Are u ready to buy what you need to fix the amp.?

 

High Res if u like


Is that it? Any other burned components.?

Do u have the know how on soldering and voltage measurements buddy.

yep. just about does it for the burnt components. and yes, I know how to solder and take voltage readings

 

U can replace the output transistors with later, better ones u know.

So, what u need to do is find new components to replace the burned ones. They are quarter watt carbon type resistors. Very cheap.

Next take out the output transistors.
Discharge all the big capacitors after disconnecting power.

Put the DMM in diode check mode and check for shorted transistors in the circuit. No need to take em out. Make sure there are no shorted driver transistors.

While u are at it check all of them transistors and diodes for dead short.

Next u need to find two resistors of around 5W 18Ω to 22Ω will do.

If u find shorted ones, u need to replace them transistors or diodes. I bet u can figure out the replacement by googling.

Replace any blown fuse with the correct type
.
If everything is A OK, then take a look at the shemaic power supply section.

I need u to isolate the PSU from the amp and test it.

Can U isolate the left right channels from the PSU ?

Or do u want me to show u how to do it ?

 

U can replace the output transistors with later, better ones u know.



Next u need to find two resistors of around 5W 18Ω to 22Ω will do.



Can U isolate the left right channels from the PSU ?

Or do u want me to show u how to do it ?

I think I know how to isolate the PSU, but you better show me how, just to be sure.

What are the 2 5 watt 18-22 ohm resistors for?

what output transistors should I use. any specific type

 

Hmm. Before tht tell me the status of the amp

 

Hmm. Before tht tell me the status of the amp

yep. the amp board is okay, besides that one burnt transistor

 

One ??

I see 3.!

Details buddy!

 

one ??

I see 3.!

everything is good besides the 3 burnt resistors and the one burnt transistor

 

If you have a DMM with cap. tester, I suggest to take out some electrolytic caps, and see if they are still OK.

The board seems to be old, 1970s, from the make of the resistors.
40 years are beyond useful service life for electrolytics.

I have seen one's from the late 1980s failing already, and causing malfunction, kind of the board worked for an hour, then became erractic, then stopped working altogether.

I would even consider to exchange all electrolytic caps.

But if you take out a few and they measure 100% OK, maybe not needed.

 

If you have a DMM with cap. tester, I suggest to take out some electrolytic caps, and see if they are still OK.

The board seems to be old, 1970s, from the make of the resistors.
40 years are beyond useful service life for electrolytics.

I have seen one's from the late 1980s failing already, and causing malfunction, kind of the board worked for an hour, then became erractic, then stopped working altogether.

I would even consider to exchange all electrolytic caps.

But if you take out a few and they measure 100% OK, maybe not needed.

my multimeter doesn't have a cap tester

 

DT9205 costs about $15. But only upto 200uF.

From what I can see it would not be an unreasonable effort to exchange the capacitors (for instance it is not a glass fiber PCB).

The larger one's you could test with a LED or small bulb
But that would be very subjective.

 

U can replace the output transistors with later, better ones u know.

So, what u need to do is find new components to replace the burned ones. They are quarter watt carbon type resistors. Very cheap.

Next take out the output transistors.
Discharge all the big capacitors after disconnecting power.

Put the DMM in diode check mode and check for shorted transistors in the circuit. No need to take em out. Make sure there are no shorted driver transistors.

While u are at it check all of them transistors and diodes for dead short.

Next u need to find two resistors of around 5W 18Ω to 22Ω will do.

If u find shorted ones, u need to replace them transistors or diodes. I bet u can figure out the replacement by googling.

Replace any blown fuse with the correct type
.
If everything is A OK, then take a look at the shemaic power supply section.

I need u to isolate the PSU from the amp and test it.

Can U isolate the left right channels from the PSU ?

Or do u want me to show u how to do it ?

what transistors would you recommend.

why do I need the 18 to 22 ohm resistors

and how do I issolate the PSU

 

I'll have to check transistor data for a suitable replacement.

The 22 ohm resistors will be used to trouble shoot the amp biasing. It will help in protecting the components if a fault still exists in the amp.

U can isolate the amp by disconnecting the ± Supply line. Keep the ground connected.

U need to cut the positive and negative rails tht powers the amplifier.

can u locate the wires from the schematic?

 

I'll have to check transistor data for a suitable replacement.

The 22 ohm resistors will be used to trouble shoot the amp biasing. It will help in protecting the components if a fault still exists in the amp.

U can isolate the amp by disconnecting the ± Supply line. Keep the ground connected.

U need to cut the positive and negative rails tht powers the amplifier.

can u locate the wires from the schematic?

Okay gotcha theres two -36V wires going to C901, and a +36V wire going to C902

I already disconnected all the other wires from the amp board. Is this okay or do i just need to plug the ground back in.

 

I couldn't find a replacement for Q756. it's 2SA706. However, I did find one similar to it. it's a NTE211. could I use this instead?

 

lemmee get back to u later after work.

 

lemmee get back to u later after work.

Ok nO prob.

 

The data for 2SA706 returns useless hits and even though some say BD139/140, so let's see if it true

But I Manage to locate the exact data specs from my database.
2SA706 - Si-P , uni-L 40V, 1A, 7.9W, 120MHz.

The exact replacement I found was the BD256. The data shows the complementary no. too

The BD139 and BD140 are suitable replacement and is available.

See to it tht u replace Q705 too, as these two are complementary pairs and should be matched for lowest cross over distortion.

U need to isolate the positive and negative wires tht connects to the out put transistors. This way u are sure tht no heavy load is connected to the PSU during testing.

Make sure tht both channels are disconnected.

 

The data for 2SA706 returns useless hits and even though some say BD139/140, so let's see if it true

But I Manage to locate the exact data specs from my database.
2SA706 - Si-P , uni-L 40V, 1A, 7.9W, 120MHz.

The exact replacement I found was the BD256. The data shows the complementary no. too

The BD139 and BD140 are suitable replacement and is available.

See to it tht u replace Q705 too, as these two are complementary pairs and should be matched for lowest cross over distortion.

U need to isolate the positive and negative wires tht connects to the out put transistors. This way u are sure tht no heavy load is connected to the PSU during testing.

Make sure tht both channels are disconnected.

Great! I just so happen to have a couple of BD139 and BD140 transistors in my parts box.

So I guess for now I should just test the power supply like you said

 

I went thru the schematic and it seems for a beginner this theory is way too much.

This amp uses ±33V for the current mirror section and ±36V for the output stage. In most amps the current mirror is powered via a resistor from the main bus. In ur case this supply is regulated.

So we will skip this stage.

What u need to do is find two 22Ω, 5W resistors. Second replace the burned components and fuses. Double check to see if no more transistors are shorted. Mainly the output devices plus it's emmiter resistors which are 0.47Ω ones connected to the emitter and speaker out put. There will be 4 of these. Two in each channel out put.

Replace all the bad ones. Double check ur work. Have the resistors on standby and post back.
Then I will tell u how to connect the test resistors and power up so you won't blow up anything if a fault exists.

Remember, Do not power up. Without me knowing or u might make it worse.

I need to get some sleep. I will get back tomorrow night.

Good night