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Post by BBB on Dec 12, 2013 9:16:33 GMT -5
There is a reported 60 Hz hum in the transmit that I do not hear in the over the speaker in the receiver.
The Mark IVa's have separate receiver and transmitter like the Mark III's, but share the main power transformer (unlike the Mark III's) that is located in the receiver. I see a multi-capacitor can on the transmitter chassis and some larger electrolytics underneath as well.
If the receiver has no audible hum, could the caps inside the capacitor can on the transmitter be causing the hum or are the voltages that come over from the receiver already filtered? Just trying to help narrow the cause. Thanks.
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Sandbagger
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Post by Sandbagger on Dec 12, 2013 11:12:10 GMT -5
There is a reported 60 Hz hum in the transmit that I do not hear in the over the speaker in the receiver. The Mark IVa's have separate receiver and transmitter like the Mark III's, but share the main power transformer (unlike the Mark III's) that is located in the receiver. I see a multi-capacitor can on the transmitter chassis and some larger electrolytics underneath as well. If the receiver has no audible hum, could the caps inside the capacitor can on the transmitter be causing the hum or are the voltages that come over from the receiver already filtered? Just trying to help narrow the cause. Thanks. Generally speaking, when a rig gets as old as the Brownings, it's probably a good idea to replace all of the electrolytic filters. If one is going south, the rest aren't far behind. And you won't have to worry about tracking down which one is the "bad" one (use a scope). |
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Post by BBB on Dec 12, 2013 19:52:01 GMT -5
Good point. Just happen to have some assorted JJ caps on the way. www.jj-electronic.sk/Yeah, and I probably need a high voltage probe for sure. |
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Sandbagger
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Post by Sandbagger on Dec 12, 2013 21:52:14 GMT -5
Good point. Just happen to have some assorted JJ caps on the way. www.jj-electronic.sk/Yeah, and I probably need a high voltage probe for sure. You don't need a high voltage probe for tube radios under 1000V. As long as it's a high impedance 10X probe, that's AC coupled, you should be ok. Generally speaking, you should have less than 3 or 4 volts of ripple on those caps. Much higher than that and you have weak caps. |
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Post by mark4 on Dec 15, 2013 9:18:19 GMT -5
I use the original replacement type. Twist lock, because I don't want to modify the chassis for a clamp mount. But I can tell you I have replaced multiple times a new
40/40/40 @ 525V multiple times in the same radio. Sometimes not even lasting a year.
The manf. Knew they had quality issues and told me it's been resolved. I hope so.
They are quite pricey.
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Post by BBB on Dec 15, 2013 10:38:40 GMT -5
For the JJ can caps, I paid $15 for the 40-20-20-20 and $10 for the 100-100, both at 500V. Still not small change, but a leading Browning parts supplier wants $45 for the 40-40-40 at 450V cap can. I'll give the JJ caps a try and report if they fail.
Mark 4, were you talking about premature failures of the JJ brand can caps in particular?
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Post by 2600 on Dec 15, 2013 16:57:27 GMT -5
Ahhh, if you're SURE that it's really a "60-Hz" hum, that would be caused by something other than a filter capacitor.
All of the rectifier circuits in a Browning Mark 4 and A are full-wave bridges. These produce two ripple "bumps" in the DC output for each single cycle of AC input power. This means that hum from a failed filter capacitor has a frequency of 120 Hz.
May seem like a fine distinction, but if the frequency of the hum is in fact 60 Hz, and NOT 120, this points to other issues.
But if it's 120 Hz, filter caps are the likeliest culprit, especially if they are the original parts, 35 or more years old. They were never meant to last that long.
The frequency of the "hum" you'll hear from the radio's receive or transmit audio caused by old electrolytic caps is 120 Hz, NOT 60. The frequency of that 'hum' can be an important diagnostic clue when trying to track down the cause of this symptom when it occurs.
73
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Post by FIXR on Dec 15, 2013 17:50:25 GMT -5
For the JJ can caps, I paid $15 for the 40-20-20-20 and $10 for the 100-100, both at 500V. Where do the 100's go? |
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Post by BBB on Dec 16, 2013 17:04:14 GMT -5
I just put the 100-100 @ 500V in the Tram D201. Real funny how they called the old style caps "snap in"... the only thing snapping was my pliers as I pulled the cap retainer out like a tooth after the factory soldered the darn thing in place. After it was out, I opened it up for your viewing pleasure:   Looking pretty nasty and burnt in there  ah, much better...  That 10-10-10-10 is out of here too! Got a 40-20-20-20 @ 500 V that is a little wider, but I'll squeeze it in there. If you had the choice, what circuit would you put the 40 uf lead on or doesn't it matter over the 20 mf leads? |
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Post by 2600 on Dec 16, 2013 23:34:42 GMT -5
That looks a lot more like a D201 than a Mark IV.
