AKAI AA-920 // Stereo lamp problem

[tcalic]

Hello everyone,

I'm trying to refresh one Akai AA-920 receiver.
Almost everything is done, and now I have some problems with stereo lamp.
Lamp lights up when I get FM stereo reception, but shining very weak ... barely noticeable.
On MPX board is Sanyo LA3300.
According to service manual PIN-4 on MPX board (lamp driver) must have 7,9V, but I only get bellow 1V when lamp is turned on.

MPX is adjusted to 0,6V on PIN-7 of MPX board.

PIN-8 and PIN-14 of LA3300 are:
- approx 1,4V when lamp is turned on
- approx 0,4V when lamp is turned off

All other voltages on LA3300 are correct.

I already replaced TR3(2SC711) and IC LA3300, nothing changed.

Any advice ?

Thanks,

 

[rayma]

According to service manual PIN-4 on MPX board (lamp driver) must have 7,9V,
but I only get bellow 1V when lamp is turned on.

Does the IC's power supply measure 9V? If so, when you connect R11 directly to the 9V supply,
does the light then work normally? And while R11 is disconnected from the IC, do you get a
higher voltage on C7? If not, try replacing C7.

 

[sgrossklass]

The problem may not actually be at of the IC (it seems to behave fine) but at the other end of the lamp here. The 7.9 V is no doubt referring to the stereo indicator being OFF. When ON, the transistor pulls its collector to ground, so <1 V seems right.

Could also be an incorrect lamp fitted at some point, maybe the original went out and someone put a 12V in. Or just a bad connection.

 

[tcalic]

Does the IC's power supply measure 9V? If so, when you connect R11 directly to the 9V supply, does the light then work normally? And while R11 is disconnected from the IC, do you get a higher voltage on C7?
If not, try replacing C7.

Hi Rayma,
IC PS is OK and it is 9V.
I'll try connect & disconnect R11 as you suggested.
Didn't mention in my first post, all elco capacitors are replaced with new ones in refresh process, so C7 i new.

 

[tcalic]

The problem may not actually be at of the IC (it seems to behave fine) but at the other end of the lamp here. The 7.9 V is no doubt referring to the stereo indicator being OFF. When ON, the transistor pulls its collector to ground, so <1 V seems right.

Could also be an incorrect lamp fitted at some point, maybe the original went out and someone put a 12V in. Or just a bad connection.

hi sgrossklass,
the other end of the lamp is connected to 25V (which is OK) directly to power supply.
the lamp is original, but I also tried with other lamp.

 

[sgrossklass]

Have you measured voltage across the lamp in operation?

 

[tcalic]

Does the IC's power supply measure 9V? And while R11 is disconnected from the IC, do you get a
higher voltage on C7? If not, try replacing C7.

IC's power is OK, 9V.
When R11 is disconnected there is no voltage on C7.

 

[tcalic]

Have you measured voltage across the lamp in operation?

yes, voltage on other lamps is 7,7V - 7,9V.

Situation on this lamp:
1.) PIN-4 of MPX board disconnected form lamp
Stereo OFF :
= 0V between PIN-4 & 25V Power supply PIN (other side of lamp)
= 0,4 V between PIN-4 & GND
Stereo ON:
= -24V between PIN-4 & 25V Power supply PIN (other side of lamp)
= 0V PIN-4 & GND

2.) PIN-4 of MPX board connected to lamp
Stereo OFF:
= -1,5V between PIN-4 & 25V Power supply PIN (other side of lamp)
= 24,5V between PIN-4 & GND
Stereo ON:
= -2,5V between PIN-4 & 25V Power supply PIN (other side of lamp)
= 0,5V between PIN-4 & GND

 

[sgrossklass]

Clearly the lamp won't do very much with just 2.5 V. It almost looks like you may have a bad ground connection there.

Please measure the resistance from TR3 emitter to various ground spots both on the same board and elsewhere in the amp. The ground connection does not happen to be carried by the mounting screws by any chance? That would be a common trouble spot, usually fixed by retightening (after cleaning off oxidation if necessary).

The problem could also be in the 25V power supply though.

