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Author Topic: Frontman 212R  (Read 1185 times)

dlbraly

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Re: Frontman 212R
« Reply #15 on: May 24, 2020, 11:59:45 AM »
maybe a dumb question, but if I read the DC voltage on the speaker outputs by putting the black lead of my DVM on P2 and the red lead P1 speaker outputs.
If I get a Negative DC reading, does that mean I need to look down the P2 path?
Like verify the DC Voltage is getting dropped across R80, R77 and back that way?

Does reading into the polarity of the DC voltage at the speaker outputs mean anything?

Enzo

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Re: Frontman 212R
« Reply #16 on: May 24, 2020, 01:16:41 PM »
P1 us the amp output, P2 is the ground return.  Note between P2 and ground is only a parallel pair of 0.22 ohm resistors.  In other words only one TENTH of an ohm.

A negative voltage on P1 means the output has swung to negative.   That could mean trouble around Q19,21, or it could mean the whole output is being driven negative back at Q12,13 or even earlier.

dlbraly

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Re: Frontman 212R
« Reply #17 on: May 24, 2020, 03:06:35 PM »
Compared DC voltage of Base, Collector and emitter on
Q21 to Q20 - Same
Q19 to Q18 - Same

Q13 - 1E= -41.3,2C= -28.7, 3B= -40.8
Q12 - 1E= 41.2, 2C= -28.9 , 3B= 41.7
Q11 - 1E= 41.5 , 2C= -39.7, 3B= 41.2

Amended this last post. deleted the bottom half, I'm learning.
I was thinking wrong about the Emitter and Collector on the schematic.
Those readings look good to me, what is next back in the circuit to check?
Q10, and Q9?

again, I'm new at this, so I post more info than is necessary just because I might be doing something wrong. I would like you guys to correct me if you see something I'm doing wrong.
« Last Edit: May 24, 2020, 05:46:16 PM by dlbraly »

dlbraly

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Re: Frontman 212R
« Reply #18 on: May 24, 2020, 07:31:49 PM »
DC voltage on
Q10 - 1E= .2 , 2C= 41.2, 3B= 0
Q9 - 1E= .2 , 2C= 40.5, 3B= .5

Looks like it starts to go negative on Q11 and Q12.

I used a DMV on diode test and checked Q12 and Q11 in the board. They seem to be OK.

Does that point to C51 being the issue?



dlbraly

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Re: Frontman 212R
« Reply #19 on: May 27, 2020, 10:59:42 PM »
Just had another eureka moment.

Enzo was talking about rail voltage.
I see that now on the schematic! Those little black triangles with the + or -16. They are attached! Guess they just did that to make the schematic easier to read like they do with the ground symbols.
Funny how that stuff just hits you... Thank you for being patient with me!

be back when I have specific questions.

Enzo

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Re: Frontman 212R
« Reply #20 on: May 27, 2020, 11:19:56 PM »
That is a VERY common drawing convention.  If we drew power supply lines all over, it would really clutter it all up.

dlbraly

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Re: Frontman 212R
« Reply #21 on: May 31, 2020, 11:40:47 PM »
Hello
I did not understand the readings I was getting on Q12 PNP. Emitter 42.3, Collector -29.4, and base 41.9. Not sure how the Collector could be negative.
So I pulled Q12, Q14 and Q15 checked with a LCR-T4-H. Also checked C51, It said they were good so I put them back.
Put it all back together and still have the same readings at the output -29.4 VDC.

Q11 The emitter reads 41.9, Collector -40.2, base 41.8 VDC, So similar to Q12 readings - Figure it probably is ok.

Q13 emitter -41.6, Collector -29.5, and base is -41.4
Those reading seem right

Q9 Emitter 164m VDC, Collector 41.1 , Base 540m VDC
Q10 Emitter 164m VDC, Collector 42 , Base -2m VDC

Do these readings look right on Q9 and Q10. is the Emitter and base so low because I'm getting in to the preamp stage?

Kind of stuck, Can you give me a little push in the right direction?
Don't tell me is you know the full on fix, but maybe a hint or something I should read on.

Enzo

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Re: Frontman 212R
« Reply #22 on: June 01, 2020, 02:19:18 AM »
Think circuits, not parts.   Q12,13 are in a circuit that stretches from V+ to V-.  If Q12 is not turned on, then the voltage from Q13 will be sitting on its collector.   And if Q13 is ON, that will be V- or similar.

The emitters Q0,10 are not low.  They are around zero, yes, but they connect to V- through Q8.  What is happening around Q8 anyway?

dlbraly

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Re: Frontman 212R
« Reply #23 on: June 01, 2020, 11:21:20 PM »
Q8 Emitter is -41.8, Collector 163m v, Base -40.5 so I think that is off.

Q7 Emitter -52m V, Base -360mV, Collector -40,3
Q6 Emitter -3.2mV, Base -223mV, collector -379mV

I still have my -16V on one side of R70, R72, and D19, The other sides are all around -360mV
by these readings, I guess my problem is not in this area.

Do I keep going back? I see a triangle that says "Mute" and another one in grid D8 by the input jack.



Enzo

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Re: Frontman 212R
« Reply #24 on: June 02, 2020, 02:04:37 AM »
Q8 is NPN, with emitter -41.8 and base -40.5, makes the base 1.3v more positive than the emitter.  Sounds like should be ON to me.

dlbraly

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Re: Frontman 212R
« Reply #25 on: June 07, 2020, 10:41:19 PM »
None of the readings seem strange to me yet then..
This far back I do not see any readings that match the DC voltage I see at the white output.
Does that mean I past where the issue is and need to move back forward to find it?
Are Q9 and Q 10 supposed to be on?