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  #1  
Old 10-13-2017, 05:00 AM
solouplay solouplay is offline
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Akai - GX630D, loud bass frequency-response

I have an Akai GX-630D, 4-tracker that produces alot of base when playing at 7 IPS. But at 3 3/4 IPS the response is flat. Is there a simple solution to fix this problem?

If I set the EQ to 3 3/4 IPS but play the tape at 7 IPS the bass-response is flat/normal, however the highs are way to sharp. This leads me to believe that the playback-circuitry is creating the increased bass-response. With some analyzing in audacity I found out that this corresponding EQ-setting is a solution that would make the Akai great again. So maybe I can implement it in a passive-circuit?

Is it possible to build a high-pass filter for the PB-circuitry to decrease the bass? Or maybe that'll just completely kill the bass?
Ideas or solutions appreciated! :)

/Solouplay
  #2  
Old 10-13-2017, 06:25 AM
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johns82 johns82 is offline
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Most decks have whats called "50 Hz head bump". This is most notable as speed increases. I just got done completely going thru my GX 270 D and yes Akai decks have a bad problem with the head bump.

I did get the play EQ adjustable BUT have to figure out how in the hell to get rid of the excessive base. Mine starts out around 100 Hz but don't see how you setting the 7 1/2 speed to 3 3/4 EQ would flatten this out. It didn't on mine as this is easy to do on the GX270D. Also on mine it's a function of "head bump", more or less.

So otherwise you have another problem or just really noticing the same thing.

By the graph you presented the slope starts off a lot sooner then what I have seen on mine so this might be a different problem.

Though I would like to flatten out the bump on mine as it does seem a tad much. I tried to find an answer from the group, none was forthcoming other then I may have some sort of feedback in the circuit.

Maybe you'll have more success then I and I will watch this thread.
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  #3  
Old 10-13-2017, 07:41 AM
solouplay solouplay is offline
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Quote:
Originally Posted by johns82 View Post
Most decks have whats called "50 Hz head bump". This is most notable as speed increases. I just got done completely going thru my GX 270 D and yes Akai decks have a bad problem with the head bump.

I did get the play EQ adjustable BUT have to figure out how in the hell to get rid of the excessive base. Mine starts out around 100 Hz but don't see how you setting the 7 1/2 speed to 3 3/4 EQ would flatten this out. It didn't on mine as this is easy to do on the GX270D. Also on mine it's a function of "head bump", more or less.
Maybe the head-bump is a weighing factor (I haven't heard about it before), but keep it mind I have a 9dB gain. I was thinking that maybe a previous owner changed some caps in it to make the 7 EQ richer. I'll go through the schematics and see if the values are legit. .. If they're OK, then maybe I'll try to mimic the 3 3/4 bass-EQ section and see if it gets any better.

I wonder if any other 630D users have the same "problem". I doubt that Akai would make it sound this way, because you need to reduce the recording-signal alot so that bass doesn't distort, which obviously increases the hiss in the higher frequencies... it's weird.
  #4  
Old 10-13-2017, 08:16 AM
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Quote:
Originally Posted by solouplay View Post
I have an Akai GX-630D, 4-tracker that produces alot of bass when playing at 7 IPS.
So this is strictly a playback problem? (not a record problem) You did not clearly state that. It is important that we know if this is a problem with the record section or playback section of your deck.

So if you play back a tape made on another deck or a commercial music tape, you have an excess bass problem? And if you record a tape on your deck and play that tape back on another deck, the playback sound fine (no excess bass)? In other words, not a record problem.
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Last edited by Tapetech; 10-13-2017 at 08:21 AM.
  #5  
Old 10-13-2017, 09:19 AM
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all the AKAIs i have serviced here dont have that particular high headbump. However i did notice some slight increased Bass response on my GX-600DB comparing it with a GX-635D i just serviced. So it seemed they changed something in the head design. As the GX-635D uses the "Smaller" GX heads comparing it with a GX-600DB that uses the larger Glass heads. Those heads are the same used in the GX-630D.

But we are not talking +9dB here. Something is horrible wrong. For starters i would check it with a different tape, like a reference tape before i start blaming the machine.
  #6  
Old 10-13-2017, 09:23 AM
solouplay solouplay is offline
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Quote:
Originally Posted by tapetech View Post
So this is strictly a playback problem? (not a record problem) You did not clearly state that. It is important that we know if this is a problem with the record section or playback section of your deck.

