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Allied 498 aka. Pioneer SX-9000 amp problem

Maddart

Maddart

Active Member
Just got an allied 498 from the trash pile. Plugged it in and the with the left channel fuse in and my light bulb equiped power strip glows.. Remove the left channel fuse and the receiver and right channel works ok, and the left channel sounds garbled without even a fuse.
Checked the outputs (2SD218) x4 and all are ok.
Appears that the amp board is almost identical to the pioneer sx-9000. I will check the transistors on the amp board next. This appears to be capactive coupled and both 2200uf caps look in tact.(No leaking)
This is all i can think of, if any one else has any ideas i'd appreciate it
Thanks
Mark
 
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Pulled all the transistors out of the left channel and all test good. Will recap the board, but i don,t know what to check next. Maybe the STV-3 ?Downloaded the SX-9000 manual and it is missing the adjustmant procedure for the Main Amp. Does anyone have that procedure?
Thanks
Mark
 
The bases of the 485 pre-drivers should not get more than 2.4 volts apart. The re-capping is a good idea, one is probably either shorted or very leaky - upsetting the balance of the circuit ( "output" voltage is 1/2 of b1 or b2) and the subsequent bias circuit.

As for the procedure, set the output terminal before the coupling cap for half of the b1 voltage using vr1, then set 15 millivolts across a 0.5 ohm emitter resistor for 30mA idle current using vr2. It's a reasonable number. Recheck for interaction.
 
Hi Mark

Awaiting parts from mouser. I assume the bases of the 485s are in fact Q5 and Q7?
The output voltage to be taken at C+ as shown on the schematic?
Recheck for interaction? Between the output terminal and the emitter resistor?

Thanks
Mark
 
Maddart said:
Awaiting parts from mouser. I assume the bases of the 485s are in fact Q5 and Q7?
The output voltage to be taken at C+ as shown on the schematic?
Recheck for interaction? Between the output terminal and the emitter resistor?

Thanks
Mark
Click to expand...

Interaction, idle adjustment affects center point voltage, or center point / output voltage adjustment affects idle. Just in case.

Yes, center point / output voltage at C+.

Yes, Q5 & Q7. I wasn't sure which channel, so they could be Q6 & Q8. But you know which channel.
 
Replaced all caps on L-channel. No fuse =L-channel=Q5 :Q7 base 1.62v and R-channel Q6:Q8 1.66v. Put the fuse back in and the makeshift variac now pulsates brightly. I removed the 2SC218s (L) outputs and the variac has a barely visible slight glow on the filimant. The 2SC218s check out good.
The Micas are delaminated and cracked. Could this be the cause of the problem?
 
Maddart said:
Replaced all caps on L-channel. No fuse =L-channel=Q5 :Q7 base 1.62v and R-channel Q6:Q8 1.66v. Put the fuse back in and the makeshift variac now pulsates brightly. I removed the 2SC218s (L) outputs and the variac has a barely visible slight glow on the filimant. The 2SC218s check out good.
The Micas are delaminated and cracked. Could this be the cause of the problem?
Click to expand...

:yikes: An ohmmeter can tell the tale. Are those TO-66's, i.e. mini TO-3's? (I'm lazy)
 
Maddart said:
These are TO-3 cases. Do I ohm the case to the heat sink?

Mark
Click to expand...

Yes, from all 3 connections on it, case and both pins - but as I posted I saw you had removed them already, thus removing the smoking gun. But check upon re-installation for problems. Have new mica insulators?
 
I replaced with new mica insulators and wakefield 120 thermal compound. Metered all 2SD218s to ground (3 connections) and readings are all OL.
Plugged back in to light bulb variac and the light bulb still pulsates low to bright. Also noticed that the dial lights are pulsating opposite of the variac. If i remove the left channel fuse the variac bulb turns off.If you look at the PCB layout it has different components than the schematic.The only transistor i didn't check on the left side is the 2SC870/2sc871 and the stv-3 I need to assume that this has to be localized to the left channel amp. I do have replacement transistors for the channel. Crosses are as follows:
2sc627= KSC2310ybu
2sc484=KSC2073TU
2SA484=KSA940
2SC871=KSC1845EBU
Exchanged left and right 2SD218s and the problem remains in the left channel. In the beginning the variac light was on constant and now it pulsates. I did replace caps on the left channel.Leads me to think the pulsating is caused by a cap
Looking at the main amp circuit could the problem reside in Q5/2SC485, D1/STV-3 and C13,100/50(new)?There is no R41 or R42 in the circuit as shown.
Thanks
Mark
 
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There have been times when there are weird interactions between the unit and the light bulb. What wattage bulb are you using and have you tried a higher wattage bulb?

