Viewing 15 posts - 1 through 15 (of 30 total)
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  • #13048
    CVRS Member

    My parts arrived today, so I ran downstairs to replace the last double cap with 2 singles on my Micro Emerson 713A Well, I install the 2 caps and shield the bare wire and think I’m good to go.
    I turn the radio on to finally hear good audio and I notice the bulb slowly get brighter and brighter. Before I can turn it off, the 35w4 makes like a flash bulb and goes out. So, from one last repair to
    now I really screwed it up. I checked the 2 caps I installed and they have survived. Nothing seems to have touched inside the chassis and I’m pretty sure I installed the caps right. I’m pretty sure somehow
    i got too much voltage on the tube heater circuit, so I guess I will go over the wiring diagram bit by bit and hopefully find the problem. It’s pretty embarrassing screwing up big time on such a simple little radio!! After doing the Globetrotter Console and it working out well, I guess I was flush with success and missed something no doubt obvious. If there is a upside, I won’t have too much trouble finding a 35W4. The panel light might be fun to find however.


    • This topic was modified 2 years, 5 months ago by garykuster.
    CVRS Member

    Part 2: the double cap I removed and replaced with 2 single caps was shorted. I checked out if I installed the new caps correctly and it appears that I did comparing everything to the schematic. I twisted
    one end of both caps together and soldered that to the common B rail (Black wire) The other two ends went to where the old cap connected using the color code on the old cap.
    I was thinking the thing I overlooked was why did the old caps short in the first place. Tomorrow, I will go through the resistors starting with R9. Here’s the schematic. The double cap is C16 (30 +50 mfd)

    Please speak up if I’m missing something obvious.

    • This reply was modified 2 years, 5 months ago by garykuster.
    • This reply was modified 2 years, 5 months ago by garykuster.
    • This reply was modified 2 years, 5 months ago by garykuster.
    • This reply was modified 2 years, 5 months ago by garykuster.
    • This reply was modified 2 years, 5 months ago by garykuster.
    CVRS Member

    This morning I replaced the resistors in the radio except for the 2 big 1w ones that I didn’t have. They both test close enough. According to the schematic, there is resistor R9 between the the caps I replaced in the schematic, only it doesn’t seem to be in that location. If I put my meter on the 2 new caps common and test the other leads, I get 80 on both caps which is the sum of both caps. This seems wrong as there should be resistor r9 between them. I have since blown up another 35W4 and need to get to the bottom of this. Is the fact that I keep blowing the 35w4 significant? or is it just the first link in the series chain. Anyone out there to hold my hand???

    John Greenland
    CVRS Member

    The main thing is to be patient.
    I think that it seems that your resistor R9 may be connected incorrectly.
    Unplug the radio and remove the 35W4 if you wish. It should not matter.
    Measure the resistance from pin 7 of the 35W4 to the common rail. Pins 2 or 6 of the 12AT6 would be a good point for the negative rail.
    The resistance should be infinite after a few moments when the 2 caps in C16 discharge.
    If my guess is correct it will probably be low.
    I suspect that one end of R9 may be connected to the common rail.
    In that case the current thru the 35W4 will be excessive.
    Good luck
    John G. VO1 CAT

    CVRS Member

    Thanks so much John.
    I measured the resistance from pin 7 of the 35w4 to the rail and it had just 1.2 0hms. I tested R9 (Brn Grn Red) 1500 ohms and it reads 1672 in situ. looks close enough for me, but I have a new one.
    I traced R9’s location in the circuit and it comes from Pin 7 on the 35w4 and connects to pin 6 on the 12BA6 where it meets up with 1 end of the replacement C16 30 MFD cap. This seems to follow the schematic as far as I can tell. This radio had the feel of an untouched radio. The screws all felt just right taking it apart, so I am pretty certain I’m the first one in there. I had a closer look and found a resistor running from a ground location on the tuner to R4 the volume control that I can’t find in the schematic. It’s (Red Purp Grn) 2,700 ohms? It reads 2,758 which I’m sure is fine. I should also add that the radio was already much improved by all the other caps being done (the big buzz was mostly gone) and there only remained a warbling effect in the audio until I changed out the C16 set of caps. I can’t be sure, but it seemed like the radio was warming up normally for about 15 seconds (the second time) and I raised the volume as the second 35w4 winked out. Could have been coincidental timing, but thought it was worth mentioning.

    Thanks for your help I really appreciate it and hope to get to the bottom of this.


    • This reply was modified 2 years, 5 months ago by garykuster.
    CVRS Member

    I spent a really long time studying the schematic to try to understand why the heater voltage is spiking and killing the panel light and 35W4. I found a Description and rating of a 35W4 (below) and don’t understand how the line voltage is transformed into 35 volts for the heater circuit. I don’t see a transformer in the circuit so is it resisters? All test O.K. or are replaced. I can only assume I’m not understanding something fundamental about this circuit as my instinct is to put a meter on pins 3 and 4 of the 35W4 and take a reading. I have looked at the base of all the tube pin sockets and in general for wiring that may have been disturbed when I worked on it. I could find nothing wrong. I have read “The fundamentals of radio”, but how much I retained in my 63 year old brain is questionable.

