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My adventure with modding a Tascam 38. (Paging Jim Williams!)
Old 30th October 2013
  #31
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yea i see the concerns.

the pdf of the service manual isn't so clear either.

but removing the transformer you have to look at what its function is:

1. step up of head signal - a gain of 18 db now the modders chose not to change the gain of the first op amp.

2. impedance matching:unfortunately, the input impedance is still high in respects to the head. now when I made a tube based pb amp one time it ended up being very similar to a mic pre I set up for really low impedance (32 ohms) I would think this tascam would be similar to the Otari I modded

3. dc isolation of the heads from the playback amp. any dc voltage would end up on the heads and could cause magnetization of the heads which would effect the tape. In worst case scenario, It could over current the head and thus be permanently damaged.

now another thing that has not been looked at would be the band reject filter located in parallel to the head and primary to the pb amp. now since the circuit parameters was changed due to the transformer removal, was this circuit analyzed for its usefulness and function? and was there a mod before on that circuit considered/tried before attacking the stage after?

here is the pdf I was talking about. its too big to post here(31mb)
TEAC TASCAM 38 SM Service Manual free download, schematics, eeprom, repair info for electronics

Last edited by drtechno; 30th October 2013 at 07:49 PM.. Reason: spelling. the one thing I'm not good at
Old 3rd April 2015
  #32
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So, even though the LME49720 draws more current, its cool to pop into the 4558 spot without taxing the power supply too much?

I'm thinking of them in my Tascam 48 as well. My 48 has (2)4559s and (2)2043s in the signal path on each channel cards. I believe they are both nearly identical to the 4558s.

In terms of the bias mod, the 38 and 48 cards are different in that they bias pot is actually a 100pf variable cap instead of a potentiometer following the 100pF (200pF if modded) cap. Any ideas on how these babies should be modded? In the 38 and 48, wouldn't this variable capacitor just be playing with the resonant frequency rather than the actual level of bias? I'd love to be able to get my machine to deliver +9 bias levels, but if not, I'll be happy with the channel card mods.

Last edited by ecs787s; 4th April 2015 at 05:45 PM..
Old 4th April 2015
  #33
Use a WIMA FKP-2 220 pf/1K V cap for the bias feed. That will get to +9. LME's are fine in there as long as you add local .1 uf psu bypass caps to each dual opamp. Use a machine socket. Other chips that may work well are AD8599 and BB OPA1612.
Old 4th April 2015
  #34
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Thank you, Jim. I've sourced the WIMA FKP-2 220 pf/1K V caps, but the problem is that I would have to replace C004, the 100pF variable cap and then loose the ability to trim the bias. Here are the schematics for the amp card and OSC amp card so you can see what I'm talking about. Since there is no pot in the Tascam 48, then how will bias be adjusted? Thank you. I appreciate any guidance.

https://www.sendspace.com/file/q33lp4

https://www.sendspace.com/file/ce1zat
Old 10th September 2015
  #35
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I've modified my Tascam 48 Record Amp cards with all new electrolytics for the power supply rails and Nichicon Muse ES between signal stages. LME49720 and LME49860s have replaced all the op amps except for the VU meter amps. .1uF decoupling caps have been added from pins 4 and 8 to ground off each of the new op amps. All is fine, but I'm having some trouble with noise and drop-outs when recording test tones to my re-calibrated unit. I'm using Quantegy 499, but this problem has exhibited itself on other tapes as well. I believe the issue is not a result of the mods I've made (happening pre-mods as well) but instead the tape path or even heads. The heads have a little wear, but nothing completely terrible. Sending signal to the machine, I've monitored via "Input", "Sync" and "Repro heads. The Input, gives me no issues... sounds perfect. I only hear the issue after listening through Repro or listening back through the Sync head. If I press the "Edit button" and scrub through, I can hear the pops, clicks and rumble, but only when scrubbing which makes me think the amp cards are just fine and the issue is, again, the heads or tape path... Take a listen and let me know if any of y'all have any ideas. OR is this something I must live with. Tones were 40Hz, 1kHz and 10kHz https://www.sendspace.com/file/cs7ilf

