Servicing Pioneer TX-520 Tuner
This unit was brought to me with the complaint that the audio output stops after playing for a few minutes. I opened the set and subjected it to a thorough cleaning and brought it to my work desk and observed that the 2SD880 Power Regulator Transistor leg had dry solder and over all there were several dry solders.
I downloaded a free service manual from http://www.hifi-manuals.com/Pioneer/TX-520/downloads and checked the input voltages at the power regulator Transistor and found it to be ok. Then disconnected the jumper wire from its output and connected my multimeter to check what was the current drawn. It was negligible, i.e., in the order of around 140mA. But the Transistor was getting hot within a few minutes. Checked the ESR of filter capacitors and a few others on random and found these to be either abnormal or at the brim. So, replaced all the electrolytic capacitors on the board and retouched all the solder joints thoroughly.
But inspite of all these, the Transistor was getting hot! Removed and rechecked the transistor using Peak Atlas, as well as analogue multimeter (x1K and x10K tests suggested by Jestine Yong in his book). There was nothing wrong with any of the components. The current drawn got reduced to around 100mA after replacing the electrolytic capacitors. (There is no mention of any current readings on the service manual) So, I decided to provide heat zinc to the Transistor. Got one from my junk collections and fixed it.
The set worked normally for hours together. Now, why the transistor was getting hot? I have no clue. (If any of you know the probable reasons, kindly do let me know in your comments) I even tried replacing the transistor with an equivalent one, replacing the Zener and Rectifier Diodes etc. Same result! So, this was the only solution! In fact, this is the second time I am coming across a similar problem in the power supply regulators. I have one BPL component system. That too had the same problem. It could be small leaks somewhere down the positive rail, probably, and it is very difficult to locate such faults, unless a failure occurs! Here are pictures of the set working as well as the capacitors replaced:
A job completed not so satisfactorily to me!
This article was prepared for you by Parasuraman Subramanian from India. He is 66 years old and has more than 30 years’ experience in handling antiques equipment Valve Radio, Amps, Reel Tape Recorders and currently studying latest techs classes conduct by Kerala State Electronics Technicians’ Association. He was a BBA graduate, retired as MD of a USA company.
Please give a support by clicking on the social buttons below. Your feedback on the post is welcome. Please leave it in the comments.
P.S-If you enjoyed reading this, click here to subscribe to my blog (free subscription). That way, you’ll never miss a post. You can also forward this website link to your friends and colleagues-thanks!
You may check on his previous repair article below:
https://jestineyong.com/12v-5a-led-driver-smps-repaired/
(82)Dislikes
(0)
15 Comments
Leave a Reply
Anwar Shiekh
November 22, 2016 at 9:52 am
Yes it is strange that a heat sink was needed when one was not around before...
Robert Calk
November 22, 2016 at 11:44 am
Good job, Parasuraman. Did you check the frequency and waveform of the transistor?
Parasuraman S
November 23, 2016 at 5:50 am
Yes, did it with an oscilloscope, which is my routine check.
skwong
November 22, 2016 at 1:28 pm
Have you check the power supply for high ripple voltage? You will encountered a lot of problems if the power supply ripple is high. Normally I check it with an oscilloscope as it easier and can showed any abnormality of the dc voltage if present.
Albert van Bemmelen
November 22, 2016 at 2:31 pm
Thank you for the download link and informative repair with unexpected results, Parasuraman. Maybe skwong's advice in his post is a good idea to check up on?
I faced something similar noticing that a TO-220 Transistor in an Akai Amplifier got hot. Although everything worked splendidly I still decided to give the Transistor an Aluminium cooling Fin just to be safe.
The Akai Amplifier must already be over 25 years old now and still works fine. But after seeing all the e-caps you had to remove and replace in the article, it probably means trouble in the years to come for me too?
James McKinney
November 22, 2016 at 2:44 pm
Maybe it is from just being old? There could be some kind of built-up resistance in the board that is virtually undetectable, from age and corrosion.
Yogesh Panchal
November 22, 2016 at 4:26 pm
Higher leakage currents may cause the overheat; as advised by Robert sir check the frequency /Voltages and waveform on suspected area.
suranga Electronics
November 22, 2016 at 8:01 pm
Hello, Mr-Parasuraman,
Nice Audio Tuner Repair.
Thanks Mr. Audio Expert.
Henrique Jorge Guimarães Ulbrich
November 22, 2016 at 9:35 pm
Hi, dear Parasuraman. The problem of low-quality e-caps again. This subject was already approached many times here. It's not difficult, I suppose, to manufacture better quality e-caps. At the time of tube-equiped equiments, this didn't happen, at least so frequently. The e-caps of that time were better. As time goes, the tendency is to improve the components, but whith e-caps it seems to be the opposite! With regard to the hot transistor, the only occurrence I encountered was with a linear power supply that, by a reason I couldn't find, was oscillating. Knowing that e-caps does not function well at high frequencies (tending to present inductance in these cases), I easily solved the problem with the addition of a non-electrolitic capacitor at the output (in this case, it would be between emmiter of Q10 and ground).
Paris Azis
November 23, 2016 at 3:10 pm
Hi Parasuraman
If this unit soon comes to you again, you will certainly find out what the real culprit was...
Anyway, in this case, if I were in your shoes, before anything else I would use the freezing spray while having connected my volt meter across R58, that 4,7 ohm resistor connected in series with the collector of the regulating transistor. This would allow me to see the changes in current draw while spraying the suspected components. Most likely the problem would have been resolved in this way. I believe that this is the most effective method for troubleshooting such failures.
(I needed to download the schematic in order to follow up your thoughts about the case and then put my comment).
Thanks for sharing the case.
Parasuraman S
November 26, 2016 at 10:10 pm
A good suggestion! Perhaps I will try it, in the event it comes back! Thanks!
Alexan Catalin
November 23, 2016 at 10:01 pm
Hi Parasuraman.
In that case we have voltage doubling rectifier. (16.5V minus 12.4V) * current flowing out emiter Q10 is the power of transistor. Perhaps due to aging the current increasing and power is overlimit. Q10 being hot. Just measure the voltage of E, C and B, be like indicating in scheme. Heat sink is very good.
Thanks for sharing !
Bulent NUR
November 24, 2016 at 12:27 am
Thanks for sharing, very skill.
Ulises Aguilar Pazzani
November 27, 2016 at 12:14 pm
Mr Subramanian , great fix keep it up Sir
Humberto
November 29, 2016 at 12:31 am
Good repair as usual, and thanks for sharing your articles.