Sony STR-DE305 Stereo Receiver/ Amp unit - no display nor output

[Chas]

Had a hectic weekend and didn't get a chance to work on the Sony until tonight.

Assembled the unit back together and set it up with the test lamp in place, switched the unit on and...



...success! The display now works and all functions can be selected via the front panel buttons!



And as you said 73's de Edd, the lamp initially comes on (not as brightly as previously), and then drops to a very dim glow.

Let's put the bridge rectifier back in . . .correct polarity . . . and leave the two bad POWER transistors out.
Then you can use the same protective series lamp, for permitting a non destructive power up testing.
Do just a 3 second power up to see if the lamp is BRIGHT . . .BAD . . .or just a medium glow and dimming.
If the latter, you can check the RED and BLACK SQUARE voltages to see if they are about the same value, since
we are operating on reduced voltage input now.
Also the RED and BLACK ARROW voltages, which should probably clear any suspicions of the Zener diodes that you tested earlier.
Get caught up on this and I will prepare a relevant schematic snippet of the audio output circuitry of the bad channel to see if some transistors that feed into the two bad power transistors might have also been crunched.

73's de Edd
.....

As requested, I measured voltages as per the red/ black arrows and squares in the schematic shown a few posts back.

The red arrow reads approx 25v

The black arrow also reads approx 25v

The red and black squares were harder to find points on the board to put the DMM probes, as the access panel doesn't open in that area. However, it looked like they were the points coming from the full wave bridge rectifier (after passing through the two large capacitors). The full wave bridge rectifier pins were easier to probe, so I probed those directly. Here are the results:

+'ve pin = 51.4v
-'ve pin = -51.4v

Under the access panel there are also some voltage values printed onto the visible board. I probed these, and the DMM results more or less matched the values that were printed on the board.

Obviously without a proper fuse in place and with two power output transistors missing, I haven't hooked up the unit to an external sound input to see if it works with headphones or speakers. I assume before I do that I should replace the faulty power output transistors and the fuse first?

Either way, progress has been made and it appears that your diagnosis is 100% spot on 73's de Edd. Thank you for helping me get this far.

Are we done with testing, and should I now get some replacement power output transistors to replace those that were shorted? They're available online so I can get some ordered if that's the next step I need to take.

Hopefully I will be able to rescue this amp after all

 

[73's de Edd]

Sir Chas . . . . .

Your post # 21's photo reveals a slow blow fuse that BLEW both VERY-FAST and VERY-HARD.
It not having had time to blow s l o w l y when your +51vdc and -51VDC . . . for a combined and summed series 102VDC . . . .were suddenly shorted out by the POWER transistor pairs . . .failures.
Thereby, that reflecting as an IMMENSE power drain on the primary of the power transformer.
With you now being back in business, with your one output channel now potentially working as normal, why not connect a speaker up to the amps good channel output and see if the lowered voltage AC voltage that you are now limping along with, will let you set the unit to RADIO mode for a test audio source and then see if you can evaluate sound .
If it doesn't have enough "oomph" it would be permisible to temporarily sub a blow drier set to high heat in place of the now somewhat power choking test lamp.
If all seems to be par excellence . . . on the potentially good working channel, we move to see if a portion of the DRIVER IC might have been backfed voltage when the bad channels two POWER transistors crunched.

73's de Edd
.....

 

[Chas]

73's de Edd, I carried out the above suggestions, and all is working!

There was no need to sub a hair dryer as the test lamp didn't appear to choke the power to stop things from functioning either with headphones or a speaker.

Thus, with the test lamp load in place I tried both an external audio input (MP3 player into the CD input via RCA jacks), and also the radio. One channel (left) up and running fine with both an attached speaker, and also via headphones. All front panel buttons work and adjust/ operate correctly. FM radio working once I hooked up a make shift antennae (piece of wire!)

The unit also has speaker A/ B select, so I also tested the unit with the speaker connected in both the A and B speaker sockets. Again, both sets of sockets working as they should, obviously only on the left channel due to right channel missing its power output transistors.

Next step then is to test the DRIVER IC? How do I do that?

 

[73's de Edd]

Sir Chas . . . . .

Let's just do an initial static testing of the output drives of the 15 pin IC701 . . .with its pin # 1 referencing being at the clipped corner of the unit.
BUT . . . we might as well do the voltage tests at the bases of the power output transistors, for ease of access..
It will involve DMM voltage testing, and you should be expecting about negative 1 Volt DC, being at the bases of the 4 power transistors.

Meter negative ground referencing will be the junction of R717 /&67, or the emitter of relay driver Q710 or the pin 7 of the UPC2581 audio output driver IC701.

If you find ~ negative 1VDC reading at the bases of Q754 and Q753, a good driver section of IC701 should be having a like voltage reading at the bases Q704 and Q703 and not being down in the hundreds of millivolts OR up in the negative 3-5 V range.
If finding the like voltages I, myself, would be pulling the LEFT channels good power transistors and transferring them to the now vacated right channel positions, ensuring by a TRIPLE check, that the NPN versus PNP parts positions were being put in their proper positions.

Then use the inserted series incandescent lamp in initial testing to assure protection, in that initial 5 second poke in of the AC cord or turn on of the unit.

If all tests out as the initial working channel did, you are ready for that semiconductor set and proper GMV . .blow slowly . . . fuse order.

73's de Edd
.....

 

[Chas]

Awesome advice Dr. 73's de Edd!

I'll get on the case over the next few days once I get a moment free (busy weekend - memorial service tomorrow day and then a fundraiser tomorrow night, then Sunday dog sitting for a hyper active terrier!)

