SX-1250 Phono EQ AWF-021-B smoke when turning up volume

[Pef]

To summarize where I'm at, I just replaced all the electrolytic capacitors (I checked everything several times) as well as all the transistors (some were reversed and others weren't), as I saw suggested in a discussion on this forum.

When I test the phono stage with dim-bulb, I gradually increase the volume and hear little cracking on one channel. I continue to increase the volume a little, and two solder joints start smoking.
My board isn't exactly the same as the ones in the schematics, it's a AWF-021-B version.
The only difference I see is that diode D6 has been replaced by a 5.1K resistor. I don't know if this is significant.

During the component replacement, I broke one leg of resistor R14 (1.54K). I managed to repair it by scraping the body and soldering two legs. Perhaps it wasn't a good idea, but the resistance measurements were correct.

I have a few questions to help me determine if you see any problems with my reassembly of this board:
- Are the transistors installed correctly?
Q11/Q12 = KSC2383
Q13/Q14 = KSA1013
Q5 - Q8 = KSC1845
Q1-Q4 = KSA992
Q9/Q10 = KSA992

- Should I replace the 5.1K resistor with a D6 diode?
- I haven't yet precisely adjusted the bias and DC offset, could that be the reason?



Thank you for your help!

 

[mrk229]

Check this thread, the resistor instead of D6 is probably correct.

Pioneer sx 1250 awh-021 wrong board

board show a D6 wz-240 but not on parts list or in schematic i assume its wrong ? can anyone confirm

audiokarma.org

When I test the phono stage with dim-bulb, I gradually increase the volume and hear little cracking on one channel. I continue to increase the volume a little, and two solder joints start smoking.

"When I test the phono stage with dim-bulb, I gradually increase the volume"
As I understand this statement you are operating the unit at some increasing volume with speakers while it is plugged into the dim bulb. That is a big NO NO.

If you run audio with speakers at any volume above a very low level while on the DBT you run the risk of voltage starving the system and potentially causing damage.
For a 1250 it could cause the soft start relay to drop out or turn on and off. This can lead to burnt out soft start components and even more voltage starving.

You did not mention but I bet the DBT is getting brighter and dimmer with the audio level?

It is wise to use the DBT after any repairs or during some troubleshooting. If the unit passes the intial power up with the DBT then it can be bypassed or removed.
As an exception you can usually get away with using headphones at reasonable levels while on the DBT.

Was it working before you made the changes?

What does "smoked capacitor negative leg" mean?

- I haven't yet precisely adjusted the bias and DC offset, could that be the reason?

It should have no bearing on the phono EQ.

Perhaps a more detailed explanation of what you are doing would help us understand.

 

[Pef]

Hello @mrk229

Thank you for all you said about testing volule with dim-bulb, I didn’t know that...

"You did not mention but I bet the DBT is getting brighter and dimmer with the audio level?"
I didn’t pay attention to the bulb, I was focus on wich channel was cracking so I don’t know.

"Was it working before you made the changes?"
Unfortunatly I didn’t do this specific test before, but sound was good on AUX at very low level with dim-bulb.

"What does "smoked capacitor negative leg" mean?"
When I was increasing the sound, the receiver was upside down (soldering side of the board facing to me) so I saw two solders smoking, one was this capacitor negative leg and the other was the solder of that kind of wire bridge.

 

[zebulon1]

General information only:
The EQ includes the +- 23v regulated voltage circuit for the tone and flat amps.

 

[rcs16]

IIRC, the 1250 phono takes +/-65V from the stabilizer, making +/-49V for itself and the volume control buffer SRPP ckt on the flat amp. The rest of the flat amp, tone, filters use a reduced +/-25V from the stablizer