Replaced the capacitors but all I get is a hum - Printable Version
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+--- Thread: Replaced the capacitors but all I get is a hum (/showthread.php?tid=7826)
RE: Replaced the capacitors but all I get is a hum - klondike98 - 11-06-2013
I use leads that have alligator clips on the ends so you can give them a little twisting when they are clipped on and ensure a good connection for the measurement. (actually its an alligator clip wire with one end clipped to the test lead; such as http://www.amazon.com/Elenco-TL-6-Standard-Alligator-10-Piece/dp/B0002JJU28/ref=sr_1_1?ie=UTF8&qid=1383790904&sr=8-1&keywords=alligator+clip+wires and the other end clipped to the resistor wire) As morzh indicates, its tough to get a good measurement with the pointed probes held on the resistor wire by hand.
RE: Replaced the capacitors but all I get is a hum - Andrew Fleck - 11-06-2013
The meter is working correctly, but I was not. I was reading the schematic wrong. Now that I have figured out the schematic I see that most of the resistors are within tolerance. The one exception so far is a resistor that it supposed to be 1 meg ohm, but is consistently measuring out to 12 meg ohm. Is this one resistor enough to disable the radio?
Thanks for all of the suggestions. I don't have the alligator clips, but I have been scraping away the old crud so I can get a good connection. The probes I have actually have a little detent near the tips that enable me to hook the leads in there. It works well.
No offense Morzh. I read a lot before I mess around with electricity, and I saw that the multimeter produces the required electricity to read ohms. It is confusing for somebody new to it like myself though.
If I have to replace the suspect resistor, how do I know what wattage to get it in? The schematic says nothing about wattage for them.
RE: Replaced the capacitors but all I get is a hum - BrendaAnnD - 11-06-2013
Without looking at the schematic, I would suggest it's most likely a 1/2 watt resistor. Resistors that size are usually in AVC or audio circuits, in low voltage and low power circuits.
RE: Replaced the capacitors but all I get is a hum - Andrew Fleck - 11-07-2013
I'm having a bit of a disaster tonight. I replaced the suspect resistor. That was easy. I then decided to use deoxit d5 to clean the contacts for good measure. I didn't spray a lot but I can see some residual film in the chassis. Now the radio will not power up at all. I noticed some of the cleaner worked it's way into the tube sockets. I took the tubes off and I am hoping this stuff will dry out. On the positive side my bandswitch operates great! My suspicion is that the cleaner will not dry out on it's own. Could I have damaged something doing this?
Andrew
RE: Replaced the capacitors but all I get is a hum - jerryhawthorne - 11-07-2013
Andrew, you sprayed the tube sockets? Well, if they are the type made from two pieces of a material and are riveted together I can tell you from experience, they don't like any "wet" material on them. It creeps between the two sheets and carries any dirt etc. with it. I found that getting rectifier sockets "wet" with almost anything will result is causing shorts followed by smoke. Need to be careful on those sockets. Doubt it killed your radio, you would have seen smoke by now. Let things dry out well.
Jerry
RE: Replaced the capacitors but all I get is a hum - Andrew Fleck - 11-07-2013
Jerry,
I didn't target the tube sockets. They were a casualty of my amateurish spray job. The lights won't even come on now. My connections are good. Nothing came loose. I double checked everything for continuity. I will let it all air out for 24hrs and see if that helps.
Andrew
RE: Replaced the capacitors but all I get is a hum - morzh - 11-07-2013
Pull tubes out, let it dru overnight.
Which Deoxit did you use - Fader Lube? (green). If yes, I think it will be OK once it dries up.
RE: Replaced the capacitors but all I get is a hum - BrendaAnnD - 11-07-2013
Check your power source. Make sure you still have power coming to the radio. DeOxit D5 won't damage anything, at least it never has for me. Generally, if nothing comes on on the radio, it's your power source, the cord, or the power switch on the radio.
RE: Replaced the capacitors but all I get is a hum - Andrew Fleck - 11-07-2013
Brenda,
You are right. I checked connections again and for some reason my connection going from the cord to the transformer needed tweaked. All is well again with the exception of the radio still not playing.
