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SX-1250........... Almost There

tsd71

tsd71

RIP Tom Petty
I have a 1250 that I got for $50 in 2008, I started right away repairing it, but put it aside for these years due to some health problems. I pulled it out of the garage last week and have spent the last few days getting it together. The receiver would not power up and would blow fuses and fry the microtemp. I corresponded through these boards with Markthefixer over that time and recapped, replaced transistors, diodes, trim pots, relay, and outputs....It is almost totally new. Last night I finished up the stabilizer board, today I put everything back together. I brought it up on a 75 watt dim bulb and both the soft start and the speaker relay engaged...I was a happy camper..this receiver hasn't powered up since 2000. Now as I went to make adjustments to the trimmers, now on full power, I noticed when I went to set VR1 it read 0.00V, I figured good thats a good DC reading. Then went to adjust VR2 on the power amp and turning the trim resistor does nothing, it reads 0.00MV and does not change at all moving it. This is on both channels.
Any ideas, both relays engage when powered, no smoke anywhere, tuner is getting signal.

Thanks:yes::yes:
 
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did you clean ALL the connector pins on all the PC boards?

check all psu output voltages.

measure and post voltages on output transistor connector pins on amp board
emitters about 0v
bases +/- 0.6v
collectors same as big cap voltages. plus and minus sixty something volts.

NOTE: ON 1250 the + and - 65 volt adjusts on the psu are done first.
 
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markthefixer said:
did you clean ALL the connector pins on all the PC boards?

check all psu output voltages.

measure and post voltages on output transistor connector pins on amp board
emitters about 0v
bases +/- 0.6v
collectors same as big cap voltages. plus and minus sixty something volts.

NOTE: ON 1250 the + and - 65 volt adjusts on the psu are done first.
Click to expand...

Mark, all connector pins were cleaned with the eraser trick. The outputs were installed the correct way with the PNP towards the front on the left side and the NPN towards the front on the right side, I also installed them with the ON-Semi logo and part number right side up, is this correct on this model?
I think they only go in one way though
 
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the outputs are to-3's, hard to reverse IN THE SOCKET.

(the two pins are offset from the center line, thus a reversal would misalign the two mounting screw holes)

what numbers (i have no data, remember?) are they mj21193g, mj21194g ?

MJ21194G
npn should have POSITIVE voltages AT THE SOCKET, THAT determines their position.... the body of the npn to-3 transistor should have plus sixty something volts, from the main cap. zero ohms

MJ21193G
pnp should have NEGAITIVE voltages AT THE SOCKET, THAT determines their position.... the body of the pnp to-3 transistor should have minus sixty something volts, from the main cap. zero ohms


if they are swapped they shouldn't have turned on, thus probably undamaged.
 
markthefixer said:
did you clean ALL the connector pins on all the PC boards?

check all psu output voltages.

measure and post voltages on output transistor connector pins on amp board
emitters about 0v
bases +/- 0.6v
collectors same as big cap voltages. plus and minus sixty something volts.

NOTE: ON 1250 the + and - 65 volt adjusts on the psu are done first.
Click to expand...

IDIOT ALERT!!!!!!!!

How stupid can I be....I went to get power supply readings and noticed when I flipped her over I forgot to connect the molex connectors to the power amp boards....:screwy::screwy:

I then adjusted the trimmers to 100MV, and 65V on the stabilizer, I'm getting 0.076V on offset and don't want to fry anything, the trim pot in way down almost at the bottom of the card, would I be able to get away with that reading without making an adjustment?

Thanks for jumping in Mark, it was exciting to see you respond so quickly....I'm glad you are on the mend and posting again, the board is not the same without you...

I'll report back when I hook up the speakers..hopefully she will sing after 12 years.
 
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.076v is 76 mv, that's too high. Did you shift a decimal point. Offset should be +/- 5 mv or better.
 
Also, if you are setting the bias on a cold unit 100mv bias will probably rise significantly as it warms up. Might I suggest 75mv initial bias setting and watch for 20-30 minutes before final setting.
 
I did the initial bias after 10 minutes then checked again at 30 minutes, I have it at 100MV now....I will insulate my screwdriver and make the offset adjustment.
By the way the receiver sounds loud and proud after 12 years of absolute silence, and 2 years as a bed for our garage cat....Sounds great and will make further adjustments.

thanks for jumping in...
 
no skipping, offset must be adjustable

INSULATE THE SCREWDRIVER SHAFT, then dial that 0.76 down to 0.000

don't skip the insulation!!!!!
 
petehall347 said:
have you got the cover back on ? it might be rf interference
Click to expand...

yeah, cover is back on, I'm investigating now, I think it's the position of my antenna, it lessens when I move it around the room.

Here are a few pictures of the 1250 all buttoned up and ready to sing again after 12 years in a coma...
 

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Nope no fluorescent lights in the room, the hum only comes on FM with volume, when I lower the volume there is no hum. No hum on phono or aux.
 
This not intended to be neg, depending on what kind of antenna is hooked up, most likely a loop antenna? that needs to be hooked to the 2 lugs that are 300ohms sometimes when it's hooked up wrong it does pickup interference. also make sure no wires are loose or broken in the antenna wire/s path or a broken ground wire on the FM board. Just a couple suggestions. May not be this but hey ....

John M
 
SicMan said:
This not intended to be neg, depending on what kind of antenna is hooked up, most likely a loop antenna? that needs to be hooked to the 2 lugs that are 300ohms sometimes when it's hooked up wrong it does pickup interference. also make sure no wires are loose or broken in the antenna wire/s path or a broken ground wire on the FM board. Just a couple suggestions. May not be this but hey ....

John M
Click to expand...

Antenna is hooked up correctly, I will have to take it apart to see if anything is loose or disconnected...Thanks
 
not clear if internal or external buzz

so

short 300 ohm fm terminals with straight 1/2" long piece of wire, see if buzz persists with all shields in place and properly torqued down.

if still buzzing, check power supply ripple, input dc and output dc voltages to 13v reg.

move unit with short in place.
also ANY modern, cheap psu dongle uses switching supply that if minor malfunction can put out a lot of crap.
 
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markthefixer said:
not clear if internal or external buzz

so

short 300 ohm fm terminals with straight 1/2" long piece of wire, see if buzz persists with all shields in place and properly torqued down.

if still buzzing, check power supply ripple, input dc and output dc voltages to 13v reg.

move unit with short in place.
also ANY modern, cheap psu dongle uses switching supply that if minor malfunction can put out a lot of crap.
Click to expand...

Mark, it's a hum in the music backround, when I lower the volume you cannot hear it anymore. You only can hear it when the volume is up. It is not an internal buzz.
 
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