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model 38-10 no reception after recap - Printable Version

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model 38-10 no reception after recap - pkingco - 02-22-2020

Hi all,

I've completed my first overhaul, replaced all capacitors including cans and bakelite block, (except mica), all resistors, new rectifier socket and tube. (this pic taken before I replaced the socket) 

The good news is that it did not explode or burn out after powering up. All tubes lit, but I do not have a way to check if they are all good. Bad news is that I get no reception on AM or shortwave. I get a little air through speaker, nothing much else. 10' of 22G wire attached to red antenna terminal. Nothing attached to black. 

What are the next steps to sorting this out? Thanks for any help you can lend this beginner.

   

RE: model 38-10 no reception after recap - Radioroslyn - 02-22-2020

Probably the easiest thing to do would be is to do some voltage checks to see if you got voltage where it needs to be. Go over the the 6F6 tube and measure the DC voltage from the chassis to pin 3 of the tube and then measure pin 4. Both pins should have abt 250v or so on them.

RE: model 38-10 no reception after recap - morzh - 02-22-2020

Touch the grid cap of 2nd detector 6q7.
See if you have hum in the speaker.

RE: model 38-10 no reception after recap - pkingco - 02-22-2020

Thanks. Pin 3 is settling in at around 283v, pin 4 278v.  When I touch a probe to the top of 6Q7 the speaker buzzes. The grid cap is loose (connected to wire, but wobbles) on 6A8.

RE: model 38-10 no reception after recap - Radioroslyn - 02-22-2020

Ok so that seems to be in good order. Have a look @ pins 3 and 4 on the 6K7 Should see some hv there too. On the 6A8 pins 3,4, and 6 will have some hv too but will be less on 4 and 6 vs pin 3.
If all that seem reasonable it's time to breakout the signal generator.
You  may want to resolder the gridcap oin the 6A8 to be sure it's making a good connection.

RE: model 38-10 no reception after recap - pkingco - 02-22-2020

tube 6A8: pin3: 179v  pin 4: 68v
tube 6K7: pin 3: 272v  pin 4: 67v
tube 6Q7: pin 3: 232v (this one is double what the schematic lists)
tube 6F6:  pin 3: 283v  pin 4: 266v
tube 6A8: pin 3: 173v  pin 4: 67v

   

RE: model 38-10 no reception after recap - Radioroslyn - 02-22-2020

Pin 6 of the 6A8?
Also have a look@ pin 5 should see a little bit of - voltage there with a modern meter.
And are #19 and 26 within tolerance?

RE: model 38-10 no reception after recap - pkingco - 02-22-2020

negative readings on 5 and 6. see pics of 3,4,5,6. 

               

RE: model 38-10 no reception after recap - Radioroslyn - 02-22-2020

Tada! we got it. It seems that either #15 is open or #5 between pins 4 and 3 are open. Which ever you need the 130v or so on the plate pin of the oscillator section of the pentagrid mixer and it's getting lost down by #15.
With the set off measure the resistance across pins 3 and 4 of #5 (oscillator coil). Should see a low resistance like an ohm or so. If not remove the coil and give it a close inspection for rot (green spots) or a break in the wire.

RE: model 38-10 no reception after recap - pkingco - 02-22-2020

Thanks, Terry. Are you referring to #15 resistor and #5 Osc. Transformer? What am I looking for... bad connections to/from either? bad component? Not sure how to rectify this.

   

RE: model 38-10 no reception after recap - Radioroslyn - 02-22-2020

>Are you referring to #15 resistor and #5 Osc. Transformer?
Correct!
You can follow the hv from the bottom end of #15 (5000 ohm resistor) to the top end of 15 then to terminal 4 of the oscillator coil then to terminal 3 of the oscillator coil/pin 6 of the 6A8.

RE: model 38-10 no reception after recap - pkingco - 02-23-2020

ok, so 180v from the top end of #15, then carries over to coil... two points on coil reading close to same, then nothing going to pin #6. Bad coil? Replaceable? Repairable?

Thanks for all your guidance.

               

RE: model 38-10 no reception after recap - Radioroslyn - 02-23-2020

It would seem that your wire is open from the coil to the tube socket or a bad solder joint where the wire is solder @ the socket. If that is ok pull the 6A8 out and measure pin 6's resistance to the chassis w/the set off. Should see a high resistance like 20 or 30K this eliminates a shorted socket. Measure the voltage @ pin6 w/o the tube in place maybe your tube is shorted.

RE: model 38-10 no reception after recap - pkingco - 02-23-2020

Got it! Seems Bad connections at these pins are the culprit. I was able to get 160v to the tube when I moved these a bit, making a connection. Can these be secured/soldered?  

I have reception!! not many channels, but a few strong ones on AM and a few weak ones on SW.  
This is great, thanks so much, Terry!

Now on to see how I improve reception (and make these connections more stable). Any thoughts welcome!!

   

RE: model 38-10 no reception after recap - Radioroslyn - 02-23-2020

Not sure which connections we are talking about but the coil you can gently scrape the wires at the solder connection and re solder. If it's the tube socket you can use a pair of needle nose pliers and gently pinch the connector on the socket w/o the tube in place. Be careful the metal can be weak and fold easily then your looking at a new socket.
To improve the performance you'll need a signal generator to adjust the IF amplifier and other stages of your set. Something simple is fine just need to verify it's accuracy.