Last edited by ecs787s; 10th September 2015 at 03:43 PM..
Old 10th September 2015
  #36
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Turns out my reel of 499 what the culprit and by recording these same tones to a new reel of 499, the issues have been fixed. No more pops, clicks and dropouts. Very strange in that the first reel of 499 was fairly new. This is a worry for me as I don't want to have to replace new tape every couple sessions.
Old 11th September 2015
  #37
NOS tape? That's a gamble. Tape was the reason I sold all my analog recorders. No tape, no music. Bad tape, a hell ride. Used tape, something we never did back in the tape days, too risky.

Nostalgia ain't what it used to be.
Old 13th September 2015
  #38
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Hello... i am new here.. ;-)
First of all i want to say "thanks" for the very useful info on this forum.

I have a Tascam 34B,an i have a question about modifications that are explained here

the steps are

1 remove the step up tranformer and bridge pin 1 with pin 3
2-replace C102 with 470mf cap
2 remove r103 and change with a 91 ohm resistor
about op amp changes.. i placed a socket for U101; for decoupling caps..it is better a 100 nano cap between pin 4 and 8.. or two 100 nano caps, from each power rail pin to ground ? i am planning to use LM4562 or BB 2132..(or JRC 4580D that in my opionion are pretty good)

many thanks in advance
Roberto
Old 17th September 2015
  #39
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Any advice?
Old 18th November 2015
  #40
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bluezplaya's Avatar
Quote:
Originally Posted by ecs787s View Post
I've modified my Tascam 48 Record Amp cards with all new electrolytics for the power supply rails and Nichicon Muse ES between signal stages. LME49720 and LME49860s have replaced all the op amps except for the VU meter amps. .1uF decoupling caps have been added from pins 4 and 8 to ground off each of the new op amps. All is fine, but I'm having some trouble with noise and drop-outs when recording test tones to my re-calibrated unit. I'm using Quantegy 499, but this problem has exhibited itself on other tapes as well. I believe the issue is not a result of the mods I've made (happening pre-mods as well) but instead the tape path or even heads. The heads have a little wear, but nothing completely terrible. Sending signal to the machine, I've monitored via "Input", "Sync" and "Repro heads. The Input, gives me no issues... sounds perfect. I only hear the issue after listening through Repro or listening back through the Sync head. If I press the "Edit button" and scrub through, I can hear the pops, clicks and rumble, but only when scrubbing which makes me think the amp cards are just fine and the issue is, again, the heads or tape path... Take a listen and let me know if any of y'all have any ideas. OR is this something I must live with. Tones were 40Hz, 1kHz and 10kHz https://www.sendspace.com/file/cs7ilf
I am in the process of adding the bypass caps to my newly installed LME49860 chips on my MS-16. I have no schematics however. Does anyone have a reference as to the best place on the card to ground the caps for pins 4 and 8?
Old 20th December 2015
  #41
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I've started doing this mod and did three cards before I realized that the schematic doesn't quite match up with the boards I have. In the schematic the op amp with r115 from pin 2 to ground is not the one connected to the transformer via pin 5, it's the one further away, but on my board it is. Am I misreading something or has anyone else noticed this?

edit: Ok I've figured it out. They're actually two halves of the same op amp, duh.
Old 20th December 2015
  #42
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Another question: The cards on my 38 already have caps running from pins 4 and 8 to ground, they're 47uf. Should these be replaced by .1uf caps, or run parallel to them, or are they not needed at all?
Old 20th December 2015
  #43
Use Panasonic FR series, 330 uf/25V. Those will fit. Add .1 uf mono ceramics at the IC power pins to a nearby ground.
Old 23rd December 2015
  #44
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Thanks for the tip! Doing this without much electronics knowledge is like trying to decipher the dead sea scrolls.
Also, does anyone know if it's necessary to replace both op amps on the cards of the 38? I ended up doing that thinking were both in the signal path but now I'm not sure, it's hard to trace the lines without printing it out and piecing it together. It looks as though U102 goes to the sync head and U101 goes to the meters, is that correct?
Old 29th September 2019
  #45
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Hello my bubbling bretheren

Thank you for all the information here. This thread is going off.