The original 40-40-40 part is wired with two sections in parallel, serving as a 2-section filter, 80+40. We use the 2x100uf 500-Volt parts like yours to repair a D201. Started using the "LCR" brand back in the 90s, then to a brand "F&T", and now the JJ brand parts from CE Distributors. We drill out the rivets from the twist-lok adapter plate. The holes line up perfectly with the saddle clamp for the new part. A ground lug under the nut under each screw holding the saddle clamp provides the chassis ground points provided by the original part.
Substituting a section rated at 100uf for the factory-original pair of 40s in parallel is not much of a change at all from the original circuit. The second 100uf section should work slightly better than the original 40uf, but you'll be hard pressed to measure the difference.
Biggest advantage IMHO is the 500-Volt rating. The D201 B+ voltage can rise to 480 Volts DC or so when turned on cold. Once the tubes warm up and begin to pull a load, it drops to the normal 410 (or so) Volts DC. A filter rated for only 450 Volts will be stressed above its rated limit while waiting for the cold tubes come to life. Tends to shorten the service life.
This seems to be less an issue for the four-section cap, but a rating higher than 450 would probably extend its service life.
73
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Post by BBB on Dec 17, 2013 9:37:59 GMT -5
The Mark IVa is up next for the filter re-cap. Then a Titan II, then a this and then a that.
Lets put it this way, I've never picked up a classic radio that had new caps in it. Those eBay descriptions should always say: "Vintage Tube CB Radio, works and is guaranteed to have a 60 Hz hum and be off frequency" ;D
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Sandbagger
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Post by Sandbagger on Dec 17, 2013 11:19:06 GMT -5
The Mark IVa is up next for the filter re-cap. Then a Titan II, then a this and then a that. Lets put it this way, I've never picked up a classic radio that had new caps in it. Those eBay descriptions should always say: "Vintage Tube CB Radio, works and is guaranteed to have a 60 Hz hum and be off frequency" ;D That should pretty much be a given for any radio that old. If you want to actually USE a vintage radio (as opposed to just "displaying" it), be prepared to do some level of restorative work. |
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Post by BBB on Dec 17, 2013 20:57:08 GMT -5
That's right, repetitive restoration work... I put the other new multi-cap in the Dam Tram 201 and lost modulation for some reason. Then put the old cap back in line to see and still nada.
Never a dull moment when fixing something that's not broken.
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Post by BBB on Dec 22, 2013 9:26:51 GMT -5
I replaced the 10-10 mf can cap in the Mark IVa transmitter with some new external caps underneath. There was less reported 60 Hz hum in the transmit audio. I left the old can cap disconnected in place. More than likely I'll find a suitable can cap to go in there so the rigs stay as close to factory as possible. The big multi-cap on the Mark IVa receiver was apparently replaced by an aftermarket/ home brew unit made by "Zack Engineering, Portland, Oregon"  Here's an informative link for folks that are thinking about replacing capacitors in their vintage radio equipment: antiqueradio.org/recap.htm |
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Post by BBB on Dec 22, 2013 14:11:50 GMT -5
After further review on the SPEW radio, I still have some hum in the transmit. Will have to check and see if it's 120 or 60 Hz.
The receiver has just the super tiniest audible hum in the speaker when normally receiving. Not really worth mentioning, but the hum really becomes noticeable in the receiver speaker when using the spot switch on either side of the center frequency peak.
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Sandbagger
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Post by Sandbagger on Dec 22, 2013 14:36:50 GMT -5
After further review on the SPEW radio, I still have some hum in the transmit. Will have to check and see if it's 120 or 60 Hz. The receiver has just the super tiniest audible hum in the speaker when normally receiving. Not really worth mentioning, but the hum really becomes noticeable in the receiver speaker when using the spot switch on either side of the center frequency peak. I had a hum issue with a D201 I was working on. It was not related to filter caps. It turned out to be a ground loop in the mic amp circuit and I fixed it by grounding the mic audio shield to chassis ground at the mic jack. You can try testing this theory by disconnecting the mic audio hot wire at the 1st preamp tube and then transmit. If the hum goes away or is greatly reduced, you have a ground loop problem. It could also be a microphonic tube. Try swapping the mic amp tubes and see it it changes. |
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Post by FIXR on Dec 22, 2013 14:58:27 GMT -5
Make sure R219 (thumbwheel) isn't turned up past halfway, and R603 isn't cranked up all the way also. Clean the pins and socket of V203 (12AX7).
I installed that "Zack Cap" about 10 years ago for a local that owned them. I used a lot of them purchased from VibroWorld, especially when CP Manufacturing was having their problems. Zack built them for Hi-Fi and guitar amp replacements. Hence the chrome can.
Alan
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Post by mark4 on Dec 24, 2013 10:43:42 GMT -5
No I was not referring to JJ brand .
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