SX-1250 smoking R34

[kevzep]

Bourns trim pots having "staying power"... good one
Blue just like Viagra

Thats right, and you won't go open circuit!!

 

[FranksElect]

yes, that will work, but then there are additional lifetime and reliability considerations in my selection.
so zeb, That is why I went with 652-3296P-1-501LF

compare the spec sheets, keep in mind 10,000 rotations versus 200 rotations is not relevant, while the stability of the resistance over time IS.

simply put - you can count on the cheap stuff for a few years, but the Bourns pots have "staying power"...
and THAT's what you want in an IDLE CURRENT pot. You don't want it to go open circuit eventually, which is why the old one is being replaced.

Think years and years of atmospheric contamination and it's effect upon the commutator (wiper to external connection) of the pot. I think Bourns puts a lot of thought and effort into solving that.

Thank yall ,,,my pots came in , i did order the Bourns , now lets put this beast back together

 

[FranksElect]

Before doing anything, measure the DC offset and idle current, the idle current is read as millivolts at service manual defined test points at pins #7 (+) and #19 (-) , for EACH channel.

The right side does tend to run hotter at the vents, that's where the power supply and it's attendant heat is located!!!

NOTE: connect your probes with POWER OFF, to avoid trouble.
note2: swapping boards like that without setting dc offsets and new idle currents is a BAAAAD idea, the idle current adjustment IS super SENSITIVE to the INDIVIDUAL OUTPUT TRANSISTOR characteristics - and obviously the other channel's transistors WILL be slightly different, which can have a BIG effect.
There are some goofs in swapping boards around that can leave you with an expensive smoking ruin of a power amplifier channel.

Mark , ok i need a litle help , VR 1 ( the multi turn pot ) i can adjust till i have 0v between 9 and ground but adjusting vr2 , (on top) i cannot get a reading on no7+ and no19- ,, , I made the 5.1k resistors for (power in) like the manual says , and no speakers hooked up ,, any ideas ? what am i doing wrong ?

 

[FranksElect]

Frank, I did rebuild the power amp boards and never had the symptom again. Since it was so intermittent I never really figured out exactly what was causing it however I suspect the ???869 transistors on the power amp boards. Those 869's control the protection circuits. One of the 869's in another 1250 caused protection issues also. I wish I could say that was it for sure but I only suspect that. I have learned that these 869's can become "noisy" and cause this, however, will test good with a DVOM.

Keep in touch, I'd like to know what you find.

Bob

Bob , can you help me with the set up on the vr1 and vr2 , i recapped bothe sides and changed out the pots with the bournes , the multi turn is for the vr1 correct ,, im doing something way wrong here ,, help would be apreciated

 

[BobbyN]

Bob , can you help me with the set up on the vr1 and vr2 , i recapped bothe sides and changed out the pots with the bournes , the multi turn is for the vr1 correct ,, im doing something way wrong here ,, help would be apreciated

Frank, Sorry it took so long to get back to you. As far as the pots go I don't remember now which is which. I would have to do a search on the 1250 here.
Can you not get the idle current where it should be and/or the DC offset?

 

[BobbyN]

Frank, Good thread here...
http://audiokarma.org/forums/index.php?threads/im-recapping-my-sx-1250.635036/#post-8423799

 

[markthefixer]

Ok, you can balance the amp by adjusting the DC offset voltage of the amplifier output using multi-turn 470 (500) ohm VR1.
The single turn VR2 is also 500 ohms and starts at zero ohms, hopefully at zero current.
IF it does not read at pins #7 (+) and #19 (-), try reading at 8 or 10 (for pin 7) and at 17 or 18 (for pin 19).

With the power off and one leg lifted, check those 0.5 ohm and 22 ohm resistors.

Measuring (with power on) between pins #11 (+) and #16 (-) should show a proper output transistor base drive voltage of 1.2 volts - and minutely adjustable by vr2. Those measuring points ARE the bases of the output transistors.

 

[FranksElect]

thank yall for getting back to me, sorry i had to get it back on the bench ,,, vr1 ( the mutiturn) is adjusting at o mv dc on pin 9 on both channels with the 5.1k ohm resistors on the power in jacks and the speaker selector swithc off on a b and c ,, now I need to figure out why i cant get any 100mv when adjusting vr2 with pins 7 and 19

 

[FranksElect]

Markthefixer or We6c , where is the best points to monitor when bringing this completly rebuilt amp board up , on the variac with the dbt in series ? all transistors and caps have been replaced with the mouser replacements yall have speced out here , both vr1 and vr2 are counter clockwise ,,

 

[FranksElect]

speaker switches are all (off) and the 5.1k ohm rca plugs are in the (power in) jacks,

 

[merlynski]

 

[BobbyN]

Frank, Were you able to get the idle current adjusted (100mV) as per the service manual? Most people on here prefer setting at 80 or so.

FWIW, I use both the DBT and a variac. Not sure if both are really necessary. Remember if you adjust the idle current with the variac at a lower voltage, lets say 110 volts, then you'll need to check and most likely readjust when going to full mains voltage.

Does the 1250 work? If so, do all the output transistors seem to be running close to the same temperature?

 

[FranksElect]

Frank, Were you able to get the idle current adjusted (100mV) as per the service manual? Most people on here prefer setting at 80 or so.

FWIW, I use both the DBT and a variac. Not sure if both are really necessary. Remember if you adjust the idle current with the variac at a lower voltage, lets say 110 volts, then you'll need to check and most likely readjust when going to full mains voltage.

Does the 1250 work? If so, do all the output transistors seem to be running close to the same temperature?

Yes , Finally got the Left side up , 80mv on the idle , left side is much cooler after complete board rebuild , and No smoke , Now i need to work on the rightr board and get it together ,, I have another 1250 setting on the shelf waiting on repair also ,, Its just been a long time since repairing audio equipment ..Thank everyone for their help ..

 

[kevzep]

Set the bias to 65mV, 100mV is way to high, the amp idles far too hot.

 

[FranksElect]

Set the bias to 65mV, 100mV is way to high, the amp idles far too hot.

kevzep , i thought the same thing , but im not an engineer , i set both sides at 75 mv after a total amp rebuild of both sides , very clean and cool on both sides now ,, but i gotta find out why i only have one side on my FM ,, been intermittant for many years on the fm ,,, thank all of yall for the info , Frank

 

[kevzep]

kevzep , i thought the same thing , but im not an engineer , i set both sides at 75 mv after a total amp rebuild of both sides , very clean and cool on both sides now ,, but i gotta find out why i only have one side on my FM ,, been intermittant for many years on the fm ,,, thank all of yall for the info , Frank

If you look through the SX1250 threads you'll find the Guru's here like Mark, Matt and Zeb etc etc they all say 65mV...

 

[mattsd]

75mV is where I've always set mine, works great. I am however (once I get my audio analyzer back from calibration) going to test a 1250 with various bias levels and see once and for all what an optimal balance between performance and heat would be.

 

[LesE]

@mattsd ... Looking forward to your test results. I did an LTSpice analysis of THD for various bias currents and the results were interesting but they seemed a little optimistic.

Here is the calculated bias current vs output transistor power dissipation for the SX-1250.

 

[BobbyN]

75mV is where I've always set mine, works great. I am however (once I get my audio analyzer back from calibration) going to test a 1250 with various bias levels and see once and for all what an optimal balance between performance and heat would be.

Also anxious to see those results.