I have recently acquired a Tascam 80-8. Going to try to do the same as described here - opamp upgrades (inc decoupling cap) and transformer removal in playback section.

Before I spend a bunch of time trying different things, has anyone attempted this?

Old 29th September 2019
  #46
I used to rebuild those back in the 1980's. Back then I used 5532 opamps to replace the 4558's. There are better choices now.
Old 29th September 2019
  #47
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nosebleedaudio's Avatar
 

Quote:
Originally Posted by catarax View Post
Hello my bubbling bretheren

Thank you for all the information here. This thread is going off.

I have recently acquired a Tascam 80-8. Going to try to do the same as described here - opamp upgrades (inc decoupling cap) and transformer removal in playback section.

Before I spend a bunch of time trying different things, has anyone attempted this?

Why remove the transformers?
I would leave them..
Replace op amp & caps first..then..
Old 4th October 2019
  #48
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ruffrecords's Avatar
I am surprised nobody has mentioned modding the bias oscillator for a lower noise waveform which seems to be a problem with many Tascam models. I have tested quite a few of these in my time. My usual method is to erase a balnk section of tape with the Tascam. Then I fit it to a Studer of the same tape width. Play the erased section and part way through drop the Studer into record for a few seconds. Replay this tape on either machine and you will clearly hear the noise drop where the Studer is erasing.

Cheers

Ian
Old 4th October 2019
  #49
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nosebleedaudio's Avatar
 

Quote:
Originally Posted by ruffrecords View Post
I am surprised nobody has mentioned modding the bias oscillator for a lower noise waveform which seems to be a problem with many Tascam models. I have tested quite a few of these in my time. My usual method is to erase a balnk section of tape with the Tascam. Then I fit it to a Studer of the same tape width. Play the erased section and part way through drop the Studer into record for a few seconds. Replay this tape on either machine and you will clearly hear the noise drop where the Studer is erasing.

Cheers

Ian
There are no doubt IMHO, several things at work.
How clean the bias osc is.
How refined the circuits are, ALL circuits.
The bias osc frequency & erase head, Record head.
I have replaced the osc frequency caps with better closer matched caps to try and get a cleaner more symmetrical waveform..The difference? Not sure how much..
But, I would NOT expect Highly refined/designed circuits in a "Low end" machine..The same way I would NOT expect the same well designed circuits in a low end console as in a high end console..And Im NOT talking about just better caps, Pots, Op-Amps..
Old 4th October 2019
  #50
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ruffrecords's Avatar
Quote:
Originally Posted by nosebleedaudio View Post
There are no doubt IMHO, several things at work.
How clean the bias osc is.
How refined the circuits are, ALL circuits.
The bias osc frequency & erase head, Record head.
I have replaced the osc frequency caps with better closer matched caps to try and get a cleaner more symmetrical waveform..The difference? Not sure how much..
But, I would NOT expect Highly refined/designed circuits in a "Low end" machine..The same way I would NOT expect the same well designed circuits in a low end console as in a high end console..And Im NOT talking about just better caps, Pots, Op-Amps..
From my tests it is clear the major contributor is bias oscillator noise or possibly record amp noise. When the test I described is undertaken, the Studer erased portion sounds audibly quieter on both the Studer and the Tascam so it is not primarily a replay chain issue. It is definitely record chain. In fact it is quite easy to demonstrate that it is bias oscillator by disconnecting the record head while doing the test.