    Also found this

    “As a cost reduction measure, especially in high-volume consumer receivers, all the tube heaters could be connected in series across the AC supply using heaters requiring the same current and with a similar warm-up time. In one such design, a tap on the tube heater string supplied the 6 volts needed for the dial light. By deriving the high voltage from a half-wave rectifier directly connected to the AC mains, the heavy and costly power transformer was eliminated. This also allowed such receivers to operate on direct current, a so-called AC/DC receiver design. Many different US consumer AM radio manufacturers of the era used a virtually identical circuit, given the nickname All American Five.”

    Lastly, to make testing the circuit easier, I thought about busting open one of the burnt out 34W4’s and converting it into a tool to test the circuit by soldering on test wires to the pin base of the tube in order to check the circuit easier with a meter and insuring the pin socket doesn’t get damaged from probing. I can’t always get underneath every tube to test it there. I made one , here’s a pic…

    Gary Albach
    Forum Participant

    Hi Gary – you have a number of subjects here so I will try and address them one at a time.

    First, you ask where does the 35 volts for the heater of the 35W4 come from? There is a good explanation on the website here:

    but in short, the heater of the 35W4 is in series with the heaters of all the other tubes and the string is across the AC line. The voltages of the tube heaters in series adds up to approximately the line voltage (50+35+12+12+12=121 volts). It’s like the old-fashioned Christmas tree bulbs in series; if each bulb is 6 volts, 20 bulbs x 6 volts each = 120 volts.

    Next, how is the dial light connected? Again, the best explanation is on the website, which says:
    ‘The tube manufacturer had provided a tap on the heater of the 35Z5/35W4to operate a 6 volt light for illumination of the dial. But the lamp filament also needed .15 amps so it just couldn’t be connected to the six volt tap. The voltage would drop way down and part of the heater would be under heated while the rest would be over heated. The currents were equalized by having the B+ current flowing through the parallel combination of the lamp and portion of the heater. So the plate of the 35Z5/35W4 connects to the tap as shown in the figure. All of the tubes had been designed so the B+ current would be right for the lamp.’

    As for working under the chassis, you will get better with practice about probing around the bottom of tube sockets when the wiring is crowded! Just take your time and maybe invest in a few different shapes of probes and wire clips.

    And finally, how to stop burning out the 35W4. John, above, has started you with the right measurement and you say that the resistance between pin 7 of the 35W4 and the B- rail is only 1.2 ohms. If you remove the 35W4 tube (by now it could have an internal heater-cathode short) and measure resistance from pin 7 on the tube socket to the B- rail, do you still get 1.2 ohms after 5-10 seconds? If so, either the 30uF filter capacitor is shorted, or the output transformer primary winding is shorted to ground (B-). You can check each of these by disconnecting them and measuring them separately. If your resistance measurement on pin 7 of the 35W4 socket increased to 1500 ohms and stopped, the 50uF filter cap is shorted. If the resistance slowly increased into the megaohm range, all is well with the new filter caps. At this point, we start looking at excessive current draw on the B+ line, starting with the 50C5. But first, tell us the results of your measurements from pin 7 on the 35W4 socket.

    Other forum participants, do you have any other suggestions to help Gary? Please join in here.

    Gary A

    Gerry O’Hara
    CVRS Member

    Hi Gary and Gary,

    I think Gary A. covered most of the bases for now. From the symptoms you describe, I would also suspect a faulty electrolytic or excessive HT current draw through the lamp section of the 35W4/bulb, as Gary says, likely originating in the output stage (faulty tube, eg. heater cathode or other tube elements shorted), shorted/leaky C14, or a physical short circuit – maybe even a blob of solder somewhere. It would also be worth checking for a short in the heater string.


    CVRS Member

    Thanks Gary, et al
    I have been reading everything I can find on the topic and already have a much better understanding of the issue. So, in short, I’ve figured out and understand the first half of your email.
    I read John’s reply and perhaps I did the test incorrectly or didn’t interpret the results correctly, not sure. But, resistor R9 seems to be in the right place according to the schematic.
    Here’s the part I don’t understand and where my knowledge is clearly lacking; When I tested to see if the 2 caps I installed had survived the overvoltage, I got a reading of 80+- mfd’s
    which seems to be the total of both 30 and 50. Although both caps have a common B+ rail connection, the other end is separated by the R9 1500 resistor. If they were just in parallel, I would expect a reading
    of 80.
    I unsoldered the plus end of the 30 cap and they test separately 30 and 50 as you would expect, so the new caps seem to be o.k. I am waiting on a complete set of tubes and 2 spare 35W4’s
    to arrive as well as some panel lights 44’s and 47’s but am having difficulty importing them for some reason. Perhaps I can find them closer to home. Also want to buy a Variac as my first piece
    of equipment to get me started.
    I will do the pin 7 socket thing again. I did have the burnt tube out when I did it the last time.

