Hum samples 60Hz vs. 120Hz & Seeburg trip -new link 9-26-16

http://www.fileconvoy.com/dfl.php?id=ge ... eabb0163de

When diagnosing an amplifier hum problem the nature of the noise can be helpful. So here are two typical examples.

This is a .wav approx 23 seconds of hum. First is the ever-popular 60Hz followed by 120 Hz.

The first was from an ungrounded preamp input and has the usual "snivets". The second is from a generator.

In listening to these it is best not to use too high a volume on small speakers as this can cause harmonics and confuse things.

60Hz Hum is typically from inadequate shielding, heater-cathode leakage and ground loops. Inadequate half-wave filtering can also cause this.

120Hz Hum is only found on full-wave power supplies when filtering is inadequate or the load exceeds the time-constant of the filters.

Hopefully these will make diagnosing amp hum problems a little easier.

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Seeburg trip switch:
http://www.fileconvoy.com/dfl.php?id=gd ... 532110fd14

Proper setup is indicated by the trip whisker snapping back to rest when reset and not following the arm at setdown. The biasing spring is set to assist the trip switch and minimize loading the arm and stylus as the arm enters the trip-off point.


Rob/NYC

Well, I can't hear the 120 Hz hum, so even if I have it, I wouldn't know it, so no need to get rid of it. I do have a hum that sounds like the 60 Hz hum. On a Wurlitzer 2150, what is the recommended process for isolating and correcting the issue?

Thanks
Brooks

Brooks, the first thing to do is find at what stage the hum is occurring. If it is 60 Hz -does it vary in level with the volume control?

If so, pull the pickup plug at the amp. If hum goes away then the suspicion falls on the wiring to the cart and tonearm.

I noticed that you've mentioned having the amp modded to take a later (non-Cobra) pickup was the tonearm rewired too? What types of new cartridge was fitted? If it possible to take a picture of the arm?

If the tonearm was rewired and still plugs into the same point in the upper-mech,arch as the Cobra did this does create a ground loop. A ground loop happens when there are two different paths to ground where there should be only one. That is a bit of a simplification, but accurate. Try running a separate cable from the pickup plug (if it is RCA) to the amp.

If the hum does not vary with volume control setting, it is possible that one of the tubes in the circuits after the volume control has heater-cathode leakage and is allowing some of the 6 volts AC that heat the tubes to leak into the audio signal. The simplest approach here is to swap-in known-good tubes.

BTW: Why can't you hear the difference between the 60Hz and 120 Hz tones in that recording?

Rob

Rob,
Even with my bad ears, I hear the two different samples just fine-- Thanks, Ron Rich

Perhaps it is the fidelity of the computer speaker. I hear the first hum and buzz but nothing for the second half of the WAV file. My wife could not hear it either and my hearing is actually pretty decent.

Based on your response, here is what I think the problem is. First, yes Bill Bickers modified the amp. I now am using a ceramic cartridge. I don't think the problem is there, even though I'm the guy that wired in the new cartridge. I suspect it is with the 6au6 tube. As you know from a previous post, I'm having sound cut in and out and based on all the various things he had me check/do, we felt it was the tube. Bill was nice enough to send me a new tube to try. Of course, when I did the swap yesterday, the new tube was DOA. No glow, no heat, nada. As I told Bill 'if it weren't for bad luck, I'd have no luck at all' So, I put the old plug back in to run through the 'tests' you outlined. Cartridge plugged in or not made no difference. Volume did increase the hum. However, I noticed if I passed my hand close by my nemesis, the 6au6, the hum would also increase. I'm guessing that is a clue and I'm hoping that the clue means, as you note, an issue with the tube such that replacement should clear things up. And the increase from adjusting the volume is a 'false positive'.

Thoughts?

Thanks
Brooks

Brooks, the 6AN8 is the first audio stage after the Cobra oscillator. In all those mono amps that is where one inputs the signal from whatever new pickup scheme (ceramic or preamp-megnetic) one chooses.

Several thoughts;

With things as they stand, what sort of sound do you get from the machine when playing. Where is the volume control positioned? Normally, Wurlitzers will be loud enough with the volume control below the midway point.

The hum your hand induces will be 60Hz -always. This tubes has reasonably high gain so given that it is unshielded some hum from proximity to your hand may be normal. The abnormal hum that I cited is from internal leakage from the heater to the cathode of the tube and would be constant although it will vary with volume control setting and AGC action. The AGC is the Automatic Gain Control circuit that keeps all records at approximately the same level.

If the hum is constant even with the pickup removed --at the amp input-- you can try another 6AN8 tube and I suggest replacing the 12AU7 for test. this is the reactance tube for the AGC (it controls the level). It can also induce hum due to heater-cathode leakage. Try removing it for test. You can also sub a 12AX7 or 12AT7 for test here.

In essence, if hum persists with the pickup removed and tubes subbed-replaced, it is likely that the amp will have to go back to the person who serviced it. Not great news -but these things happen. If it comes to that, I suggest taking the tonearm off and sending that with the amp.

