[morzh]

To Mr/Fixr's remark.

When you troubleshoot a problem like this one, you should remove all non-native devices. Including bluetooth.
You need to confirm it is the radio that does that. Bluetooth is an extra variable.
I was not aware you had it (maybe I missed it).


Where and how is it connected? Can you sketch the exact circuit mod? (the switch, the jack, everything that connects the BT to the radio). I imagine it is at the circuitry involving vol. ctl, and this is close to the detector. Any subtle mistake might affect it.


The diodes (no, I do not think you need to do it, as the tube has been swapped) go the same way as the diodes of the tube: A to A, K to K.
And you only need one, as the two of the 6H6 are paralllelled. You can leave the filament circuit alone, it is not doing anything.
Again, at this point this is unnecessary.




John

He said he swapped the 2nd detector tube, else I would also say "swap it".

[dconant]

Tuner has been taken out and thoroughly clean. Radio still playing normally. AVC varies between -2.7 and -5.6 from on strong station to no station. I'd say AVC is working correctly. Radio just crapped out AVC is at -2.6. Volts on the antenna coil starting at the top are -2.6, -2.7, -2.7, -2.6, -2.6. I am able to pass a 456 signal nice and loud.

With radio playing normally now my transmitter gives me -14 AVC. Reception is poor when radio in crapped out mode with AVC at -2.6.

[morzh]

What about the bluetooth?

[dconant]

I have tried that before. Bluetooth plays fine while radio is crapped out.

[morzh]

No. The question was, is Bluetooth plugged or unplugged when theradio craps out, and I mena, physically unplugged and removed from the radio.

[tbone]

I think we can rule out the antenna coil secondary windings based on the AVC voltage being the same at all taps. I’m assuming it’s the same at the 6A8 cap which is important.

To me the AVC voltage dropout to the -2.6V level does indicate an issue in the RF section, front end and IF stage of the set.

If the stations still come in on the correct dial location, but just weak, I would also think the oscillator is good. I believe this was verified already.

If you can put your finger on the 6A8 grid cap and signals drastically improve as it previously did, then your problem is still likely between the antenna connection and the 6A8. You should be able to put your finger on the A terminal with nothing else hooked up and hear signals better than with nothing.

I understand that the IF signal passes through loud and clear, but there are too many variables to just say the IF chain is good. Not sure where the signal is being injected and at what level? You should be able adjust the level low enough to just be heard like you would do when performing an alignment. If the generator pumps too much signal, it can make you think everything is OK. Brute force.

If you can measure AVC as a reference when the radio is working vs not, while injecting the same IF frequency signal generator level, you would have confidence if the IF gain is stable. Maybe set it to get 4V AVC when the radio is good and see if it’s the same when the radios is weak.

Another puzzling thing is that even your strong transmitter fails to lower the AVC below the -2.6V reference. A strong signal should bleed through the radios front end and still be pretty good, hence my above IF stage comments.

I am curious as to whether or not the connected antenna is actually stable, and maybe the problem is not with the radio? You can connect up your signal generator to the A and G terminals on a broadcast frequency, not the IF, with a fixed level to see if the radio is stable or still drops out. This might be better than using over the air stations.

On a side note, I have seen tube sockets go intermittent. Sometimes the solder terminal looks solid, but the actual connection to the tube is bad. Also, wires soldered to grid caps can actually break inside the insulation and become intermittent.

All the advice you have been getting from Mike, Aaron, and MrFixer55 is spot on with troubleshooting. I’m simply thinking outside the box and throwing out some ideas that may or may not be beneficial.

[morzh]

Tony


Good insight. And the sockets have been brought up. And the sockets and heat could do exactly that.
One thing that has been done, I think - tthe tubes have been wiggled and poked; if the socket had one pin somewhere disconnecting, wiggling the tubes would likely do something to indicate bad connection.
It is almost like something get to a pretty stable state after 2-3 hrs. Like a flip-flop.

Forgot to ask: have we done the freeze spray yet? Has been suggested.
If not, we should, if yes, we should also try it on the tuning cap and the associated trimmers on it.

