.27 ohm emitter resistors all measure between .38 and .42 ohms

Hmmm

Wow, I even did the Sunn Concert Series power amp stability mods at the end of this service manual and the amp performs exactly the same !!

I'll let you know if I ever figure it out

My Concert Lead at home does 87W into 8 ohms and 132W into 4 ohms. Power supply rails are +/-24 as well. This is what I usually see.

Thank you all for your input so far.

Of course you are all more knowledgable than me but I've worked on a couple dozen of these amps and I really do believe it should do about 130W into 4 ohms if it works right. Maybe I am hardheaded or LT can verify

These all (earlier silverface ones and later red knob ones) had insterstage transformer to couple the output transistors. I do however have the early silverface one that does not have the symmetrical clipping trimpot on the output board like LT mentioned.

My power supply rails at output transistors are +/-24V. LT could you please elaborate on what you mean by 35V flying bridge and the signifigance of R305 and R307 connection to ground? They do go to ground on the amp and check fine for resistance.

I will have to take my Concert Lead apart at home and compare but I am pretty certain it clips at 23V into 4 ohms. I am really missing something, unless interstage transformer could be faulty?

I have replaced Q1-Q5 with new RCA2n3055 and there was no difference.

The thing is I usually clock these amps in at about 130W or so at clipping into 4 ohms. I actually have a concert lead that does 132W at clipping into 4 ohms. There are these weird stragglers every once in a while, hmmmmm

Just to clarify, both versions that I have seen this scenario with are the silverface versions that I believe were made from 1972-1975. The one with interstage transformer

No big DC on the output. There is about a volt as you say with no speaker attached. I have some 2N3055 I could swap some and see if there is a difference.

Will check into this amp more tomorrow thanks LT

What is the scenario that causes this?  I have seen it twice. One Sunn Concert Bass that did 112W into 8 ohms and only about 84 into 4 ohms (at clipping) and now I have a Sunn Concert Lead that will do again 30V into 8 ohms for 112W but only 16V into 4 ohms for a pathetic 64W at clipping.

Is this a scenario of driver transistors not being able to supply enough current to output transistors? I really don't get it unfortunately 

Sunn Concert Lead schematic is attached

You guys are great thanks for explaining further. Really need to sharpen my skills in SS realm. Slowly getting there.

Enzo I'll be putting in a parts order in the next few days. Heat sinks, dummy load resistors, 40408s, the whole deal. Thanks.

Of course I believe you but I must be misunderstanding something. Monitoring collector voltage of Q5 with an analog meter I only can get it to vary from about 35-50VDC when I turn the amp up and down between full volume and no volume.

2N5320 is 10W power dissipation and RCA4048 is 1W, so it seems ok. Has put out full power without a hassle since my last post at 4:05PM, so about 2hrs and 15 min of full power.

I still want to buy the Dale dummy load resistors thanks Enzo. Getting 3 parts orders this week and trying to finish many repairs. Should have some $$$ for you soon

Hmmm, well with original transistor being 90v 700mA and 2N5320 being 75V 2A wouldnt this be pretty over spec'd as is?  Transistor in circuit only has 40V on it so no worries there.

I am running the amp now with 2N5320 now. So good so far

Ok the wizards have helped sort the problem once again and I am truly appreciative. Q5 has a thermal issue. I used an ice pack and a Q-tip. I assume I am homeward bound now. I will replace RCA 40408 with 2N5320 tomorrow and let it run for a while to make sure, but I bet it will maintain full power.

I really do appreciate it. Thank you

https://www.youtube.com/watch?v=ajAP4Ff1HrI&feature=youtu.be

Hi Enzo thanks for your reply. I will try going through some of this and report back.

In the video I am turning the volume amp up and down to show how it responds

I don't have freeze spray but a straw is a great idea. Let me see what I can find.

Power rails seem to sag. the main cap has 85V but when you turn up volume it goes down to about 76V, then settles about 79V with signal still applied. isolating cap at the speaker does the same, about 40V at idle, dropping to about 35 when amp is cranked then settling around 38 with signal applied.

Using the power amp in jack I get the same signal after the amp has been on for a bit, about 10 minutes.

When I just turn the amp on the power amp in jack produces a very symmetrical and powerful signal that gets weaker over time.  The same thing happens when I use the regular input jack, but the output signal has much more variance, basically modulating shape as it's turned up and then settling in some sort of stable wave after about a second or two.  If this is hard to understand I can make a quick vid.

The amp performs good when it's first turned on. If you turn it on and apply no signal for about 10 minutes it will already have a weak output when you apply the signal.

Ok I have hit the wall and need assistance from the real pros   :dbtu:

This Acoustic 230 has weaker and weaker output over time.

When you turn the amp on output looks roughly like this


After about 10 minutes it will be something like this


It seems to be some sort of thermal drift problem because when the output is weak I can literally blow it back up by blowing on the PCB with output section on it

Q8 and Q10 have been replaced because I thought they were not able to drive the output transistors. Still the same. With new output transistors the result is the same too.  I have clipped caps in parallel over every cap in the output section and the result does not vary.

Here's a video of the output. It bounces all over and is not stable, let alone losing power over time. See how when I turn it up it has some sort of settling effect as it deals with trying to put out a big signal?
https://www.youtube.com/watch?v=Jq_UwCRaG6w&feature=youtu.be