#16
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Observation on aged caps
I noticed on these caps they all showed evidence of expansion and contraction causing the cap to deform
I think this is caused by the cold, expanding the caps When I see this I automatically make the cap a candidate for replacement I believe cold weather storage is worst than hot weather storage Any opinions on this? |
#17
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Checking B+ and rectification
I put a DC meter on my + and - of the selenium rectifier and got a reading of MILLIVOLTS?
Could the lights be on, the tubes heat up and I get some sound and radio if the B+ was so low? If the B+ was very low could it be the cause of the muddled sound? Should I measure the ~ AC on the rectifier I think I will change the original selenium rectifier for a full wave bridge rectifier and a 100ohm 10watt resistor Last edited by omegaman; 01-15-2012 at 09:39 AM. |
#18
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Lights and tube heaters are AC so wouldn't depend on the B+. Check the plate voltage on the output tube(s) vs. B- and compare to schematic voltage, or if no schematic, compare to typical ratings of that tube from a tube chart. That would be a clue as to the health of the high voltage supply.
__________________
Reece Perfection is hard to reach with a screwdriver. |
#19
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On the bench with a proper meter connection the B+ is 264vdc and the schematic calls for 270 so I am now under the impression that the rectifier is working properly.
I am going to check the resistors and change the last two caps, especially the NP 20uf in the crossover circuit I have to read up on checking plate voltages on the output tubes, since I have not done that before and I like to understand what I am doing. |
#20
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Connect another known good speaker to the extension terminals on the radio and check if that sounds good indicating radio speaker trouble. Is the "muddled" sound on AM, FM, or both? If you play an external source through the phono input, how is the sound? The external source could be a transistor radio with cable plugged into its earphone jack and patched into the phono input. If sound is good this way would point to the audio/speaker section being OK.
Tube plates will have B+ voltage on them, varying by design of the radio to suit the tube. Your - meter probe will go to common B- and the + probe to the plate pin of each tube (look up tube basing diagrams.) The schematic will show approximate voltage at each tube plate or if not, then a typical range of voltages can be read from a tube chart. If the voltage is way high or low the circuit around the tube needs to be checked.
__________________
Reece Perfection is hard to reach with a screwdriver. |
Audiokarma |
#21
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Determining exactly which pin to connect the DC meter to on the output EL95 to measure plate voltage was surprisingly challenging with my limited mental capabilities.
Finally, Wikipedia showed me that the anode was the plate After reading up on measuring plate voltage (mostly guitar amps) and looking at EL95 diagrams, all I could do was match the plate symbol to the pdf diagram of the EL95 pinout and determine that EL95 PIN-5 the 'a' pin was the pin that I would connect my probe to I have those clip probe connectors so I will connect to pin 5 of the EL95 and measure the DC voltage Target voltage is 260 Does it matter which EL95 I test first? Last edited by omegaman; 01-21-2012 at 02:32 PM. |
#22
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Why am I seeing no voltage on the anode plate?
My first ever attempt at reading plate voltage
I set my DMM to DC ___--- I connected the black lead to the - on the rectifier I connected the red lead to the EL95 PIN5. At first I counted wrong but after I figured it out I got the connection correct Last edited by omegaman; 01-28-2012 at 12:52 PM. |
#23
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From the terminal side of the tube socket, starting at the wide space, and counting clockwise, pin 5 is the plate. You should have high DC voltage between the plate and the negative side of the filter caps. Using the schematic, follow the high voltage from the power supply section through every component until you arrive at the output tube plates. Something is open or shorted keeping power from those tubes.
__________________
Reece Perfection is hard to reach with a screwdriver. |
#24
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From the terminal side of the tube socket
The terminal side being the side where the tube plugs in? Thats how I counted |
#25
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I was 99.18436572% wrong in calculating the correct pin to test
Terminal side of tube socket, where the wires terminate, counting clockwise from the open space between pins PIN 5 Now I get it Makes a big difference PIN 5 measures 246 on both EL95s Still sounded too bassy through the regular speakers So I connected an external speaker and it sound much better, matter of fact it sounds great A lot of bass but I can control it The woofer is now suspect. I measured 5.1 ohms from the 10 inch woofer, which is what I would expect I'll pull the woofer out and check it out. It is wrapped in a sack cloth Last edited by omegaman; 01-28-2012 at 03:03 PM. |
Audiokarma |
#26
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Theres your problem right there Vern
Nice speaker from Grundig
Looks OK from the back Total failure of the foam surround Well, It's a GOOD thing I went ahead and did a total re-cap of the whole receiver AND bought new tubes when it probably just needed a new surround in the first place yeah... a good thing... Now I have to pull all the speakers and check them out Last edited by omegaman; 01-28-2012 at 04:11 PM. |
#27
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Good for you, you got it going! That's a good speaker and it's been matched to the set to give its characteristic performance. You may have replaced foam surrounds before but if not it's not difficult. Here is one source, there are others:
http://www.matelectronics.com/acatal...ories_149.html
__________________
Reece Perfection is hard to reach with a screwdriver. |
#28
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Well, It turns out replacing the foam surround/gasket is a manual task that does not include a ready built kit.
I conferred with my pals in AudioKarma and one found this: http://www.radiomuseum.org/forum/gru...rtschrank.html So I have to think about this a bit as I want a cleaner solution for the new surround But Hey! I guess the radio part works Okay and the re-cap was successful. First time I measured plate voltages First time I re-strung a tuner I still have work to do but all-in-all a great experience Last edited by omegaman; 01-29-2012 at 12:17 PM. |
#29
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You should be able to do this taking a little care and ingenuity. The dealer that I referenced has I believe a $25 minimum which is a lot if all you want is a surround worth a couple of bucks. You could modify one of their surrounds to fit your speaker. Or, I used to re-surround speakers using 1/4" "hobby" foam, cutting my own surround "donuts." You'd want to be careful not to get any debris down inside the voice coil area while working: your speaker has no dust cover there. You could cut a round piece of black felt (fabric store) an inch or so in diameter and glue there first. Get fabric glue while you're there: similar to white glue but stays a little flexible and is waterproof as it's meant to glue patches on clothes that will be washed. When glueing on a surround, I used to glue it onto the cone first and then next day glue the outer part to the speaker frame, all the while making sure that I was not bending the cone to make the voice coil rub on the magnet pole piece. One way to do that is to play a radio low through the speaker while you're working on it and listen to be sure there's no rubbing distortion. Hold down the outer edge of foam with bunches of clothes pins to dry.
__________________
Reece Perfection is hard to reach with a screwdriver. |
#30
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Replaced the surround AND gasket
I got some thin flexible foam and cut a surround out 9" inner 10.5 " outer.
I used some adhesive from another surround I did (good stuff) After several hours spent clearing old adhesive off of the cone, I glued the surround onto the cone, using a small plastic ruler to press up from below the surround. This process took a couple of hours as I had to do a little at a time. Let that dry overnight Next day I hooked up an amp and CD player to the speaker, applied the glue to the outer surround-to-frame edge. Playing the 30Hz test tone allowed me to align the cone as to not touch the coil. Each ten minutes I test again to make sure. Also test in horizontal position as this is the normal position of the speaker I also cut out a 4mm thick ring of open cell foam to use as the gasket I'll re-install the speaker after it cures VICTORY! It sounds fantastic The multi-sonic control is effective and the base is sweet and controlled The gaskets are important to flush the speaker against the cabinet I think I spent 30 hours of labor on fixing the woofer alone Last edited by omegaman; 02-05-2012 at 01:24 PM. |
Audiokarma |
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