Quantcast Panasonic 850 linear audio: hiss and dropouts, cause? - digitalFAQ Forum
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  #1  
04-04-2021, 03:20 PM
Koen Koen is offline
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Hi there, over the past few years I've used intensively my trusted Panasonic 850. In recent times, it has developed some linear audio issues that are nagging me.

1) On quite a few (but not all) recordings there are cyclic audio 'brightness' issues, i.e. high frequencies (including noise) come and go (to an extent) at a set rhythm with something like 1 or 2 second interval.

2) This machine is more prone to pick up audio drop outs that are probably caused by minor tape damage (the typical loss of background hiss for a split second so typical for low quality linear audio... you get it with audio cassettes all the time).

3) The background noise level is noticeably higher than two other machines I've just compared this recorder to (a spare Pana 850 and a JVC 7611).

I have a few theories but my knowledge is limited, especially in the field of electronics.
Cyclic brightness + drop outs -> worn pinch roller? It's pretty shiny and has microscopic cracks all over. I see it can be sourced from Ebay, and replacing it shouldn't be a big deal - I've done that before.

Background noise -> I've been adjusting the A/C head as a matter of course for individual recordings for years with what I'm only now realising is a magnetic screwdriver... maybe not a very good choice. Could the noise be a result of this unwise choice?

However, another thing I notice is that the linear audio output is pretty low - the same recording loaded into audacity can be amplified with 12 dB when played with my backup Pana 850, but with 20 dB when played with the 850 I'm having trouble with! Possibly the high noise level I notice is caused by a bad signal-to-noise ratio.

I notice the VCR has a circuit board with nine adjustment screws and perhaps one of them can be used to adjust linear audio volume, but I can't find a service manual for this machine and I'm not very keen on changing these settings on the off-chance one of them may do what I want.

Any suggestions on what I could do would be gratefully received... thanks.
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  #2  
04-04-2021, 03:55 PM
latreche34 latreche34 is offline
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Post a sample before you start screwing the factory adjustment that can never be brought back without an alignment tape.
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  #3  
04-04-2021, 04:24 PM
Koen Koen is offline
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Oh, I screwed up A/C head factory settings years ago (as I said, I often find it useful to adjust because so many tapes are recorded out of spec).
Samples... two quick and dirty YT uploads, one from the problematic Pana 850 and another one from a JVC. Note the JVC too has a slight cyclic brightness issue, but it's far more pronounced on the Pana (which, as said, also has a for more unappealing signal to noise ratio).

Pana:
https://www.youtube.com/watch?v=d9238QZWRbM

JVC:
https://www.youtube.com/watch?v=A4hLrD_xnRs

(Tried a few tricks to embed videos but no luck, sorry. I'm not very good at these things.)

Audacity capture. Problematic 850 first, another 850 second.

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  #4  
04-04-2021, 07:31 PM
latreche34 latreche34 is offline
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I hope you didn't screw up the JVC one too, The JVC video and audio is way better, why bother with the Panasonic then? To me it sounds like the Panny audio level is clipping the capture card's audio input, does it have gain pots for audio output inside the VCR PCB?
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  #5  
04-04-2021, 10:21 PM
timtape timtape is online now
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Yes the Pana 850 YT upload has bad dropout due to "spacing loss". As you say it's not consistent but waxes and wanes.

A dirty A/C head, worn A/C head, and especially since you say you have played around with the head alignment, a misaligned A/C head (head height or tilt) are also possible contributors.

Note that careful adjusting of only the head head azimuth does not cause this symptom. Misalignment of head height/head tilt can contribute.

Correcting head height and tilt is normally dealt with in the relevent service manual. Head tilt is usually the most difficult to adjust. It often involves visually aligning the head's face to the tape guides on either side of the A/C head. The three have to be in agreement.

Sometimes a VCR with too low tape back tension can also contribute to spacing loss just as it can at the video heads. The adjustment is a tradeoff between minimising spacing loss and minimising head wear.

Possibly tape damage such as creasing or mild stretching at the tape's top edge is also a contributor. Then of course foreign matter on the tape surface can contribute to spacing loss dropouts.

