William Sommerwerck wrote:
wrote in message
...
William Sommerwerck wrote:

I'm guessing that the 7.5V specs are for the lowest voltage
the designers felt gave acceptable performance.

I feel that Grundig assumed the service person would connect
their bench supply between rail ground and "battery +",
not between "battery -" and "battery +".

Unfortunately, they call "power rail voltage" and "battery voltage"
[the same thing. They are different by 2 V in MW mode! ]

aren't the same thing. They are different by 2V in the MW model.

Thanks for fixing that William, but that's not what I
said or meant.

The MW and FM models are the same radio model.

When testing in Medium Wave *mode* the radio
requires 7.5 V between system ground and the power rail and is
specified to draw ca 20 mA. In FM *mode*, the radio is specified
to draw ca 22 mA from that same 7.5 V rail.

This is *not* the same as a 'battery voltage' of 7.5 V.
Our friends at Grundig have the same phrase to describe
'rail voltage' and 'battery voltage', which are normally
ca ~2 V different on the 'return' side.

I now understand that it makes perfect sense to attach our service
supply between 'system ground' and 'power rail' so that the
'system ground' can be bonded to earth ground via the power
supply, for safety. This is not the same as attaching the
supply across the battery, however.

While we're at it... It's perfectly normal for amplifier stages -- tube or
transistor, in any kind of device -- to be fed through a small resistor,
with a largish capacitor to ground.

Yup. In this case, they are decoupling the final audio
stage from the rest of the radio.

This "decouples" the stage from the
power supply, to prevent feedback. The resistor /is not/ present to reduce
the power-supply voltage.

Yet it does, by about 2 V from the battery to power rail.

--Winston