So if you play back a tape made on another deck or a commercial music tape, you have an excess bass problem? And if you record a tape on your deck and play that tape back on another deck, the playback sound fine (no excess bass)? In other words, not a record problem.
I shouldn't have said that I suspected the PB because now that I think about it, it could just as well be the recording circuits. But yea, I really need to know where the problem is originating from.

I don't have access to any other RTR-deck right now and the only commercial tape I have is Black Sabbath's first album, but it's in 3 3/4 IPS. I'll just digitize it in both speeds and then reduce the speed of the 7-digitization digitally. This'll show if it's a PB or recording issue :)

Quote:
Originally Posted by Ebzen View Post
But we are not talking +9dB here. Something is horrible wrong. For starters i would check it with a different tape, like a reference tape before i start blaming the machine. :)
I'd get a reference tape, but it's too expensive :(
  #7  
Old 10-13-2017, 09:57 AM
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Quote:
Originally Posted by solouplay View Post
I shouldn't have said that I suspected the PB because now that I think about it, it could just as well be the recording circuits. But yea, I really need to know where the problem is originating from.
Yes, you have to know which (record or play).

In case this is a record problem, then you need to first carefully clean (with D5) the speed EQ switches , the two record mode switches and the "wide range" switches. Do that and see if it changes anything.

Has anyone messed with the internal calibration adjustments (like the bias)?

.
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Last edited by Tapetech; 10-13-2017 at 10:07 AM.
  #8  
Old 10-13-2017, 10:36 AM
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And how do you know for certain that the problem is too much bass and not too little treble? They both can give similar curves (frequency response).

Speaking of frequency response: give us the record/play frequency response of your deck at both speeds. At -10VU record level using 1K as the zero reference. So indicate dB levels of 50Hz, 100Hz, 1KHz, 5KHZ, 10KHz and 15KHz.
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Last edited by Tapetech; 10-13-2017 at 10:42 AM.
  #9  
Old 10-13-2017, 10:40 AM
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Quote:
Originally Posted by solouplay View Post
I have an Akai GX-630D, 4-tracker that produces alot of base when playing at 7 IPS. But at 3 3/4 IPS the response is flat. Is there a simple solution to fix this problem?
<JOKE>Sure, use 3 3/4 ips</JOKE>

Different people understand different things in "alot of bass".

However, I think the diagnostic is correct, as 9.5cps has less head bump than 19cps. It may also be a defective cap in the EQ but chances are that only one would go defective so the issues will focus on a single channel not on both (I assume both are affected).

There is no cure to this.
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  #10  
Old 10-13-2017, 01:58 PM
solouplay solouplay is offline
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Quote:
Originally Posted by Ghitulescu View Post
<JOKE>Sure, use 3 3/4 ips</JOKE>
Was waiting for that one ;)

Quote:
Originally Posted by Ghitulescu View Post
However, I think the diagnostic is correct, as 9.5cps has less head bump than 19cps. It may also be a defective cap in the EQ but chances are that only one would go defective so the issues will focus on a single channel not on both (I assume both are affected).
Yup, both channels affected.

Quote:
Originally Posted by tapetech View Post
Has anyone messed with the internal calibration adjustments (like the bias)?
I don't know for sure since I'm not the original owner.
Personally I've balanced the left and right channel and tweaked the bias so that a 1 and 10KHz tone had the same volume-relationship (on the recording circuitry). If I increase the bias-pot setting it'll just sound very bright, without any mids.

I just did some testing, and can confirm that it's the recording-circuitry that's creating the excessive bass-response. I'll clean the switches as you advised me to do.

Quote:
Originally Posted by tapetech View Post
And how do you know for certain that the problem is too much bass and not too little treble? They both can give similar curves (frequency response).
I guess it could be either. I was just thinking since the bass is the undesired element that would be the problem.

Quote:
Originally Posted by tapetech View Post
Speaking of frequency response: give us the record/play frequency response of your deck at both speeds. At -10VU record level using 1K as the zero reference. So indicate dB levels of 50Hz, 100Hz, 1KHz, 5KHZ, 10KHz and 15KHz.
I'm on it, will post the results soon
  #11  
Old 10-13-2017, 02:33 PM
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Quote:
Originally Posted by solouplay View Post
Personally I've balanced the left and right channel and tweaked the bias so that a 1 and 10KHz tone had the same volume-relationship (on the recording circuitry). If I increase the bias-pot setting it'll just sound very bright, without any mids.
If cleaning those switches changes nothing, then try a different record bias setting: Record a 1kHz tone at 7.5ips and adjust the bias controls for maximum output level of that tone (at line out). See if that changes rec/play response.
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  #12  
Old 10-13-2017, 02:58 PM
solouplay solouplay is offline
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Here's the result from the analyzing. As you can see from the graph the 3 3/4 IPS setting is also affected, however not as much compared to the 7 setting. Sorry for misinforming you all.



