Capacitor Question bugging me

[thinkchronicity]

Getting into the habit here of ripping out old power supply filter caps in various equipment, not just BBs, and making that stinky hum go away. Doesn't always work in boomboxes due to the stray magnetic fields of the transformer being close to the tape head...but I diagress. So I know there are the 3 tests you can do on a cap: uF reading, esr and dc leakage current (that one i do with a power supply, ammeter and a watch). Is it true that an old leaky cap can read to spec on capacitance, but actually be well below due to the excessive leakage current fooling the multimeter by virtue of taking longer to perform the measurement? So many big old caps seem to measure ok on uF.

if anyone has shielding tricks for BB transformer s then please let me know - since my neighbour hired a hot tub this summer I got seriously sensitized to low hums!

 

[hopey]

Don't forget the op amp caps they can introduce noise as well. I only replace caps if I really have to and can be bothered finding which one.

 

[thinkchronicity]

Well I can answer my own question. I spoke to a vcr engineer with decades of experience today. Yep, slightly leaky caps CAN make you think the uF reading is still to spec as it tricks your multimeter.

I'll get back on the other issue if I have news.

 

[hopey]

Ground Loops - Eliminating System Hum and Buzz

Got an annoying hum or buzz in your audio system? You likely have a ground loop. This article helps you diagnose the problem so it can be eliminated. Don't put up with hum. Read this article!

www.audioholics.com

 

[thinkchronicity]

Hopey, you give me hope! Have you got any thoughts on the hum that the tape head picks up thru the air from the mains transformer which in all boomboxes seems to be way too close for comfort? I'm only thinking about this as I've just discovered my hifi separates need separating badly when it comes to the amp and cassette deck, which live on top of each other and create this proximity problem. Press play on a BB with no tape and crank up the sound - it's not pretty is it?

 

[Wes125]

there are two types of hum..mains volt hum and audio hum. if its a boombox put just the radio on and turn down volume. stick ya ear near each speaker. usually the speaker near the power supply will have more of a hum..but that dosen't mean its coming from the speaker. it could be the transformer hum making the speaker cone viorbrate..hums are a right b---tard to detect..but going back to audio hum thats not a problem long as its not over powering the sound..but mains hum from the speakers is not good. ac it getting through and things start to cook over time. depending on how loud that hum is..and a noisey transformer can throw hums all over the unit. i have a sanyo stereo with one and it hums like fk out of speakers. and thats got all new caps in it as well

 

[hopey]

Hopey, you give me hope! Have you got any thoughts on the hum that the tape head picks up thru the air from the mains transformer which in all boomboxes seems to be way too close for comfort? I'm only thinking about this as I've just discovered my hifi separates need separating badly when it comes to the amp and cassette deck, which live on top of each other and create this proximity problem. Press play on a BB with no tape and crank up the sound - it's not pretty is it?

Current follows path of least resistance, With your separates it probably just bad RCA cables. With Boomboxes the hum doesn't Travel though the air, Its a ripple in the power supply (Bad Power caps) or ground loop (ground current entering the audio path). You can eliminate the first one by running on batteries, if the hum is still there disconnect the Tape head to PCB audio Cable.

 

[thinkchronicity]

Current follows path of least resistance, With your separates it probably just bad RCA cables. With Boomboxes the hum doesn't Travel though the air, Its a ripple in the power supply (Bad Power caps) or ground loop (ground current entering the audio path). You can eliminate the first one by running on batteries, if the hum is still there disconnect the Tape head to PCB audio Cable.

Sorry, i didn't make myself clear enough in my explanations. When I talk about hum travelling thru the air I mean inductive coupling from the transformer's stray magnetic fields (in the air) reaching the very sensitive tape head coils. This can happen in a separates system as I've found out (hum goes away when I lift my amp off the deck) and must happen a lot in boomboxes as everything has to be squashed in. With music pumping it's not a problem of course, but can be in quieter sections of music. The perfectionist in me is thinking about how to stop the unwanted magnetic flux reaching the tape head.

