[Guest]

Question: Is it normal to hear substantial background noise on a Philco 70-A ? and what are the options to eliminate this problem.

Symptoms: Background noise is present even with the volume control completely turned down and the antenna lead has been physically disconnected.

Set known characteristics: Canadian model & has no AVC.

Performance: Although there is background noise, this set will received well on the AM band with plenty of volume.

The only occurrences where the background noise completely disappears are:

• When the 2nd detector tube is physically remove from the set;
  
• Plate voltage disconnected from the 2nd detector tube; and
  
• The coupling audio capacitor .1 uf is disconnected.

I have tried the following option: the background noise can be significantly be reduced by altering the 2nd detector tube overall gain performance. To achieve this goal, the suppressor Grid bias capacitor .09uf has been temporaly disconnected, which will reduce the 2nd detector tube overall gain with the result of significant reduction of background noise.

I currently ran out of troubleshooting options.

Any advices on how to remedy this set will be most welcomed.

Recepteur

[AI2V]

It depends what the noise sounds like- static crashes are often a bad tube.

I would first check the cathode bias resistor, #27- the 50K. If the bias on the AM detector is too low all sorts of noise will pass through.

The screen grid cap #31- .09 must stay in the circuit- it keeps the audio, RF, and IF signals from crosstalking to other tubes.

My only problem with the 70 is the audio distortion at higher audio levels; next time I'll buy a 90!. Good troubleshooting......

[Guest]

Hi Pete,

Thanks for the prompt response and advices.

As part of the troubleshooting process, the 2nd detector tube 24 was replaced and noticed no changes.

As you recommended, I will swap the cathode bias resistor, #27- the 50K with new one.


Cheers

Recepteur

[PTurney]

Hi,

The volume control on this set is at the front end, in the antenna circuit, so no adjustment of it will reduce noise introduced in later stages of the set. Since the noise disappears when you "kill" the 2nd detector it is likely being introduced in one of the stages between antenna and (including) the 2nd detector.

Have you tried swapping out tubes, or even changing the 24s around? Tubes can generate unpleasant noises even when they test out correctly (even if caused by degraded solder connections in the base or grid cap).

In addition to the things already suggested, ensure there are no dry solder joints. Although not on a model 70 (actually a Philco 38-12), I've had noise introduced by poor connections to the little socket that pushes onto tube grid caps (what's the proper name for that??), even when the connection has looked ok. Re-solder these to be sure.

Does the noise get worse if you gently tap the chassis? Going to be a process of elimination.

Good luck!
Paul

[Guest]

Hi Paul,

Thanks for the sound advises.

I have changed the 24's around with the same results.

I will acquire some new 24's and will see the impact on this receiver.

As you recommanded, degraded solder connections could be the cause.

My next step will be to resolder all the joints and clean the tube sockets.

A lenghty but mandatory troubleshooting process.

Cheers

Daniel

[Guest]

Gents,

Final update on the status of a Philco 70A.

After completing a detailed checkout of the wiring, solder joints, components and collecting voltage readings on all the tubes, I have pinpointed that the voltage reading at the 2nd detector screen Grid were way to high and confirmed that the 2nd detector Screen Grid capacitor and resistor were missing.

Obviously, I was not the first one who troubleshooted this receiver and someone omitted to insert these critical components.

Lesson learned: Do not trust any previous repairs.

These two vital components are essential to provide proper bias to the 2nd detector tube.

After proper insertion of equivalent component values, the 2nd detector tube voltage readings were brought to the established parameters as indicated as per the Philco 70A diagram voltage table.

The frying background noise is now unnoticeable and the set receives very well on the whole AM band. Please note this set has no AVC, therefore the local station are very loud and clear. Distant stations reception level are comfortable.

I wish to express my sincere thanks and acknowledge the fine support of thoses who provided me with sound technical advises in order to cure this set.

Regards

Recepteur.