Turn vr3 and vr4 down to zero ohms (minimum bias) and pull both fuses

Cycle power with neither fuse in.

Cycle power with left fuse in only

cycle power with tight fuse in only

Cycle power with both (and the minimum bias).
 
Replaced 60w bulb with 100w bulb.

VR-4 lowest attainable 16 ohm-VR-3 reads OL in all positions.(Open)

Neither fuse=Variac bulb off- dial lights on- garbled music both channels (Headphones)

Left fuse=Variac bulb pulsates to constant on,dial lights out,no music only pulsating hum.

Right fuse=variac bulb off,dial lights on, music clear right side, garbled left side

Both fuses= same as left fuse only.
Mark
 
Maddart said:
Replaced 60w bulb with 100w bulb.

VR-4 lowest attainable 16 ohm-VR-3 reads OL in all positions.(Open)

Neither fuse=Variac bulb off- dial lights on- garbled music both channels (Headphones)

Left fuse=Variac bulb pulsates to constant on,dial lights out,no music only pulsating hum.

Right fuse=variac bulb off,dial lights on, music clear right side, garbled left side

Both fuses= same as left fuse only.
Mark
Click to expand...

Ding ding ding, We have a winner!!!

Vr3 should not be open, a quick and easy test is to short vr3 out and see if the pulsation goes away - the music will sound like ass, but get the right resistor in there and the bias will come back where it should be.

Zero ohms is minimum bias in that circuit, the maximum of 50 ohms is maximum bias. Open circuit is what cooks output and driver circuits.
Be glad the dim bulb was in circuit.

IN fact R41 (and 42 on the other side) looks like a safety measure in case the stv-3 bias diode opened up, but nothing was done about the pot opening up.
 
I have a 8 ohm 10watt resistor to put in place of VR3 to check circuit.
I did check stv-3 and it was 1.44. R41 and R42 don't exhist on this PCB.

Placed 8ohm resistor into the circuit and the variac went off and the music returned with the left fuse in.
I guess i will have to makeshift a potiometer as the one i took out has lead spacing of .405"=10mm and .550" =14mm. Any Ideas?
This is a nichicon 500 pot.

I found mouser 531-pt15d-100 which with a little manipulation looks like it will fit. It is a 100 ohm trimpot. I can then place a 100 ohm 1/4w carbon film resistor in parallel with the pot. This should afford protection from the circuit opening again. Will this work?
 
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Maddart said:
I have a 8 ohm 10watt resistor to put in place of VR3 to check circuit.
I did check stv-3 and it was 1.44. R41 and R42 don't exhist on this PCB.

Placed 8ohm resistor into the circuit and the variac went off and the music returned with the left fuse in.
I guess i will have to makeshift a potiometer as the one i took out has lead spacing of .405"=10mm and .550" =14mm. Any Ideas?
This is a nichicon 500 pot.

I found mouser 531-pt15d-100 which with a little manipulation looks like it will fit. It is a 100 ohm trimpot. I can then place a 100 ohm 1/4w carbon film resistor in parallel with the pot. This should afford protection from the circuit opening again. Will this work?
Click to expand...

sorry I missed this, you need to figure out what resistance will get you the idle current you need, then double it if you want to add a parallel resistor. Try to get a range around it or use a larger value pot like:
531-PT15D-200 at 200 ohms or
531-PT15D-1K at 1000 ohms

that series resistor will complicate the calculations more than it's worth imho.
 
Actually I have the 531-pt15d-100 at 100 ohms and a 100 ohm resistor for parallel connection.
I believe the formula would then be R1 x R2= A and R1 + R2= B and then A/B= Resistance.
The original pot is 50 ohm. I believe the range is then 1 ohm to 50 ohm
Does this sound correct?
 
Maddart said:
Actually I have the 531-pt15d-100 at 100 ohms and a 100 ohm resistor for parallel connection.
I believe the formula would then be R1 x R2= A and R1 + R2= B and then A/B= Resistance.
The original pot is 50 ohm. I believe the range is then 1 ohm to 50 ohm
Does this sound correct?
Click to expand...

Yes, I agree, go with it.
 
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