Sears 7108

5/18/2009 7:20:45 PMPaul K.

I have a Sears 7108 Battery radio that I love because I can receive stations from great distances. This month I am only getting static. Have good 90 volts and 1.5 volts off my batteries. Replaced Caps a few years ago, all still check OK. Tubes check OK, only notice the output tube is a bit weak. (1C5) Schematic has 1A5, but radio has been working with the 1C5 in the past. Any ideas on why only receiving static?

Thanks all, really love this radio.

5/18/2009 9:46:00 PMBill VA

:I have a Sears 7108 Battery radio that I love because I can receive stations from great distances. This month I am only getting static. Have good 90 volts and 1.5 volts off my batteries. Replaced Caps a few years ago, all still check OK. Tubes check OK, only notice the output tube is a bit weak. (1C5) Schematic has 1A5, but radio has been working with the 1C5 in the past. Any ideas on why only receiving static?
:
:Thanks all, really love this radio.

Get back to 1A5, 1C5 can't hold up because of difference in filament current? They aren't really interchangeable.
Bill

5/18/2009 9:46:44 PMEdd

That seems to be quite a poser, with initial consideration being given not to the radio , but instead the onset of a new
unexpected source of interference in your house. . .listening position.

Now initially agreed that the battery operated set would be less subject to static interference than an AC line connected set.

Could you put another radio in the same physical position as the 7108 now and see what its situation is, also is your
7108 now even picking up ANY stations ?

As for the marginal AF output, I could only attribute that situation as to its producing weaker static. . .lol.
Another question would be the TYPE of static ?
I am having two distinct types coming to mind one of which is the choppy / staccato static such that you could be hearing
with an AC/ DC motor type of appliance / tool being nearby.
Then there is the other type, which would be the 20-30-40 khz whine which could be picked up from a product using an
internal switched mode power supply in its design.
Or from a TV, computer monitor, or nowadays, the more likely situation of the nearby use of a CFL replacement lamp used in
place of an old generation common incandescent lamp.

Just my initial thoughts without any further input info. . . .

73's de Edd

5/18/2009 9:51:46 PMNorm Leal

Hi Paul

Touch top of the 1H5 tube. You should hear hum. If you do audio circuits are operating, 1H5 & 1A5/1C5 tubes.

Does touching top of 1A7 bring in stations?

Measure plate & grid #2 voltages on tubes. Check coils to be sure they haven't opened.

http://www.nostalgiaair.org/PagesByModel/680/M0017680.pdf

Norm

5/18/2009 9:57:18 PMGeorge T

:Hi Paul
:
: Touch top of the 1H5 tube. You should hear hum. If you do audio circuits are operating, 1H5 & 1A5/1C5 tubes.
:
: Does touching top of 1A7 bring in stations?
:
: Measure plate & grid #2 voltages on tubes. Check coils to be sure they haven't opened.
:
:http://www.nostalgiaair.org/PagesByModel/680/M0017680.pdf
:
:Norm
:
::I have a Sears 7108 Battery radio that I love because I can receive stations from great distances. This month I am only getting static. Have good 90 volts and 1.5 volts off my batteries. Replaced Caps a few years ago, all still check OK. Tubes check OK, only notice the output tube is a bit weak. (1C5) Schematic has 1A5, but radio has been working with the 1C5 in the past. Any ideas on why only receiving static?
::
::Thanks all, really love this radio.
:
:
Hi, that 1C5 is fine in a parrallel circut but not in a series set. The 1A5 can also be replaced by a 1Q5 in a parrallel set, a 1Q4 can be used for a temporary repair. Good Luck

5/19/2009 9:25:27 AMNorm Leal

Agree with George. 1C5 draws 100 ma filament current while 1A5, 50 ma. Either tube will work in a parallel wired filament circuit. 1T5 and 1Q5 will also be ok. Battery life will be reduced with 1C5 and 1Q5 since filaments draw more current.

Norm

::Hi Paul
::
:: Touch top of the 1H5 tube. You should hear hum. If you do audio circuits are operating, 1H5 & 1A5/1C5 tubes.
::
:: Does touching top of 1A7 bring in stations?
::
:: Measure plate & grid #2 voltages on tubes. Check coils to be sure they haven't opened.
::
::http://www.nostalgiaair.org/PagesByModel/680/M0017680.pdf
::
::Norm
::
:::I have a Sears 7108 Battery radio that I love because I can receive stations from great distances. This month I am only getting static. Have good 90 volts and 1.5 volts off my batteries. Replaced Caps a few years ago, all still check OK. Tubes check OK, only notice the output tube is a bit weak. (1C5) Schematic has 1A5, but radio has been working with the 1C5 in the past. Any ideas on why only receiving static?
:::
:::Thanks all, really love this radio.
::
::
:Hi, that 1C5 is fine in a parrallel circut but not in a series set. The 1A5 can also be replaced by a 1Q5 in a parrallel set, a 1Q4 can be used for a temporary repair. Good Luck

5/19/2009 7:03:26 PMPaul K.

