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Author Topic: Technics RS1500U: Recording Level adjustment  (Read 11262 times)

Offline Arnold_Layne

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Technics RS1500U: Recording Level adjustment
« on: January 09, 2010, 03:40:28 PM »
I need help. My 1500 is recording about 5db lower in the left channel than the right. Is there an internal pot to adjust the recording output levels? I have no service manual.

Steve

Offline Tim

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Re: Technics RS1500U: Recording Level adjustment
« Reply #1 on: January 09, 2010, 06:16:00 PM »
It's highly unlikely that the internal record gain adjustment or the overall gain of your record amp has changed over time, so you need to look at other causes of low record level.  High on the list is a defective record head. Is is absolutely clean?  What does the head wear-pattern look like?  It should be a perfectly-shaped rectangle. Sides of this rectangle should be parallel, straight lines (not wavy). Other things to check are oxidized switches and pots (the front-panel line-in pot).  Head alignment does not change or drift by itself, so don't mess with any head alignment screws (it's not the cause of your low record channel). If you ever do fiddle with any adjustments (electrical or mechanical), always carefully mark where the original adjustment was. 
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Offline ironbut

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Re: Technics RS1500U: Recording Level adjustment
« Reply #2 on: January 09, 2010, 06:19:51 PM »
Hey Steve,

I guess the first thing to ask is whether this is a recent problem or if you've just started trying to record with this machine?
Another thing is are you basing this inbalance on the VU meters while recording or during playback or what you're actually hearing?

One way or the other, do a very thorough cleaning of all your heads and tape path. Be particularly meticulous with the corners of the guides and the outside edges (farthest from the machine face) of the heads. It's pretty hard to clean the outside of both of the playback heads since they're obscured by the headshields. It doesn't take much to hold the tape far enough away from the head surface to lower the level (consider that drop outs are caused by the distance that a speck of dust holds the tape away from the head).

Also, clean all the contacts of the connectors from the source you're recording and work the input pots back and forth while listening to "source" to see if there's any scratchy noises coming from a dirty pot. Those input pots see so little use on many machines that they can be pretty grunged up until you need them.

Finally, if you have a test CD that has some test tones on them, try and record them to confirm just how far off the levels are. If you don't, I'm pretty sure you can download some and play them directly out of a media player on your computer to your Technics (you may have to run to Radio Shark to pick up an 1/8th inch stereo plug to RCA pair adapter cable).

I see that Tim has covered some of this stuff already.
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Offline Arnold_Layne

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Re: Technics RS1500U: Recording Level adjustment
« Reply #3 on: January 09, 2010, 06:46:49 PM »
I can definitely hear the imbalance. Pre-recorded tapes play fine, and levels are accurate on the VU meter. I have pro-recorded tapes with test tones and during playback the test tones are equal in both channels with the output level knobs set at the same settings for both channels. It is a recording issue. It is possible it's a problem with the record head, as it seems to have gotten worse over the years. I did take the back panel off, but after seeing the location of the circuit boards, I knew I was in for more trouble than it was worth, so I left the guts alone.

Here's how I figured out it was in the recording path:

Using a Nak Dragon, I output the built in 400hz test tone to the technics. I set the monitor switch to source, and adjust the input levels to 0db on the metes. While recording the test signal, I switched the monitor to tape, adjust the output dial till I get 0db on the right channel, but the left is only reading -5db. Switching the monitor back and forth, not only does the VU meter move, but you can here the differences in the level. The right channel stays at the same levels in both monitor modes.

The deck is large and rather expensive to ship. I'm wondering if changing out the headblock would be easier and would alignment still be needed after the change out?

A_L

Offline Tim

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Re: Technics RS1500U: Recording Level adjustment
« Reply #4 on: January 09, 2010, 07:08:42 PM »
You could ship the headblock to JRF if you think that is the problem.  If the record head is defective, then both record and play heads should be replaced.  They then do an optical alignment of the heads and guides.  When you get the block back all you need to do is a minor touch-up of the play and record head azimuth.  We can explain how to do this.
« Last Edit: January 09, 2010, 07:17:06 PM by Tim »
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Offline Arnold_Layne

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Re: Technics RS1500U: Recording Level adjustment
« Reply #5 on: January 10, 2010, 08:36:59 AM »
I did some more cleaning of the record head. Seems to have helped. Now I'm getting a lot of "wow and flutter" in the left channel. I believe there is an alignment or tension issue causing the problem as the tape moves over the record head as the issue is not there on pre-recorded tapes even those with test tones. If I ever get enough funds together, I'll send the deck in for a "tape project" overhaul of the tape path. For now, I'll use my back up deck, my Otari MX5050B2II.

A_L

Offline Tim

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Re: Technics RS1500U: Recording Level adjustment
« Reply #6 on: January 10, 2010, 12:31:59 PM »
Wow & flutter are terms for speed errors, so that's not what you are hearing.  You're having drop-out/head-to-tape contact issues with one of the record head channels.  It's possible your play head is ok in this respect which is why the pre-recorded tapes sound better.

Yes, you are probably having both tape-alignment and tape-tension issues. (notice I said tape-alignment and not head-alignment...your head is exactly where it should be, but the tape is taking the wrong path).  This is a common problem with the Technics "isolated-loop" series.  Due to many factors, the tape skews (has a thrust-vector 90 degrees to the direction of travel) within the headblock, causing the tape to buckle and not lie perfectly flat across the head. This eventually causes uneven head wear, which results in the dropouts you are hearing in one channel. The best indicator of this defect is to observe the head-wear patterns.  Do the heads show uneven wear as I earlier described? It should be a perfectly-shaped rectangle. Sides of this rectangle should be parallel, straight lines (not wavy)

 
« Last Edit: January 11, 2010, 03:11:59 AM by Tim »
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Offline Arnold_Layne

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Re: Technics RS1500U: Recording Level adjustment
« Reply #7 on: January 10, 2010, 03:44:15 PM »
Wow & flutter are terms for speed errors, so that's not what you are hearing.  You're having drop-out/head-to-tape contact issues with the record head.  It's possible your play head is ok in this respect which is why the pre-recorded tapes sound better.

