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Philco 70 volume issue - Printable Version

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Philco 70 volume issue - Radiowilbur - 02-10-2018

I recapped my Philco 70 and it plays well. The volume control,however is not working properly. at full it is loud, a little distorted, but loud and basically clear. 1/4 turn brings the volume gradually down to zero, but theres still 3/4 of the turn with silence..... my total range of volume is in that small 1/4 turn at the full end...... any help or hints would be appreciated.

RE: Philco 70 volume issue - bzadel - 02-10-2018

You probably have a very dirty volume control.  

I had a similar problem with mine. It had a lot of dirt and grease cake'd up on the contact surface.  Try removing the control completely from the chassis and pry up the three tabs that hold the control together.  The control will come apart in two pieces.  I used a Q-tip and denatured alcohol to clean the contact surface.  You'll be surprised how much crap builds up in those controls over the years. I had to repeat the process a few times until my Q-tips no longer turned black after wiping down the contact surface.

That solved my volume control problem.  I hope it works for you.

Bill

RE: Philco 70 volume issue - Radiowilbur - 02-10-2018

I will check the control, but I do not think dirt or crud is the issue. The level goes from zero to full volume at the top 1/4 rotation. It gently fades to no volume without pops, scratches, dropouts or intermittants-but at that top 1/4 rotation only and silence below that point. I do not think the control is dirty. I am wondering if a single winding of one of the controls went open, if the radio would still fundtion in this manner.

RE: Philco 70 volume issue - Radioroslyn - 02-10-2018

Hi Wilbur
It's a dual pot on the older model 70's common to have one of the section bad (open). As Bill mentioned a good clean may put it right.

GL

RE: Philco 70 volume issue - Radiowilbur - 02-10-2018

terry, i didn't see your response before I replied to the note above. I will indeed check for an open section. I do not think it is dirt within. It operates quietly and well, but it is just at that top 1/4 turn of the control and silent below......

RE: Philco 70 volume issue - morzh - 02-10-2018

This is a wirewound pot. They don't suffer from dirt as much as carbon ones do.
Do clean it, but then desolder he antenna and check for open both sections.

RE: Philco 70 volume issue - Radiowilbur - 02-10-2018

i dismantled the unit and cleaned up the rear section of the control. i didn't disassemble the front section of the control. The front section seems to be OK, measuring 0 to about 5K ohms from the center tab to both ends nice and smoothly. The rear section also seems to be OK, measuring 0 to 280 ohms from center tab to both ends. I did, however read open on one of those sections prior to removal of the control but it seems to work OK now. I am gonna put it back and see if the radio works any better.

RE: Philco 70 volume issue - Radiowilbur - 02-10-2018

No change....same issue.

RE: Philco 70 volume issue - morzh - 02-10-2018

Same issue, meaning? The distortion or the open having returned?

RE: Philco 70 volume issue - Radioroslyn - 02-10-2018

>No change....same issue. 
Ok next thing to do is with the set off and the volume full up measure the resistance from the ant post to the chassis. Should see a fairly low resistance like less than 50ohms. If not the primary on the ant coil is bad and will need to be rewound.

GL

RE: Philco 70 volume issue - Ron Ramirez - 02-10-2018

Hi Wilbur! Icon_wave

In my experience, the 250 ohm section of those dual Philco volume controls are prone to going bad as Terry mentioned above. That section of the dual control is very important as it controls the cathode bias of the RF and IF tubes. If this control goes bad (open)...the RF and IF tubes receive no cathode voltage and your radio goes dead (silent).

The 5000 ohm section controls the amount of signal getting to the first RF coil. It actually isn't as important as the 250 ohm section.

A person could effect a repair by connecting a 5000 ohm resistor across the antenna coil (2) where the 5000 ohm section of the control was, connecting the antenna terminal direct to the top of the coil primary where the wiper arm of the control currently connects. Then, if you could find a 250 ohm 5 watt rheostat with a suitable shaft, use it in place of the original control. It will turn the cahode bias of the RF and IF tubes up and down, increasing and decreasing the amplification in those tubes, and rendering the radio usable. The trouble is, rheostats can still be purchased but they are extremely expensive these days and in a quick search of Mouser and Allied, I couldn't find any of that resistance below 12 watts which is overkill for the 70 but would certainly work.

I suggest you disconnect the wires going to the 250 ohm section, then place the leads of a multimeter from one of the ends to the wiper arm (center terminal). Then slowly turn the control, watching the meter. If it goes open at any point, reclean. If that doesn't help, it's bad.

Good luck.

RE: Philco 70 volume issue - Radiowilbur - 02-10-2018

....i tested both controls with an ohm meter out of the circuit, i even dismantled the back section of the control. each operated smoothly from 0 to their maximum and with correct readings. if you connect to the center pin of either control and measure from either end and then the other, all the readings were smooth and without interruption, glitches or intermittants. zero to 250 and zero to 5K. i believe the control is in good shape. it is a wire wound type and it is clean Philco part by CTS. I think the problem lies elsewhere.

RE: Philco 70 volume issue - Radioroslyn - 02-10-2018

If your control turns out to be defective if you have a spare model 20 or 21 the control seems to be the same value should work. Mark O sells a replacement  single pot w/a resistor to replace the original.

RE: Philco 70 volume issue - Ron Ramirez - 02-10-2018

Hmmm...that's strange. Your symptoms sound like a bad volume control but if it is measuring OK, then it probably is OK. The next step would be checking coils as Terry mentioned.

RE: Philco 70 volume issue - morzh - 02-10-2018

Well... how's that #40 wirewound, does it check out?