So to sum up what I have done so far,
1. Recapped radio
2. tested resistors; replaced one that was way out of tolerance
3. definitely cleaned the bandswitch
So, I will try and best describe how the radio acts when I try and play it. The hum that I included in my title to this thread is not a loud one. I would describe it as a soft hum. Tuning the radio results in no audible difference *unless I move the volume control and the tuning at the same time. I then hear static, but it doesn't get louder or quieter. The static is just as loud at the volume low end as the high end. I can only produce this static if I move volume and tuner at the same time. Any thoughts? I want to get the tubes tested, but from what I have been reading I should still be able to hear something. Maybe if they are all faulty maybe not.
I appreciate all the help from everyone. I'm sorry I'm all over the map with this. I should have caught the bad connection. I will try and investigate more thoroughly before I run over to the computer and post every little problem. Short of getting the tubes tested I'm not sure what to do next. My gut is telling me that something with the volume is not right, but I don't know enough about radios to be certain.
Andrew
RE: Replaced the capacitors but all I get is a hum - morzh - 11-08-2013
Andy
1. Measure the voltages across all tubes, compare with the Voltage chart. Report it here. Pay most attention not to some discrepancies but to drastic ones. Like voltages on plates being 0V or close instead of say 100+ Volts like they should be.
2. In case most turn out OK,
What you should do first is to try to do the "grid cap touch" test.
You have to be VERY careful as I understand you are not well familiarized with things yet.
You can use a screwdriver with an insulated handle for that. Most of us use our hands but for you I'd recommend starting with the screwdriver.
Unfortunately In your case there are no grid caps so it makes it a bit more challenging.
Start touching with the screwdriver the grids of the tubes going away from the audio like the G1 in this tube.
http://www.radiomuseum.org/tubes/tube_7v7.html
Listen if the buzz is there when you touch.
See when it stops.
then start checking the continuity of the interstage RF transformers after the tube where it stopped.
3. Overall continuity check on all inductances (RF transformers mostly what they are) should be done.
RE: Replaced the capacitors but all I get is a hum - Andrew Fleck - 11-08-2013
Morzh,
I need some help checking the voltages on the tubes. Where do I put ground? I'm assuming anywhere on the chassis. Do you have any links that can show me the whole process. I am a complete beginner at this and am learning as I go.
Andrew
RE: Replaced the capacitors but all I get is a hum - Andrew Fleck - 11-08-2013
Also, is it safe to power the radio with the chassis upside down?
RE: Replaced the capacitors but all I get is a hum - Andrew Fleck - 11-10-2013
Please disregard my previous questions. I have figured out how to test voltage on these tubes. The finding is not great. The only tube that has any real voltage is one that is labeled 6X5G. It measures out at 451 volts only on the pin that one of the electrolytic capacitors is connected to. The largest voltage that I got off of any other tube was 1 volt, but I can see light in all of the tubes when it is powered on. I do measure voltage on the pins on the underside of the chassis correct? Any thoughts?
Andrew
RE: Replaced the capacitors but all I get is a hum - BrendaAnnD - 11-10-2013
Time to check the resistance across your speaker field coil. That's the next thing in line for your B+ chain. If that's open, it will not allow power through to anything else in the set (filaments are a different winding on the transformer, so the tubes will light regardless).
If the field coil is open, you can bypass it temporarily with a 1000 ohm, 10 watt resistor for test purposes. Be aware, though, that your speaker will not perform as it should, since it will have no magnet for the voice coil to work against. Your volume will be extremely low and distorted.
Your 451 volts sounds pretty high, but it is likely due to not being under load. It should drop to about 250 volts once you have the rest of the radio connected to it.
You can get a voltage reading connecting your negative lead directly to chassis ground, but take note that the large can type resistor mounted to the chassis forms the circuit from the transformer to chassis ground (and creates the negative bias for some parts of the radio). Make sure this resistor is OK, should be about 180 ohms from one end to the other.
RE: Replaced the capacitors but all I get is a hum - Andrew Fleck - 11-12-2013
Thanks Brenda. I'm not the greatest with the schematics. What does the speaker field coil look like exactly. I tested voltage on the tubes without the speaker connected. Should I have connected it first?
Andrew