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Hi: Put in some new caps and the low voltage bridge and now the amp section is working but the tuner is intermittent. Something is pulling down its B+ rails. It will work for a short time then all the B+ gets sapped and the neon lights go out and there is no radio. Amp continues to work fine. Opened up a IF can and all looked fine: no dead bugs shorting stuff out. Let it rest overnight, maybe it will work for a few minutes then die again. Wiggled cans, removed all FM tubes: still bad. Am wondering about removing the tiny caps to ground (.005) one by one or freezing them with freon spray? Ideas appreciated. Thank you. --Tom
Follow Ups:
Tom
Was the tuner working before you put in new caps and did you change the filter can that supplied the tuner as well? Does the preamp section continue
work when the tuner quits along with the amp section?
On three occasions,I have seen brand new CE branded filter caps develop an intermittent open or an intermittent short.It is rare but it did happen. Make sure your solder flowed well to the pins on the cans.Make sure a bare lead component isn't laying close against another connection.
Forget the freon spray at this point.Disable the MPX B+ supply lead and
see if the FM still dies.How much has the B+ dropped on the IF tubes as well as the oscillator?
"For every complex problem there is an answer that is clear, simple, and wrong" H. L. Mencken
Michael: My friend gave this to me saying it had redplated on the output tubes and "I could never get the tuner to work."
First I put in new bridge for the DC filaments and bias. Still heard hum on phono and FM with no actual radio functionality so replaced last two filters to phono and tone plus all five on the tuner B+ string. There had not been any hum in spkrs with volume down so I left big B+ and screen filters alone. Phono and EXT now worked and tuner would play for a few minutes then quit. That is when I started chasing all this seriously.
With all three tuner B+ loads removed from B+, the plates on 7591 stay at 430 as called for. Within two seconds of putting any one FM load in place, 7591 B+ drops to more like 300. What was over 200 unloaded B+ for tuner drops to around 60.
The new capacitors I put in are panasonic from mouser; all discrete under the chassis. Thank you for insight. --Tom H
what a odd problem. Did you look at everything with a magnifying glass for something small bridging a connection?
Yes I have examined all the connections for B+ as it goes throughout the set. I turned it upside down and sideways and shook it: nothing rattling around. Sprayed in the IF cans, wiggled the cans, wiggled wires all while it is on mind you. Tried hitting it all at once (not slow on variac) still no closer to solution.
The 100 some Volt B+ that is at the plates of the tuner tubes? What does it drop to? Nothing is smoking or smell anything getting hot when it fails? Then probably a bad connection or cracked resistor. Trace the voltage drop in failure mode starting at the PS filter capacitors there are resistors in series starting with R311, R310, R309, R308, R205... if it is the feed marked 5. Add R307, R508 to that if it is the feed marked 4...
What you will probably see is the voltage on one side of one of these resistors and not the other, so bad connection/solder joint. Or one of the resistors is getting hot and something is pulling it down on the other side (less likely).
Hi Fredtr: Thank you for ideas. Nothing is getting hot or smoking or smelling bad. The various resistors 307-311 in that path all measure fine and sends the B+ right through provided there is no load. If I lift off 4, 5, 6 the red orange and blue loads to the radio section, the available voltage is over 200. Adding any one of the loads, let alone all three, it will drop to under 50V. I do not leave it on very long when it is doing this. THe DC resistance on each of these tuner load wires is around 30K and that is with filter caps (all brand new) not connected. That seems low to me. The total watts draw from AC is right at 100W pretty much regardless of whether tuner is working or not. And did I mention: this fault condition of sapped B+ will occur with all tuner tubes out of sockets. The person who gave me this stated: "I could never get the tuner to work." So past history on how this developed is unknown.
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