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Messages - gbono

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1
Amplifier Discussion / Re: Acoustic 370 Output Level Issue
« on: March 31, 2021, 10:17:12 PM »
Replaced Q301/2 with 2N4248

voltage around diff pair

Q301
VB=7.5

Q302
VB=7.8

VE=8.2

2
Amplifier Discussion / Re: Acoustic 370 Output Level Issue
« on: March 31, 2021, 09:04:14 PM »
UPDATE - I found that Q303 was mismarked and has been replaced with NOS 40408

Voltage around Q303
VC=7.2
VE=.15
VB=.75

Q304
VC=9.5
VE=7
VB=7.7

Q301
VB=7.6

Q302
VB=7.8

VE=8.2

I have the amp on a light bulb limiter and at turn on you see a bright flash (charging caps) and then a very very slight glow. I haven't tried to drive a signal into the board yet and it still looks like Q303 collector is no where near VCC/2.

3
Amplifier Discussion / Re: Acoustic 370 Output Level Issue
« on: March 31, 2021, 07:23:14 PM »
Understood but if the node at R317/339 is at 4V how does this condition apply to the differential pair Q301/2?
As I understand it the difference in base voltages at Q301/2 are why VE is measuring at around 3V. I'm missing the connection (no pun) between the diff pair and the voltage at R317/339?

4
Amplifier Discussion / Re: Acoustic 370 Output Level Issue
« on: March 31, 2021, 03:38:37 PM »
Voltages around Q301/2

Q301
VB=9.1V

Q302
VB=2.8

VE=3.5

So Q302 base is getting the lower DC voltage through the voltage divider set up by R314, etc. Question still remains why is voltage at collector of Q304 so low??? So if there is 4V at node R317/339 then there is a fault (over current) flowing through R323/3? So I could have a short in some of the output transistors Q307-314??

5
Amplifier Discussion / Re: Acoustic 370 Output Level Issue
« on: March 31, 2021, 12:07:28 AM »
One side of R316 is at 83V and between R316/7 I get 43V but on the other side of R317 it's sitting at 4V. I have lifted one side of CR308 (zener) which doesn't change voltage at node - what is dropping the voltage?  All measurements referenced to minus terminal on C403.

6
Amplifier Discussion / Re: Acoustic 370 Output Level Issue
« on: March 30, 2021, 05:43:29 PM »
Rechecked the board and found Q3010 was replaced but not soldered.
Voltages around Q303 are:
VC=4.3
VE=1.9
VB=2.6

I'm still seeing LT 3V at C404.

Soldering station is grounded and RH in Northern California isn't usually an issue (40% here) - always check mains connections at the start and ground/neutral/hot all properly connected.

So to summarize: bias transistor Q404 is functioning - Q303 is biased on and differential pair is also functioning but unbalanced BUT amp will not pass signal beyond Q301. With no signal applied: DC voltages around bias transistor and output cap are 4V (VC of Q304) and 2V at C404.

frustrating...

7
Amplifier Discussion / Re: Acoustic 370 Output Level Issue
« on: March 29, 2021, 02:07:40 PM »
Rechecked voltages around Q303
VC=1.9
VE=.18
VB=0

I have replaced this transistor several times with 2N3440...

Voltages around Q204
VC=4.37
VE=1.9
VB=2.67


8
Amplifier Discussion / Re: Acoustic 370 Output Level Issue
« on: March 28, 2021, 10:35:07 PM »
Voltages around Q303
VC=.8
VE=2.5
VB=.2

note: Q303 previously replaced by 2N3440 250V/1A/TO-3 very similar to 40409/8

Voltages around Q310
VC=.8
VE=2.5
VB=1.9

Voltages around Q305
VC=82
VE=3.7
VB=4.3

I had replaced Q310 and Q305 with  2N4033 and 2N3440 respectively -Q310 and Q303 are off??. VE on Q310 should be around 40V. Issue with replaced Q310?

9
Amplifier Discussion / Re: Acoustic 370 Output Level Issue
« on: March 28, 2021, 04:44:19 AM »
I thought that if the base voltages vary then the collector currents would also vary but the emitter current (total) would stay the same.
With 3.6V on the emitters of the dif pair then Q301 would be off and Q302 would be on - correct?

How is Q303 biased? Isn't Q301 supplying bias to Q303?

Still not able to explain why the voltage at C404 isn't at VCC/2?

10
Amplifier Discussion / Re: Acoustic 370 Output Level Issue
« on: March 27, 2021, 08:31:26 PM »
Lifted the zener and no change to voltages. Also noticed that the emitters of the long tail pair are at 3.6? Base of Q301 is around 10V and base of Q302 is under 3 volts.

Yes Q304 is keeping 2 volts between VC/VE but what happened to the 40 plus volts that should be at the collector?

11
Amplifier Discussion / Re: Acoustic 370 Output Level Issue
« on: March 26, 2021, 07:36:01 PM »
Okay now the amp is doing this: voltage at C404 is 4V and voltage at collector of Q304 is 2.4V and emitter is 2V .... The longtail pair has 10.2V on base of Q301 and of course base of Q302 is 2.4V. Only Q305/10 are connected in the output all the other transistors are out of circuit. The amp does not draw excessive current. I am completely stumped! The bias circuit has an issue but where? Could the filter cap be bad? It looks like the zener diode in the base of Q305 is conducting so I assume that is the 4.2V on the collector of Q304 ???

12
Amplifier Discussion / Re: Acoustic 370 Output Level Issue
« on: March 26, 2021, 02:44:11 AM »
Yes Q303 collector has 83V and base is sitting at 53V. Assume Q301 is biasing Q303?

13
Amplifier Discussion / Re: Acoustic 370 Output Level Issue
« on: March 25, 2021, 11:55:50 PM »
Replaced Q303 with original transistor since there was no change when a substituted in a 2N3440. Even with Q303 in place I get 83V at C404 and 81V at the collector of Q304.

14
Amplifier Discussion / Re: Acoustic 370 Output Level Issue
« on: March 25, 2021, 07:56:45 PM »
I removed Q303 and replaced with 6.8k and voltage on C404 rose to 28V. Replaced Q 305/310 with usual alternatives:2N3440 and 2N 4033 . Now voltage at C404 is at ps rail 84 V. Also had removed all other transistor in output. The amp is connected to a light bulb limiter which is not showing any excessive current draw. I'm stumped. I should be looking at complementary pa configuration with a single ended supply rail. It's as if Q305/314 are not connected?

15
Amplifier Discussion / Re: Acoustic 370 Output Level Issue
« on: March 25, 2021, 03:11:07 AM »
The bases of the long tail pair are both at 10.5V. The voltages around Q310 are VB=10.1 VE=10.7 and VC=0 Q310 is off..

What does the zener do on the base of Q305 - shouldn't it keep Q305 at 4.1V? (1N4731A)

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