[dconant]

AVC when acting up is -2.6 on 6A8 grid cap.
Stations come in weak in their correct positions when acting up.
Did not try finger on the A terminal.
Signal was injected at the antenna terminals with .5 cap in series with A term.
I will check with signal generator with antenna disconnected and see what happens.
I have no reason to doubt my antenna because just the act of removing the antenna ground and placing it back on bring the radio back. This is not enough to even wiggle the A connection. The connections are via alligator clips.
I have cleaned the tube sockets and resoldered the pins. I will clean again and tighten the sockets.
Tubes have been wiggled and poked. Tube sockets are quite tight in this radio making tube removal a little difficult.
Freeze spray has been done.
Tuning cap was also freeze sprayed.
Last thing I have done is rob the glass 6A8 from my 37-116 and that too crapped out. All other changes of the 6A8 were metal tubes. Yes pin 1 is grounded. Yes I used a tube shield and verified it was grounded.
So since the antenna is in question is there still a chance the primary of the coil proved to be good even though resistance reading are the same normal and acting up with power on. Also I used another wire for an antenna and that also caused the problem.
As for the Bluetooth question there was nothing plugged into the jack. I have even tried with the Bluetooth disconnected from the radio.

I really do appreciate you guys hanging in there.

[morzh]

One thing caught my attention, which I am not sure is consistent with this conversation.

>>>Using my signal tracer is no help as soon as I touch the RF probe to anything the signal returns to normal.

Does this mean that when the radio craps out and you touch the probe of your tracer to ANYTHING??? - the radio starts receiving well again?

[dconant]

Yes that was true Mike. I have not tried it since then.

[tbone]

Here is a link to the whole schematic which includes the tuning info and voltages.

6S254 W Tuning Info

When I read the last post, it was stated that the generator was connected to the A terminal with a .5uf cap. That does not jive with the instructions. The only time a .5uf cap is used is for the IF alignment, and that is at the grid of the 6A8. The rest of the time either a 200pf cap or 400 ohm resistor is used at the A terminal. Regardless, that is not the root cause of the issue.

I also noticed that the voltage listed at the 6A8 and 6K7 grids is listed as -0.5 volts, not -2.6 volts. However, the first plate of the 6H6, the source of AVC voltage shows -2.5 volts. The -.5 volts could be a schematic error? My guess is that the -2.6 volts you are seeing is a symptom and not a problem. It simply means the 6H6 detector is not getting a strong enough signal to create more negative AVC. I suspect this voltage is the quiescent state no signal voltage.

Isolating which RF stage that’s going bad, RF or IF, would be my suggestion. You could run the radio with a modulated IF signal injected at the 6A8 grid per the alignment instructions, then wait and see if the radio quits. It is possible that the 6H6 tube could be an issue. They are known finicky tubes.

It was mentioned that there is a Bluetooth device somehow connected to the audio section. It is possible it could cause an issue. There are many components after R7 and C10 that could have a loading effect. I think it was mentioned before to disconnect additional components with this circuit. I know it’s a long shot, but still worth mentioning.

Obviously this is not a simple problem, but breaking the radio down section by section, would be how I’d tackle this problem. Usually one works backwards from the audio section, then the IF, followed by the RF section. I suggested the IF stage because we are confident the audio is good.

Just some more thoughts and suggestions. Maybe someone else can chime in with more ideas.

[morzh]

Tony,

>>However, the first plate of the 6H6, the source of AVC voltage shows -2.5 volts

Both plates and cathodes are in parallel.


>>It was mentioned that there is a Bluetooth device somehow connected to the audio section.

He answered this in the post #98,

"As for the Bluetooth question there was nothing plugged into the jack. I have even tried with the Bluetooth disconnected from the radio."


Of course there is a question of where does jack go and does it have anything else between it and where it is plugged that is ot original to the radio (was asked before).

[RodB]

Tony, Dan also posted that when the problem occurs he can send a modulated IF signal through the radio with no problem. Figure that!! It's really weird.

[dconant]

Tony, I failed to note where to connect the sig gen for the first step and thought it odd to be on the antenna.

I have changed the 6H6 multiple times. I have several more to try.
I have disconnected the Bluetooth wire. Sig gen is running through .5 cap to 6A8. Set to 100mVpp. AVC is at -2.5. While typing this signal dropped. AVC is at -.8. Increasing the sig gen voltage barely increases the AVC.

[dconant]

Measuring the AVC at grid of 6K7 for above post. I also note when I measure the AVC at the 6A8 grid it starts very high > -15 and it slowly goes down to -2.5.