Master Electronics Repair !

Welcome to Master Electronics Repair! website.Here you will find:Service Modes, Circuit Diagrams,Service manuals, Firmware Update procedure, Disassemble procedure, Universal remote control setup codes, Troubleshooting, Diagnosing, Measuring, and more.... of many of LCD/LED TV/Displays,others various electronic apparates,trough many technical notes and tips/tricks to help repairing/fixing Troubles/ faults,for electrical technicians.This site helps you to save the Earth from electronic waste!

Richtige Fernseher haben Röhren!

Richtige Fernseher haben Röhren!

In Brief: On this site you will find pictures and technical information about Service Modes, Circuit Diagrams, Firmware Update procedure, Disassemble procedure, Universal remote control set-up codes, Troubleshooting and more....

If you go into the profession, you will obtain or have access to a variety of tech tips databases HERE IT IS Master Electronics Repair !.

These are an excellent investment where the saying: 'time-is-money' rules. However, to learn, you need to develop a general troubleshooting approach - a logical, methodical, method of narrowing down the problem. A tech tip database might suggest: 'Replace C536' for a particular symptom. This is good advice for a specific problem on one model. However, what you really want to understand is why C536 was the cause and how to pinpoint the culprit in general even if you don't have a service manual or schematic and your tech tip database doesn't have an entry for your sick TV or VCR.

While schematics are nice, you won't always have them or be able to justify the purchase for a one-of repair. Therefore, in many cases, some reverse engineering will be necessary. The time will be well spent since even if you don't see another instance of the same model in your entire lifetime, you will have learned something in the process that can be applied to other equipment problems.
As always, when you get stuck, checking out a tech-tips database may quickly identify your problem and solution.In that case, you can greatly simplify your troubleshooting or at least confirm a diagnosis before ordering parts.

Happy repairing!

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Monday 11 March 2019

ROTEL RA 980BX REPAIR ASSISTANCE

Fault: No output from left channel. The right channel is working fine.

Added some photos and circuit for reference

While testing the amp noticed that the left channel is giving switch on off thump while powering ON and OFF the amplifier. It happens when there is DC voltage flow through the speakers. It shouldn't be there. And the thump is there only on the left channel. While testing the amplifier section found that all the transistors where idling at mentioned voltage. All Base pins where around +/- 0.5volt. and the output is almost 0.012mvdc.

POWER AMPLIFIER BIAS ADJUST

PRIOR TO ADJUSTMENT RUN THE AMPLIFIER FOR ABOUT 5 MINUTES WITH LAOD RESISTANCE OF 8ohms

SETP	PLUS LEAD	MINUS LEAD	ADJUST	ADJUST FOR
1	TP3	TP1	VR601	DC MILLIVLOT 10mv
2	TP4	TP2	VR602	DC MILLIVLOT 10mv

As the amplifier had an external Pre -IN ,Pre -out coupling it was easy to use an external pre amp to test the amplifier section.

While testing with an external pre amp found that the amplifier section is OK. There were no switch off thump too while working through the external pre amp.

After testing the amplifier stage and confirming it is OK , turn to the next stage. The internal pre amp (tone control) section.

The internal pre amp of Rotel RA 9808X is built with two AD712JN ICs. Where each AD712 is used for the left and right channel separately. Hope Rotel did it to reduce the bleeding between the left and right channel. Its possible to reduce the bleeding between two channels. If we use two op amps separately for each channels than using the half of each.

While testing the pre amplifier its found that there was a DC voltage of 15V at the pin no 7 (output pin) of one of the AD712 which is used for the left channel. It shouldn’t be there. The output, non inverting and inverting pins of an op amp which is used in the tone stage circuit should be at 0vdc ,if it’s working in dual supply rail (+/- supply). And there was no voltage found on the pin 7 of other AD712 which is used for the right channel. While further treating found that the supply voltages to AD712 where ok its there it +/- 17.5V.

As there where nothing else to check replace the AD712 with a new one. Then power the amplifier and found that it’s working perfectly. There were no voltages where found on the pin no 7. And also there were no switch of thump too. Also replaced the power filter capacitor and coupling capacitors near the AD712 of each channel as its found too old.

If AD712 is not available then Instead of AD712 you can use TLO82. Its an equivalent to AD712. Or you can use NE4558 , it will work too but there will be some difference in frequency response of the amplifier, but not that much noticeable. If your customer is not an audiophile then it’s ok.

CIRCUIT DIAGRAM OF ROTEL RA 980BX

TONE 1/2