[SteveJB]

This radio was not playing when I got it. I did a complete recapp and checked/replaced resistors as needed. After I got it up on a variac, it played well, volume seemed good. I tuned the IF cans by ear but havent done an alignment yet.

The next day I was playing it on my bench while cleaning up. After a while the volume dropped to a wisper. I looked for bad connections (tap tested tubes and components in case I had a bad joint) but could not locate one. It has remained at low volume since then.

I decided to check voltages today and see if I could locate a problem. All tube voltages are very low. I dont have a tube tester, but have some spare tubes from a parts chassis, so I started subbing tubes in case I had a bad one. One thing I noticed while replacing tubes is when I pulled either 6L6 with the other left in, the volume went up to what seemed normal. Whenever I replaced the tube so both were installed, the volume dropped after a second or two. This happens no matter what socket is empty. The pin voltages are lower than they should be with only one 6L6, but much lower with both of them in.

How should I trouble shoot this? I am still in the learning stage and would like to get this working.

Thanks,
Steve

[rghines1]

Think you may have lost negative bias on the 6L6 push-pull stage. Should see about -20 volts on the G1 grids of both tubes. Otherwise you will draw excessive current pulling down everything.

Suspect cap 140A that connects to the center tap of the high voltage winding was installed with wrong polarity. Positive side connects to the chassis unlike the other electrolytic caps. Remember the center tap is the most negative point in this radio.

Next would check resistors 134 and 135.

http://www.nostalgiaair.org/Resources/234/M0013234.htm

[jerryhawthorne]

+1 on the latest response. I will attach a link to the schematic for others. Should you be able to post a picture, I would love a close up of your tuning dial. I have a friend who is missing a part. Like to see what it is. I too may be working on one in the future. Good luck. See my post here on a 116.

Jerry

http://www.nostalgiaair.org/PagesByModel...013234.pdf

[SteveJB]

I didnt say this was a code 125, sorry. The cap you mentioned is 123a on this model. I did check the center tap voltage. It is -99v. I checked and all the E caps are in correctly with respect to polarity.

I checked all the other tubes and all voltages are low by about 75v. They are 100 v low on the 6L6's (180v reading) until I pull one. Then they go up to 235v on each tube socket. Should be 290v.

I did hear some arcing when I initially turned it on and the set was playing well. I dont get that sound anymore. Maybe a component failed?

[Radioroslyn]

Shorted 6L6?? As mentioned measure the voltage on pin 5 of BOTH 6L6's. Should see -20v or better on both. If only see - voltage on one of the 6L6's You have a bad driver transformer.
Terry

[SteveJB]

I don't get any measurable voltage on either 6L6 at pin 5. The volume is louder when one 6L6 is in the radio. It can be either side, with the other pulled out. Only when both 6L6's are in does it reduce volume.

Would that be a symptom of a bad driver transformer?

[Radioroslyn]

Say I just looked at the schematic that things got a lot of little parts don't it? Check voltage at the junction of 115 and 116. If you have - voltage there then check both sides of 100 and 101. If you have - voltage there then it's time for a driver transformer. Seems to be a common problem with 38-116 after 74 yrs
Terry

[SteveJB]

I will check the transformer for continuity and the voltages you recommend.

But, would that cause all tube voltages (not just the 6L6's) on the Philco schematic to be low?

[codefox1]

Already stated that lack of proper bias on 6L6 will cause heavy load on power supply. The bias is set on pin 5 (control grid) and must be negative with respect to ground. Follow the volts! Triple check to see that power supply is wired correctly. Bias or negative circuits can be a little tricky and not so intuitive.

The driver transformer may or may not be bad.

I would also recommend you replace the small carbon resistors, as they are probably drifting up, and may do funny things under power that you can't test for.

[SteveJB]

All the resirtors were in spec or replaced. The trans had six pins, three on each side. I did check the transformer for continuity, but its hard to match up pins to the schematic as there are six pins and a lot of wires. I do get 400 ohms on one side of it (right bottom and right middle pins of the trans). I am prerry sure this is primary based on connections. It should be 400 ohms, so it checks.

From either 10 k resistor/trans connection (parts 100 and 101) I get a 160 and 179 ohm reading which is correct though I cant completely make out the other pins. Form the top left pin (one side of a 10K to the middle pin on the left side I get 179 ohms), the schematic calls for 175. From the other 10k resistor/trans connection I get 160 ohms (top right pin to left side bottom pin on trans), it should be 150.

I do not get voltage on either side of 100, 101 or the junction of 115/116. No voltage on pins 5 of the 6L6 either. I am measuring from chassis ground to the pins/connection points here.

Thanks for all your help. I really want to get this playing. I am refinishing the cabinet now!

Steve

[rghines1]

The driver xfmr should be Ok based on those resistance readings.

Said earlier there was -99V on HV winding center tap but no voltage at the R115-R116 junction. Seems to only leave R115 open or wiring error of same to center tap of HV winding.

Like the "follow the volts" advice mentioned earlier.

http://www.nostalgiaair.org/Resources/vi...013235.pdf

[SteveJB]

Hi All,

Thanks for all your advice. After I had time to check the circuit wiring and check resistors, I found a very high reading on R115 which was in the Meg ohm range and should have been 3k.

I replaced it and the radio works well now. I have to align and refinish the cabinet next and I'm done!

Steve