Help Fixing receiver - Page 1
Forum section: electronics / beginners

author: AngraMelo
author_rank: Regular Contributor
timestamp: June 08, 2018, 01:59:04 am
content: Hey guys, how are you doing?I have been studying electronics for about 2 years now. Mainly trying to fix some equipment and slowly understanding the theory behing them.The issue in question is a receiver Sony Legato Linear.Ive been struggling to find the problem with no success.What I do know is that D803 is conducting (and I dont think it is supposed to, it seems like a fail safe diode of some sorts), it is bringing the ground to around -15V. Also, the voltage on the + and - of the bridge rec is around 24V and -55V. Which is really weird and I have no idea how that can happen.When I remove D803 the voltages on the Bridge Rec stabilizes and become around 39V and -39V.The Transistor Q803 is showing 0.5V on its base so it is not letting the collector to open.I already changed D803 and Q803 with no success.I tested all the resistors coming from the positive rail up to the base of Q803 and they are all fine, which is insaly weird for me because I dont understand how can almos no Voltage get to the base of Q803 if its not the resistors fault.Can anyony help me?My main question is, why would D803 start conducting? and is it his fault that the rails are messed up and not equal?Im uploading the whole schematics but the components Im talking about are on the first page (if I uploaded correctly)Thank you so much for your time, please let me know if you want me to make any tests or probe something.

author: AngraMelo
author_rank: Regular Contributor
timestamp: June 08, 2018, 02:00:06 am
content: Here goes the other pages.Also, please note the original post is talking about the file numbered 03

author: Dukov Ahzrukhal
author_rank: Regular Contributor
timestamp: June 08, 2018, 02:11:33 am
content: It looks like D803 and Q803 are making up some form of voltage regulator, and D803 is being used as a voltage reference. can you find the part number of D803? could you remove it and test it?

author: AngraMelo
author_rank: Regular Contributor
timestamp: June 08, 2018, 02:15:50 am
content: I already removed and changed D803 and Q803. Still the same situation.

author: AngraMelo
author_rank: Regular Contributor
timestamp: June 08, 2018, 02:31:45 am
content: Quote from: Dukov Ahzrukhal on June 08, 2018, 02:11:33 amTo be exact, the 803 part number is EQA01-16R it is a 16V zener, which I already changed. appears to be working because it is conducting 15.5V at this moment.Q803 is a 2SD880 which was also replaced today.

author: JohnPen
author_rank: Regular Contributor
timestamp: June 08, 2018, 07:32:46 am
content: Have you tried replacing the bridge rectifier as if one diode fails you can get some odd effects? Also check that the capacitors in parallel with the diodes are ok (disconnecting them all temporarily would probably be ok in this situation). Another thought check if the transformer secondary winding centre tap is actually making proper contact with the Ov/reference earth rail.

author: AngraMelo
author_rank: Regular Contributor
timestamp: June 09, 2018, 06:14:48 pm
content: Quote from: JohnPen on June 08, 2018, 07:32:46 amThe bridge was good the first time I checked, now that I changed those 2 components I will check it again. Also for the capacitors, I replaced all the electrolytic caps of the power supply section including the 2 main large filter capacitors.

author: AngraMelo
author_rank: Regular Contributor
timestamp: June 12, 2018, 04:26:13 am
content: Quote from: JohnPen on June 08, 2018, 07:32:46 amI just removed the Bridge rectifier and tested it out, the issues persist. I even changed for a new one and got the same result

author: JohnPen
author_rank: Regular Contributor
timestamp: June 12, 2018, 08:30:52 am
content: Starting back at the beginning check that the transformer centre tap is actually connected to the chassis. Another thought check that D806 and D807 are still diodes. A simple check might be to lift one end of each diode to eliminate any effect they might have on the regulated supplies. The transistors Q802-5 are set up as simple voltage regulators using a Zener diode on the relevant transistor base to set their appropriate output voltage on their emitters. Check each of the voltage regulator outputs to see if they have the correct value. In each case it is the emitter of the transistor that has the regulated voltage output. Q803 should have +15V relative to the chassis. Q802 should have +6.6v but only if Q803 is providing +15v to it. Q805 should be -21v relative to the chassis and Q804 should be -15v relative to the chassis. Similarly the QR01 should be providing +28.6 v relative to the chassis.Hope this helps.John

author: AngraMelo
author_rank: Regular Contributor
timestamp: June 12, 2018, 04:17:42 pm
content: Quote from: JohnPen on June 12, 2018, 08:30:52 amThank you very much for your help, what I did noticed from probing around is that Q803 is not receiving any voltage at its base. the Zener D803 is also conducting. Center tap of the transformer is connected to the chassis and given all of this, Im completely lost. I have no idea what might be causing this. Do you know why would D803 start conducting? is it supposed to be conducting all the time?

author: JohnPen
author_rank: Regular Contributor
timestamp: June 12, 2018, 06:37:22 pm
content: Q803 base should have +15.6v relative to the chassis, the voltage rating of the Zener diode. If it was a normal silicon diode or perhaps a Zener diode connected inreverseQ803 would have only +0.6v relative to earth. If the capacitor C803 was s/c that would also lower the voltage below +15.6v.Even though you originally measured only +24v from the bridge rectifier there should still be enough volts to provide the +15.6 voltage on D803/Q803 base. This does assume that R806 and R811 are both OK.John

author: tpowell1830
author_rank: Frequent Contributor
timestamp: June 12, 2018, 06:49:10 pm
content: Quote from: AngraMelo on June 12, 2018, 04:17:42 pmYou stated earlier that there is 15.5 V at the D803, yet you have no voltage at the base of Q803. Is there a break in the board trace?

author: lordvader88
author_rank: Frequent Contributor
timestamp: June 18, 2018, 11:13:33 pm
content: As for a common zener diode rated 15V and 1W, that would probably be passing around 10mA from cathode to anode (conventional current). So yes if it's working it would be passing a fairly small current with a big voltage drop.Lower voltage zener's like a 3V usually pass a lot more current, like 80mAhttps://www.vishay.com/docs/85816/1n4728a.pdf

author: mzacharias
author_rank: Frequent Contributor
timestamp: June 19, 2018, 12:17:44 pm
content: I think either a short or near short is pulling down a voltage line, or a main filter cap is open circuit, so that any load lulls it down, or the ground reference is faulty, such as a burned foil run or resistor.I would check resistance to ground at any of the DC voltage regulated outputs, like +5, +28. +15, etc.One of these may have unusual low resistance.Does Q803 0r Q802 get hot very quickly at power on?Make (Sony I presume - Legato Linear is NOT the Brand) and model number PLEASE.