[R-390] BFO Problem

[Flowertime01 at wmconnect.com]

Harold,

I put this back out on the reflector to share it with every one for several 
reasons.
First you have a real problem in your BFO and more input on solutions is 
always good. Second there are many more readers of the reflector that collect the 
wisdom and insight for their own knowledge. Third the reflector gets archived 
by date and subject. Many people who are not regular subscribers of the 
reflector will read the archives for days to find posting about a problem they are 
having and find tips and inspiration to help them solve the problem they are 
having. I hope you do not mind your subject being posted this way.

Roger.

-----------------
Harold, You sent me the following post.

I still have the BFO problem. I have determined that it is an intermittent 
problem that was running me crazy before I found that it is intermittent. It is 
more off than on. Problem is that at V-505, the BFO tube, should be 86  volts. 
>From cold start it is OK but then it slowly climbs to 223 volts! I have been 
able to sectionalize to the IF sub chassis and that is about it. With a few 
exceptions, voltage and resistance measurements are OK. I need to follow-up on 
those that are out of limits. Obviously, some component is changing value with 
heat.

I certainly appreciate your interest and assistance.

 Harold,

----------------------
Harold,

BFO problems are mostly bad tubes and cold solder joints. This gets followed 
by the old caps going leaky. This gets followed by more cold solder joints 
from the cap replacement. I have had the shaft in Z502 seize and thus fail. My 
other problems with the BFO have all been solder joints. Today the big plastic 
caps are getting leaky and causing problems. The small caps are holding up 
well. IF you suspect a small cap is going open with heat as the receiver warms up, 
you can just touch a cap across it for a test. If the circuit comes back to 
life you may have found an open cap. 

My schematic notes are that the BFO B+ feed line voltage is 220 volts. Your 
reading of 223 volts is well within reason. But you are reading this voltage on 
the plate of  V-505, the BFO tube. As you say it should be 86  volts. From 
cold start it is OK but then it slowly climbs to 223 volts.

So the plate goes to B+. This tells you the tube has stopped conducting. 
Mostly experience is the tube has stopped oscillating the control grid has 
accumulated a charge that leaves it below the cathode voltage and the tube is thus 
cut off. Oscillators are some of the most problematic circuit in electronics 
vacuum tube or other stuff.

First eye ball the filament. The R390A BFO tube has a filament in series with 
the ballast tube and the VFO. As you do not report the whole receiver loosing 
signals as it warms up we can expect the filament string is in operating 
order.

You get two choices for the next procedure. Option one is a tube extender. 
Option two is to set the receiver up on the IF deck end and swing the IF deck 
out so you can probe the BFO tube socket. This is where you were needing to 
remove the BFO shaft and bellows coupling so you could just get to the tube 
socket. Working with the equipment you own you do what you need to do. You are 
likely going to need to solder something around that tube socket anyway so you may 
as well get the receiver up on end and the deck swung out for inspection.


Second consider the cathode. If the cathode circuit is going open as it heats 
up. the tube will stop conducting and the plate will go to B+. We expect the 
cathode to be at zero volts and conducting some current. If the cathode has 
more than a volt or two when you meter it, you are looking for a cold solder 
joint on pin 7 of the tube socket, pin 2 or 3 of Z502 (BFO coil assembly) or at 
the ground lug where pin 3 of Z502 is grounded. Cold solder joints are the most 
common problems in the BFO circuit. The pins of Z502 do not take a tin well. 
The pins are a stainless steel and got tinned as part of their manufacturing. 
Some times some acid solder is in order to get a good bond. Remove the whole 
assembly, then tin the lugs and clean every thing up good before you reinstall 
Z502. You can then solder the circuit back to gather with proper solder and 
flux.

The ground lugs in the IF deck are becoming problems. They are causing all 
sorts of problems depending which lug and which circuit is faulting. The 
solution is to just loosen the hardware and retighten it a couple time to break up 
the oxide in the mechanical joint. 

Check pin 2 of the tube socket. The suppressor grid is grounded. Again you 
are looking for a ground lug oxide problem. It will likely meter OK with an ohm 
meter. But after you massage the hardware, the problem just goes away if that 
was the point of failure. Other wise the hardware massage is just one more 
inspection that is inconclusive.

Check that C533 and C534 have been replaced. If replaced consider a cold 
solder joint. If the caps are the original plastic devices of ill repute, get then 
changed out. These caps are just B+ line filters. A larger value cap would be 
OK. A voltage of 250 volts would be OK. 600 volt orange drops are nice. 
What's there is there and likely OK. But if you need to do a replacement consider a 
.1 verse the .033. The replacements will be smaller than the originals and 
leave some room for test probes.

Back on topic, if C533 is shorting it will pull the screen grid down in 
voltage and the tube to cutoff. The plate will go to B+. The shorted cap will not 
likely pull enough current char the resistors. IF C534 is leaking you would not 
expect to see B+ on the plate. It would pull B+ down some but again not 
necessarily enough to char resistors. Over time these leaks could get bad enough to 
cause resistors to char. This is what we see from receivers that have set for 
long times while the caps keep degrading from age. Then when the receiver is 
power up, the caps leak enough to char some limiting resistor. You bring a new 
to you receiver home and power it on. After a week or so it quietly dies. 
Inspection discloses a smoked resistor. You likely do not even detect the smoke 
or smell as it resistor gets slow roasted.

Next step is the grid circuit. Again a cold solder joint is likely. As Z502 
works from a cold start we expect the problem is not in the can its self. In 
young receivers (68 -75) a bad Z502 was an open coil and could be found with an 
ohm meter.

First check all the mechanical connections at the ground lugs.
Second check all the solder joints on Z502.
Third check all the solder joints at the replaced capacitors.
Forth of course replace the tube.

Before you go into Z502 you may want to just replace C526 and C527. One is 
100pf and the other is 5pf. Theses are not known problems and getting good parts 
can be hard to do. If you need these parts, ask here on the reflector. 
Someone may offer you the parts in an envelope from their stock for a couple dollars 
back in the mail.

If all this fails, then remove Z502 and open it up.
There are a some caps inside and solder joints to inspect.
There could be a cold solder joint in side.

If this does not lead you to a solution, send some more mail, Let us know 
what you have inspected and found. 

Hang a meter on some of the grids and watch what happens as the tube warms up.
You likely cannot hang a meter on the grid without pulling the tube out of 
oscillation. 

For sure send a post back with what you find the problem to be. 

Good luck and have fun.

Roger AI4NI



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