On Mar 12, 11:59*am, klem kedidelhopper
wrote:
On Mar 12, 10:29*am, Winston wrote:









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

I was using my RCA Senior Voltohmyst in the beginning. It has either a
10 or 11M impedance, however I built it from a kit I think in 1964. So
being almost 50 years old it's calibration could have been suspect. I
then switched to my Fluke digital. In any case I found that the
readings on both were very close anyway. (I don't use the VTVM much
anymore unless I get an old tube set in here to work on).

My initial reading on R24 before I made any adjustments, with 7.50V
applied to the battery terminals was 1.00V. *That voltage rose to
3.60V when C38 would fail. Remember, (after replacing C38) that I
noted an approximate *.015 V rise in VR24 when I increased the supply
from 7.50V to 9.0V. Assuming the engineers meant for this voltage to
be set with 9.0V applied, then perhaps I was reading what would be a
perfect bias point (with 7.50V applied), as measured on VR24
initially. It would be nice to know for certain after such an
exercise, but at this point I doubt very much that even someone at
Grundig would be able to shed any further light on this. But then what
you're saying makes perfect sense too Winston. I'm just really
surprised at the ambiguity of the statement on the schematic. In any
event there seems to be an amount of latitude available here, and
given (and I'm assuming) that the radio appears to have been working
that way for many years, perhaps it's a waste of time to fret over .
015V. Maybe I'll set it somewhere in between, enjoy my radio once
again and step back into reality....That would make my wife, who has
failed to see the sense in fixing a 40 year old radio very happy
indeed. *Lenny


Well I couldn't resist, so to take this a bit further I checked the
power supply supplied with the radio. It is rated at 9.00V .200A. The
no load voltage is 9.50V. With it connected to the radio and with the
radio playing softly its output drops to 9.25V. In looking at the
schematic again and rereading the statement a few more times I now do
believe that in spite of the ambiguity, the original intention was to
supply 7.50 V to the battery terminals, however reference all
adjustments and voltages to the positive side of C59, (system ground).
So I did that and set the bias on FM to -1.18V. (AF126 I E) I then
recorded the following voltages:

Voltage readings
Test point Radio working:
------------------- --------------------

E. AF178 -1.15V
B. AF178 -1.48V
E. AF124 -1.14V
B. AF124 -1.37V
E. AF121 -0.90V
B. AF121 -1.24V
E. AF126 (1) -1.18V
B. AF126 (1) -1.47V
E. AF126 (II) -1.14V
B. AF126 (II) -1.34V

The voltage across C59 is 6.65V
So now although the radio has been working fine on both AM and FM, I
notice that the first two stages are reading a bit high. I wonder if I
may have another leaky cap. I was looking at possibly C12, C19 and
C32. Assuming I haven't driven everyone nuts with this project yet I
wonder what is the general consensus with this new revelation is?
Lenny