strange amplifier warbling?

[ALBATROS1234]

ok so i restored a 1946 stromberg carlson 1121  receiver/amp and its doing good as far as radio reception. at first when i got it if i plugged a record changer into the phono input it was distorted and shite sounding. so i looked at the schematic and there was another input  for "television/wire recorder" when i plugged phono into this input it would sound ok for a few minutes then it would produce a warbling/low output signal. i looked at the schematic and i noticed the phono input joined with the tele/wire recorder input via a 470kohm resistor so i snipped the bastard. when i then plugged a record changer into the tele/wire input it sounded good. but.....but.... after approx 20 minutes  or about 3/4 thru an album side it was starting to warble/drop out a bit. i dont get it because to me it lloks like since i snipped the 470k resistor off it should connect the same as the radio which has no issue in the same path. so i dont get why it sounds great for 20 or so minutes then gets wonky.see schematic below.

[shooter]

I'm not gonna get the crayons out for tracing:icon_biggrin:
the 3 r's form a voltage divider +, the line that wanders off tween the 470k n 130k probably should be followed, all the caps should be replaced, section 6 should be well cleaned, tensioned n lubed, might wanna do all the sections in time. or you can hot-wire it to the left side 13? 15? .01uF n play the other side

[ALBATROS1234]

i replaced every paper/wax cap i left in a few silver mica types, which were the ones in the phono input circuit, but when i clipped the 470k ohm that connected the tele/wire input to the phono input it started working well except after 20 to 30 miniuted this warbling drop out appeared. the tele/wire input now connects to a switch which gets routed to the tone/volume portion before the 6sq7 audio preamp tube. now that the phono input circuit has been removed to me it looks like the input goes strait to a switch and gets routed if selected to the tone/volume pre 6sq7 circuit. which works fine if the radio is selected with said switch but if i select the input eventually it sounds wonky. from what i see since i clipped that 470k there is nothing between the tone/vol circuit which is different from the radio input.

[shooter]

cut the red, jump the blue, can you make it through the otherside without wonk?

[ALBATROS1234]

thats what i did pretty much except i cut the 470k off of the tele/wire input next to your left red x so it goes strait to the switch. i need the switch because it selects between sw,am,fm, and phono/tele/wire recorder. but thats what i was saying, i removed that mess on the phono input with by clipping the 470k whic attached to the rca input for tele/wire. now basically the rca tele/wire input goes thru a shielded wire to a switch, thru a .01 cao to the tone/vol network which goes to the grid of the 6sq7. and thru that switch the am/fm/sw sounds fine thats why i was confused theres nothing i can see thats causing this, specially since it sounds great for a while then starts sounding shite. it actually varies. earlier i started up after 20 or so minutes. it did that a few times bout 3/4 thru an album side. later i played hank williams sr. double album and the shite didnt start until the 4th album side so it is intermittant and does not happen when using the radio . all inputs go thru this switch to the input of the 1st audio stage. is there some reason why radio might work fine but the record player would cause some kind of weird thingy in the tone circuit?

[ALBATROS1234]

but i literally replaced all paper caps plus some suspect resistors, then i eliminated the mica mold types on the phono input when i severed the 470k from the tele/wire input. before i did that anything at anytime sounded cracky and shitty. i looked at the schem and saw that pile of crap on the phono input and said to myself theres the culprit. which now it mainly sound good for a while instead of sounding crappy all the time. idk, thats why i posted i eliminated what you suggested which seemed like it must be the problem then after i thought i had fixed it now it only sounds crappy after it plays for a while. which before i clipped the 470k and adjacent phono input off it sounded crappy 24-7. idk

[shooter]

all inputs go thru this switch

if you jumper'd past the switch then the same point in space will play a radio fine, but nothing else?  I didn't put a lot of time studying so when the swamp weather leaves, maybe 

[ALBATROS1234]

Thank you much for putting in the time to take a look kind sir. Like I said before I even posted I looked thru the schematic and like you I thought that parallel phone input array was the culprit so I clipped it off at the 470k. Before I did that it was garbled the whole time. Now that the phone input is removed I have an intermittent issue where I plays fine for 20+ minutes and then sounds bad.

