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1122DC Offset Issues

Geek

Geek

Nerd
Hi Folks,

So I have this 1122DC in for service. Everything is all good, bias, no oscillations, etc. but the DC offset is 6.5V one channel, 2.4V the other. Pretty far out, too out to trim.

What my revision had was for R735 (and respectfully in the other channel) instead of 560R, was 270R and 390R in series with one of them miserable STD-100 thermistors accross the 390R. This was a factory revision change. Thermistor coating was cracked, so for the heckuva, I just changed the circuit to the schematic 560R.

Dial-up on the VARIAC and the offset was better - 500mV one channel, 180mV other channel.

Here's the kicker.... after 3mins warmup, offset whent right back to 6.5V one channel, 2.4V the other.

Could the thermistors on the bias transistors be out of balance? Problem is replacing them.... I did a T25/100 test and did the scary math and came to about 550K - fat chance finding anything under 2700K nowadays :idea:

So, what am I missing?

Thankees in andvance!

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I'm not gonna be much help, but you could check the zener regulated voltages on each end of the offset control to make sure they are OK.

You are aware that the 'ground' reference is split, with the input and output sections of each power amp having their own 'ground', right?

Tom
 
Thank you, sir! There was a floating ground connection that cleared up the left channel back to 0mV offset. Right still sits at 3V, full VARIAC and goes bouncy, bouncy about. A tipoff as to the issue was a sawtooth right up until about 112V, while the left cleaned up higher than about 70V. Tells me there's some sick and leaky caps that channel.

Just waiting on the estimate for a "Go!" and we'll have a final report :thumbsup:
 
On the right track.....
Several resistors in that channel (don't ask me why the other channel resistors 3" away weren't affected) have become loose tolerance thermistors in their own right. Replacing R702, 704, 724, 726 and 742 cleared up the offset bouncing around.

Found them with freeze spray.

Caveat - using 5% carbon film resulted in the same problem. Using closest 1% metal film cured it.

The offset is still a nasty 2.5V, but doing a quick mod on one side of the offset pot brought it to 0.00mV, holding within +/-20mV over warmup.

Will run her for a week now and see if the ol' gal can keep it together :thumbsup:
 
Update:
Bias kept going haywire. Thermistors! Digikey 495-2156-ND thermistors will replace the SDT-100 thermistors in these. I suspect the 300DC as well.
Cheers!
 
Geek said:
Update:
Bias kept going haywire. Thermistors! Digikey 495-2156-ND thermistors will replace the SDT-100 thermistors in these. I suspect the 300DC as well.
Cheers!
Click to expand...

I am exited about this fix!
How did you determine that the 495-1256-ND parts match the Beta curve of the SDT-100?
Tom
 
Thank you!
Another substitute is that finicky old 2SK109 - I just dropped in a 2SK2145BL with adapter (the 1122DC uses a common source) and it needed zero tweaks. Mouser 757-2SK2145BLTE85LF is the sub for it.
Cheers!
 
Hi Tom,

I will test it dynamically today or next week. Did let it sit on the bench for a couple hours and alternated fan and heatgun and it stayed within 2mV
(which did uncover some DC offset driftiness... not enough to click the protector but enough to make me replace some C1775 and A872's with KSC1845 and KSA992 to fix it)

Cheers!
 
catrafter said:
This is good to know! Did you monitor the bias over a large temperature range to see if it (the bias) was tracking?
Click to expand...

Final report on this particular amp - bias tracks perfectly. In real everyday use and on the bench craking 100W/ch into a dummy load for an hour and getting her toasty a-la toasty! Bias stays within 3mV of original condition under such severity.

Found the final cause of +/- offset driftiness - just replace those resistors on the ceramic standoffs. All of them. They become intermittent varistors.

Also you'll have to completely clean the glue goop off the trace side of the board and add a pile of jumper wires as the glue eats copper (Japan used the stuff by the truckload to hold in capacitors and other parts. Brown stuff. Becomes corrosive as h.e. double hockey sticks and leads/traces disappear under it. Not just Marantz, but Sansui, too).

Cheers!
 
Geek said:
Final report on this particular amp - bias tracks perfectly. In real everyday use and on the bench craking 100W/ch into a dummy load for an hour and getting her toasty a-la toasty! Bias stays within 3mV of original condition under such severity.

Found the final cause of +/- offset driftiness - just replace those resistors on the ceramic standoffs. All of them. They become intermittent varistors.

Also you'll have to completely clean the glue goop off the trace side of the board and add a pile of jumper wires as the glue eats copper (Japan used the stuff by the truckload to hold in capacitors and other parts. Brown stuff. Becomes corrosive as h.e. double hockey sticks and leads/traces disappear under it. Not just Marantz, but Sansui, too).

Cheers!
Click to expand...

Hi Geek! thank you for all of the tips!! much appreciated! May I kindly ask you a question? I currently have a 1180DC (one in my personal collection) and after a full recap, and all new transistors (except output transistors) and diodes, it takes at least 15 minutes for the DC offset to stabilize (needs to warm quite a bit). For example, when I power it up, initial Left DC offset goes to 250mV, then drops to zero and back up to -50mV and right settles at -30mV and stays like that until it gets up to running temperature. After 15 minutes of idle, it goes down to approximately 5mV and stays there within +-2mV on both channels. I re-set it to 0mV and stays there, within +-1mV. However when i cycle it a couple of times (on, off to cool, and on again) it goes up to 5 or 6mV approx, after it is fully warmed up. I am trying to understand if this is normal or not as i never had this issue on other amps before. At first i thought the differential pair is freaking up, so I replaced the 2SK109 FETs with LSK389B (which seems to work well) and it made no difference. I also made sure to match the other stage of differential transistors (originally 2SA872 which were replaced with KSA992). Preamp was also furnished with new, matched differential transistors. Disconnected the preamp but problem persists. It seems that it is quite slow to settle on the dc offset. I did measure all the resistors after i let the amp get to temperature and they seem to be within spec. ... Is this normal on these power amps? they also seem to run a bit too hot for my liking.

Any help would be greatly appreciated!

Kind regards!!
 
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