SX-1250 question

[ocdecio]

Trial sets of idle current (one channel at a time!!) under DBT ("tickling the dragon's tail") are important to tease out any faults.
Just remember that on DBT the current must be returned to zero at the end of that channel's test,
if that setting were allowed to receive straight line power, it would be way too high - probably damagingly so.

Also dialing up idle current on the DBT makes DBT glow brighter and as that happens the ac TO the unit is strangled more.
Protects have been known to be triggered with the lower ac voltage.
You can increase the wattage of the DBT, there is no reduction in what the DBT is protecting from.
Another lamp in parallel with the first lamp.

Excellent, thank you again, @markthefixer . I didn't realize I should return to zero and will do that (didn't plug it on the wall yet).

I have a good assortment of lamps including a 300W and dual sockets. They may not protect the small signal components but I hope they will avoid something more catastrophic.

I am going to review the right channel and add the missing resistors before trying to bias it.

 

[ocdecio]

I am testing the right channel. I get good bias control but DC goes all over the place and protection goes on and off so I am going to replace the small signal transistors (current mirror/diff pair) hope that will help.

 

[ocdecio]

@markthefixer both channels now are stable and they can reach the spec'd idle even under a 200W dbt. I've watched and read different opinions about how much to set idle - factory specs are 100mV but I've seen some people set it lower say 75mV claiming it would give the same THD and run cooler.

Any advice?

 

[markthefixer]

The only effect of excessive idle current beyond what is needed to minimize crossover distortion is more heat.

Some claim it keeps the amp in class A mode a bit longer, and thus class A is supposed to be lower distortion.
some day i'll be in a position to test that.

I think the factory was leery about the Vbe spread of the available (technology, production variances etc) transistor pairs, and didn't want insufficient idle current to affect some units.
especially when the penalty is just warmer heat sinks. warmer, not hotter....

 

[ocdecio]

The only effect of excessive idle current beyond what is needed to minimize crossover distortion is more heat.

Some claim it keeps the amp in class A mode a bit longer, and thus class A is supposed to be lower distortion.
some day i'll be in a position to test that.

I think the factory was leery about the Vbe spread of the available (technology, production variances etc) transistor pairs, and didn't want insufficient idle current to affect some units.
especially when the penalty is just warmer heat sinks. warmer, not hotter....

So I assume it would be safe at this point to plug it on the wall and adjust bias/DC? I only repaired the amp boards and this unit still needs a full rebuild, the stabilizer board gives the required +65V and -65V. Or should I first restore before any further adjustments/testing?

Thank you, really appreciate your advice.

 

[markthefixer]

It should easily withstand further adjustments and testing.

Just don't short circuit anything....

 

[ocdecio]

It should easily withstand further adjustments and testing.

Just don't short circuit anything....

Last question (I hope) on this thread - I plugged it on the wall, trimpots CCW (replaced them with Bourns). Set both channels to 80mV and they hovered around that for about 15 minutes. Stable DC as well.

Power consumption on the killawatt shows 92W give or take - is that too much or about right at idle?

Thank you again!

 

[markthefixer]

probably just right, my wattmeter is harder (older) to set up than a killawatt so I don't routinely use it.

I should get a kill a watt.

 

[ocdecio]

probably just right, my wattmeter is harder (older) to set up than a killawatt so I don't routinely use it.

I should get a kill a watt.

Most definitely I'd recommend. It is simple, probably not very accurate but it gives you what you need to know. I connected a separate preamp to it, both channels sound the same to me, a little lifeless if I may say it, wonder if a full restoration would make any difference in the final sound. I also replaced the speaker protection relay. I was a little hesitant since it is my first but from what I can see it seems to be, when working properly, a very stable and powerful receiver.