[Eric]

I need of help!

What I have is a Stromberg Carlson 1120PL Have tested the tubes and there good so I replaced the electrolytic's and all the wax paper caps. I did not touch the molded mmf caps or any resistors. Powered it up and no smoke :.-)
Now my problems
1) I get some stations but their week
2) This part puzzles me I get the same stations in two locations on the dial
3) I get a hum through the speaker which is constant (I was very careful to replace with the same value's on the E-caps should I go higher?)

When checking the voltages per Riders 15-1 they all are with in 1-10 volts of the published values and the heaters are all 6.3-6.5vac when taking the voltages like on 6sk7 pin 8 I get loud (good volume) crackling until the probe is firmly attached should I look for a bad resistor?

Some of the other readings that are questionable
6SA7 pin 5 calls for 7.5 I get -7.2
6SF7 pin 2 .6 I get -.44

Eric

[rghines1]

Hi Eric,

The -7.2 voltage is good. That means your oscillator is working. The voltage chart is incorrect with no minus sign preceding the 7.5V. If the oscillator was not working it would explain stations weak and at different spots on the dial. You can also use a AM transistor radio near the radio to hear the oscillator beat.

Using a 22uF or 33uF cap for C-45 will help with hum. A slight hum is usually OK.

The crackling noise is good sign on the 6sk7 tube. You are creating white noise which is theoretically all frequencies which is able to pass through all the stages.

Check that the antenna coils show continuity too.

Richard

[Arran]

There can be any number of sources of hum other then bad filter caps, one of the most common being a heater to cathode short in the power output tube, another may be the poor routing of wiring in the AVC circuitry near tube heater wiring or open ground connections. If you are getting a station at two places on the dial this is called imaging and may be caused by the IF being out of alignment. It's also possible that the oscillator circuit has a problem and is running too fast or too slow, a leaky mica or a drifted resistor can cause this. I would also go through and check all of the carbon resistors and replace any that are more then 20% out, even some that read fine cold but are in a plate or screen grid circuit. When I overhaul a radio now I always figure on changing at least 1/4 of the carbon resistors while I change the capacitors, in a set with body end dot code resistors I figure on changing them all, resistors are just too cheap not to change.
Regards
Arran

[Eric]

Richard,
Did you mean C-35 There is nothing showing as C- 45

Thanks Eric

[Eric]

Arran
Thanks for the input and will start checking the resistors first then the mica caps.

Eric

[rghines1]

Richard,
Did you mean C-35 There is nothing showing as C- 45

Thanks Eric

Should be C-34 (15uf) left of L-13.

[Eric]

Richard and Arran
Changed C-34 to a 33uf that took care of the hum then I went checking resistors and noticed some with sensitivity when touched with a wood dowel so I changed them ( value's were ok) but it made no change. I also noticed that the 6sk7 seemed to crackle alot and stations came in better when touching it with a probe so I thought I would run through what it would take to put a signal generator on it so I thought I would check out the adjustment screws on the IF transformers and see if the were frozen or would move and I just moved the one screw ( with the set on ) on the 1 IF and it came right in. Works great!

Thanks for all the help now on to the record player

Eric

[rghines1]

Sweet!
May have disturbed some contamination shorting the trimmer.

Richard