Simple question (for the one who knows)

Alex SE

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#1
Long story short:
Trying to adjust peak meter (FL). It indicates 0db instead of +3 (Dolby levbel properly adjusted, so it's only peak meter)
My question is which resistor need to be replaced. Have marked 4 resistors but not sure which one of them.
If that (probably) matters, there are no more components between that point "9" on the right side of a picture and a display.
Thanx in advance!
1717330917739.png
 

Makymak

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#2
Technics RS-BX707/727/808/828?

Are you sure there is a real problem with the meter levels? I have 4 of these decks and the meters are very accurate. At 388mV at the Dolby chip they peak exactly at Dolby mark.


Though, if I wanted to alter the peaks, I would start with a 150-200 kohm trimmer in place of r67.
 

Alex SE

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#3
Technics RS-BX707/727/808/828?

Are you sure there is a real problem with the meter levels? I have 4 of these decks and the meters are very accurate. At 388mV at the Dolby chip they peak exactly at Dolby mark.
I'm sure. Have 555 and 565 which are as you say very accurate. Have 2x 965, one was +4db on fine scale, the other at +6db (ther're not identical from the factory).
The one in question is 727 and is peaking to 0db. Only mods I did are adding J-fet preamp between a head and PCB connector and replacing original 4066 with MAX4066.
Generally, I have seen a very strange things with all those decks. For example (nobody believed me that) when properly adjusted dolby level on 965, the output is higher than it should be. Not disaster, but when somebody want every detail to be as perfect as it can be, those things (outpuyt level, peak meter) are very irritating. At least for me.
Somebody said that peak meters are just eyecandys. I dont agree with that, there is a purpose with them.

Though, if I wanted to alter the peaks, I would start with a 150-200 kohm trimmer in place of r67.
Will do so. The reason why I was asking is that sometimes, resistors in combination with capacitors are working as a filter and I definitely don't want to play with that.

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* Thanks to Vince for providing me a great book in Italian "La registrazione magnetica del suono"
 

Makymak

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#4
For me, Technics have good meters. As for the 965, I definitely believe you because I have the same problem with both of mine. The fault on these is the wrong calculation for the voltage going to the aa class stage and not at the meters themselves.

So, at the 727 when you have 388mV at the Dolby chip, what is the voltage at the deck's output? Is it the expected 545mV?
 

Alex SE

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#5
Havent touched that deck for about a year so I don't remember the details.
I have a 4 days off starting at thursday and will definitely check everything.
Will report here of course ;)
 

Alex SE

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#6
Dolby level 380-386mV on both channels, it can't be stabile.
Output is 550-555mV which is a 5-10mV over and its OK.
Regarding peak meters, I was wrong (easy to forgot after one year). I have peaks at +2 fine scale, so its only 1 db is missing.
Most of people would say its nothing but for me it's kind a challenge to adjust everything as close as possible to perfect. Otherwise I would never test or open a deck but just use it as it is.
Have to find correct values of R67/R68 and to adjust internal calibration pots after that.
It's almost midnight here so I'll leave it as it is for a moment but will fix everything until the end of the week.
Funny thing is that by some reason I love that 727 more than both of mine 965 which are far better than it.
 

Makymak

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#7
Funny thing is that by some reason I love that 727 more than both of mine 965 which are far better than it.
I have a 707 and two 808 I would happily exchanged with your two 965!!!

Of course I'm just kidding. The 606, 707 and 808 are great machines after the minor modifications A. N. T. has documented.

I believe raising a few Ohms the r67/68 you will gain this one dB to the meters. But before doing so, I would suggest you to recalibrate the pb levels at 385-390mV (I know it is not steady) and see if this one dB now is there.
 

Alex SE

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#8
I believe raising a few Ohms the r67/68 you will gain this one dB to the meters. But before doing so, I would suggest you to recalibrate the pb levels at 385-390mV (I know it is not steady) and see if this one dB now is there.
You probably have right about rising dolby level a bit but one thong I'm not sure (actually lot of things but in this case only one):
Should it be as close as possible to 388mV but not over 388 OR as close as possible to 388mV while some mV +/- is accepted?

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Alex SE

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#10
Those Pb gain trimpots are way too sensitive :) Got it to about 385-391mV and peaks are still at +2.
Adjusted R67/R68 (now trimpots) to 78K and it's there, exactly at +3 (Dolby mark).
Will order fixed 78K next time, until then trimpots will do a job.
 
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