Further investigation of Eton(Grundig)S-350 drift problem.
 

Folks, I dismantled this silver puppy. And here is what we found! The
RF board dated 2004.4.15 . TWO PVC tuning capacitors. PVC1 has 4
built-in trimmers,PVC2 got none . PVC2 appears to contain more plates.
IC1 is TA7358AP. Circuitry is indeed purely LC based analogue- there is
not a single crystal in the oscillator/mixer area . So, if the variable
capacitors are using pvc as an insulator I can imagine that would
partially explain such wild frequency drift due to temperature changes
How stable is TA7358AP?

Arthur Pozner wrote:

Folks, I dismantled this silver puppy. And here is what we found! The
RF board dated 2004.4.15 . TWO PVC tuning capacitors. PVC1 has 4
built-in trimmers,PVC2 got none . PVC2 appears to contain more plates.
IC1 is TA7358AP. Circuitry is indeed purely LC based analogue- there is
not a single crystal in the oscillator/mixer area . So, if the variable
capacitors are using pvc as an insulator I can imagine that would
partially explain such wild frequency drift due to temperature changes
How stable is TA7358AP?

That's why the Panasonic RF-2800 & RF-2900 had a drift problem. The
S-350 is apparently modeled after those two receivers. They used
miniature tuning capacitors like the kind found in small analog
transistor radios. They have thin plastic insulators, probably
polyethylene. The drift can be minimized by adjusting (relaxing) the
compression on the stack of plates and insulators but I don't advise
doing this if you've never had one of these variable cap's apart. There
are many small parts.


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starman wrote:


That's why the Panasonic RF-2800 & RF-2900 had a drift problem. The
S-350 is apparently modeled after those two receivers. They used
miniature tuning capacitors like the kind found in small analog
transistor radios. They have thin plastic insulators, probably
polyethylene. The drift can be minimized by adjusting (relaxing) the
compression on the stack of plates and insulators but I don't advise
doing this if you've never had one of these variable cap's apart. There
are many small parts.

The tiny Grundig G1000a has a bad drift problem too. The wide bandwidth
on MW is pretty bad, with a strong AM station being audible over a 20
khz spread. Weak stations tend not to spread themselves out so far.

I'm glad I bought this thing cheaply (used). It was worth the fifteen
dollars. Even at that it was a gamble, as it only worked on the wall
wart and not batteries. The coiled spring end of the battery holder
needed to be soldered back on the printed circuit board.

It's not a serious radio by any means, but it can be fun..



mike

Found specs on TA 7358AP; rated from -25 to +75*C . Obviously it is
not the problem. The PVC1 and PVC2 on the other hand are most likely to
cause the terrible drift,especially now, in the hot summer days. Their
internal structure contains polystyrene dielectric,as the tmperature
rises plastics expand as far as I know. This will inrease the
capacitance very minutely, but apparently enough to be seen on the
digital display . One way to tackle this is to add a component with
opposite thermal rating- capacitor or inductor in the oscillator/mixer
circuit.

"Arthur Pozner" wrote in message
...
Found specs on TA 7358AP; rated from -25 to +75*C . Obviously it is
not the problem. The PVC1 and PVC2 on the other hand are most likely to
cause the terrible drift,especially now, in the hot summer days. Their
internal structure contains polystyrene dielectric,as the tmperature
rises plastics expand as far as I know. This will inrease the
capacitance very minutely, but apparently enough to be seen on the
digital display . One way to tackle this is to add a component with
opposite thermal rating- capacitor or inductor in the oscillator/mixer
circuit.


I know I'm picking nits on this one, but I'll guess the capacitor's
dielectric is polyethylene, not polystyrene. Polystyrene is actually one of
the best plastics as far as temperature coefficient goes. Very good
quality, low temp coefficient, low loss fixed capacitors are made with
polystyrene. Unfortunately, polystyrene is also somewhat brittle and
scratches easily. The plastic in variable capacitors is polyethylene, as
far as I know. Polyethylene is soft and sort of waxy, and forms a decent
bearing material for the capacitor's plates. It also has a higher
coefficient of expansion than polystyrene.

The expansion will reduce the capacitance, rather than raise it. This will
cause the frequency to drift up. It's possible the radio already has
temperature compensation, and the compensation is over aggressive. If a
radio's IF is above the radio's oscillator frequency, and upward drift in
oscillator frequency will cause a downward drift in received frequency. I
have the impression the s-350 is single conversion, 455 kc or so, however.

Inductors show the same effect. Expansion lowers the inductance.