The reference is -10 dB. Also I recorded this with the tape-switch set to "wide" instead of "low-noise". But there's almost no difference what-so-ever between them. If I apply the EQ-filter from the first picture I showed you all, the waveform levels out and becomes flat.

Again, I'll try cleaning the switches tomorrow. If that doesn't work I'll try adjusting the BIAS with a 1KHz tone. ... If that doesn't work I'll put a high-pass filter on it! :)

Last edited by solouplay; 10-13-2017 at 03:03 PM.
  #13  
Old 10-13-2017, 04:28 PM
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macman007 macman007 is offline
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I didn't see where you posted what type and brand of tape you are using, and if this deck has been calibrated to it using the proper record/playback speed/EQ switch position. When calibrating the record section, there should be a notable difference in the performance numbers when changing the EQ switch on the front from one position the other. Only one EQ setting will be closest at a given speed before any record adjustments are made.

You really need to get a playback (called Reproduce or Repro) alignment tape. MRL makes the best ones, at the proper flux level for your deck. I would get the 185nWb/m tape, but no hotter than the 200nWb/m tape for your Akai. Trying to set the Akai up for a higher flux level is often be nothing but problems, making it more difficult if not impossible without extra modifications to the circuitry.

The Repro section MUST be set up FIRST, before any meaningful diagnosis can be carried out. The Repro calibration tape lets you calibrate the deck's playback section to the same standards used by all other similar decks . You'll want to pick a 7 1/2 ips NAB equalized tape, one with 4 or 5 test tones at either 185 or 200nWb/m. You can also use this tape to check and adjust the repro section at 3 3/4 ips , the conversion numbers for setting up @ 3 3/4 ips using the 7 1/2 ips calibration tape is on MRL's website pages.

Once you have the tape, you can evaluate the repro section after calibrating the it, to make sure your deck is operating within it's advertised playback specifications. This is the "foundation", you need to build on this before you can properly calibrate the record electronics. When you do calibrate the record section, always use the tape type you plan to record on most often.

You will need a good copy of the service manual, if you don't already have one. It may be available free online, however the best quality and most readable, complete manuals with all the factory updates in them will be the paper manuals you can buy from Rick over @ www.stereomanuals.com, a Tapeheads sponsor. Follow the service manual without skipping any steps when aligning your deck. You will want to do all the indicated checks and adjustments published in the manual, leading up to the Repro and Record calibrations to achieve the best most consistant sonic results.

If for some reason you cant get either electronics section to calibrate properly, you will still be in the best possible position accurately diagnose and then correct whatever is deficient with the deck.

This is the best and most empirical way to accurately pin down whether or not you have problem with the deck, it's calibrations , which one, and/or the tapes you are using themselves. Of course, it goes without saying that for optimum results, you should always be using fresh new modern tape to calibrate the record section with, and to record your music on once finished as well
  #14  
Old 10-13-2017, 05:11 PM
solouplay solouplay is offline
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Quote:
Originally Posted by macman007 View Post
I didn't see where you posted what type and brand of tape you are using, and if this deck has been calibrated to it using the proper record/playback speed/EQ switch position.
I'm using RTM Recording the Masters (RMG) SM911 tape. I have not touched the PB-section on this unit, but I'm fairly sure it's to spec. The recording circuitry has been tampered with and it's certainly out of spec considering the bass-response.

Quote:
Originally Posted by macman007 View Post
When calibrating the record section, there should be a notable difference in the performance numbers when changing the EQ switch on the front from one position the other. Only one EQ setting will be closest at a given speed before any record adjustments are made.
Just to clarity, are you talking about the wide/low noise EQ-settings or the speed EQ-settings (3 3/4 and 7)? Changing from wide to low-noise and vice versa barely does anything at all from what I can tell by ear. It's probably broken, really.

Quote:
Originally Posted by macman007 View Post
The Repro section MUST be set up FIRST, before any meaningful diagnosis can be carried out. The Repro calibration tape lets you calibrate the deck's playback section to the same standards used by all other similar decks . You'll want to pick a 7 1/2 ips NAB equalized tape, one with 4 or 5 test tones at either 185 or 200nWb/m. You can also use this tape to check and adjust the repro section at 3 3/4 ips , the conversion numbers for setting up @ 3 3/4 ips using the 7 1/2 ips calibration tape is on MRL's website pages.

