I have removed all the output devices and R304 and R303 are still heating up. I did find a short that the previous "tech" had snuck in - connecting node 2 to 4. The VC (collector voltage) on all the output sockets is now correct at 1/2VCC = 48/2= 24V. I replaced all the bias resistors on Q2-Q5 for good measure (some had drifted) but that's not the fix.

What else should I look for to see if the bias is balanced? Since the secondaries on the interstage transformer are floating - I'm assuming that, unless there is a short to ground on S1,T1 or T1,S2, they don't figure into this. Yes it did run a continuity check on the above secondaries and they don't appear to be shorted to ground. What else he asks??
:duh

Playstation what's that - the last video game I played was Tetris

Okay I measured the voltages on the output transistors and I'm seeing an issue with the bias on all devices.

        Q2    Q3    Q4     Q5
VCE 4.6V   46V  46V    3.9V
VBE 3.4     4.8V  3.9V  4.8V
VE            4.2

Okay the voltages above are referenced from either the "-" terminal of C2 or the "-" terminal of C1.

The values for VCE should read around 23V for Q2 and Q5 but they're not. The base bias is really wrong - should be 1/2 of VCC. I didn't measure IE (didn't have time). I'm almost 100% certain that the transistors are fine - they're new. I did notice that the TO-3 sockets have loose pins (low insertion force) for the emitter and base but the devices are making contact.

Check the grounding scheme and found no issues - though it looks like this design lands chassis ground at the output connector. All the bias resistors check out good and I replaced a few for good measure.

Frustrating since this is a simple common emitter circuit - where's the problem? It has to be simple but I don't see it :grr

Hello JM

  All the emitter resistors check out (spec is .27 ohm they read .3 on my HP DMM) - I question the the fact that the emitter voltage isn't about half of VCC (almost appears that the 22 ohm resistors are not connected in the base bias). Could there be a ground issue??

I will look at the other transistor bias values - something simple that I'm missing

http://i284.photobucket.com/albums/ll33/bonosurf/conLeadPA.jpg

The issue is Q3 is getting very hot at turn on with no signal input or load. R303/304 get really hot (+200F) in about 30 sec.

I don't measure 1/2 VCC at emitter - only around 4.6V with the base at 5V. Supply voltages correct and all new transistors in the output but only Q3 gets hot at idle (yes I swapped another device and it still gets hot). Bias trim pot is measuring correct resistance but has on change on the issue. Something is wrong with the bias but I can not see it - changed or inspected all components on this board  :grr

This poor amp was a pig's breakfast when I received it - someone pulled off a stack or dropped it and then some tweeker tried to "fix" it.

I'm assuming you compare the other amps using the same speaker cabinet? Your Sunn should drive a 4 ohm system fine - don't try anything lower in impedance unless you like the smell of burning components. My guess is that you have a damaged output transistor or your bias has drifted (only adjustable on some of the black/silver faced versions).

Not an expert on hearing loss but it's usually loss in the upper frequency range for normal aging - if you shoot fire arms, work with explosives, etc all bets are off on the above.

Hey JM I bet there's plenty of good wine in your backyard

Bonarda = charbono in Cali

Though I'd rather visit your speaker shop.

Okay - I thought you needed someone to build the design out and test it for you. I'm located in Petaluma CA - 45 min north of San Francisco. You might know it as the "wine country" if that's your thing. Mesa Boogie and Magic Parts are about 2 miles from my house.

Okay, you have the GERBER files, what about the components? Are you asking someone to purchase materials? No problem with an NDA or whatever. IMHO patents are a waste of resources unless you have the $$ to defend them. http://www.tinaja.com/glib/when2pat.pdf

I worked for a company that had the laywers and $$ to get what ever they wanted and unless you were able to spend serious money to defend yourself in court you lost and they won - they won a lot.

Do you have GERBER files for these boards? I might be interested in building and testing your boards or you - do you have all the components or a usable BOM?

bumping this back up....

Still happening - On power up the amp will briefly light the red fault LED and then shut down except the fans come on at low rpm?

I checked out the transformer (torroid) and all secondary winding/voltages are correct - out of circuit. When I re-solder the secondaries back on the PCB and power up the amp I do not see correct voltages at the +-15v and +-5 supplies. I'm assuming that the issue is with the pre-amp and PA driver sections of the amp.

Checked limiter transistors Q5/6 and they appear to be okay.

JM's comments regarding the relays: REL3A/B are interesting but I'm not sure how they are disabling the low voltage supplies?

Mmmmm, maybe you haven't dissected any consumer/computer electronics lately?

Okay, shielded mains cables do make sense when you are trying to get your design through FCC type 16/15 testing. Unintentional radiator - more of an issue with Class D (PWM) amplifiers and power supplies.

Skin depth for copper @ 70F is 8.5mm - much more of an issue, skin effect, at higher frequencies. Permiability of the material is also in the equation - iron/steel wire is only used at the center of high energy transmission lines - can you see why?

Note that there is a gradual change in phase as you increase "depth" into conductor. For more fun google skin effect and super conductor. Now I would pay $1800 for SC mains cables  

Okay when I measure the voltage on several of the low voltage supplies I get very low readings - 1.95VAC, .5VAC, etc. Remember that the amplifier is just sitting there with no power LED lite on the front panel - there is 120VAC on the primary side of the PT and 120VAC on the larger (60V) supply and the readings above on the smaller (15V) supplies.

Two things to consider - on the later version of this design they put a PTC thermistor on the PT - just a mistake in the earlier design or was the PT getting into trouble in the field? This amplifier definetly has seen some heating of the neutral connection on the primary side of the PT. Not sure what caused this and have asked the ownere if he had a problem with utility power in his area - it does happen here in Northern California - the local utility has equpiment problems and the neutral goes away and you get 240V or some such on the mains. Though I would assume that this type of damage to the PT would be blowing fuses. :grr

I'll check the supply rail voltage on the output devices. Yup, this amp just isn't happy with some fault that's going on. I would "assume" that the red front panel fault LED would stay on if the amplifier had a problem with the relay REL-KX or some other light problem as you say.