Frank Dresser

"Frank Dresser" wrote in message ...
"Arthur Pozner" wrote in message
...
Found specs on TA 7358AP; rated from -25 to +75*C . Obviously it is
not the problem. The PVC1 and PVC2 on the other hand are most likely to
cause the terrible drift,especially now, in the hot summer days. Their
internal structure contains polystyrene dielectric,as the tmperature
rises plastics expand as far as I know. This will inrease the
capacitance very minutely, but apparently enough to be seen on the
digital display . One way to tackle this is to add a component with
opposite thermal rating- capacitor or inductor in the oscillator/mixer
circuit.


Frank, don't take this the wrong way, but there is something sexy
about concision. You're talking dirty to even the dumbest electronics
layman such as myself.

Regards,

Grumpus


I know I'm picking nits on this one, but I'll guess the capacitor's
dielectric is polyethylene, not polystyrene. Polystyrene is actually one of
the best plastics as far as temperature coefficient goes. Very good
quality, low temp coefficient, low loss fixed capacitors are made with
polystyrene. Unfortunately, polystyrene is also somewhat brittle and
scratches easily. The plastic in variable capacitors is polyethylene, as
far as I know. Polyethylene is soft and sort of waxy, and forms a decent
bearing material for the capacitor's plates. It also has a higher
coefficient of expansion than polystyrene.

The expansion will reduce the capacitance, rather than raise it. This will
cause the frequency to drift up. It's possible the radio already has
temperature compensation, and the compensation is over aggressive. If a
radio's IF is above the radio's oscillator frequency, and upward drift in
oscillator frequency will cause a downward drift in received frequency. I
have the impression the s-350 is single conversion, 455 kc or so, however.

Inductors show the same effect. Expansion lowers the inductance.

Frank Dresser

Frank Dresser wrote:

"Arthur Pozner" wrote in message
...
Found specs on TA 7358AP; rated from -25 to +75*C . Obviously it is
not the problem. The PVC1 and PVC2 on the other hand are most likely to
cause the terrible drift,especially now, in the hot summer days. Their
internal structure contains polystyrene dielectric,as the tmperature
rises plastics expand as far as I know. This will inrease the
capacitance very minutely, but apparently enough to be seen on the
digital display . One way to tackle this is to add a component with
opposite thermal rating- capacitor or inductor in the oscillator/mixer
circuit.


I know I'm picking nits on this one, but I'll guess the capacitor's
dielectric is polyethylene, not polystyrene. Polystyrene is actually one of
the best plastics as far as temperature coefficient goes. Very good
quality, low temp coefficient, low loss fixed capacitors are made with
polystyrene. Unfortunately, polystyrene is also somewhat brittle and
scratches easily. The plastic in variable capacitors is polyethylene, as
far as I know. Polyethylene is soft and sort of waxy, and forms a decent
bearing material for the capacitor's plates. It also has a higher
coefficient of expansion than polystyrene.

The expansion will reduce the capacitance, rather than raise it. This will
cause the frequency to drift up. It's possible the radio already has
temperature compensation, and the compensation is over aggressive. If a
radio's IF is above the radio's oscillator frequency, and upward drift in
oscillator frequency will cause a downward drift in received frequency. I
have the impression the s-350 is single conversion, 455 kc or so, however.

Inductors show the same effect. Expansion lowers the inductance.

Frank Dresser

There is also the possibility of frequency drift caused by mechanical
instability (binding) in these plastic dielectric tunings cap's. My
RF-2900 used to drift a few Khz after the tuning was changed, especially
when the new frequency was many Mhz from the previous one. It took a
minute or more for it to stabilize again. I improved the mechanical
stability by disassembling the tuning cap (layer by layer) and carefully
reassembling it, to minimize any binding in the rotation.


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"Grumpus" wrote in message
om...
Frank, don't take this the wrong way, but there is something sexy
about concision.

Well, Patrick Turner seems to have his following, but maybe that's a RAT
thing.


You're talking dirty to even the dumbest electronics
layman such as myself.

Now I see your point. A one sentence reply, especially if it includes terms
such as a**h*le or the Cheney word, is practically an invatation to the
long-term group relationship here on RRS.

Not that there's anything wrong with that.


Regards,

Grumpus


Frank Dresser

Frank Dresser wrote:

"Grumpus" wrote in message
om...
Frank, don't take this the wrong way, but there is something sexy
about concision.

Well, Patrick Turner seems to have his following, but maybe that's a RAT
thing.

You're talking dirty to even the dumbest electronics
layman such as myself.

Now I see your point. A one sentence reply, especially if it includes terms
such as a**h*le or the Cheney word, is practically an invatation to the
long-term group relationship here on RRS.

Why is it necessesarily the 'Cheney' word? Kerry used the 'F' word in a Playboy
interview.

Bush-Cheney '04