That's six possible contributors that I can think of.

It's good that you had another VCR to make the audio comparison and didnt simply assume the problem was solely "the old tape".
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  #6  
04-05-2021, 12:35 AM
Koen Koen is offline
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Thanks to both of you.

Going through a few suggestions.

Clipping - isn't that something you get when audio is very loud? In this instance it's very quiet...
I'm attaching the Audacity picture showing sound coming in from the problematic 850 followed by a clip from the same recording played on the spare 850. (I hadn't previously noticed you can add files to a post.)

Pana 850 is worth sticking with if it's working properly - audio ok, and sharper picture than whatever JVC I've come across (although the JVC's clearly have their own advantages).

Pots - ah, is that what they're called. I can see a couple of tiny metal rods on one of the print plates, marked (amongst other things) audio out left and audio out right. They just like like pins stuck on one side of the board and coming out at the other end with seemingly very little surrounding them, but perhaps they're more performant than I give them credit for. :-) Hope to tamper with them on a basic, related Pana I have (see below).

Dirty A/C: probably not, the thing gets more alcohol than I get myself. :-)

Worn A/C: couldn't say. Would be a first for me, but then I don't know everything!

Misaligned A/C: possibly, although in my experience you have to lose a lot of high frequency before there's a noticeable drop in volume. I tend to adjust azimuth only, I basically never touch height.

Thanks for suggesting looking at A/C and tape guides agreement, hadn't considered that before. (In this instance I can see only a tape guide at the right of the head.)

Back tension - I considered that briefly but discarded it on the possibly naive assumption that something so far removed from the A/C could have great influence. Also can't immediately find screws for adjustment where I'm told to expect them, perhaps I need to look a bit closer into that.

Damaged tape - all I can say is that I don't see anything untoward and the tape is clearly playing better in another 850 and a JVC.

Dirt - all looks clean to me, but I haven't carried out a microscopic investigation.

I have a more basic Pana with issues from the same line that probably shares a lot of the 850's mechanism and electronics and I hope to try a few things on that one rather than playing around with the 850. There's a good chance I won't be able to solve the issue, but worth having a go anyway and any further comments are gratefully received!


Attached Images
File Type: jpg Capture.JPG (36.2 KB, 6 downloads)
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  #7  
04-05-2021, 11:53 AM
latreche34 latreche34 is offline
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It may sound quite to your ears but that's not how gain works, Gain is not the volume it's the audio level in dB, While the Pana sample from youtube link has almost twice the audio level of the JVC, I have no doubt that what you are hearing is distortion from clipping as indicated by excessive tape hiss, I don't hear any drop outs. Try a preamp in the chain and lower the level and see for yourself.
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  #8  
04-05-2021, 12:38 PM
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gain = input
volume = output
Both are pressure, loudness.

levels = affects loudness (gain/volume), but is neither.
And as an example, a 50% level can be louder, more volumetic, than a 70% level. (Note: % based on waveforms, most do it by db, but I prefer % since I visually work waveforms.)

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  #9  
04-05-2021, 05:00 PM
timtape timtape is online now
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It's good to refer to the audio examples provided. I'm assuming this is a linear audio track. Tape dropouts are short, momentary drops in level, especially noticeable in the treble. Dirt on the tape can cause it. Whereas on a vinyl record, the same dirt would cause clicks and pops, with tape there are no clicks or pops.

The JVC example has some short audible dropout around 0:3.

The Pana example's "dropout" is so bad that it shouldnt be called dropout. In most of the sample, the treble is muffled. But listen from say 0:40. For just a second at 0:47 the music jumps back to full clarity but then reverts back to what it was. From 0:48 to 0:57 it's jumping up and down.

0:47 is our reference here. That brief 1 second of clarity at 0:47 is the giveaway that something is wrong in the rest of the audio. It gives us a short glimpse of how the audio should sound all the way through the 57 second clip but doesnt.