 

[llwhtt]

Check R4 on the power supply board, it should be 560 Ohms @ 1W. If that resistor has gone way up in value you'll still read 25VDC unloaded BUT once it's loaded there won't be any current available for the lamp.

Craig

 

[tcalic]

Check R4 on the power supply board, it should be 560 Ohms @ 1W. If that resistor has gone way up in value you'll still read 25VDC unloaded BUT once it's loaded there won't be any current available for the lamp.

Craig

Hi Craig,
So, I checked R4 and it is 560 Ohms.
Just to be 100% sure, I also replace it for testing (with 380 Ohm 1W, didn't have 560)... nothing change.
Measuring 2V on Lamp - between PIN-4 and 25V power supply.

 

[tcalic]

Clearly the lamp won't do very much with just 2.5 V. It almost looks like you may have a bad ground connection there.

Please measure the resistance from TR3 emitter to various ground spots both on the same board and elsewhere in the amp. The ground connection does not happen to be carried by the mounting screws by any chance? That would be a common trouble spot, usually fixed by retightening (after cleaning off oxidation if necessary).

The problem could also be in the 25V power supply though.

Hi,
So, I measured the resistance of TR3 emitter to various ground spots... It is 0 Ohms.
Also, There is no screws for ground connection .
Boards are mounted on plastic girders.

 

[tcalic]

Additionally,
I disconnected R11 & C7 from IC on MPX Board and measured voltage between PIN-8(14) of IC and GND:
- Lamp ON = 0,9V
- Lamp OFF = 0,1V

 

[llwhtt]

.9VDC is enough to turn on TR3, once turned on the collector should be close to ground or 0VDC and the lamp should be lit brightly. What I don't understand is when TR3 is off there is 7.9VDC on the collector and the lamp should be off. Assuming a 28 volt lamp since there is 25 volts across it when on, it should still be dimly lit even when it's supposed to be off, maybe it's dim enough to appear to be off. There is no mention of the Stereo lamp specs in the service manual that I could find. Also since R4 is rated at 1W it indicates some good current thru the resistor. A 30ma lamp would cause R4 to dissipate a 1/2W which seems reasonable derating of the resistor.

When you measure across the lamp when it's on what is the voltage? Should be close to 25VDC. Pin 8 of the PS board to Pin 4 of the MPX board. Measure both mono and stereo.

Craig

 

[tcalic]

.9VDC is enough to turn on TR3, once turned on the collector should be close to ground or 0VDC and the lamp should be lit brightly. What I don't understand is when TR3 is off there is 7.9VDC on the collector and the lamp should be off. Assuming a 28 volt lamp since there is 25 volts across it when on, it should still be dimly lit even when it's supposed to be off, maybe it's dim enough to appear to be off. There is no mention of the Stereo lamp specs in the service manual that I could find. Also since R4 is rated at 1W it indicates some good current thru the resistor. A 30ma lamp would cause R4 to dissipate a 1/2W which seems reasonable derating of the resistor.

When you measure across the lamp when it's on what is the voltage? Should be close to 25VDC. Pin 8 of the PS board to Pin 4 of the MPX board. Measure both mono and stereo.

Craig

Model AA-920 (which i have) & model AA-930 use same MXP board, as I can see in service manual.
But AA-930 do not use same 25V from power supply for other side of lamp... strange to me.

 

[llwhtt]

PS board 92-5004 and MPX board 94-5008. Try grounding Pin 4 of the MPX board, lamp is bright now? If not measure Pin 8 of the PS board, should still be 25VDC ref. to grd. If you have 25VDC and ground to the lamp and it's dim maybe the lamp is bad/wrong OR that's the way it is.

Craig

 

[tcalic]

PS board 92-5004 and MPX board 94-5008. Try grounding Pin 4 of the MPX board, lamp is bright now? If not measure Pin 8 of the PS board, should still be 25VDC ref. to grd. If you have 25VDC and ground to the lamp and it's dim maybe the lamp is bad/wrong OR that's the way it is.

Craig

Tnx once again.
I'll try it tomorrow, it's almost midnight at my side.
Take a look at this example , it should be bright.
http://img.canuckaudiomart.com/uploads/large/1036684-mint-akai-aa-920.jpg

 

[tcalic]

fyi...

Lamp was bad