Hey Guys. This is my first resto and I have no audio output. I'm looking for suggestions.
Could be a bad trans but I'm really not sure what I got.

I had no output when I got this radio but it crackled when I changed bands.
All the tubes fire up and tested good on a multimeter. I dont have a tester.
All the paper caps were changed to Orange drops. Equal value or higher. And 5 resistors.

Got bumpkis. So I hooked up the variac at 50v (dont want to destroy anything) and got these pin values.
Grounded the multimeter neg to the chassis. Changed to AC as indicated * .

attached schematic.

Welcome to the forums.
Good that you changed out the paper caps. Have you changed out the electrolytics?
If not, do that before applying power again. A shorted electrolytic can cause much damage.
As for the transformer, with the plug removed, a resistance test should give you a quick go/no go across the output leads (you'll need to remove tubes and lamps for the filament tests).
But get that C33a,b,c replaced!
73,
-marc

Typo in the title. This radio is a "L-915"?
http://www.nostalgiaair.org/Resources/197/M0008197.htm

Yes, it's important to swap out the old electrolytic capacitors. Most radios would play even if some resistors are way out of tolerance, so that would be a lower priority task.

Voltage check points in the schematic is for when full 120VAC power is applied.

The suggestion to measure resistance between the two blades on the plug is a good one. The dim bulb tester is reliable. It's worth the trouble to build one.

Don't overlook stuff like oxidized tube pins, switch contacts. cold solder joints, etc. Double check all tubes went back to the correct sockets. This radio needs a long wire antenna.

We often give out same suggestions repeatedly because most of the time when radio doesn't play, it's one of those causes.

Take your time. This is not a simple 5-tube radio that many first timers learn/practice with; which means there are more opportunities for things to go wrong

Text from "Elements of Radio Servicing" Chapter 8. https://antiqueradios.com/archive.shtml

Thanks for the tips guys

I'm building the dim bulb this weekend.

Also the caps never seem to build any power. Like its all running to ground. just weird.

Good to hear from you Patrick. Thought we'd lost you.
So you have replaced the C33's? If not, do that first!
AJJ has provided a really great primer!
73,
-marc

Be aware that the DC voltages in the radio will not follow the AC line voltage in anything like a linear way. At 43% of the normal AC voltage going in, the B+ voltages will probably be much less than 42% of normal.

However, the AC voltages for the tube heaters and rectifier plates will vary pretty much 1:1 with the input AC.

Due to the tube type rectifier, the B+ will stay close to zero until the filament starts to get a little hot.. Then it's a pretty hard to predict how fast B+ will rise with increasing AC.

You need to watch the AC current as you crank up the voltage. If the current stays in line at the normal AC line voltage, then you measure the DC voltages in the chassis.

Ted

Aside from the ohmmeter. One of the quickest ways to test an output transformer & speaker in on go is to crack a 9V transistor radio battery across the the primary. On 6V6 that's normally on the screen pin and the plate. No noise generally indicates bad transformer, open speaker, or the 6V6 plate bypass cap shorted. IF it fails it's of no value, powering the set again, until that issue is fixed.

One hopes the crackle mentioned was from the speaker, rather than it being the sounds from arcing or the demise of a part? Some electrolytic caps will crackle a bit just before they explode.

Marcc

Well got some good news.

I have audio.

Whoever was in the chassis before me gutted the C-33 cap and wired in a 16uF 450v another 16uF 450v and daisy chained a 22uF 450v + a 16uF 450v. What a bonehead. The schematic calls for a 10uF 300v a 15uF 300v and a 300uF 350v. So that was WAY off.

So I got a 300uF 350v cap and replaced that mess and wired up some old speakers and voila! I got an AM station. I get nothing on the 14 inch magnetic speaker thats mounted in the cabinet. Continuity checks out on the speaker pins and magnet.

Now phase 2.
I cant go over 65 volts on the variac. Signal gets blown out with noise. So I have some issues to work out.
And the speaker issue.

I'm chasing down a signal tracer as a next step to get some direction on where its going sideways on me.

The problem you have may be something else, but....

There's a good chance that's a typo in the schematic. The previous replacement may be more correct. Especially that 5Y3 tube data sheet specifies a 10mfd.