Rob

I should get my new tube today and will keep my fingers crossed. The 12AU7 is bypassed for the ceramic cartridge. Keep it in, remove it, it makes no difference. The same is true for one of the 12AX7's. The volume is not an issue at all. I've never tried to crank it up. I'd say 50% is really really loud, so I keep it at probably 20%. Maybe that's a sign of old age.

When I first had the issue, Bill had me try a couple of things then told me to send it back to him. However, the amp was really badly handled in shipping it back. (As you know, the thing weighs a ton and even though it was marked fragile and had a plywood 'bottom' on the box for protection and rigidity, just one toss on a truck can cause damage) So, I am really hoping I can get everything sorted out without shipping it back to him. Plus it's a good education for me (appreciate your patience in that regard).

Old Goat wrote:I should get my new tube today and will keep my fingers crossed. The 12AU7 is bypassed for the ceramic cartridge. Keep it in, remove it, it makes no difference. The same is true for one of the 12AX7's. The volume is not an issue at all. I've never tried to crank it up. I'd say 50% is really really loud, so I keep it at probably 20%. Maybe that's a sign of old age.

When I first had the issue, Bill had me try a couple of things then told me to send it back to him. However, the amp was really badly handled in shipping it back. (As you know, the thing weighs a ton and even though it was marked fragile and had a plywood 'bottom' on the box for protection and rigidity, just one toss on a truck can cause damage) So, I am really hoping I can get everything sorted out without shipping it back to him. Plus it's a good education for me (appreciate your patience in that regard).

Brooks, from your description it appears your technician has both eliminated the Cobra oscillator (good idea) and disabled the AGC circuits IMO: Bad idea. When you eliminate the AGC you end up with levels all over the place from one record to the next. It can also lead to distortion if precautions are not taken to reduce the gain structure in the amp. A lot of people get rid of the AGC, Personally I consider that to be stupid. Even for home use.

I agree re: shipping. This situation has gotten worse over the last 15 years -especially with UPS. It is yet another reason why I don't take-in work.

Get your "new" tube and we'll see from there.

Rob

Guyz,
Just a word covering my "experiences" with UPS--Insurance is useless in a damaged package case, unless you allow "the UPS store" to pack it at their exorbitant rate ! They will pay if it is vandalized, or lost, without too much hassle. Ron Rich

The new tube seems to have addressed the issue, but I will need to run it some more. Talk about UPS, the tube was sent by first class mail with tracking. I picked this seller because he is about 40 minutes north of me. Label printed Wednesday, picked up Thursday and did not arrive until Monday. The package was first sent 100 miles due East, sat for two days and was then sent to me. Amazing efficiency.

Oh well. That having been said, you (Rob) said "Brooks, from your description it appears your technician has both eliminated the Cobra oscillator (good idea) and disabled the AGC circuits IMO: Bad idea. When you eliminate the AGC you end up with levels all over the place from one record to the next."

Well, guess what, I have noticed that phenomenon with a couple of records, i.e., the volume is much lower. How would one go about restoring the AGC circuit and which tube is it that controls it? Is it just a matter of 'restoring the traces' with wire?

Thanks
Brooks

Brooks, that depends on what was eliminated and how capable you feel at reconstituting the circuits.

Were the traces on the board removed? If not, things are a little easier.

Take a look at these two pages and see if they make sense to you:

http://www.verntisdale.com/schem/532a.jpg You may have seen this one already.

http://www.verntisdale.com/schem/532.jpg

Everything to the -left- of C6 is for the Cobra oscillator tank and NOT needed -but the 12AU7 is as half of it is the AGC reactance (it acts as a voltage-controlled resistor and reduces the volume when a loud record is played).

The 12AX7 will be needed as this samples the audio, converts some to a DC voltage and then controls the reactance tube (half of the 12AU7).

Since the new pickup is a ceramic it needs a high impedance input and you need to find out what, if anything he (tech) did to change the input point (probably at the 6AN8).

You'll may need to contact your tech and ask what mods he did in removing the AGC section and there is some chance that the level from the new pickup may be a bit low, or high as compared to the Cobra circuit. This probably won't be a problem. Ask also if the tech did anything to the squelch circuit (besides disconnecting it) . This is just a connection to the mechanism switches that preconditions the AGC with a bias voltage equivalent to a loud record at the beginning of each record. This prevents a momentary blast of volume while the circuit gets control of the gain.

This may sound a bit daunting, but it is really pretty straight-forward if you understand the basics of electronics servicing; can read schematics and solder properly.

Rob/NYC

Hi All,
Thanks to a posting on the "jukebox list", by Steve Whal, here is a cool site for "noise" --
http://www.shure.com/americas/support/t ... identifier
Ron Rich

Ron Rich wrote:Hi All,
Thanks to a posting on the "jukebox list", by Steve Whal, here is a cool site for "noise" --
http://www.shure.com/americas/support/t ... identifier
Ron Rich

You want noise?
I'll give ya noise!
Listen to my grunting and assorted choice words as I work on an antique radio in the shop.

No Matt--that's "Gar--barge"
Ron Rich