 

[Superduper]

Transformer magnetic flux strength is directly related to the power (size) of the transformer. On hifi home gear, they often use very high powered transformers so if improperly designed, could lead to some noise issues. However, the transformers on most boombox is so small that it's really not going to cause any issues with the tape heads... the flux really doesn't extend that far. Transformers usually are mounted at a corner with decks usually in the middle of a set. Most likely any hum you are hearing is either from the transformer vibrating because the windings were not potted in epoxy (not uncommon, since they cost more and are larger). Or your heads aren't properly grounded. This issue is very common. Or, perhaps you are confusing the hum with a noisy circuit. Boomboxes are notorious for having low S/N ratios. Finally, make sure that the cord is inserted in the proper orientation. Some boomboxes don't care. Some use polarized cords that are intended to be plugged in only 1 way. If you plug it in wrong, you can get hum.

Now as far as home equipment.... I have scores and have not observed the issue you mentioned. Most of my gear had been stacked and either the sets were designed in such a way that the flux does not affect others or they have proper shielding. Also inductive coupling requires extremely close contact. I really can't see it as being an issue. I forget the formula for calculation but the effect diminishes rapidly upon the introduction of any distance. See "near field effect" for an explanation about this. Perhaps this can be a case with magnetic heads however since they are intended to pickup miniscule signals and have massive amplification but the heads practically needs to be right above the transformer I would think.

You didn't mention if you are hearing cleaner sound using batteries?

 

[hopey]

One important point audio cables are shielded by ground wire wrapped around Signal cores. When they are brand new there will be no ground loop or external interference possible. However once they are used or abused the insulation can deteriate and you have problems.

 

[thinkchronicity]

Interesting thoughts thanks for that. It makes sense about the much smaller transformers in boomboxes. I don't have any batteries, but I just tried an AC adapter in both my Goldstars - the 800&581 - cranked up the volume with no tape in and pressed play. No mains hum seeping thru the speakers when powered this way as expected. With the mains cord in, the 800 is quite acceptable regarding mains hum, the 581 is awful. So this makes me think the inductive hum I suspected may be a red herring - I think I have grounding issues. This box came with a lot of rust on the speakers and beach sand inside! I've read articles about grounding which have been quite confusing to fully grasp. The main takeaway i took from one article about turntable grounding was that noise signals cannot exist where all grounds (tonearm, turntable chassis, amp chassis, signal ground wire) are at the same potential. Now that makes sense to me. So with a boombox do I have to solder a wire from the tape head case to ground in extremis? I've also noticed with this box that sometimes if I touch the metal tape control keys I get an extra buzz thru the speakers.

My home stack set up is definitely a case of inductive hum however. It's a NAD 3120 sitting on a Nak CR-2 deck. I found only a few forums where this issue is known about and discussed, so it does seem quite rare, but it happens. I'm surprised the manufacturers don't have a warning in their manuals tbh. When you consider that the magnetic strength on a cassette tape is about 24uT and the earth's field at 50deg latitude is 58uT...tape heads are very sensitive.

I've been considering emf noise from cassette motors too. I've seen a kind of shielding tape wrapped around some small mabuchi motors in pics online. I've been looking at these products - might experiment at some stage.https://www.electrosmogshielding.co.uk/product-category/magnetic-field-shielding/ Thinking more about my Nak here as that seems quite noisy even when I eliminate the hum issue by repositioning it. The thing is I don't want to reposition it - there's nowhere for it to go, so I wonder if some mu metal sheets placed between my amp and deck would block the flux?

 

[thinkchronicity]

try that link again https://www.electrosmogshielding.co.uk/product-category/magnetic-field-shielding/

 

[thinkchronicity]

Update for anyone curious: Just got my hands on 1mm thick Mu Metal that covers the whole of my Nak deck plus a bit more. The hum has gone completely.