Touching the 1H5 tube and did Not hear a hum. Checked voltages at the tubes and all seem OK. Getting more of a crackle, than static. Now it sometimes crackles and sometimes nothing. (dead) Not getting stations. Should I be betting on the 1C5 as the problem?

Thanks to all for the info, you're the best! Paul...

:Hi
:
: Agree with George. 1C5 draws 100 ma filament current while 1A5, 50 ma. Either tube will work in a parallel wired filament circuit. 1T5 and 1Q5 will also be ok. Battery life will be reduced with 1C5 and 1Q5 since filaments draw more current.
:
:Norm
:
:::Hi Paul
:::
::: Touch top of the 1H5 tube. You should hear hum. If you do audio circuits are operating, 1H5 & 1A5/1C5 tubes.
:::
::: Does touching top of 1A7 bring in stations?
:::
::: Measure plate & grid #2 voltages on tubes. Check coils to be sure they haven't opened.
:::
:::http://www.nostalgiaair.org/PagesByModel/680/M0017680.pdf
:::
:::Norm
:::
::::I have a Sears 7108 Battery radio that I love because I can receive stations from great distances. This month I am only getting static. Have good 90 volts and 1.5 volts off my batteries. Replaced Caps a few years ago, all still check OK. Tubes check OK, only notice the output tube is a bit weak. (1C5) Schematic has 1A5, but radio has been working with the 1C5 in the past. Any ideas on why only receiving static?
::::
::::Thanks all, really love this radio.
:::
:::
::Hi, that 1C5 is fine in a parrallel circut but not in a series set. The 1A5 can also be replaced by a 1Q5 in a parrallel set, a 1Q4 can be used for a temporary repair. Good Luck

5/19/2009 7:40:05 PMNorm Leal

Hi Paul

Touching top of the 1H5 should have made a lot of noise/hum in the speaker.

There is a small cap from plate (pin #3) of 1H5 to ground. Cap C22, identified as .0001 in the schematic. Cut one end lose and see what happens.

The radio will operate without this cap. If it becomes leaky will cause static crashes to be heard.

http://www.nostalgiaair.org/PagesByModel/680/M0017680.pdf

Norm

:Touching the 1H5 tube and did Not hear a hum. Checked voltages at the tubes and all seem OK. Getting more of a crackle, than static. Now it sometimes crackles and sometimes nothing. (dead) Not getting stations. Should I be betting on the 1C5 as the problem?
:
:Thanks to all for the info, you're the best! Paul...
:
::Hi
::
:: Agree with George. 1C5 draws 100 ma filament current while 1A5, 50 ma. Either tube will work in a parallel wired filament circuit. 1T5 and 1Q5 will also be ok. Battery life will be reduced with 1C5 and 1Q5 since filaments draw more current.
::
::Norm
::
::::Hi Paul
::::
:::: Touch top of the 1H5 tube. You should hear hum. If you do audio circuits are operating, 1H5 & 1A5/1C5 tubes.
::::
:::: Does touching top of 1A7 bring in stations?
::::
:::: Measure plate & grid #2 voltages on tubes. Check coils to be sure they haven't opened.
::::
::::http://www.nostalgiaair.org/PagesByModel/680/M0017680.pdf
::::
::::Norm
::::
:::::I have a Sears 7108 Battery radio that I love because I can receive stations from great distances. This month I am only getting static. Have good 90 volts and 1.5 volts off my batteries. Replaced Caps a few years ago, all still check OK. Tubes check OK, only notice the output tube is a bit weak. (1C5) Schematic has 1A5, but radio has been working with the 1C5 in the past. Any ideas on why only receiving static?
:::::
:::::Thanks all, really love this radio.
::::
::::
:::Hi, that 1C5 is fine in a parrallel circut but not in a series set. The 1A5 can also be replaced by a 1Q5 in a parrallel set, a 1Q4 can be used for a temporary repair. Good Luck

5/19/2009 9:30:47 PMJim l

becomes leaky wi more of a crackle, than static. Now it sometimes crackles and sometimes nothing. (dead) Not getting stations. Should I be betting on the 1C5 as the problem?
::
::Thanks to all for the info, you're the best! Paul...
::
:::Hi
:::
::: Agree with George. 1C5 draws 100 ma filament current while 1A5, 50 ma. Either tube will work in a parallel wired filament circuit. 1T5 and 1Q5 will also be ok. Battery life will be reduced with 1C5 and 1Q5 since filaments draw more current.
:::
:::Norm
:::
:::::Hi Paul
:::::
::::: Touch top of the 1H5 tube. You should hear hum. If you do audio circuits are operating, 1H5 & 1A5/1C5 tubes.
:::::
::::: Does touching top of 1A7 bring in stations?
:::::
::::: Measure plate & grid #2 voltages on tubes. Check coils to be sure they haven't opened.
:::::
:::::http://www.nostalgiaair.org/PagesByModel/680/M0017680.pdf
:::::
:::::Norm
:::::
::::::I have a Sears 7108 Battery radio that I love because I can receive stations from great distances. This month I am only getting static. Have good 90 volts and 1.5 volts off my batteries. Replaced Caps a few years ago, all still check OK. Tubes check OK, only notice the output tube is a bit weak. (1C5) Schematic has 1A5, but radio has been working with the 1C5 in the past. Any ideas on why only receiving static?
::::::
::::::Thanks all, really love this radio.
:::::
::::Could be volume control. Try turning volume up and down. See if static/noise changes. A squirt of good ole WD40 will clean out volume control if dust has gotten inside

5/19/2009 10:00:10 PMNorm Leal

Not likely a 1C5 tube. Tubes are good or dead. They can get weak but don't usually cause crackle. Nothing wrong with using a 1C5 in place of 1A5 if you have the filament current.

