SX-1250 right channel "WHAP" speaker distortion

Tried that Mark to a 'T", but got nothing from either side on the pos. speaker connector. Neg. yes on all. I believe after looking at the protector circuit schematic, power must be on in order to energize the relays in order to complete the speaker circuit.

markthefixer said:
The easiest way to find out is WITH THE RECEIVER OFF, connect one ohmmeter lead to one of the emitter resisitors and then probe the speaker connections (no speakers connected, you could turn off a & b and use c) for continuity.

That'll figure it out very quickly.


ARRRRGGGGGHHHH, how could I say THAT in the MIDDLE of the DAY?????????? :withstpd:
 
ARRRRGGGGGHHHH, how could I say THAT in the MIDDLE of the DAY?????????? :withstpd:
Glad we agree on that, I thought I was going nuts until I thought about it for awhile. On top of that one of my leads on my meter decided to break free on one of my probes so had to re-solder it at 4 am this morning. This is bugging the heck out of me. I added the 8 power transitors and gaskets to my Mouser cart with the cap replacements but haven't submitted yet. I'd love to say it's just isolated to a certain CD issue but it has occurred with some others and LP's also. Always that right channel. :tears:
 
Tried the speaker/amp board continuity check with the power on to determine which side drives the right or left speaker channel. Got continuity across all 8 emittter resitors with the probe connected to the positive "C" speaker terminal. AARGGHHHH
 
Power off, too many stray voltages for what the ohms test of a dmm does.

The protection board's pin 6 is the left channel input
pin 7 is the left channel output after the relay to the speaker switches.

pin 9 is the right channel input
pin 8 is the right channel output after the relay to the speaker switches.

so hook onto pin 9 on the protection board, and find which card it is connected to.
 
Mark,
Parts are here from Mouser with the 8 power transistors. After installing the new transistors and checking for shorts to the heatsink, should I do any other precautionary measures before powering up. Like turning down the idle current currents a bit? Or should they be okay where they were last set? Don't want any magic smoke.
 
Mark,
Parts are here from Mouser with the 8 power transistors. After installing the new transistors and checking for shorts to the heatsink, should I do any other precautionary measures before powering up. Like turning down the idle current currents a bit? Or should they be okay where they were last set? Don't want any magic smoke.

With new outputs (or ANY work on the amp) you ALWAYS ZERO THE BIAS before applying power.

Always bring it up on a light bulb tester (with bias set to zero after amp work) first. Then do a PRELIMINARY bias adjust to TEST it with the dim bulb tester, in case turning UP the bias causes a problem. BUT using the dim bulb causes a brownout condition, that will mean that if the bias was set while it was connected to the dim bulb tester, the bias will be WAY TOO HIGH when plugged directly into the power line.

So on the dim bulb, turn the unit on, set the bias on a channel - JUST to see that turning up the bias doesn't cause a FAULT, and then turn the bias back DOWN to ZERO. Then do the same for the other channel. The bulb WILL probably get a bit brighter as the bias is turned up, the unit IS drawing a bit more power....

In your case, once the dim bulb bias is set, read the voltages across ALL of the emitter resistors for that channel, making the voltage readings across the individual resistor's leads, the readings should be almost identical. If not, stop and post. If they are, zero the bias setting in preparation for doing the other channel or removing the dim bulb tester.

THEN take out the dim bulb tester and plug it straight into the line.

THEN dial up the correct bias. The position the pot ends up in with the correct bias will be lower than the dim bulb set position.
 
Mark,
What bulb wattage should I use on the dim bulb tester?

Like EW said, 100w


On your Avatar, is that your vehicle? A plum crazy Charger ca. 72-74 ? I don't think that was a stock color...
 
Like EW said, 100w


On your Avatar, is that your vehicle? A plum crazy Charger ca. 72-74 ? I don't think that was a stock color...
Thanks for the info so I don't let the genie out of the box!

Very close Mark. The Charger was mine way back for many a year, but no longer. :tears: It was a 1971 factory stocked with a 383 engine including the purple paint. Fun to drive and an eye popper, tho it didn't beat any speed records with a 2 barrel. Fun to work on to get a little more HP out of it once you got to know it. Wish I still had it now as I only paid $2300 with under 30,000 miles on it back in '74.
 
Thanks for the info so I don't let the genie out of the box!

Very close Mark. The Charger was mine way back for many a year, but no longer. :tears: It was a 1971 factory stocked with a 383 engine including the purple paint. Fun to drive and an eye popper, tho it didn't beat any speed records with a 2 barrel. Fun to work on to get a little more HP out of it once you got to know it. Wish I still had it now as I only paid $2300 with under 30,000 miles on it back in '74.

I should have realized that, the belt line at the the rear wheel did look a little high for the 72-74 models, PLUS the plum crazy was factory in 71.

I got a second hand 73 Charger Se in 79. 400 2BBL. Put in a 69 383 (10.5:1 0.030 over) 4 BBL in the early 80's. It met an untimely end in 84 during rush hour traffic. That's when I stopped putting money and extreme effort into cars.
 
