Cassette deck power supply droopy

[BeBuLamar]

Tape decks may have the 0 dB calibrated at different flux level 160nWb/m, 200, even 250. However, the Dolby level is always 200 nWb/m. So using the 160 nWb/m tape the level is about 2 dB low.

[Quantumplate]

Tape decks may have the 0 dB calibrated at different flux level 160nWb/m, 200, even 250. However, the Dolby level is always 200 nWb/m. So using the 160 nWb/m tape the level is about 2 dB low.

Well i agree, but Pioneer seem to do things their own way. See page 25 of the service manual. It states their STD-341a tape, 333Hz, 0dB, 160nWb/m is to be used, and emphasises the importance of this adjustment for determining the Dolby level.

[wasedadoc]

Try bridging each of the audio coupling caps (not power rail decouplers) with another capacitor in turn to see if one has gone very low capacitance or very high ESR.

Good tip thanks. I wonder if you still have your homemade Dolby unit....

Am i right in thinking that if i can only reach under voltage at the Dolby test point, then when i play a prerecorded Dolbyfied album, more of the sound will be 'swallowed up' so sounding not right? That is certainly the experience i'm hearing.

Yes still have it.  Been in the loft for years.  Used it with an Akai GXC-40D cassette deck.  That has been boxed up for years but I did fire it up about 2 years ago to check its belt.  It was all fine unlike the belts in a much, much younger Sony 3 head Dolby B/C/S deck that I sorted out for a colleague.  Belts had turned to a gooey black sticky mess.

Yes, the sytem works by attenuating low level, high frequency components.  So if the playback level of that channel is too low it will sound less "bright" than the other one.

[Alex Nikitin]

Tape decks may have the 0 dB calibrated at different flux level 160nWb/m, 200, even 250. However, the Dolby level is always 200 nWb/m. So using the 160 nWb/m tape the level is about 2 dB low.

That is not quite correct. The Dolby level for Compact Cassette is 200nWb/m ANSI = 218 nWb/m DIN (according to Dolby Lab information), so it is roughly +2.8dB from 160nWb/m DIN.

The CR860 Dolby chip used in Pioneer is an equivalent of NE545 and the Dolby reference level is 580mV on pin 3 (playing back a true Dolby level cassette, like Teac MTT-150 or my L04) .

Cheers

Alex

[Alex Nikitin]

I've looked at the service manual, the tape used was 250nWb/m, not 160nWb/m, no surprise you've run out of the adjustment range! 650mV on pin 7 is roughly correct for 250nWb/m DIN level (the accurate figure is 665mV). Use a proper level cassette and it should adjust just fine.

Cheers

Alex

P.S. - you can see from the manual that for 160nWb/m the voltage is 410mV (the accurate figure is 425mV) - it is clear from the level diagram.

[Quantumplate]

I've looked at the service manual, the tape used was 250nWb/m, not 160nWb/m, no surprise you've run out of the adjustment range! 650mV on pin 7 is roughly correct for 250nWb/m DIN level (the accurate figure is 665mV). Use a proper level cassette and it should adjust just fine.

Cheers

Alex

P.S. - you can see from the manual that for 160nWb/m the voltage is 410mV (the accurate figure is 425mV) - it is clear from the level diagram.

Hi Alex and thanks!
That's altered things a bit.
Before i read your post i just checked pin 3 using your dolby tape and got 860mV. Oops.

But what happened to the 580mV you quoted originally? out of interest..
I've scoured the manual but i can't see where they ever mention a 160 or 250nWb/m. What page was it on? I found the level diagram ok.
 
Head's spinning a bit as this deck is 1978 - i thought that was pre the 250 cal level established in 1981? It also means Gennlab have the wrong information on their page. But i'm sure you're right. It makes sense now, the numbers - it seemed crazy to run out of pot adjustment.
Btw what do you mean by 'the accurate figure'?
Cheers
Tim (thinkchronicity)

[Alex Nikitin]

As I’ve said, the voltage should be 580mV for the Dolby level tape, 665mV for 250nWb/m DIN, 425mV for 160nWb/m DIN. By “accurate “ figures I mean derived from the Dolby level (580mV) and 160-218-250 ratios.

Cheers

Alex

[Quantumplate]

As I’ve said, the voltage should be 580mV for the Dolby level tape, 665mV for 250nWb/m DIN, 425mV for 160nWb/m DIN. By “accurate “ figures I mean derived from the Dolby level (580mV) and 160-218-250 ratios.

Cheers

Alex

Yep, my mistake. Brilliant thanks.

[Quantumplate]

650mV on pin 7 is roughly correct for 250nWb/m DIN level (the accurate figure is 665mV).

Actually, 650mV on pin 7 (the test point) is right as this pin is ~0.3dB lower than pin 3.
Just for the record!