Studer A80RC 1/4" Distorted Output

[Manitou]

Hi Folks,

Got an old Studer A80 2 track here which I have run an MRL tape on. I'm hearing a lot of distortion on the lower frequencies (50/100Hz) when calibrating the repro cards. It's less noticable from 1khz up and on playback of speech/music.

I have swapped around and cleaned all the card contacts for repro cards, stabilizer card & even tried a different headstack. It still sounds the same.

Anybody got any suggestions as to what might be the problem?

 

[abbey road d enfer]

Hi Folks,

Got an old Studer A80 2 track here which I have run an MRL tape on. I'm hearing a lot of distortion on the lower frequencies (50/100Hz) when calibrating the repro cards.

Are you sure it's not the speakers distorting? What speakers do you use to listen?

 

[Walrus]

I've had it before where one of the transistors that do the speed EQ switching has been leaky, and if the bass happened to be turned up on the other speed, then I got distortion.
Only had this on one channel at a time though.

 

[Manitou]

Are you sure it's not the speakers distorting? What speakers do you use to listen?

Listening through my studio monitors / headphones, all are showing up distortion at the low end. I've tried different inputs on the desk as well as the 2 track options. All sound the same, so I'm concluding it's the machine itself.

I've had it before where one of the transistors that do the speed EQ switching has been leaky, and if the bass happened to be turned up on the other speed, then I got distortion.
Only had this on one channel at a time though.

This is definitely coming through on both channels. At first I thought it might be low voltage on the +/- DC supplies on the "stabilizer/audio pcb" card. I've measured them at 11.9v / -12.1v. `As specified on their schematic.

Also, I tried swapping repro cards as I have 2 spare. All sound the same. I'm thinking it would be rare to have leaky transistors on all the cards. Although I've been wrong before 

I've got a service manual for this model, but I can't find the schematic for the audio cards in it.

Anyone know of anything between the repro cards and xlr output I can check?

 

[Walrus]

Does it distort on the Sync output??

 

[Manitou]

Hi Kevin,

No sync mode on this model, or did you mean if i record a pure tone to tape does it com back distorted?

I'll try that next. I've setup playback with the tones on the MRL regardless.

Let you know how it goes!

 

[abbey road d enfer]

I've got a service manual for this model, but I can't find the schematic for the audio cards in it.

Anyone know of anything between the repro cards and xlr output I can check?

The manual is available on-line at:
http://thehistoryofrecording.com/Manuals/Studer/A80RC_MkI_MkII_Service_Manual.pdf
Download is very slow (about 30 min)
p.219 Repro amp
p.221 & 223 Mono/stereo switching
Check block dgm at p. 195; tyhe M/S switch is between the head amp and the line amp.

 

[Manitou]

Thanks for the schematic link & page numbers!

Well, I've got the repro side of the recorder setup nicely now & the machine is playing back my old tapes perfectly.

At the risk of sounding mental, I think it is actually the mrl tape that is the source of the distortion I was on about. I've noticed a bit of print through on the spoken section at the start & I'm wondering if the distortion I'm hearing is something to do with that when the longwave 50Hz & 100Hz tones are running.

Anyway, I've run into another problem lining up the record cards.

Only channel 2 is recording and the problem is not following the record amp cards. I have traced it back to the channel 1 signal to the record head from the oscillator card in the attached circuit diagram. All the test voltages on the diagram check out correctly. So I scoped the ac volts on R17 & R18 whilst recording. There is no signal on R17. R18 shows a healthy ac signal of 21v.

Is this simply a problem with the relays K1/K2? Or is the ac signal leaking off somewhere before it reaches R17? C8 measures 78v rms, whilst C9 has only 8v. Might that be a leaky cap at C9, or bad contact on a relay?

 

[abbey road d enfer]

Is this simply a problem with the relays K1/K2? Or is the ac signal leaking off somewhere before it reaches R17? C8 measures 78v rms, whilst C9 has only 8v. Might that be a leaky cap at C9, or bad contact on a relay?

There are many possible causes but, yes, I would definitely check the relays.