Hi all,
Well I must have blown something in the re-cap. I got this 48D given to me and it was playing quite well. So I tackled it starting with a nice laquer re-finish, looking great, and then went to the caps and a few resistors, pretty straight forward. Not much had been done to it in the past, had orig. electrolytics. Several .05 caps had been replaced long ago.
So I fire it up and I have no signal, just noise, a strong hum/growl not affected by the volume control. Not unusual for me, the trouble shooting starts and it's pretty exciting when I find my goof and the signal comes through. Not this time. This time I haven't been able to find my good and I don't know how to go forward, maybe you can give me some direction.
First, made sure all the dimmers were off in the house, then cleaned all the tube pins (I had used a naval jelly type product on the chassis and thought some remnant could be insulating somewhere), swapped out a couple of tubes that I had seconds of, the others going by my Hickok tester. Because it worked before the re-cap of course the first thought is an incorrect wiring, but I took pictures and drew sketches as I went and with those and tracing everything with the schematic, cannot find an error. Checked the tuner fins for short. I attached an ipod to the volume pot and recieved great sound so the af section is ok? All the while there is this loud hum/growl uneffected by the volume control. I can put an rf signal from a sig. generator into the antenna wire and get a very low signal out of the speaker again with the hum/growl overwhelming it. Plate and heater voltages are all there. There is no voltage on the secondary of the 2nd I.F. transformer.
Because it worked before I started it's likely something I did, like short something. Please, I hope that you can send me in a good direction.
Thanks,
Alan
maybe you are in a rare situation where you might have received a bad "new" capacitor???
can you remember if a particular capacitor size that was installed did not match the schematic?,, meaning there was a schematic error but the actual installed cap was correct but.. you installed the size that was on the schematic?
I am an amature for old radios and know less than the rest of the people here however i am electrically savy so ..... just asking things that i think of first that have happened to me.
i see some shielded welded volume control wiring,,,, i had a rare instance where out of the blue,, the actual conductor inside that shield was arching to the shield causing similar issues. i temporary bypassed the shielded wiring and the set worked so i bypassed it.
You have 2 coils to the left their is a tube socket bottom picture a red colored cap close to a orange body dog bone. Top picture red cap has been replaced by a nice new yellow cap looks like the wire from the yellow cap could be touching the wire from the orange dog bone???
Start retracing the steps. Everything you've done so far, not just the caps.
Any disconnected wires. Or not reconnected.
Wrong cap values.
Bad caps.
Trace every cap of the non-working part to the schematic.
Use sig gen to pass a signal through, see where it stops.
three brown Caps in right corner,,,only 2 new electrolytic,,,1 yellow cap,,,but 1 brown cap missing on second tie terminal strip,,,,hope that helps
Thanks for your replies, you're keeping me going.
I'm sorry the after pic didn't show the third electrlytic in bottom right corner, so nothing seems wrong there, it's two 10 mf and a 22 in place of two 8's and a 16.
Checked all caps for capacity and place in the circuit against the schematic, all good. Started checking the cap values, good those that I checked (ya have to disconnect an end to check em).
I get an AF signal loud and clear from the 2nd IF transformet on.
Couple of odd things, I can connect the sig gen to the antenna wire and get a faint signal through overwhelmed by the hum/buzz. The antenna var. cap is grounded on both sides. So the antenna wire has continuity with the chassis. Seems to ground somewhere in the push button band switch mechanism. Looked around in there but it's tight and involved disconnecting a lot, still don't understand that switch well. I keep thinking that somethings grounded that shouldn't be.
I've gotta figure this one out, my neighbor gifted me the radio and I can't tell him this is how it ended.
Thanks everyone,
Alan
Unless you mean the antenna tuning cap is grounded on both sides of the base, (I construed it that both sides of the cap are grounded) - only one side should be. It could ohm out as grounded due to the low antenna coil resistance, but it should not be grounded.
I am also not sure what you mean by:"I get an AF signal loud and clear from the 2nd IF transformer on."
