the one end disconnected, was there really voltage across it, or just a voltage drop calculated by measuring voltage to ground from each end?


We tend to think of our meters as invisible to the circuit, but they do have impedance and will interact with the circuit, plus when used referenced to ground, they will draw some small currents.

One place this really shows up is trying to neasure the grid voltage around phase inverters. A typical circuit has a 1 meg resistor from the bottom of the cathode resistor to each grid. If you measure say 90 volts at the cathode, you might then measure 60 volts at each grid. (I am making the numbers up), leading you to think there was 30 volts across the 1 meg resistors. But there really is not 60v on those grids, your meter is onscuring the readings. By going back and measuring from one end of the 1 meg to the other end, we find almost no voltage drop, and the real grid voltage is whatever the drop across the cathode resistor was subtracted from the 90v.