1073 clone diy keeps burning resistor

[tedsorvino]

Hi everyone.

Had built this 1073 clone with acceptable performance a couple of weeks ago but the noise floor could be improved.
So i reopened the box and i started experimenting with trafos placement, insulation and copper tape.

After a bit the results were alright and just before finishing it off, apparently by shorting part of the output trafo with a copper sheet i burned a couple of resistors, one on the pcu board and the other on the in of the pre amp board.

Replaced the resistors, the power suply gives me the 24v needed (don’t know about the quality of voltage) but the power input resistor R21 (12 ohm 1/4w) on the preamp pcb keeps burning. I disconected the transformers, but it keeps burning.

I wonder if the output power transistor is gonne (such a hassle to disconect it) or any other transistor. The capacitors seem ok. Is there a good chance that the resistor stops the bigger damage by being burned ( since) the current flows across it.The other 25v input resistor at the other side of the circuit stays intact, so i guess the trouble is around the output stage, or at the end of the power supply board.

Any other idea what might have happened and what should i replace?

I have to add that yesterday, while testing and fixing stuff, i was testing the output bias, and the values were not good at all. The bias must be set to 130mv and i couldn’t go above 34mv. Even if the trimmer itself was just fine.

 

[RoadrunnerOZ]

Supply to the output stage tail end comes via the output transformer - no transformer primary, no feed to 2N3055, so if it’s disconnected (the primary) and the resistor keeps burning then one of the capacitors may have failed - C29 or C30, if C25 has failed and going short then R3 would also get hot. Are you certain you disconnected the trafo primary? 1/4W seems a little bit small for R21 - what is listed in the build parts list?
Edit: from looking at the bias adjustment they say to adjust for 130mV across R6, then they indicate this gives bias current of 130mA which equates to .2028W - getting close to max for .25W resistor - but you’re getting burning without the transformer connected so something other than the output transistors Q1 and Q2 is drawing current, Q3 is fed by a 68K resistor so not likely that.

Edit: Q2 is fed by 68K and Q1 Q3 fed directly

 

[Voyager10]

My first suggestion would be to look for shorts from the 'bottom' of R12 to ground, as Roadrunner suggests.

However, I'm very suspicious of the circuit around Q11/R74 and the LED in the PSU circuit you posted. The base of Q11 is tied to +48v and the collector's connected to +24v via a 1ohm resistor? How can that be right?

Can you beg, steal or borrow a bench PSU with adjustable voltage and current limit? Might be safest to use a known-good power source with a low current limit before powering up again, and debug the PSU later.

 

[RoadrunnerOZ]

However, I'm very suspicious of the circuit around Q11/R74 and the LED in the PSU circuit you posted. The base of Q11 is tied to +48v and the collector's connected to +24v via a 1ohm resistor? How can that be right?

The LED must be a higher voltage one with inbuilt dropper resistor or the value of 1 Ω on the schematic is incorrect. Maybe they supplied the LED with something like a 1K resistor already fastened and in its own bezel??

Edit: the power on indicator is driven by the 48V turning on Q11 but getting power from 24V line - if either supply rail is not working the LED won’t turn on - it’s the only rationale I can find for that arrangement.