The bias oscillator in the Studer is not significantly more refined than that of the Tascam. In fact on quarter inch machines you can do this test using s plain Revox A77 with the same results. It should not be too hard/expensive to upgrade the Tascam bias osc and obtain a useful improvement in S/N ratio.

Cheers

Ian
Old 5th October 2019
  #51
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Brian M. Boykin's Avatar
Quote:
Originally Posted by ruffrecords View Post
From my tests it is clear the major contributor is bias oscillator noise or possibly record amp noise. When the test I described is undertaken, the Studer erased portion sounds audibly quieter on both the Studer and the Tascam so it is not primarily a replay chain issue. It is definitely record chain. In fact it is quite easy to demonstrate that it is bias oscillator by disconnecting the record head while doing the test.

The bias oscillator in the Studer is not significantly more refined than that of the Tascam. In fact on quarter inch machines you can do this test using s plain Revox A77 with the same results. It should not be too hard/expensive to upgrade the Tascam bias osc and obtain a useful improvement in S/N ratio.

Cheers

Ian
Is this something worth looking into on the MCI JH16 I’m restoring? As in, are there better bias oscillators now than there were in 1977 even for 2” decks? Are they interchangeable? I haven’t made it that for with this deck. I’m still working out the quirks MCI’s are known for. Then it’ll get a full recap and metal films in the audio path. The transformers are staying.
Old 6th October 2019
  #52
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ruffrecords's Avatar
Quote:
Originally Posted by Brian M. Boykin View Post
Is this something worth looking into on the MCI JH16 I’m restoring? As in, are there better bias oscillators now than there were in 1977 even for 2” decks? Are they interchangeable? I haven’t made it that for with this deck. I’m still working out the quirks MCI’s are known for. Then it’ll get a full recap and metal films in the audio path. The transformers are staying.
To be honest I don't know. Can you post a schematic?

Cheers

Ian
Old 6th October 2019
  #53
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Brian M. Boykin's Avatar
Quote:
Originally Posted by ruffrecords View Post
To be honest I don't know. Can you post a schematic?

Cheers

Ian
I could, but I have way to many other fish to fry with this deck before I get into the bias oscillator. Like it actually recording. The transport is 99% complete. Then off to testing all the cards on the channels and then recapping them. The heads need to go out to be evaluated also. I’ll start a separate thread one day when I get there.
Old 6th October 2019
  #54
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ruffrecords's Avatar
Quote:
Originally Posted by Brian M. Boykin View Post
I could, but I have way to many other fish to fry with this deck before I get into the bias oscillator. Like it actually recording. The transport is 99% complete. Then off to testing all the cards on the channels and then recapping them. The heads need to go out to be evaluated also. I’ll start a separate thread one day when I get there.
No problem. Looks like you have your work cut out but it is a very worthwhile venture. Best of luck.

Cheers

Ian
Old 6th October 2019
  #55
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nosebleedaudio's Avatar
 

Quote:
Originally Posted by Brian M. Boykin View Post
I could, but I have way to many other fish to fry with this deck before I get into the bias oscillator. Like it actually recording. The transport is 99% complete. Then off to testing all the cards on the channels and then recapping them. The heads need to go out to be evaluated also. I’ll start a separate thread one day when I get there.
The heads would have the been the first thing I would have tested..
Old 6th October 2019
  #56
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Brian M. Boykin's Avatar
Quote:
Originally Posted by nosebleedaudio View Post
The heads would have the been the first thing I would have tested..
On a deck coming out of a studio that was a daily driver and going straight into service, absolutely, this was a deck in storage that hadn’t seen the light of day in over 15 years. I made a calculated decision that a 1977 JH16 would need a complete rebuild including replacing the heads. No sense in the expense of heads if the deck never gets to pulling tape and recording and playing back music. JFR has replacement heads and they’ll last my modest little studio a lifetime if I have to fork up the money for a new set. They won’t be genuine MCI heads, but the deck already isn’t genuine.
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