Sunn Concert Lead PA

[gbono]

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.

[J M Fahey]

If it gets hotter than others, it's dissipating more than others (no, not duh!, we must know why)
Turn on the amp only 15/20 seconds for each measurement, then turn it off to avoid bigger damage:
ah !! no speaker !!!
1) make a small table as in:
         Q2  Q3  Q4  Q5
Vce      ?     ?    ?    ?
Vbe      ?     ?    ?    ?
Ie         ?     ?    ?    ?
being: Vce= Voltage collector to emitter; Vbe=voltage base to emitter; Ie=Voltage across emitter resistor divided by 0.27 ohms (or whatever you have there)
I suspect a bad or very overbiased Q2. Of course I may be wrong.
Previously check that those emitter resistors are healthy.
Your multimeter can't be trusted on such low ohms value, but you should get roughly the same across al 4 (probably around 0.5 to 0.8 ohms)

[gbono]

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

[J M Fahey]

Hi gbono.
So far so good, but now we need the other voltages.
I suspect Q2 is overbiased , probably R306 open, or a very base-collector leaky Q2, which will almost saturate making poor Q3 handle a lot of voltage and a lot of current, which in my book means a lot of heat.
Of course we can't be sure until you measure so leave that Playstation alone and go back to the bench ... now !!! :lmao:

[gbono]

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

[gbono]

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

[J M Fahey]

Hi gbono.
PLEASE measure what I ask you to and answer that, not "something else".
With amp on, no signal, no speaker, no transistors.
1) DC Voltage across (that's to say, measured from end to end):
R303   (I expect around 30V)
R304   (I expect around 30V)
R301   (I expect around 30V)
R307   (I expect around 30V)
then across:
R302   (I expect around 0.65V)
R305   (I expect around 0.65V)
R322   (I expect around 0.65V)
R306   (I expect around 0.65V)
Then, with the black probe touching chassis (which is *not* the negative of C1) measure the voltage of test points (TP):
TP 16/17  (I expect around +30V)
TP 12/21/22 (I expect around -30V)
Thanks.

[gbono]

1) DC Voltage across
R303  = 24V
R304  = 24V
R301  = 24V
R307  = 24V
R302  =.58V
R305  =.58V
R322  =.58V
R306  =.58V

TP 16/17 =24.5V
TP 12/21/22 = -24.6v
With amp on, no signal, no speaker, no transistors.

[J M Fahey]

Perfect !! :tu:
Now the biasing string is fine.
Diode test all output transistors or use new ones and then replace them on the amp.
If everything is properly connected, you should still measure those 24V; base to emitter you might have a little less, perhaps 500 mV; nothing should overheat.
After checking that, we will move on.
Good luck. :tu:
JMF

[gbono]

Okay maybe it's not clear but though the above bias readings appear "balanced" I'm still seeing significant (>100F in 45 sec) heating of T303/304/307 only. This is of course without any active devices installed.

Something is wrong but what is it?

[J M Fahey]

R 303/4/1/7 (I think you missed one) *do* heat because they dissipate 3/4W and should be warm/hot to the touch.
+If* one of them stays cold , it's open, although you simply forgot onbe.
What tells me that none is open is that all four bias voltages (580 mV) are present.
I suggested to check previously the power transistors is to nbe able to trust them.
Use a series lamp for safety , mount those transistors and turn it on.
Good luck.

[gbono]

I have new On Semi 2N3055's that I matched on a curve tracer at 1A/20V (use pulsed supply mode and work FAST). They are going in as soon as I can get around to this project.

I have repaired about 6 of these amps and have never noticed that the bias resistors get as hot as these do. (My original issue was that Q3 was getting very hot in about 45-60 sec.) We'll see what happens - thanks for the advise and I'll post the results.

[J M Fahey]

In the beginning that transistor was overbiased (you said there was a strange connection there) and overdissipating.
You also had 4 or 5 Volts across the two lower 910 ohm resistors, which causes the two upper ones to stand, continuously, almost the full power supply voltage, which means around 3 or 4 times the normal dissipation.
I trust now it will work.
Good luck.

[gbono]

Hey it works and it puts out +21Vrms into a 3.2 ohm load. The only issue now is the preamp is generating some even order harmonics. Thanks

[J M Fahey]

Good !!  :tu:
Your amp is putting out around 140W; not bad for a "150W" amplifier. :tu:
Congratulations.