Airline 74BR-1510b problem

[legenbass77]

Hi everybody,

I've been working on a Airline 74BR-1510B for a relative. They said it had been playing, but I don't know how well, and what damage may have been done. When I powered it up on the variable isolation transformer, I could hear it receiving stations, but the familiar 60 hz hum was very pronounced, and didn't change with the volume control, so I proceeded to recap the radio. After a complete recap, it behaves exactly the same way. I've checked everything I can think of, rechecked my work, component values, transformers, volume control, speaker, tubes on my tester, and by substitution. Also checked voltages noting ac, and dc voltages. I compared them to the schematic, and everything was very close to the schematic. I ended up re-stuffing the 40, and 20 mfd multi-section can with a 47, and 22 mfd since there is not much room underneath. 40 is a square, and 20 mfd a triangle, and I wired it back up the same with 47, and 22 mfd modern caps. I did manage to get it turned a different direction, but it's still wired up the same, confirmed with my meter. Also made sure it's isolated from the chassis, and all the negatives are going to B-, not ground. There was also a 27 ohm resistor across pin 2 and 5 of the 35z5 that had gotten hot, and soon crumbled. I replaced it with 22 ohm (all I had in stock) and it's not getting hot anymore after the recap. All other resistors tested in spec. There was also a .15 mfd @400 volt going from the volume control to pin 6 of the 50L6, and it has a ground side that went to pin 6 to the rf choke coil to ground. Printed on the cap is Cornell Dublier, Tiger, .15 MFD, 400 VDC, and a black band with ground. Not quite certain what kind of cap that is, but all I had was a .1 600 volt non polarized replacement. I substituted a 2 mfd electrolytic in, and it made no difference in sound. There aren't too many parts in this chassis, so I didn't think it would be so hard to get going!
As always I appreciate the help!

[stusnyder]

Originally Posted by legenbass77

Hi everybody,

There was also a .15 mfd @400 volt going from the volume control to pin 6 of the 50L6, and it has a ground side that went to pin 6 to the rf choke coil to ground. Printed on the cap is Cornell Dublier, Tiger, .15 MFD, 400 VDC, and a black band with ground. Not quite certain what kind of cap that is, but all I had was a .1 600 volt non polarized replacement. I substituted a 2 mfd electrolytic in, and it made no difference in sound.

Thats a plain paper cap, non electrolytic. The black band means nothing. The 2mfd electrolytic does not belong in there.
Go thru the IF stages. I usually use a external antenna lead and touch the top of the tube caps and listen for changes. No change and the problem is in that area. Also check your coils. one might be open.

[dieseljeep]

I looked at N/A under 74BR1510B, not listed. I did find it under 1501B.
The 22 ohm resistor is close enough in the circuit. The .1@600 is OK, as it's a line isolation cap.If you look at the schematic, you'll see, there was a few revisions in the production.
When I recap a set of this type, first thing I do is temporarily bridge the electrolytics, to see how well the set performs. Then I go on with the restoration, slow and easy.
I seems that you have a slight wiring error, that's overloading the B+ circuit.

[Electronic M]

Black band on a non-polarized cap means outside foil. If the outside foil was connected to ground it would act to shield the cap from picking up stray RF energy.....It it not important to observe that convention with replacements, but it may make the original circuit easier to follow...

[init4fun]

You mentioned testing the tubes , and also substituting . Did your test include a check for Heater to Cathode short , especially on the 50L6 ? If all your checking is Cathode emission , and your pool of substitutes only includes a couple of each type , the possibility exists that more than one of your 50L6 tubes could have this defect (It's a somewhat common failure mode in the 50L6 type) .

[Reece]

+1 on subbing a known good 50L6. Also as a test: temporarily lift C12 from the center terminal of the volume control. If now no hum, you know the problem is ahead of this point. If hum persists, trouble is in the audio section or rectifier circuits.

[legenbass77]

Thanks for the replies! I just noticed my typo in the title it is 74BR-1501B, not 1510B, sorry for the confusion. I must have been thinking of my Lowrance LRG-1510B depth finder. Good to know about the .15mfd, just never seen one like it before. I don't have the 2 mfd soldered in, I just clipped it in to see if it made any difference before I soldered the .1 mfd in place. I checked the coils labeled 33, and 34 on the schematic and they had continuity, but will check them again just to make sure I didn't miss a section, schematic calls for around 40 ohms on the primary, and secondary windings. I will take a break from this project for a few days, and then recheck my work again. I did perform a short test on each 50L6, and they showed no heater to cathode shorts, or any other shorts. One is at the high end of the bad range on emission, and the other is on the low range of good, and I have that one installed. I'll test, and try another to be sure.