Once you have the tape, you can evaluate the repro section after calibrating the it, to make sure your deck is operating within it's advertised playback specifications. This is the "foundation", you need to build on this before you can properly calibrate the record electronics. When you do calibrate the record section, always use the tape type you plan to record on most often. ...
An MRL would be very useful, but for this particular case I'm not going for a complete respec and I'm just assuming that the PB-circuitry is in spec. Tapes that have been recorded on a different machine sounds great, so I'm OK with PB section, but whenever I try to record something it adds excessive bass. I only want to flatten the recording-circuitry so I can record tapes at a reasonable level.. you know, get the overall EQ-response within 1dB at 20 Hz - 20 KHz. :)

I think the underlying problem is a bad component/components somewhere in the recording or EQ-circuit or maybe a bad connection
  #15  
Old 10-13-2017, 05:55 PM
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Quote:
Originally Posted by macman007 View Post
. When calibrating the record section, there should be a notable difference in the performance numbers when changing the EQ switch on the front from one position the other. Only one EQ setting will be closest at a given speed before any record adjustments are made.
Yes, there should be, but not on this particular deck. The record difference between LN and wide-range settings is very small. Just the way it was designed. Nothing is broken. Just set it to wide-range.
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  #16  
Old 10-13-2017, 06:05 PM
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Quote:
Originally Posted by solouplay View Post
The recording circuitry has been tampered with and it's certainly out of spec considering the bass-response.
Exactly what parts have been changed (tampered with) in the record section? Give part values.

Or are you just guessing on this?

NAB standards for 3.75 and 7.5 require a 3dB boost at 50Hz in the record amp and a 3dB cut at 50 Hz in the play amp. You can measure those and see which is not correct. You always start with the record amp and see if it has that boost.

Clean the switches first and re-set bias as I indicated before checking bass EQ.
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Last edited by Tapetech; 10-13-2017 at 06:07 PM.
  #17  
Old 10-13-2017, 06:06 PM
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dowload audiotester for your PC, and connect the AKAI directly up to line in and line out of the PC.

Run a frequency response test (Sweep measurement) where you monitor the recording of tape. I would like to see the graph coming from it.

If you set the level at 0VU on the meters at the 1 KHz step, then they should peg out in the red as soon you go from 20 Hz - 200 Hz correct?

Run a test at both speeds and post the graphs here.

Use a direct connection only.
  #18  
Old 10-14-2017, 04:15 AM
solouplay solouplay is offline
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Quote:
Originally Posted by tapetech View Post
Exactly what parts have been changed (tampered with) in the record section? Give part values.

Or are you just guessing on this?
No, I'm talking about the pots, I tweaked them a little bit. I still haven't checked out if the values are correct on the components and still haven't had time to clean the switches, but I'll do it later today.

Quote:
Originally Posted by Ebzen View Post
dowload audiotester for your PC, and connect the AKAI directly up to line in and line out of the PC.

Run a frequency response test (Sweep measurement) where you monitor the recording of tape. I would like to see the graph coming from it.
I did a similar test with Audacity, but I can do another one with that program after I've cleaned the switches / adjusted the bias. I'll post the results later today.

Quote:
Originally Posted by Ebzen View Post
If you set the level at 0VU on the meters at the 1 KHz step, then they should peg out in the red as soon you go from 20 Hz - 200 Hz correct?
Yes. When I ran the frequency-response test with audacity they maxed out. As you can see from the results I posted it's about +6 to 7 dB over reference.
  #19  
Old 10-14-2017, 04:21 AM
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Awesome!

We will standby.
  #20  
Old 10-14-2017, 06:49 AM
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I cleaned all the switches, but it didn't fix the problem.

I did some sweeping measurements in audioTester and came up with this graph. The upper signals (blue and red) is recorded at 0 dB. The lower signals (pink and green) is recorded at -10 dB. As you can see both measurements is following the same pattern, boosting the bass-frequencies, peaking at ~80 Hz. It seems that the signal goes back to reference at around 400 Hz.

For some reason the -10 dB recorded signal goes 6 dB over reference at the end of the sweeping tone (18 KHz), but that's fine.
The bass-peak seems to be around 80Hz, close to 8 dB over reference.

I'm going to try fiddling with pots again to see if I can better the results (record-levels and bias). I was gonna go over the components on the board, but I just remembered that I've done a complete recap on the REC and PB-board, the values are correct and the caps are surely in spec. Also this problem was present before I did the re-cap. So instead I'm going to overlook the speed-EQ circuit-path and the tape-EQ board.

Last edited by solouplay; 10-14-2017 at 07:06 AM.
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