I'm still troubleshooting garbled audio on my 38-116 that I've recapped and replaced out-of-tolerance resistors. I've been checking tube voltages and find that they are significantly low. The lowest measurement was on the 6J5G driver tube. I measured 48 vdc on it's plate instead of the specified 260 vdc. I tried another 6J5 tube and got the same result. I started checking heater circuit pins 2 and 7 on various tubes. I found 6.3 vac on pin 7 but 0 on pin 2. I checked the volts out of the transformer on the two black wires for the heater circuit and found that the output of the transformer for pin 2 was 0. I assume the problem is in the power transformer. But, I wonder why the wiring from the transformer for 6L6G pin 7 would first go to a chassis ground terminal on the bakelite block. Shouldn't it just go direct to pin 7? Note: Dial lamps work normally.
One side of the filament of all tubes except the rectifier is tied to chassis ground. The other filament pin runs from tube to tube and ultimately is tied to one of the wires of the filament winding of the transformer (at your "2" in the pic). The other side of the filament winding is tied to chassis ground along with pin 7. Your pic shows that. Measuring from chassis to one of the filament pins (pin 7) will give 0 volts AC. From chassis to the other should give somewhere around 5.8 - 6.3 volts AC (pin 2) as you indicated. If the tubes and pilot lights are lit, then that part of the transformer is good and the wiring is correct. What is the plate to plate AC voltage on the rectifier (careful, probably is nearly 600 volts AC without the rectifier in the circuit)? Also, what is the DC voltage at the positive terminal of cap 120?
(This post was last modified: 09-20-2019, 09:22 PM by rfeenstra.)
I'm glad to hear that the heater circuit is ok. The socket voltage diagram I have shows 6.3 vac at both heater pins; so, I thought something was amiss.
The plate-to-plate voltage with the rectifier tube (5Y4G) removed measured 723 vac. I'm using a digital multimeter. The + side of cap 120 (and 121) is 275 vdc; supposed to be 315 vdc based on the diagram I have.
I'm wondering if I have misconnected something with the new electrolytic caps. For cap 123A I had installed a 10 mfd rated at 160 volts (with + side going to chassis ground). The voltage rating on the original can was labeled as 50 volts. But after a couple minutes of operation, I heard a small pop and saw a wisp of smoke. That cap had apparently overheated and discharged a small amount of oily residue. I've replaced it with a 12 mfd rated at 450 volts (closest one I had at higher voltage rating). I've attached a couple photos showing how I installed the new caps. I tried to mark on one photo how I interconnected the - sides of 120, 121, and 123 with the + side of 123A.
I noticed that the socket voltage diagram shows a 60 vdc measurement for the 10 mfd at 50 volts cap. Seems like that cap's voltage rating is too low.
Thanks for taking the time to help me with this!
Larry
Re-check your 123A. It appears that you have it wired with the negative tied to ground. The positive must be tied to Chassis ground and the negative to the negative terminals of 120, 121, and 123. This junction typically rides at about NEGATIVE 25 to 50 volts DC referenced to chassis ground. This negative voltage will vary depending on the model of the radio. A 50 volt cap is usually sufficient. I will typically, though, use a higher voltage cap just because I have them. If 123A is wired backwards, it probably will fail as you described.
(This post was last modified: 09-21-2019, 12:13 PM by rfeenstra.)
I re-checked voltages. Still 723 vac between the plate sockets of the rectifier. Voltage at cap 120 is still low. 279 vdc from + to chassis ground and 331 vdc from + to - terminal on original cap. The voltage diagram says first should be 315 vdc and second should be 385 vdc. The voltage on cap 123A is now -54 vdc; but, the voltage on cap 123 is only 318 vdc (should be 370). Voltages on caps 70, 70A, and 70B are just slightly low; 245 vdc vs 260; 76 vdc vs 90; and 218 vdc vs 225. The plate on the 6J5 driver tube still reads low at 54 vdc.
I checked resistance between rectifier sockets 7 and 8. Supposed to be less than .1 ohm but I get much higher readings, from 3 to 10 ohms, depending on how the probes are positioned. Also checked the vac between those sockets and got 5 vac.
By the way, I have the input voltage to the radio set at 115 vac.
Added edit: I very much appreciate the time you've spent educating and helping me with this radio. I thinking about just setting it aside for now. There is a much nicer 38-116 that will be sold at an auction near me this coming Saturday. The cabinet is in much better condition than mine and the chassis has all its tube shields which means it hasn't been hacked to accept metal tubes.
(This post was last modified: 09-21-2019, 04:18 PM by LarryFS.)
Your 5X4 may be tired. Or, you have something else drawing too much current. The biggest power suckers are going to be the 6L6's. Make sure the grid bias is ok. Given your -54 volts at one end of resistor 115, you should see about -21 volts at the junction of 115 and 116. This supplies the bias for the grids of the 6L6's and about -20 volts on the grids should be good. If they get too close to 0 volts, the tubes will draw too much current.
What is the voltage on cap 70B? this voltage supplies the driver plate through the driver transformer. Does the primary of the transformer measure around 400 ohms?
Also, check the candohm resistor 117. These are often bad.
(This post was last modified: 09-21-2019, 08:00 PM by rfeenstra.)
The rectifier tube is a 5Y4G. If I continue with this set, I should get a 5X4G. I measured -17 vdc on each 6L6 grid pin 5. I also measured -17 vdc at the intersection of resistors 115 and 116. I had to replace both of those and didn't have exact values for replacement so I used a 3300 ohm resistor for the 3000 (115) and a 2200 ohm resistor for the 2000 (116). Also, both 6L6's tested on the borderline of acceptable (70% on my Jackson tester) and they both rattle. I think the 5Y4 also tested weak. I read that the 5Y4 is only for moderate dc requirements. Maybe a weak one just won't hack it with this radio.
The 5X4 max continuous draw is 675 ma. The 5Y4 continuous draw is 400 ma. If the radio calls for a 5X4, you must use the right rectifier. It will strain the heck out of a 5Y4, and could be causing your problems.
In case you need them...
5Y4 data sheet: https://frank.pocnet.net/sheets/049/5/5Y4GA.pdf
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