I would gather more comments before powering up the radio again with that 300mfd capacitor

https://frank.pocnet.net/sheets/049/5/5Y3G.pdf

Here:
http://www.nostalgiaair.org/Resources/198/M0008198.htm

This version of the schematic makes mroe sense.

Between the two versions. One has a speaker field coil (L9) while the other has a resistor at the same position (implying a permanent magnet speaker). But the difference shouldn't change the value of that capacitor by that much, if any.

Not a good idea to use 300 mf connected to filament of the 5Y3. This large of capacitor will cause excessive surge current and damage the 5Y3 rectifier. Standard value here would be 10 mf @ 450 volts. Could use up to 22 mf @ 450 volts.

Best to used 450 volt rated capacitors for each section of #33.

...the previous repair was with 16 and 22 in series. That calculates to about 9.3mfd. The last guy probably didn't have a 10mfd on hand so he used this combination.

There may also be a second error in the schematic: C33 is drawn as "a, b, c" when it probably should be "c, b, a" from left to right.

Unfortunately I did not get to see what was in the original c-33 but I dont want to take out my 5Y3 either.

I will order 3 new caps and take another run at this.

But wow. A 15uF 300v vs. a 300uF 350v ? Thats no rounding error. I know the war was on but c'mon now.

The 5Y3 data specifies not over 32mfd so 15 / 15 is reasonable, lots used 16 /16uF and many use 22uF. If the "B" voltage is a bit high you can reduce the first ones value. I will not go below 500V with 5Y3 if it is feeding heater tubes.

Marc

I replaced the C-33c with a 30uF 350 cap. So everything should be to spec now.

I then Checked Plate and Grid voltages today.

All seem to check except the 6J5 AF - Plate reads high and increases with voltage. Grid stays at 0.

The other 6J5 Phase inverter - Plate reads normal Grid is positive.

So things are going sideways after the detector 6SQ7 tube.

There is the original mica cap in the mix that I have not replaced or tested. So thats the next task.

So: What were the cathode voltages? Normally when a control grid goes positive, either the coupling cap is shot, or the cathode is going open; in which case all grids on that tube will be positive and approaching that of B+: That includes the cathode.

Check resistors & wiring, including control grid.

Marc

Update:
- Went thru the entire chassis and checked all connections.
* I have a Iso Transformer now (modified for test bench - no earth ground)
- Variac is next then my floating chassis
- verified power with MM - all good. all voltages were checked at 117 AC on the chassis
- connected the speaker and replaced frayed speaker wires. good continuity checks.

I am getting negative volts on the Plates on the 5Y3. Filters have voltage.

Plate voltages are all over the place as indicated. Zero voltage on the Cathodes. Bad Rectifier?

Voltage on plates of the rectifier is AC. Set your meter to read AC voltage.

There is an error on voltages listed. Print has 75 volts on pin 3 of 6V6 tube. This is plate and your reading of 203 would be correct.

Be sure you replaced electrolytic caps and they are in the correct polarity. Check R30 and R31. These resistors drop voltage to rest of your radio. Are these resistors very hot?

Beware as to what you are reading and how? Most of the voltages are referenced to ground and often the schematic will tell you that.

5Y3 is a little different. If horizontal in the wrong orientation, it will likely short & suffer a catastrophic failure.

When you start getting into plate resistors of hundreds of Kilo ohms, Digital meters will present less circuit loading, than the early analogue ones & read higher. It is important to understand (learn) what test equipment is telling you. All voltmeters will lie if applied incorrectly.

Now! The 5Y3 filament, which is also the cathode, has 5V on it: But! it also is the source point of B+. It is therefore imperative that is filament winding does not ground. In other words it "floats" above the chassis. Relative to ground / centre tap, pins 2 & 8 will be positive. Filament voltage (AC) is measured pin to pin, not pin to ground.

The plate voltage of 5Y3 is measured at centre tap, or chassis (ground) from pins 4 (anode 2) and pin 6 (anode 1). That voltage being Alternating Current. Do not measure pin 4 to 6 unless the meter is capable of withstanding more than 1kv

Marcc

I got 4.7 volts AC on the Filaments Pin 2 to 8 on the 5Y3.

I only have 700 volts AC on my MM so I did not try measuring plate voltage.

I made a spreadsheet on all the measurements and blocked in blue everything that is under.
Could a bad rectifier cause the under on all the plates down the line like that? or is this a bad tranformer?