[R-390] BFO Problem

Fri Dec 28 19:50:01 EST 2007

No sir, no problem. I am getting closer. I got off on a tanget for a while 
because I began to wonder if all the B+ was high. It is not. I am back in 
the IF Chassis. Most all resistances are ok. I am double checking one or 
two. Most all voltages are a little high but B+ on the BFO Tube, V-505, is 
about three times what it should be. I am assuming that the correct B+ for 
this chassis is 240VDC. The 150 V Regulated is right on the money (Though it 
is not involved in this sub-chassis). This problem really bugs me because 
with this much voltage discrepency, I would expect to find an obvious 
component failure.

I fully appreciate your help. I sent this  back to both lists.

Harold

----- Original Message ----- 
From: <Flowertime01 at wmconnect.com>
To: <r-390 at mailman.qth.net>
Sent: Friday, December 28, 2007 1:53 PM
Subject: Re: [R-390] BFO Problem

> 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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