I should have realized that, the belt line at the the rear wheel did look a little high for the 72-74 models,
No, you're right on the rear angle. They were all the same stock. I installed air shocks in the rear after I bought it. Tried the torsion bar trick to raise and lower the front end before and handling was scarey at high speed tests. Reset the torsion spec to factory and installed the air shocks and adjusted the rear height to what you see in the pic. Just enough to maintain control and traction. Looked mean too and what fun! My parents had a '73 Charger SE with the opera windows and 318 they purchased brand new. Those damn choke coils in the intake well were famous going bad. I can remember my dad bitching every winter when mom's car wouldn't start. I learned the cleaning trick but they were still cantankerous in cold weather because of the design. The old muscle cars now are way out of my reach $ wise so I've turned to my other love of good affordable audio gear from the past for audio, hence my quest to keep my 1250 up to par. Glad we have more than electronics in common.
 
100W will do fine.

Thanks Echo. I'll have to search through my stash of boxes of home electrical gear and wiring to build my tester. Goat67 has a Variac which I can borrow but don't know if that would replace the dim bulb tester for this purpose. Yours or Mark's input?
 
No, you're right on the rear angle. They were all the same stock. I installed air shocks in the rear after I bought it. Tried the torsion bar trick to raise and lower the front end before and handling was scarey at high speed tests. Reset the torsion spec to factory and installed the air shocks and adjusted the rear height to what you see in the pic. Just enough to maintain control and traction. Looked mean too and what fun! My parents had a '73 Charger SE with the opera windows and 318 they purchased brand new. Those damn choke coils in the intake well were famous going bad. I can remember my dad bitching every winter when mom's car wouldn't start. I learned the cleaning trick but they were still cantankerous in cold weather because of the design. The old muscle cars now are way out of my reach $ wise so I've turned to my other love of good affordable audio gear from the past for audio, hence my quest to keep my 1250 up to par. Glad we have more than electronics in common.

The belt line is basically the line across the bottom of the window glass. I looked at it at the rear wheel well as it swept there and continued with the trim over the rear side windows. Later models had less "rise" back there, and I recognized the air shock stance immediately. At first I used air shocks to correct for some spring sag, until I did it right with Super Stock springs, and stiffer torsion bars. It was super stable at speed, but a bit of a kidney buster at slow speeds.... typical sport vs. passenger suspension stiffness, while still keeping the shock's action.
 
The belt line is basically the line across the bottom of the window glass. I looked at it at the rear wheel well as it swept there and continued with the trim over the rear side windows. Later models had less "rise" back there, and I recognized the air shock stance immediately. At first I used air shocks to correct for some spring sag, until I did it right with Super Stock springs, and stiffer torsion bars. It was super stable at speed, but a bit of a kidney buster at slow speeds.... typical sport vs. passenger suspension stiffness, while still keeping the shock's action.
Tried that too but like you said a belt buster. I went through all of the rear spring stiffening/rise mods then turned to the air shocks. Problem was the seals on the shocks aren't compatible with cold weather and would leak. Monroe finally came out with some new ones that supposedly fixed the problem but I never gave them a try. Fell in love a friend's GTX with a 440 magnum short block, 4 speed. Original suspension it would handle like a race car and the engine torque would put the lower water hose clamp through the oil filter if not installed right. We did it racing a sleeper Sebring off of the stop light for 1/2 mile. We had a Mopar collection amongst friends, Super Bee, Challenger, Duster, my Charger and the GTX. We'd get together and create our own Mopar row at sporting events where we could tailgate and party. Besides my purple Charger the lime green Challenger stood out in the crowd. Aahh the memories!
 
Tried that too but like you said a belt buster. I went through all of the rear spring stiffening/rise mods then turned to the air shocks. Problem was the seals on the shocks aren't compatible with cold weather and would leak. Monroe finally came out with some new ones that supposedly fixed the problem but I never gave them a try. Fell in love a friend's GTX with a 440 magnum short block, 4 speed. Original suspension it would handle like a race car and the engine torque would put the lower water hose clamp through the oil filter if not installed right. We did it racing a sleeper Sebring off of the stop light for 1/2 mile. We had a Mopar collection amongst friends, Super Bee, Challenger, Duster, my Charger and the GTX. We'd get together and create our own Mopar row at sporting events where we could tailgate and party. Besides my purple Charger the lime green Challenger stood out in the crowd. Aahh the memories!

The Monroe's worked fine, we had some pretty cold winters around then.
 
My dim bulb tester is assembled and tested. Cost me just under $8 from my local home Inprovement store. This weekend I will be removing old power transistors cleaning heatsinks and installing the new ones and checking things over B4 power up. Time to clean and organize the bench tonight and get familiar with my new DMM. I have two at my disposal now with one of Goat67's I borrowed. My testing arsenal is slowly getting upgraded from analog to digital. What works the best for removing the old thermal paste?
 
I use elbow grease, a thick soft cloth and am very careful not to bend it at all. It's silicon grease with zinc oxide (the white stuff in mondo sunblock).
 
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