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Motorola 6y ca. 1937 Problem

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Motorola 6y ca. 1937 Problem Empty Motorola 6y ca. 1937 Problem

Post by mushroombrew on Thu Sep 03, 2009 7:00 pm

I acquired a Motorola 6y radio with the typical loud 60hz hum a while back. It used to belong to my grandmother and I remember that it used to work perfectly about 15 years ago. Once I opened it up I found a mess of incorrect modern capacitors used in combination with the old electrolytics. So I removed the improvised "repairs" and rebuilt the power supply stage with correct value electrolytics using the schematic as a guide. Problem is now that I get no audio except for a very very faint hum. All of the filaments light and the voltage is about 2/3 of a volt high. The Plate voltages are about 40 volts high however, and the 6v6-g power output tube gets VERY hot. Everything that I have checked seems to be in good condition, and I can't identify what the problem is. Does anyone have some insight as to what I should try? Any help will be appreciated.

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Post by sprman on Thu Sep 03, 2009 7:53 pm

First check all solder joints,then check all tubes,then check all resistors.Then check all voltages to each tubes.As a rule of thumb 10% tolerance is ok with voltages and resistance.Make sure all the grounds are good too.Sprman Smile

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Post by sprman on Thu Sep 03, 2009 7:55 pm

Try reversing the power plug to and check the antenna too.Ionce had a bad tube do that to me.The tube checked fine ,found it by replacing the tube.Sprman Cool

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Motorola 6y ca. 1937 Problem Empty come to find out...

Post by mushroombrew on Sat Sep 05, 2009 2:14 pm

Come to find out, two resistors are missing from the radio. A 2.7k 2 watt and a 47k. Both leading from one of the 2uf electrolytics to the 6v6-g output tube, among other things. Since the 2.7k is supposed to be a 2-watt, I'm assuming that it's supposed to be acting as a voltage dropper. I'm going to be replacing all of the resistors and adding the two which were removed. It took me a while to figure out the color codes. I've never had a radio this old, and the majority of the resistors are the wire wound type with a dot of paint on the body. The few carbon composition resistors have a different type of code painted on them. Different from the 1950s radios I'm used to. I used my IR thermometer to measure the temp of the 6v6 tube and it reached 300 degrees F in about 3-4 minutes! Ouch! I hope it's still okay.

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Motorola 6y ca. 1937 Problem Empty Re: Motorola 6y ca. 1937 Problem

Post by sprman on Wed Sep 09, 2009 8:36 am

6V6s will run very hot as will 6L6,6F6,6K6.Make sure voltages are ok on them.2.7K is a bleeder resistor.The resistors should be ok if they are within 10% of value.Sprman Cool

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Post by DannyG on Wed Sep 16, 2009 1:31 pm

Hows the repair on this Motorola coming? Have you been able to breath any life into it yet? The resistors with dots are probably what is known as dog bone resistors. I use the BED method to read these. They are read Body first, End dot 2nd and Dot in center is the multiplier. So if you have a body painted red with an end dot yellow and center dot is orange you have 24000 ohms. Long time ago M was used to designate Kilo so don't let that fool you. Like the man said, go by the color codes on the resistor. Just wondering where you were at with this one?
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Post by mushroombrew on Wed Sep 16, 2009 2:04 pm

Thanks for the reply. I've got it completely re-capped (with the exception of the little micas), and I have new resistors on order from "justradios". Should be here soon I hope. I had a time finding out how to read those codes, but you're right on about them. There are a few of a different type. Plastic body but with the code so sloppily painted on that it's hard to distinguish the body color from the stripe. Their color codes are more or less identical to the "dog bone" resistors you mentioned earlier though. Took a little patience, but I managed to identify them all. Still the same few problems though. 6v6-G output tube plates over-volted by 40v and gets super hot, and just a faint hum from the speaker. We'll see...

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Post by sprman on Wed Sep 16, 2009 9:25 pm

What are the voltages on the 6V6s,if still high go back look at power supply and check all resistors there.Most radios were made for 110 volts AC 120 will give you higher readings. Sprman Cool

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Post by mushroombrew on Wed Sep 16, 2009 10:27 pm

Well, I checked my line voltage and it's 121.1vac here. Is there some way I can limit that without getting a variac? The plate voltages for the 6v6, as indicated by the datasheet from nostalgiaair, should be 255 and 260vdc on pins 3 and 4 respectively. Turns out that pin 4 goes to a grid though. The voltages I get are 290 and 300vdc. The resistors I checked are consistently out of tolerance by at least 10%. Some 20+%. I ordered metal film replacements for each resistor in the radio. Admittedly, that might be a bit of overkill, but anyways... Better to have it and not need it than the other way around. Oh yeah. Filament voltage should be 6.3v, but it's also high at 6.8v. Not sure if that's "too" high, but it's worth mentioning anyway. I appreciate this feedback! Smile

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Post by DannyG on Thu Sep 17, 2009 10:31 am

No need to drop the line voltage. Radio will handle 121.1v just fine. Filament voltage is fine @ 6.8v. Have you replaced the 2.7K and the 47K resistors that you said were missing? Understand that when these radios were manufactured they were using meters much less sensitive than the ones made today so your readings will be higher for that reason and as pointed out earlier the line voltage is a bit higher also. Beleave me, the 6V6 will run hot normally. Justradios in Canada is a great place for components but if you didn't pay extra for shipping, it can take up to two weeks to get your parts. Shocked
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Post by sprman on Sat Sep 19, 2009 2:49 pm

Some where on the ARF forum is a post on hum bucking transformer that drops 120 to 110.Helps radio run cooler and lowers voltage.RCA manual shows max plate voltage is 315 volts and 285 volts grid.Looks like you should be ok once resistors are replaced.Dont forget to check field coil of speaker.you should get 800 ohms and 5.5 ohms.Sprman : )

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Post by Alfred Corbin on Fri May 14, 2010 11:35 am

Check the DC voltage on pin 5 of the 6V6. That should be several volts, maybe 10v, negative. If that voltage is zero, the tube will draw much too high plate current and overheat. That negative voltage comes from the power transformer center tap going to ground through two resistors, bypassed by an electrolytic capacitor.

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Post by mushroombrew on Fri May 14, 2010 11:53 am

I had no idea I'd get so much feedback. Good surprise. Smile Anyways, I haven't messed with this one for several months. Hoping to get back to work on it soon. Looks like I have a few new things to try and I'll be sure to update when I know something. Can't wait to get that awesome warm sound out of it again. Thanks for the responses!

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