    Thanks Gary K.

    CVRS Member

    With the meter connected between pin7 on the 35w4 and pin 2 (B rail) of the 12 AT6 , I got the result below. 25.70 Mega Ohms. I left the meter attached while I was typing this and it’s now reading: OL M Ohms.
    I pick up the tubes tomorrow, but no panel light yet. so the cap 30 looks o.k.? Is that right? What’s next he asked with only a meter and a modicum of knowledge at hand.

    Gary In Montreal (63 year old newbie)

    Jean Marcotte
    Forum Participant

    One easy way to check if you have a problem in the filament string is to measure the resistance between the two pins of the wall plug (unpluged of course) with power switch on. You should read aroud 120 ohms more or less. If you have a significantly lower value, you have a short somewhere. It can be any of the filament lug (pin 3 and 4) that is in contact with the B- . A careful visual inspection can reveal this. It can also be a filament to cathode short in one of the other 4 tubes. The short may only appear when the tube is hot enough (like after 15 seconds maybe). Also check wich portion of the filament in your two burnt 35W4 is open. If it is between pin 4 and 6, B+ is not involved. If it is between pin 3 and 6, then it is definetly a B+ problem.

    Jean (in Montreal also).

    CVRS Member

    Fantastic Jean! Merci bien!
    I did want to check the filament string next and that’s a good way to do it! Gerry had mentioned to check that as well and now I know how. I am leaving now to see Claude St. Onge to pick up my replacement tubes (tube bazaar ) I will also check the burnt tube as you recommend. Nice to know there is a radio brother here in Montreal!. O.K. , I’m back. I replaced the burnt 35W4 with a
    new one and put the meter across the plug, turn radio on and I get 120 Ohms. So far, so good. Will have more time later today to test the burnt tube.

    The tale of two tubes : First I tested the heater circuit Pin3 to Pin4 (35W4) on the bad tube and it’s open. Then I compared the heater circuit on the new tube and have 35 Ohms.
    Next I followed Jean’s instructions and took the following measurements:

    Bad 35W4 , Pin 4 to Pin 6 = OL. Ohms open, Pin 3 to Pin 6 =30 ohms

    Good 35W4 Pin 4 to Pin 6 = 9 Ohms, Pin 3 to Pin 6 =27 Ohms

    So I guess it isn’t a B+ problem? I checked over and over for blobs of solder, or any crossed up wiring and all is like it should be. This is the cleanest chassis I have ever worked on without any corrosion at all.
    Thanks, Gary in Montreal

    • This reply was modified 2 years, 5 months ago by garykuster.
    CVRS Member

    More sleuthing time today. I tested all the tubes heater circuit individually and they tested good as expected. Next, to eliminate the possibility of a cathode to filament short, I replaced all 5 tubes one by one
    carefully watching the 35W4 and killing the power before it fries.(waiting for a variac) All tubes warm up quickly with the same intensity (including the 35W4), then after around 8 seconds, the rest remain normal, and the 35W4 quickly brightens up as the heater filament goes over voltage accompanied by the first barely audible buzz from the speaker. The 6v panel light replacements haven’t arrived, but in the short term, I don’t think that matters too much as it burned out at the same time as the first 35W4, so it’s not the cause, just another casualty. I am sorely tempted to reinstall the shorted cap again to see if the radio will work again as it was fine albeit garbled on the audio before I ‘fixed’ it. Lastly, I have not replaced the 5 caps? resistors? contained in this device (pic below) because I read these are usually o.k. Should I go there next?

    Sterling Spurrell
    CVRS Member

    Well not sure what I would do next but in my trials I always replace one cap at a time and try the radio again to see what changes. So if this is the only work that you have done and it did not burn up your tube then I would look at this area for sure again. Maybe post a picture of this work and all of the chassis and maybe someone will see something you are missing.

    CVRS Member

    I decided to do a Hail Mary today and reinstalled the defective double cap. Turn the radio on and it warms up normally and Works fine on all stations except for a warble on the audio and a slight hum.
    So, back to where I was before changing out the double cap. Now why did this happen?? Both new caps test perfect at 30 and 49 uF
    The old wax double cap has Green+50 MFD. 150 WV. and Red + 30 MFD. 150 wv. The common black wire goes to the B rail.
    I twisted 1 end from each new cap together and soldered to the B Rail, the 30uF cap to where the Red wire went and ditto 50uF cap to the Green both are 160 V.
    Where did I screw up?

    Thanks, Gary in Montreal

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