I'll check out those voltages and then swap the transistors over if all is OK.

Onwards and upwards!

 

[Chas]

Sir Chas . . . . .

Let's just do an initial static testing of the output drives of the 15 pin IC701 . . .with its pin # 1 referencing being at the clipped corner of the unit.
BUT . . . we might as well do the voltage tests at the bases of the power output transistors, for ease of access..
It will involve DMM voltage testing, and you should be expecting about negative 1 Volt DC, being at the bases of the 4 power transistors.

Meter negative ground referencing will be the junction of R717 /&67, or the emitter of relay driver Q710 or the pin 7 of the UPC2581 audio output driver IC701.

If you find ~ negative 1VDC reading at the bases of Q754 and Q753, a good driver section of IC701 should be having a like voltage reading at the bases Q704 and Q703 and not being down in the hundreds of millivolts OR up in the negative 3-5 V range.
If finding the like voltages I, myself, would be pulling the LEFT channels good power transistors and transferring them to the now vacated right channel positions, ensuring by a TRIPLE check, that the NPN versus PNP parts positions were being put in their proper positions.

Then use the inserted series incandescent lamp in initial testing to assure protection, in that initial 5 second poke in of the AC cord or turn on of the unit.

If all tests out as the initial working channel did, you are ready for that semiconductor set and proper GMV . .blow slowly . . . fuse order.

73's de Edd
.....

Had a hectic week thus a slight delay in getting back on the case with this unit. Managed to fire up the DMM and get testing tonight. Here's what I found:

Q703 and Q704 are the two (good) output transistors that are currently installed on the board. I checked the voltages with my DMM, test lamp in place and using pin 7 of the IC701 (pin numbers conveniently printed on the board) for the DMM meter negative. At the base of each transistor, the voltages are:

Q703 = +1.1VDC
Q704 = -1.09VDC

I checked the solder pads for Q754 and Q753 just because, and there's no readings there that came up on the DMM. I assume that the output transistors need to be installed for readings to be present.

Thus now we can establish that Q704 is indeed giving ~ negative 1VDC, however, Q703 is doing the opposite and giving ~ positive 1VDC. However, with these transistors in place, the unit functioned and output to both speaker and headphones.

What should be the next step? Should I go ahead and swap them into Q754 and Q753 and test them there?

I feel that I am getting so close yet so far to getting this unit up and running again.

 

[73's de Edd]

There is a schematic error on the voltages on the output transistors bases. What you have is correct . . .
proceed to the transistor swap between channels . . . . being sure that the NPN---PNP pair don't accidentally get swapped.

 

[Chas]

Thank you Dr. Edd!

I have already ordered two new pairs of transistors as they weren't expensive at all.

Before then (they're on the slow boat from China). as soon as I get the chance, I will swap the current good pair into the other channel just to ensure all is working in the other channel. I will report back as soon as I have done that.

 

[Chas]

More success!

I swapped over the good output transistors from the left channel to the right channel, and all is working in the right channel. Test lamp load in place of fuse behaves the same, all functions of the amp are working. Thus some new transistors and a fuse should be all it needs to be back up and running again.

I ordered the output transistors very recently, and they arrived much quicker than normal considering how slow Chinese post can be. I ordered some fuses from a USA seller yesterday, and they should be here within days. I also ordered some thermal past at the same time I ordered the transistors, though that hasn't arrived yet.

Once the thermal paste has a arrived, I will be able to install the new power output transistors. I will give the unit a final check with the test lamp in place, and if all is good, I'll install the fuse and test again. If all continues to work, then another Electronics Point repair success could be on the cards, with much credit to Dr. Edd!

I'll report back as soon as I have the parts and get everything installed.

 

[Emiliano]

More success!

I swapped over the good output transistors from the left channel to the right channel, and all is working in the right channel. Test lamp load in place of fuse behaves the same, all functions of the amp are working. Thus some new transistors and a fuse should be all it needs to be back up and running again.

I ordered the output transistors very recently, and they arrived much quicker than normal considering how slow Chinese post can be. I ordered some fuses from a USA seller yesterday, and they should be here within days. I also ordered some thermal past at the same time I ordered the transistors, though that hasn't arrived yet.

Once the thermal paste has a arrived, I will be able to install the new power output transistors. I will give the unit a final check with the test lamp in place, and if all is good, I'll install the fuse and test again. If all continues to work, then another Electronics Point repair success could be on the cards, with much credit to Dr. Edd!

I'll report back as soon as I have the parts and get everything installed.

Did you have any success? I'm having the same problem, I was driving the unit too hard and then I saw a light coming from the amp (probably the same fuse blown)... My unit is slightly the same STR-DE310

 

[73's de Edd]

Sr Emiliano . . . . .



Did you have any success?

Assuredly he did . . . . as he had "pre tested " the bad channel with the good pair of power output stage transistors from the other working channel.
Then, he just needed a set of good, new transistors and a proper fuse.

I saw a light coming from the amp

The semantics used there seems improper . . . as it would be "I saw a FLASH coming from the amp".

Since that fuse is hidden away down inside the unit and it is being a one time / one shot . . . .milliseconds of duration event.
Never to be seen again, unless being stupid enough to put in another fuse without INITIALLY confirming WHAT overload
of developed bad part(s) caused the original unit to blow . . . . . unless you just REALLY taxed its audio output capabilities, considering that to be the only easy fix hope.
Thus . . . you really should use a 60 W incandescant lamp in place of the blown fuse for making an initial exploratory operational evaluation.

Thaaaaaaassit . . . . .

73's de Edd . . . . .


Claustrophobic people are more productive when thinking out of the box.

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