In the tuning cap, antenna side, there is continuity between rotor and stator. The wire attached to the stator comes from the push button band switch mechanism, the other side of the mechanism is the antenna long wire. There's a ground somewhere in the band switch mechanism. Come to think of it, there are some coils attached to that mechanism. I better look at those. I thought that I had seen continuity between rotor and stator before? Are you sure that's simply wrong? I can get an AF signal all the way through from the antenna wire and an AF all the way through the audio sections, but no signal is going through the tuner, no variance by the tuner when inputting an AF signal with the sig. gen.
Thanks!
Alan
Maybe there's a clue there. With the sig gen I send a audio signal in after the 2nd IF transformer and the tone is loud and clear and variable with the volume control. I send an RF signal of any frequency into the long wire antenna and it's heard very low and not variable through the volume control although it is controled by the sig gen level.
The bottom left corner some of the wires look like they have been moved just a little could be with old insulation something is touching. The blue wire going around towards a brown wire near one of the 2 coils there is a spot on the brown wire with no insulation that the blue wire could be making contact.
Man, I still haven't figured this thing out. The long wire antenna is somehow grounded. It has continuity with the chassis. This is absolutely indicative of a problem, right?
The long wire antenna attaches to the underside of the band switch. There are terminals on the switch that are purposely grounded. Really hard to disassemble or get an eyeball in there to trace the circuit. It seems futile because I'm looking at things that I didn't touch, and the radio received fine before my re-capping. I've poked and prodded connections, checked coils, retraced the schematic, sketches and photos and I'm finding nothing.
Any insight into that grounded antenna wire?
Thanks everyone,
Alan
Are you seeing the low resistance to ground through your radio's antenna coil? That would be normal.
The schematic for this has a lot of symbol for the band switch that makes things a bit obscure for me but I've looked at some others that show an antenna clearly going to ground. So I guess that's fine. Other radios that I have seems to have a cap keeping the antenna wire from being grounded, none of the other radios show continuity between the antenna wire and chassis.
So back to staring at this thing. I've checked the coils but I guess I'll give them another look. When I send an RF signal into the antenna wire it comes out the speaker but really low, it's not getting amplified?
Thanks,
Alan
this whole thread bothers me because it happens to me all the time. i go in on a working set and end up with a problem after simply doing some preventative maintenance work. typically in all cases i have ended up dealing with a problem i did not cause, the problem just simply decided to show up!
so.... i want you to pay attention to something that happened to me out of the blue.
go to your middle knob.
observe the conductors that are "passing through that chassis welded / soldered shield.
***there must be no continuity from that shield to the wire inside it.
de-solder the far end "wires" up near the tube socket or at the what appears to be your volume knob.
set your meter to check to see if this wire has any resistance to the outer shield.
~~this happened out of the blue in my minerva tripic master set.
if you have continuity, disconnect the conductor on both ends and add another wire bypassing it un-shielded for the time being.
another thing.. you must try
flip the radio back around with the tubes facing up.
grasp each end of the chassis while playing and twist the chassis itself slightly ,, your checking to see if there is an intermittent cold solder joint somewhere.
you are going to have to get your hands on a schematic that shows the voltages you are to have with the set playing , these voltages would be for each and every pin usually while you are in AM mode.
sams photo fact info can be had for online purchase and downloadable pdf's.
next..
while the set is on... use a plastic tool to wiggle test all your connections while the set is dialed in to a station you would expect reception on. i would focus heavily on that push button section myself, just by it being upside down, gravity might be playing a role in your making good contact ,, wiggle test your push buttons fairly aggressively.
pick up the powered *OFF* radio and shake it to listen to see if you hear things rattling around in one of the IF's.
rig yourself up a dim light bulb tester which is nothing more than a light bulb in series with the radio sets HOT leg of power.
light should be going bright as the set powers up and then goes almost out when the tubes come on line.
if anything other than this happens, report back.
i would imagine you could rig up a dim light bulb tester on the output leg of your onboard transformer as well., this would give you a way to help isolate which circuit is being affected, yet it would be up to you to figure out what size bulb would apply to which lead,, since some of the secondary may be high voltage and others being very low. your secondary high voltage output might have a center tap you can manipulate to utilize only one of the hv legs at a time to find the right bulb.
if ive given bad advice i expect to hear about it,,