Fender HRD III - changing to 240v - confirmation needed

Philtre

After @ICBM's helpful advice on changing my Fender 65 DRRI from 230v to 240v I'm going to do the same for my HRD III.

The schematic shows that P4 needs to be swapped with P10 on the board. At the moment P4 is white with a black stripe and P10 is black.

Just need to double check with someone who's done this!

ICBM

Yes, thats correct.

Philtre

@ICBM cheers, I've done it now and it's all working fine.

It's much easier to access the board on the HRD than the DRRI. The hardest part (if you can call it hard) is snipping the cable tie(s) to re-route the cables and popping another cable tie on.

I'm feeling rather pleased with myself for doing this, knowing that it will be easier on the amps overall and, in the case of the HRD, makes it slightly quieter for home use.

Philtre

Would it be expected that the bias reading at TP30 is now lower? It's at 45mV now. Normally at 60.

ICBM

Philtre said:

Would it be expected that the bias reading at TP30 is now lower? It's at 45mV now. Normally at 60.

Lower, but not normally by that much - it was probably a bit low before.

It might even be worse than that if you're metering from the test point to the chassis - there's enough resistance between the ground end of the resistor and the chassis to spuriously increase the reading slightly. Either meter across the resistor itself directly, or meter from the chassis to the other end of it (directly behind it, the resistor is parallel to the end of the PCB) first and then subtract that number - it's easier to do that rather than try to get the two probes onto it at once.

Philtre

@ICBM Measuring across the 1ohm resistor at TP30 I get 53mV versus 60mV when the negative probe is attached to the chassis.

ICBM

Philtre said:

@ICBM Measuring across the 1ohm resistor at TP30 I get 53mV versus 60mV when the negative probe is attached to the chassis.

It's normally about that. The error is always in the safe/conservative direction so it doesn't really matter, but if you prefer you bias it to 60mV across the resistor or 67mV to the chassis.

Philtre

That's a really good point about connecting the ground side of the resistor rather than the chassis. The whole point of measuring a voltage drop is to have a 1 ohm resistor, and if it's a bit more than 1 ohm then the reading won't be accurate.

I've gone for 57.5mv across the resistor which is about 65mv to the chassis (aiming for longevity of valves rather than a hot tone)

The measurement from the ground end of the resistor to chassis is about 7mv.

Thanks once again for your help, @ICBM !