Does anyone else hear this?
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  #10  
04-06-2021, 12:33 AM
Koen Koen is offline
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Latreche - I'm not sure what YT has done to the videos, it may have cranked up the gain. See the Audacity picture I attached to the 6th post in this thread - the output from the problematic recorder is - according to Audacity at least! - much quieter than a bona fide machine. Perhaps I don't quite get what you are trying to explain, as I said I can only go by what I notice but I don't have a lot of theoretical background. My impression is bad signal to noise ratio.

Timtape - Yes, linear mono, and what you describe is pretty much what I noticed, that's good to know.

Today I was tired plus a four hour tape was occupying the limited tech I would have needed to perform checks for most of the period I had time for it. So no new information from me this time but I'm hoping to be back before the weekend, if nothing serious intervenes, with fresh observations. Would f. ex. like to know what happens when I hold a pencil next to the A/C to gently increase tape/head contact...
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04-06-2021, 02:13 AM
timtape timtape is online now
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Quote:
Originally Posted by Koen View Post

Timtape - Yes, linear mono, and what you describe is pretty much what I noticed, that's good to know...

...Would f. ex. like to know what happens when I hold a pencil next to the A/C to gently increase tape/head contact...
A small artist's paint brush or even a cotton bud/swab would be much better as it's gentler and conforms better to the shape of the head. The linear audio track is at the top edge of the tape so somewhat difficult to access, even to view. Be careful.
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04-06-2021, 02:21 AM
Koen Koen is offline
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Thanks for that, I intended for the pencil (byro... whatever) to act as a guide, something like an inch before the tape reaches the head, to slightly pull back the tape, thus ensuring better contact between head and tape. However, your brush/swab suggestion may well be better! If I use a brush that's wide I could exert pressure on the entire width of the tape... that may be the best option.
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  #13  
04-06-2021, 02:40 AM
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Youtube sometimes attenuates gain, in an effort to normalize audio on the platform. For better or worse (usually better).

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04-06-2021, 12:39 PM
latreche34 latreche34 is offline
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That's why it is always good to attach audio/video samples to the forum even if they are few seconds short, Youtube is never the right tool for sampling.
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  #15  
04-06-2021, 02:23 PM
Koen Koen is offline
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Here's the Pana file, now as an attachment (I initially didn't know you can attach files to the forum, many places don't want that.)


Attached Files
File Type: mkv Sample Pana 850.mkv (75.97 MB, 7 downloads)
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  #16  
04-06-2021, 03:16 PM
Koen Koen is offline
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And here we have the JVC sample.


Attached Files
File Type: mkv Sample JVC 7611.mkv (62.34 MB, 1 downloads)
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  #17  
04-06-2021, 04:11 PM
Koen Koen is offline
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Some rather limited success this evening.

Careful A/C adjustment has clearly diminished the muddled sound problem, but sound continues to be less than stellar. I am, however, now noticing the same tape also doesn't sound entirely right on the backup Pana 850, so possibly its soundtrack isn't all that good anyway and JVC is in this instance just better compared to a Pana in whatever condition.

That still doesn't explain why the audio output of the first Pana is noticeably lower than that of the second, resulting in a rather high noise level.

Back tension adjustment and gently pushing the tape against the A/C head didn't change much. And while I unfortunately can't measure the back tension, the tape didn't seem slack at all.

The 'rods' which I was hoping to be gain pots can't be adjusted as far as I can tell (they won't love easily and I'm afraid of damaging the circuit board if I exert too much pressure) so that possible solution to adjust audio out seems to be off the cards.

Have now opened up a more disposable Pana, but it doesn't have the gain pots (?) seen inside the 850 and I'm afraid to change carefully calibrated settings if I start turning the ones inside the 850.


I'm afraid I've now taken this about as far as I can go without spending more money than the machine is worth (and even if I would be prepared to do that, I know no trustworthy repair people in this area.) Still open to suggestions, if anyone has further ideas. :-) There's also only so much time I'm prepared to spend on technology, especially knowing there's a high likelihood of me not having the skills and knowledge to solve these kind of issues - I think I have so far spent something like six or seven hours, and that doesn't help to get rid of the mountain of tapes I'm hoping to get through...
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04-06-2021, 07:37 PM
hodgey hodgey is online now
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Hm, if you have two identical ones one thing you could try to would be to swap the mechanisms to see if it's the audio head being bad (or borrow an audio/control head or mech (- drum) from another cheaper deck with the same mechanism.)