[Ritchie200]

Maybe it just likes country music!

Like Shooter said, replace those caps.  Try 15,16,17.  Do the cheap basic stuff first.

Jim

[ALBATROS1234]

yea so 15,16,17 were replaced when i rebuilt the bugger as well as all of the paper/wax caps and electrolytics as well as a dozen or so out of spec or darkened resistors. am i not being clear? i do appreciate all responses but i did a pretty standard refurb/rebuild with regard to consumables. when finished the phono and tele/wire inputs sounded fucked so i studied the schematic and eliminated the whole phono input from the 470k back. it then began sounding good......for a while then after shit gets warmed up or something the weirdness happens. and i cant seem to understand why because when the am and fm go thru the same path they sound fine, when i put a record changer thru the wire /tele input which goes strait to the tone/volume array which has had all caps and suspect resistors changed it begins to sound garbled with some volume drop and signal intermittant drop out. i dont know i am gonna go study the schematic more and see what i can find.

[jjasilli]

I can't make sense of the schematic; but Rule 1 is suspect tubes first. 


Because the problem is intermittent, it's hard to identify the problem by guessing at source, or signal path.  It might be just a coincidence that the issue has not occurred while using the radio.


I suggest lying in wait for the issue to occur.  Be ready with a scope or listening amp to systematically test signal quality at each stage from one end of the amp to the other, while the issue is audible in the speaker.

[bmccowan]

From your description, it does not seem that the issue is randomly intermittent, but always occurs after the amp gets warm? If so, since I do not use a scope, my next step would be a listening amp/signal tracer and find where in the signal chain the issue starts. Something is not liking getting hot, a tube, a socket, a resistor, a cap, a pot, a solder joint? In reworking old amps, I have been surprised how many issues I trace back to an old tube socket. Why does it happen with one signal type? Good mystery to solve; aggravating during the chase, pleasing/funny when you solve it, eh?

[jjasilli]

Heat - that was my initial reaction.  But because the issue is intermittent, it's most likely to occur after the amp's been On for awhile.  Hence, the seeming correlation to heat might also be a coincidence.  Educated guesses, and the shotgun approach, are fine -- if they produce a quick result.  But if they fail to work, it's time to get scientifically organized.

[shooter]

sometimes I try logic, but 
since you bypassed the switch, injected at exact place the radio enters, it's still "left" of that point which leaves source signal (try country :), cable, or signal gain since everything "right" is common to both

[ALBATROS1234]

Thanks guys thus one has me stumped. Last night u.played 2 whole albums both sides and it sounded fine.  Today it started warbling 20 min into the 1st side of an album. I can play the radio either am or fm for hours and it never does it. This one is a head scratcher for sure. I have tryed different power tubes and rectifier (6v6 pair and 5u4) but have not switched the 6sq7 or 6sc7 so I can try that. And then study the schematic and try to come up with some sort if plan. Thanks again

[bmccowan]

This is curious. A question, I suspect the phono has lower output, eh? So if so, you are turning up the gain to reach the same volume level - is that right? If so, is the warbling associated with the gain stages?
Regardless, my next move would still be a listening amp/signal tracer to pinpoint where in the circuit the warbling is being introduced.
Alternate - you are not listening to Tiny Tim LPs are you?

[shooter]

I suspect

well
 
source signal (from couple replies back) 

[bmccowan]

Right, not many variables left on the table.

[ALBATROS1234]

About to go do some work. When I get a chance I may try to inject a s one wave onto that tele/wire input then wait and see if it will do it,at which time I will test different points in the signal path and perhaps I can isolate the prob. My luck it won't do it