Tascam 34B Headphone Output

[aokikamawara]

Hi there -

I read in the manual that accidentally inserting a stereo 1/4" cable to the headphone jack could blow it out. I'm pretty sure that's what happened with the headphone jack on my Tascam 34B. I can't find an instruction manual that goes beyond page 35, though - so I'm uncertain about how to service it. I imagine it might be a simple chip replacement..but I need more details and I'm hesitant to open it up without knowing what I'm doing.

Anyone have any ideas?

Thanks,
Aoki

 

[SkywaveTDR]

Plugging in a Mono 1/4" jack can cause problems but not a stereo one. Besides that every good designer put current limiting resistors in the output in case of shorts. In this case the R size is 22 ohms. Headphone do not require much current. The Op Amp is the amplifier and the current is delivered by some buffer transistors. More likely you have a broken solder joint or dirty (tarnished jack contacts). The HP amp is on the Monitor amp board.

 

[aokikamawara]

The instruction manual said that 2 to 3 minutes of a mono plug could burn out the op amp. That seems conceivable to me - but I'll take a closer look at the jacks.
Just to be safe, though, is the TL072 a pin-for-pin replacement for the U101?

 

[SkywaveTDR]

I would put in a compatible IC not a Bi Fet as they have way too high bandwidth. You need to then bypass the power supply with small caps with no better results. It is also possible that the feedback resistor will need a cap across it to limit the bandwidth of the amp. Bi Fet op amp just love to oscillate at 3 MHz and burn themselves out. That also results in hiss to the perception of the listener until the amp finally cooks. Unless an EE it is best not to do this.
There are current boost transistors in the circuit after the op amp which is a NJM4558DD which is U262. I would place an 8 pin IC socket on the Monitor board and put in a like op amp no more than a 4559. This headphone out appears to be mono anyway.

I wondered why they did this- better to take a headphone out in stereo from a mixer than to take it from the deck in mono. I guess another last minute idea was shoved into the unit.

 

[SkywaveTDR]

The TLO72 is pin for pin the same but the insides of the op amp are not. Plus it makes no sense to change the op amp when it is isolated from the headphone jack by the current buffer transistors which are more likely to be damage than the Op amp but with the 22 ohm resistor in series the pair might heat up but should not fail and if designed right the resistor should open. The Op amp in this case does not drive the headphone jack directly. But all this conjecture is all a waste of time- go in and see what has failed then we will know.
The Headphone IC is NOT U101 it is U262.

 

[Evoroadster]

You can purchase the complete manual, operating instructions, theory of operation, service instructions, full schematics and illustrated parts breakdown figures and parts list, all for just $6, from the following link:

http://www.vintage-electronics.net

 

[Jim Agans]

Hello there. I have the exact same problem with my headphone jack. Can either of you render a step by step repair for this problem? It has been a long time since I had this machine apart, and I frankly do not remember all I did to it. It appears that I replaced Q261 and Q262 with an NTE 152 and a 153 respectfully. Does this sound right? Are there any components that typically blow out on the monitor ampl PCB? The phone jack shows a cap across the tip and the ground C361 and the tip and ring are shunted together on the headphone PCB, so somebody is confused re the instructions regarding the phone plug as this would moke the jack mono, would it not? Could you guys help me on this dilemma?

 

[SkywaveTDR]

Hi Jim,
The Tascam 34B is always a major pain to look for in the service manual department. Most of it is like the 34 but the 34 has no headphones outs so the actual 34B manual is needed. I have one in a comb type paper attached binder. NO schematics or none that you can easily read. In that book there is a page describing that the tip and ring are connected together and the output is not a stereo type. If you want a real stereo headphone out that would have to be obtained from the RCA outs via a model 1 mixer or a external headphone type device. The 34B was a upgraded machine of the 34 but not much good documentation has been found on it. I think there is a service manual that was made but no one I could find ever scanned it or has copied it. The only thing you find is the Owners Maintenance which is NOT at all like the Service Manual. The mono outputs on deck like the A3440 and like are just a convenience item for getting something into the headphone. If they were implemented correctly there would have to be at least two knobs and more likely 4 or more as in the TEAC Model 1 which has 8 inputs with pan pots and then a final Headphone level out. I would suggest that this may be a much better solution than the output of the deck. I use Model 1 units on my bench for level matching Audio signals to the units under test for audio checking beyond the sine wave tone outputs.
As best I can tell the Q261 is a 2SC2655Y and the Q262 was a 2SA1020Y
These are both current boost drivers from the output of the Op Amp U262 a 4558DD IC
The output to the phones jack was from pin 7 of the J204. The motherboard on this line has been known to have broken solder joints and bad feed through pins.
If you need additional help- skywavebe@sbcglobal.net and I will assist the best I can.

 

[Jim Agans]

Thank you kindly Skyman. I am really impressed with your turnaround response of my post. I have a service manual ai downloaded from where you suggested, thanks for that lead. You are quite right on the headphone jack, the tip and ring are shunted and the shield goes to ground through a .01 50V ceramic capacitor. I agree with you wholeheartedly, they could have done a lot better, considering what this unit must have cost. According to the SM Q261 is a 2SD880Y and the Q262 is a 2SB834Y. It looks like I installed a NTE 152 qnd 153 respectively. I am still scratching my head wondering why the hell I stopped mid stream and put the whole damn machine back together. Maybe I just thought I would do as you said and go off the RCA outputs Very weird behavior on my part The wires from the phone jack run to plug 142 from what I can see. I am just going to use a standard stereo jack and put in shunt the tip and ring and pray for success. Does this sound like the thing to do? I am attaching a jpeg of the jack PCB so it might help others.

Thank you again, sir. PS did I pit in the right transistors with the 152/153? I must have looked for a cross reference for what was there originally. Do you know what ither components tend to burn out by installing a mono plug? Kind of difficult to understand since the ring and tip are shunted anyway, don't ya think?

 

[SkywaveTDR]

The transistors I have listed in my manual do not show a 2SD880 or a 2SV834Y. I think that according to this that someone changed them before this time. If you look at the schematic the output of the push pull buffer section is isolated from the headphone jack by a 22 ohm fuse resistor so even if shorted that resistor would absorb the heat for a period of time. It doe snot matter what jack you put in - is the one you have broken. Jack do not fail except that when plastics get hard they may crack apart. The headphone signal comes into the amp circuit from off the pot to pin 5 of the U261 goes out pin 7 and then through a low Pass Filter into pin 3 of the U262. The output from pin 1 goes to the transistor buffer push pull circuit and the output of the amp goes out from the emitter joint of the two transistors- to the 22 ohm safety resistor then to the connector J204 pin 7. This feeds a connector called P185 which is red that then goes to the jack.
The 2SC2655 Y cross to a lot of transistors but not a 2SD880. See this site for the list-
http://alltransistors.com/crsearch....=5&ic=2&tj=150&ft=100&cc=35&hfe=120&caps=TO92

The 2SA1020Y is crossed to this list here-
http://alltransistors.com/crsearch....=5&ic=2&tj=150&ft=100&cc=40&hfe=120&caps=TO92

If you have a scope you can trace the path I gave you and see where it stops.
This item on E bay might be of more use to you all the way up to an 8 channel deck-
http://www.ebay.com/itm/TEAC-MODEL-...961076?hash=item3613f387b4:g:q~4AAOSwSlBYvmvu

I have two of them here in my shop.
Sam,