I would still lift one end of that cap from plate of 1H5 to ground.

Norm

:becomes leaky wi more of a crackle, than static. Now it sometimes crackles and sometimes nothing. (dead) Not getting stations. Should I be betting on the 1C5 as the problem?
:::
:::Thanks to all for the info, you're the best! Paul...
:::
::::Hi
::::
:::: Agree with George. 1C5 draws 100 ma filament current while 1A5, 50 ma. Either tube will work in a parallel wired filament circuit. 1T5 and 1Q5 will also be ok. Battery life will be reduced with 1C5 and 1Q5 since filaments draw more current.
::::
::::Norm
::::
::::::Hi Paul
::::::
:::::: Touch top of the 1H5 tube. You should hear hum. If you do audio circuits are operating, 1H5 & 1A5/1C5 tubes.
::::::
:::::: Does touching top of 1A7 bring in stations?
::::::
:::::: Measure plate & grid #2 voltages on tubes. Check coils to be sure they haven't opened.
::::::
::::::http://www.nostalgiaair.org/PagesByModel/680/M0017680.pdf
::::::
::::::Norm
::::::
:::::::I have a Sears 7108 Battery radio that I love because I can receive stations from great distances. This month I am only getting static. Have good 90 volts and 1.5 volts off my batteries. Replaced Caps a few years ago, all still check OK. Tubes check OK, only notice the output tube is a bit weak. (1C5) Schematic has 1A5, but radio has been working with the 1C5 in the past. Any ideas on why only receiving static?
:::::::
:::::::Thanks all, really love this radio.
::::::
:::::Could be volume control. Try turning volume up and down. See if static/noise changes. A squirt of good ole WD40 will clean out volume control if dust has gotten inside

5/20/2009 7:40:40 AMEdd

Sir Paul. . . . . . .K

Seems like my consideration of external static did not come into play, with the situation more likely to be just sounding like no receive action at all , with the possible exception of the minimal noise that you are getting.Or, if the sets local oscillator section is not active. Any chance that the noise is only there / being created when the volume control or tuning condenser setting is moved ?

The sets design is forward in its utilization of a DUAL IF stage, vice the common single stage, and probably only to have been bested by the additional utilazation of a Tuned RF stage up front of the mixer. At any rate, you are happy with the sets performance with that dual IF stage enhancement.

For sure, you positively confirmed the set working before, with the current tube lineup.

Looking at the schema and seeing the fact that the sets filaments are all wired in parallel operation. I can only see the present tube used in the output stage begging and getting itself another 50 ma of current consumption, however in comparing the specs between the two tubes, there is a wide disparity
in each units parameters.. . . . . . . . .
[ Comparative. . . . . AF output tube spec sheets placed at bottom page. ]

Specifically, the matching impedance to the output transformer , the transconductance,the power output capabilities, the 1st grid biasing parameters, etc.
Now, when Sir Norm mentioned the commonly used test of quickly evauating the audio stages + its power supply feed presence and the output transformer + speaker .

With that being done, by the touching of the first grid of the 1st audio amplifier and inducing the stray 60 / 120 cycle being picked up from house wiring by the human body acting as an "antenna" in capturing it.
There is one difference here in using a stand alone battery powered set in the respect of there not being a ground connection as is present when one is using an AC line powered set and the resultant less differential effective level of AC test signal, then capable of being presented to that 1st grid. Therefore the sound level might not seem so pronounced, along with a battery sets inherent power output capabilities being scaled down into the 100's of milliwatt levels.

Now,considering either, that you might not have an audio test generator, nor want to pump in the signal from a CD-DVD player, Walkman, M-peg player opr phonograph in that 1st grid as referenced to chassis common ground. Could you further the "simple" audio test aspect, by another means, requiring but one test lead with end clips.

In looking at that sets schematic, you will see the units tone control on the output stage in the form of merely switching an .005 ufd capacitor to ground in order to slough off a bit of higher treble response and fool the ear into thinking that there is actually more bass presence.
Place the switch such that the cap is NOT being grounded, then take one clip end and connect to the tone switch side of that .005 and the leads other clip end goes to the high side of the volume control.

Place the volume control to its CCW, no / low volume position, now when you power up the set you should find that the volume control should give you a variable frequency oscillator. Starting with the varying stacatto of a " cricket " then sweeping all thru the audio spectrum, until there is no longer adequate feedback level to sustain oscillation. Somewhere, within that range, the hitting of resonance of the speaker and cabinetry and really shaking out any dust and cobwebs.

Ergo ,you actually DO have a working receiver from the output of the sets detector stage onwards.

73's de Edd