The NV-HS850 is one of the few models with part digital video decoding (aka "Digital Process") stuff, so I think the PCB components and layout differs a bit from the standard vhs models (unlike with e.g JVCs from the time period where they shared much of the video ICs and other stuff between the SVHS and Hi-Fi models.). They did share mechanisms though, in the Z mech decks like I think this one is, with only some minor differences other than the video head drum. EDIT: For European models at least, in the US many pana decks have a different mech not seen in the Euro ones.

There is always some possibility that there is a bad component somewhere in the audio chain too.

Last edited by hodgey; 04-06-2021 at 07:48 PM.
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04-06-2021, 08:06 PM
Koen Koen is offline
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Thanks for your input.

Perhaps I should have mentioned before I'm in Europe (Belgium) so we're talking EU models here.

I had been toying with the idea of just swapping the A/C head, although I'd need to take into consideration the possibility I would somehow create havoc and end up with a machine I can no longer use at all. Am also wondering whether lower audio output could have something to do with electronic components on the boards going bad, but that would really be beyond my capabilities.

Swapping over an entire mechanism is something I've never done before... perhaps it would be asking for trouble if I tried to do that.

The machine I opened earlier today is a bog standard 630, and its electronics indeed look quite different. At least people buying these expensive recorders back in the day got something extra for the money they spent. :-) I have a variety of JVC's and while I'm currently using a 7611 (a Digipure one) I generally struggle to see much difference in output between it and much cheaper JVC's, assuming they can be set to edit mode (whereas with Pana the difference between, say, 850/88/1000 and more basic models is quite clear).

(Come to think of it, I'm not all that impressed with the Pana 88 I got last summer. It's sharper than the 850 but is very weak on colour and contrast is too harsh for my taste. I'm wondering whether these observations are generally true or whether my machine has issues. Considering the great praise this model (and especially its reportedly related brother 200, which I don't have) generally get, I fear I may have a duff recorder. That's actually something for another topic, though.)
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04-06-2021, 10:36 PM
timtape timtape is online now
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Quote:
Originally Posted by Koen View Post
...I had been toying with the idea of just swapping the A/C head, although I'd need to take into consideration the possibility I would somehow create havoc and end up with a machine I can no longer use at all.
Yes that's wise, but more fundamentally, is the head worn? I would only remove a head if careful visual inspection (while it was still in the machine) revealed it was worn. And even if you did source another head, how would you know it was in better condition than the one you had removed, unless of course you could source a "new old stock" head.

Another factor is that after many tapes played the tape wears a tape sized groove in the head. Adjusting azimuth can actually make things worse because tapes with incorrect azimuth are now misaligned to the wear groove. The tape can become distorted or even pop out of the groove, even though the azimuth angle is now strictly corrected. The tape could become permanently damaged. So adjusting azimuth on a worn head can make the sound worse, and possibly damage the edges of the tape. I only adjust azimuth on tape heads with little or no wear. But that's easy for me because I've been relapping tape heads for 50 years.

But as almost nobody adjusts A/C head azimuth in video transfers, the subject of this complication with the wear groove rarely comes up.


Quote:
Originally Posted by Koen View Post
Am also wondering whether lower audio output could have something to do with electronic components on the boards going bad, but that would really be beyond my capabilities.
If the sound is otherwise clean and undistorted, the fact that on one deck it plays back 4 db quieter than another should be of little or no importance. It doesnt have more noise, it's just a little lower in level. In transferring, something like the dropout you reported, must be sorted at the VCR end because it cant be fixed later digitally. But the level disparity in the digital capture file is trivial because the level can easily be adjusted later, digitally.

Last edited by timtape; 04-06-2021 at 11:07 PM.
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