Yes, you are probably having both tape-alignment and tape-tension issues. (notice I said tape-alignment and not head-alignment...your head is exactly where it should be, but the tape is taking the wrong path).  This is a common problem with the Technics "isolated-loop" series.  Due to many factors, the tape skews (has a thrust-vectors 90 degrees to the direction of travel) within the headblock, causing uneven head wear, which results in the dropouts you are hearing in one channel. The best indicator of this defect is to observe the head-wear patterns.  Do the heads show uneven wear as I earlier described? It should be a perfectly-shaped rectangle. Sides of this rectangle should be parallel, straight lines (not wavy)

 

You are correct, wow and flutter is the wrong term. Tape alignment and tension issues sound more likely. I have done my best to take a picture of the record head. I'll post it here:



A_L

Offline docb

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Re: Technics RS1500U: Recording Level adjustment
« Reply #8 on: January 10, 2010, 07:23:40 PM »
Here's my guess: You don't hear the problem with pre recorded tapes, but you do when recording. I bet the tape you are recording on is sticky. Try a different tape.
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Offline Tim

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Re: Technics RS1500U: Recording Level adjustment
« Reply #9 on: January 11, 2010, 02:37:07 AM »
Sorry, but the wear patterns on the heads don't show up in the photo.  The lighting has to be just right.  I'll post a photo as an example.

Just recently, I repaired a Technics RS1500 that had the exact same symptom (low level in one record channel).  The record head was somewhat new and only a moderate amount of wear. The head was, in fact, installed and aligned by JRF.  Anyway, the audio and bias voltages going into the head were identical in both channels, yet one channel was down 5 db.  I installed a new head and the defect was repaired. The coil resistance and inductance was identical in both channels, so I'm guessing there was a defect with the gap in one channel although it looked perfect under a microscope. I don't know if this defect head was a fluke or a common problem. Just another possibility.
« Last Edit: January 11, 2010, 03:28:47 AM by Tim »
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Offline ironbut

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Re: Technics RS1500U: Recording Level adjustment
« Reply #10 on: January 11, 2010, 12:25:21 PM »
I'm with Tim on this one. It's almost certainly head/guide wear that's causing your problem.
Removing and sending your headblock to JFR will be pretty painless. It's an unfortunate reality that even if you hardly ever use the recording head, it get worn whenever you play a tape. But, in the larger scheme of things related to audio, getting your heads relapped and the headblock check out isn't very expensive.
Once you've taken the headblock off (3 screws, the silver colored ones, one on top and two below) examine the tape path with a magnifier closely. It's the only practical way to get a close look at the outside edges of the path.

Here's a picture that Tim just emailed me of a worn head which doesn't have well defined, straight borders to the wear or is correctly centered.


By ironbut at 2010-01-11
steve koto
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Offline Arnold_Layne

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Re: Technics RS1500U: Recording Level adjustment
« Reply #11 on: January 12, 2010, 06:26:23 PM »
I did some more tests. I have some various tapes of different grades etc. It would seem the problem with with old tapes. First was some old used reels of Ampex 631. I had a solid 5db drop on the left channel while recording. Next, I tried some Maxell UD35 which was a previously used tape as well.  Left channel was almost perfect, but would have intermittent drops to -5db. So I watched the tape as it passed by the record head, and noticed the tape looked deformed when the dropouts occurred. Next, a brand new never used reel of RMGI SM 911 tape. Now the VU meter registers only a hair difference.

My conclusion:

Stretched for deformed tape. The SM911 tape is backcoated, and seems to pass over the heads smoothly vs non-backcoated tape.


I never really noticed this before until I shined a light on the tape as it passed over the record head.

Thanks for everyone's time in helping to locate and fix the problem.

A_L

Offline docb

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Ouch!
« Reply #12 on: January 12, 2010, 06:35:57 PM »
That was me, hurting my shoulder while patting myself on the back for guessing that it was old tape that was the problem.
Dan "Doc B." Schmalle
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Offline ironbut

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Re: Technics RS1500U: Recording Level adjustment
« Reply #13 on: January 12, 2010, 06:46:01 PM »
That's cool Arnold!

The left channel deficit is a common problem with pre recorded tapes with questionable long term storage, 1/4 track tapes in particular. The left channel is on the outside edge of the tape width with 1/4 track no matter which side you're playing and since the outside is more exposed to elements such as heat and humidity it always gets damaged first. It sounds like your older tapes have curling which is always worst on the outside. A lot of times, this is caused by long term storage with an improper/irregular tape wrap. The natural force on the tape pulls it in toward the hub and if the layers don't have the support of the layer underneath, the tape edge "curls" toward the hub which becomes permanent after a while. This process is accelerated by heat.
So, storing your tape with a "played wrap" rather than a fast wound wrap is always better (fast winds tend to increase the air pressure between the layers as it's winding and are never quite as good as a slower played wrap). Some machines have a "library wind" feature which is a good compromise between fast and play speeds.
steve koto
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Offline ironbut

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Re: Technics RS1500U: Recording Level adjustment
« Reply #14 on: January 12, 2010, 07:06:25 PM »
Nice going there Doc!
I'm actually kinda amazed how many of these little mysteries we can solve here.
If only you'd started a household appliances forum we could both be retired by now!
steve koto
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