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RCA Q-103 tube ? - Printable Version

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RCA Q-103 tube ? - Eric - 05-06-2016

I have this RCA  Q-103, http://www.nostalgiaair.org/pagesbymodel/596/M0015596.pdf  The correct schematic is the one on top for the Q103

And I have a question on the alignment page 16-11 the chart on the right hand side for the Q-103.
#1 12SK7 IF grid?  Do they mean pin 2,4 or pin 6? I think 6?
#2 12SA7 1st Dect grid. Do they mean 1,4,5 or pin 8? I think 4?

Thanks Eric

RE: RCA Q-103 tube ? - Phlogiston - 05-06-2016

On most mixer/oscillator tubes (converter) the grid closest to the cathode is the oscillator, pin 5. Then the detector is going to be the grid that is between the 2 grids that are tied together, pin 8 between grids on pin 4.

On a lot of these tubes you can test the performance of the 2 parts separately, though they aren't really separate.

   

Speaking from my recent experience with crappy 6A7s.

If you are asking about the control grid (input) on the 12SK7 it is closest to the cathode. Pin 4.

You can always remember that on a pentode, one grid is going to be at or near the cathode voltage (near 0) and one grid (screen) is going to be at or near the plate voltage. So you can eliminate them as being the control grid (signal).

So your tuning stages will be on the control grid (input) and on the plate circuit (output).

RE: RCA Q-103 tube ? - Radioroslyn - 05-07-2016

Hey Russ,
You know you just let out the top secret about how a pentagrid mixer tube works! Some folks it takes them years and years to figure that out and why that tube has so many grids.

Terry

RE: RCA Q-103 tube ? - Eric - 05-08-2016

Thanks, I have the hardest time picking out the grids and the various books and prints seem to call them by different names!

Eric

RE: RCA Q-103 tube ? - Radioroslyn - 05-08-2016

If it helps think of it this way. Grid 1 is the control grid for the oscillator(pin5). Grid 2 looks like grid but acts like a plate. It has hv on it and acts as the plate of the oscillator (pin4).
Grid 3 is the control grid for the mixer (pin 8 ). This is where the rf input is connected, antenna coil and such. Remember grid 2? acts like a plate but is a grid which allows the electrons to pass though it. If it was constructed like a plate the electrons wouldn't pass though it they would stop. This is where the mixing takes place the oscillator signal leaking though G2 and mixing with the rf signal on G3. G4(pin4) is the screen grid for the rf part of the mixer. Notice the G2 and G4 are connected together internally. This way hv can be applied the grid the act as the oscillator plate and the screen grid of the rf part of the tube all at once. Grid 5 is the suppressor grid (pin 1 or 6) Usually tied back the the cathode and is pretty much at ground potential.

Another way to look at it is Grid 1,2 and the cathode make up a triode oscillator. Grids 3,4,and 5 and plate make up a pentode rf ampilfer. With some leakage from the osc in the rf pentode it mixes the two signals together.

Simple right????

Terry

RE: RCA Q-103 tube ? - Eric - 05-11-2016

Looks as though I'm going in circles, After the alignment all I get are whistles when I try and tune. So I check voltages and I seem to have a problem with the 12SK7 unless it's a misprint. All other voltages are good on the 12SK7 I get
pins 1,3,5 are tied together and go to chassis
pin 2, 7 are heaters I get 23acv and 34acv
pin 4, -.6vdc
pin 6 88vdc
pin 8 83vdc

Print says Pin 3,4 should have 82vdc???
Tired of chasing my tail! ( to old)

RE: RCA Q-103 tube ? - Radioroslyn - 05-11-2016

Ok voltages look good so connect you generator to the ant connection not the grid of the IF amp. Set for 455kc and adj the level so you can hear it in the spkr. Adj the IF trimmers for max loudness. As you peak these trimmers turn the generator down so you don't overload the set with signal.
Next would be the RF alignment which you can do using the RCA info.

Terry

RE: RCA Q-103 tube ? - Eric - 05-12-2016

Terry,
I hooked it up and had no problem finding the signal and peaking the IF's for the 455 then I moved on to the AM band skipping the SW bands for now. ( is that a problem doing it that way). have no problem finding the signal at 1300 and 600 but when I start going up and down the scale after peaking the trimmers and slugs all I get are whistles. I can land it on either 680 or 750 our strongest stations and play with the slugs and get one or the other I do this with a antenna strung in my garage, take the antenna away and the signal is lost. The coils in both IF cans show the correct value's could I have a problem with the caps inside? In the print 17-6 it shows some changes the things outside the cans have been done don't know about inside.

Eric

RE: RCA Q-103 tube ? - Radioroslyn - 05-12-2016

So does your set cover broadcast band and 2SW band or a longwave, bcb, and sw band??? The diagram show both.

Terry

RE: RCA Q-103 tube ? - Eric - 05-13-2016

Terry
Three bands
1 - AM 540-1600 kc
2 - Medium wave 2.3-7 mc
3 - Short wave 7 - 22mc
The print your looking at with the long wave is for the Q103X Page 16-10. Mine is the Q103 Page 16-9
Eric

RE: RCA Q-103 tube ? - Eric - 05-17-2016

I did the alignment (several times) with the same results lots of whistles so I attached my garage antenna (about 30 feet) and played with T1 and T2 till I got the two strongest stations then did some with the trimmers for steps 8 & 9 and got a few more, but if I remove the additional antenna the stations drop off. Any thoughts? Also if I use the Phono with my MP3 player and adapter it works fine.

Eric

RE: RCA Q-103 tube ? - Radioroslyn - 05-17-2016

Well You could try tuning the bcb oscillator by monitoring it's signal in a well calibrated receiver. Use C-6 to set the osc signal heard in the well calibrated receiver at 1755kc (1300+455kc) with the dial set at 1300kc on the RCA. Same thing for the low end of the band. Use L-9 to set the osc heard in the well calibrated receiver at 1055kc (600+455kc) with the RCA dial set at 600kc. It's not uncommon to have so interaction between these two adjustment so you may have to adjust these a few times to get it on the nose. If you can't get the osc to tune from 1055 to 1755kc and your bandswitch is good and clean them I'd replace C-8 and C-11.
The other thing that comes to mind is interference from other electrical devices like cell phone/lap top chargers or computers. They use switching mode power supply and generate a lot of rf noise which can be very strong.

Terry

RE: RCA Q-103 tube ? - Eric - 05-18-2016

Already changed C-8 & 11 (grasping at straws) Also have turned off everything I can think of.

bcb oscillator? Not sure what you mean?
I'm using a Knight KG-650 signal gen, I do have an old boat anchor receiver that seems to be accurate it's a Hammerlund HQ-140-XA. Remember I'm still some what new at this!

Thanks Eric

RE: RCA Q-103 tube ? - Radioroslyn - 05-18-2016

Sorry broadcast band oscillator. The oscillator operates at the dial frequency + the IF frequency. So if your dial is set at 600kc the oscillator should be oscillating at 600kc+455kc=955kc.

Terry

RE: RCA Q-103 tube ? - Eric - 05-19-2016

In the change page 17-6 it shows in T2 There are changes to the mmf's will this make a difference ? Think I should open T2 and see if this was done?