Kieth, why are you thinking of replacing the Micamold cap #14? That's the oscillator padder for the shortwave band and is a critical tolerance part. It should be 2250 pF, which is not a standard value. Unless it is definitely bad there is no reason to replace it.
Don't go by the color code marked on the cap. Its not the standard color code for resistors and standard mica caps.
Thank you Mondial, I was going to replace the micromold because I thought the wrong value was installed based on the blue-blue-yellow color code. Based on the link you attached it is the correct value.
Keith
Sorry I meant to write micamold and made a typo. The cathode for a 6A7 tube is pin 6 correct? I've got a single resistor attached to this pin that doesn't match the schematic so I think I need to write Chuck to get a better schematic. I believe I should have at least two resistors attached to this pin and I have just one. The wiring of the 6A7 varies between the two nostalgia aire schematics and I think I may have a third variant. I can't be confident checking the coils without a proper schematic. Next time I'll order the schematic from Chuck before I begin.
Keith
Keith, the cathode of the 6A7 (pin 6) is grounded on the second type schematic. There may be one, two or even no resistors actually connected to the socket pin since any convenient chassis ground will serve the same purpose. Is your pin 6 directly connected to chassis or is there a 300 ohm resistor to chassis ground?
When they drew the schematic, it was easy to draw the resistors as shown, but it does not necessarily imply that they were actually located and connected there.
(This post was last modified: 03-13-2015, 07:46 AM by Mondial.)
(03-13-2015, 06:41 AM)Mondial Wrote: Keith, the cathode of the 6A7 (pin 6) is grounded on the second type schematic. There may be one, two or even no resistors actually connected to the socket pin since any convenient chassis ground will serve the same purpose. Is your pin 6 directly connected to chassis or is there a 300 ohm resistor to chassis ground?
When they drew the schematic, it was easy to draw the resistors as shown, but it does not necessarily imply that they were actually located and connected there.
Pin 6 is not connected to the chassis and it has a single resistor that connects it to ground. The resistor measures 20 ohms. It appears to be brown (body), black(end), black (spot). But the color isn't clear and it's possible that it's a orange, brown, brown that got hot and changed colors. However wouldn't the resistor go up in value over age and not down in value? Here's a picture cathode resistor . From the picture you can see that it's not clear if the body is brown, red or orange, it looks like a mixture of the three colors.
That looks like a 20 ohm resistor to me (red, black, black). It appears that you have the second type chassis, with the cathode of the 6A7 grounded through the 20 ohm instead of directly. This resistor was removed in late production, and the cathode was then grounded to the chassis. See the last page of the nostalgia air info on the 610. It mentions the change removing the resistor.
I would use that second type schematic for reference, as it shows the resistor already removed. It should not change other aspects of the schematic wiring or connections to the osc coil
Mondial, I usually panic when I try to follow schematics through rotary switches and I have a hard time following schematics in general. Having said that I think the oscillator coils for the broadcast band are circled as shown below. Is that correct? I've marked some test points and recorded the resistance readings. I'm thinking my coils are good but I'd like another opinion. I've got a 32K resistor where the schematic shows a 51K resistor and I've got the 20 ohm resistor from cathode to ground as discussed earlier.
Thanks,
Keith
I've tried listening for the osc in another radio several times and haven't heard anything. I'm not very good at troubleshooting oscillators. Any idea what to try next?
Thanks,
Keith
Try measuring the voltage on pins 4 and 5 of the 6A7. Pin 4 is the oscillator plate where you should have around 150V. Pin 5 is the oscillator grid and there is a negative voltage there of a few volts if the tube is oscillating. Connect a 100K resistor in series with your DVM probe to measure voltages on these pins to prevent loading of the circuit at RF frequencies.
As Terry mentioned, the coils seem to measure ok, so the problem may be elsewhere.
There are very few weak links:
1. The Tube (checked)
2. The tube panel.
3. The coils.
4. The switch that commutates the coils.
Now, to check the switch you do not need to trace it: simply measure appopriate coils's continuities from the beginning of the circuit to the very end with the switch in a partiular position. If the switch is not closing properly you will see the open or large resistance where it should be a low coil's resistance.
People who do not drink, do not smoke, do not eat red meat will one day feel really stupid lying there and dying from nothing.
(This post was last modified: 03-14-2015, 05:38 PM by morzh.)
Well, I just checked the voltage on the pins, I've got 6.2V AC between 7 and 1, on pin2 I get 236VDC, pin3 56VDC, pin4 I get 2.2VDC, Pin5 I get -0.43VDC and on pin6 I measure .2VDC. It looks like I've got a problem on pin 4. I will check the switch next.
Thanks,
Keith
Check the 9K resistor 39 and the 32K resistor 42. Both these resistors will drift high in value or open, causing low voltages on the 6A7 and no oscillation.
