This morning, from a cold start, it took about a minute for the hum to appear. The 10 ohm emitter resistors on module D and E measure between 188 and 210 mV and the transistors (TR571~TR574) seem way too hot.
So a shot of compressed air on module E (mostly), makes the hum disappear. Even blowing on module D with a straw removes the hum for 15 seconds or so.
To answer one of your first questions, "how does one voltage affect the other". This regulator is a dual tracking regulator and only the + regulator has a reference, D715. So it is the controlling regulator. The - regulator gets it's reference from the the + regulator by way of voltage divider R731 and R732. Whatever the + regulator does the - regulator will track the + regulator. If the output of the + reg goes up so too the - regulator. Now back to your problem.
Craig
Craig
Yes ,back to problem ... Power supply is supplying +-26v to modules A B C D E ,and also to Phono preamp ,and one of them drawing too much current ,because 3 volts voltage drop on 15 ohm means ~200ma current . C806 C805 should have +-37V at each ,or more ,if voltage will be close to regulator output ,maybe they are bad and lost capacity . Check voltage drop on R341 ,R344 (22ohms) ,R514 on modules A and B (22ohms) ,R550 ,R551 (10 ohms) module C ,R583 R584 (10 ohms) module D and E and write here . You wrote previously 188-210mv drop ,this is 18ma , should be 5ma if schematic has correct values . So all modules transistors are hot ,or one of them ? D and E are identical ,A and B ,you can swap them for testing .
All 8 emitter resistors measured around 200 mV My + and - supplies are actually +31 and -32.8 V......could this be causing the modules to over-heat?
Higher voltage may increase transistors current ... Why voltage is higher ,thats the question .Negative regulator just repeats positive voltage ,so need to look at TR703 TR712 TR713 D715 R727 R728 R729 R730 .Probably there's no room left for regulator to regulate ,so the ripple at output is high and you hear it as a hum ... For regulation itself responsible TR703 , what voltages it has .So you saying regulator output is +31V , what voltage is present at TR712 collector ( regulator input ) ? Maybe TR712 713 are shorted ?
Thanks Ximikas;
C805 and C806 have been replaced. There is 39 V on Collector of TR712 and 38.2 V on the emitter of TR715.
Thanks llwhtt;
I was wondering why the neg side does not have a zener.
C805 and C806 have been replaced. There is 39 V on Collector of TR712 and 38.2 V on the emitter of TR715.
Thanks llwhtt;
I was wondering why the neg side does not have a zener.
Negative side made as tracking to avoid only one polarity present in case of faults ,to avoid dc at preamp outputs. Lets say ,there is short circuit in positive side ,or overcurrent ,and such regulation guarantees that both voltages are similar ( not +7 and -26 volts).
So looks like positive regulator outputs too high voltage , because of this preamp draws too much current ,this causes regulator to operate in current limit mode (high drop on 15 ohms),not regulating output voltage itself .
Voltage measurements
TR703 B: 7.84 C: 35.2 E: 7.31
TR707 B: 31.04 C: 35.3 E: 31.02
TR712 B: 34.7 C: 37.9 E: 34.2
TR713 B: 35.3 C: 37.2 E: 34.7
TR703 B: 7.84 C: 35.2 E: 7.31
TR707 B: 31.04 C: 35.3 E: 31.02
TR712 B: 34.7 C: 37.9 E: 34.2
TR713 B: 35.3 C: 37.2 E: 34.7
Looks like TR703 collector not able to shunt TR713 base to zeners voltage ... Check TR703 , maybe very low hfe ,or C E reversed,or dead ,collector pin open,try to change with another npn at least 40v . Also test with 1k resistor in paralel TR703 C-E pins ,both regulators voltages should drop to 7-9 volts.If its true ,means tr703 not working . Also in schematic C713 displayed in wrong polarity ,check how its in reality , R717 R719 junction should be positive pin.
Tr707 base has resistor divider , some negative voltage applied ,so it may look like bad B-E , but its role is to output current limiter .If shorted B-E (unlikely) ,output would not limit current , if C-E shorted , then output not operate at all ,TR712 713 not able to open,output will not handle more than 1ma load .According to schematic ,Tr707 has -0,8V B-E under normal condition ,to begin operate at higher voltage drop on R725. But offcourse check TR707 ,to be sure its ok .
It looks like emitter of TR707 goes to the plus side of the cap----I guess I should replace that cap!
I was told about a error on the cap board but WOW!
I was told about a error on the cap board but WOW!
After lengthy checking, it seems that C713 although marked polarity in schematic is wrong, the "+" is correctly marked on the pcb...and the cap is correctly installed:
positive to junction of 10k and 18k resistors.
I will check/ replace TR707 next....day job is interfering. Thanks guys, for your help!!
positive to junction of 10k and 18k resistors.
I will check/ replace TR707 next....day job is interfering. Thanks guys, for your help!!
Another possibility, if you can't figure out whats wrong, is use lm317 or 7824 voltage regulator for positive side.
Peter S, I know it’s been a few years, but did you ever find a solution for your hum problem? I’m studying your thread as I believe I have the exact same issue with my CR-2020.