Tascam Porta02mkII Hiss

[Leonid]

Hi,

I have recently got a bargain Porta02 mk2. The are a couple issues that machine has. Most annoying one is it has a very high gain hiss once I press play on the machine. I believe this is due to previous owner trying to repair it himself after plugging wrong polarity power supply. What was done is on channels 1&2 op amp was changed to TL074. When I got the machine there was no output on those channels and after changing capacitors this hiss appeared. Also it seems that the machine does not record at all, or the level of hiss is higher gain than what I was trying to record. below is the link with sound sample of "tape hiss" and playback without cassette hiss a few seconds after.

https://soundcloud.com/cdsmirno/tas...d&utm_medium=text&utm_campaign=social_sharing

1. Can it be that hiss is introduced by non original opamp TL074 instead of NJM4565LD? or am I looking in a completely wrong place?

2. What else should I have a look at if machine was used with wrong polarity?

Thank you very much for your help in advance.
Leo

 

[SkywaveTDR]

Hi Leo, I a case like this you would need to diagnose and work on it one thing at a time. The hiss is most likely due to a oscillating op amp that has been damaged as most op amps do not want a negative polarity applied to them. Then the Large Electrolytic caps on the power supply would be next as well as anything that looks damaged as in puffed or leaking. If you put a scope on that particular channel you will probably find a high frequency oscillation going on. If a person was inside the unit most the time they do more damage than good as many of the wire sets are going into connectors that if stressed will crack easy- a lot of headers as they call them in these Porta studios need to be resoldered with a real hand solder job using Kester 44 and a Weller- I spend a lot of hours resoldering some boards in decks I work on as I can not warranty a unit with half soldered parts. My solder joints are bullet proof. I worked at Hughes and we had to repairs electronics on aircraft.

 

[Leonid]

I've managed to fix the buzz, and playback is great. Now I can't figure out what happens to recording part. When recording onto tape and giving it hot signal, when the playing back that signal is very low, barely can hear it. Any ideas where I should look?

 

[SkywaveTDR]

Well a start at the calibration section of the service manual will get you to the point that you can adjust it but you can not start in the middle of the process as all playback section must be calibrated correctly to do the record section. One builds on the other.

 

[Leonid]

Have no luck finding service manual unfortunately. Do you have any idea where I can get one by any chance?

 

[SkywaveTDR]

I checked a lot of places and the only place might be E bay where an original one might be sold. They are rare and I don have one either. Porta Studio 02 yes but the Mk II is totally different.