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Messages - dlbraly

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1
Amplifier Discussion / Re: Frontman 212R
« 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.



2
Amplifier Discussion / Re: Frontman 212R
« 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.

3
Amplifier Discussion / Re: Frontman 212R
« 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.

4
Very true, good point.
I'm learning, slowly.

Learned a lot about transistors today that I did not know and how they relate. I still don't feel confident about them.
Its fun learning.

Vacation is just about over.
Hope you all had a good Memorial day.

Back to work tomorrow.

5
Looks like you can watch a few of the videos, then you have to buy a subscription for more advanced stuff.
Might be worth it to someone.

6
I don't have a tight grasp in reading circuits. I don't fully know what to expect when looking at a circuit, unless something is really messed up.
Had the amp on and kept smelling a burning smell! Took me a while to figure out it was just the light I was using to see.  :duh Bad enough I have to keep changing my glasses to see the amp, to the circuit to the computer monitor.

Found this YouTube channel that seems pretty good.
"This guy was an aerospace engineer and electrical engineer for NASA he started doing vids in his garage..."
There are a TON of sharp guys in this field on this forum as well, but figured I would pass it along. Might be of some use. Maybe this will help me from asking so many questions.

https://www.youtube.com/user/mathtutordvd/videos

He has a lot of videos from basic stuff to more complex.

Lesson one starts very basic descriptions and builds from there.
https://youtu.be/OGa_b26eK2c

7
Amplifier Discussion / Re: Frontman 212R
« 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?



8
Amplifier Discussion / Re: Frontman 212R
« 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.

9
Amplifier Discussion / Re: Frontman 212R
« 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?

10
Amplifier Discussion / Re: Blackstar ID Core 40 'Bricked' Problem
« on: May 24, 2020, 11:07:38 AM »
This is an old post, but pretty neat YouTube video you linked.
Clyde,
I would also be interested in hearing more details on what you did.

11
Amplifier Discussion / Re: Frontman 212R
« on: May 23, 2020, 08:57:48 PM »
LOL I am confused.
I got confused with the term Normal/Drive - I was thinking SW2 switches between normal and Drive. There are 2 different readings when S2 is IN or OUT. I was not sure if it should be IN or OUT. So I measure all the positions of both S2 and S1.
I guess since SW2 IN gave me the correct reading of 0V, it should be measured IN.

Thank You for helping me.

yes, excessive DC Voltage at the speakers output.
Problem with Power Amp.
I have good rail voltage 42V, 16V after the drop resistors.

Next step? Check for transistor short? Can I do that without removing the board?

12
Amplifier Discussion / Re: Frontman 212R
« on: May 23, 2020, 05:43:43 PM »
each side Dropping resistors:
R144 - 41.4 VDC one side and 16.4 VDC on the other
R145 - -41.7 VDC one side and -16.4 VDC on the other

TP20 (@D42) Panel SW2 OUT .67 VDC - SW2 IN 2.19 VDC (Looking for .75, and 2.3)

TP21 (@D45) SW2 OUT 1.17 VDC - SW2 IN 1.16 VDC (Looking for 1.2, and 1.2))

TP22 (@D50) SW2 OUT 15.94 VDC - SW2 IN -15.28 VDC (Looking for 16, and -16)

TP23 (@R127) SW2 OUT -16.48 VDC - SW2 IN -.03 VDC (Looking for -16, and 0)

TP24 (@R133) SW2 OUT and SW1 OUT 1.11 VDC - SW2 OUT and SW1 IN 1.10 VDC (Looking for -2.4 and -9.3)
      SW2 IN and SW1 OUT -4.5 VDC - SW2 IN and SW1 IN -4.18 VDC

TP25 (@R134) SW2 OUT 15.59 VDC - SW2 IN -14.87 VDC - SW1 IN 15.65 VDC (Looking for -16 and 16)

TP26 (@D47) SW2 OUT and SW1 OUT -2.29 VDC - SW2 IN and SW1 OUT -2.3 VDC
   SW2 OUT and SW1 IN -8.48 VDC - SW2 IN and SW1 IN -7.8 VDC (Looking for -11.8, -5.14, and 4.82)

TP27 (@R136) SW2 OUT and SW1 OUT -16.37 VDC - SW2 IN and SW1 OUT 0 VDC -
   SW2 OUT and SW1 IN - -16.44 VDC SW2 IN and SW1 IN -16.42 VDC (Looking for 0, 0, and -16)

Thank You

13
Amplifier Discussion / Re: Frontman 212R
« on: May 23, 2020, 02:26:15 PM »
Speaker is unplugged

Circuit board is still in place. For test points I tried to find available wire on top. Example TP26 I tested at D46, and TP27 at R136

I read that (#6) and used chassis ground on my measurements the first time. Chassis/source ground is what I interpreted that as. Started seconded guessing myself after reading something about circuit ground, or floating ground. Thank You cor clarifying.

Using Chassis ground, verified I get 119.7V AC going into the transformer to make sure I had a good ground for my DVM. I did not do that the first time, should I go back and retest the points?

Not sure which side is the collector. Measured DC  left, center, right legs
Output Transistors? - Is that Q20 and Q21?
Q20  -28.5, 41.9, -29
Q21  -28.6, -42, -29
After those readings, Is the center one the collector?

Which ones is the dropping resistors?

Thank You

14
Guitar News / Re: Teisco coloring book challenge
« on: May 23, 2020, 01:03:28 PM »
Neat  :dbtu:

15
Amplifier Discussion / Re: Frontman 212R
« on: May 23, 2020, 11:44:52 AM »
I get 29 VDC measuring across the speaker leads.
Can I leave the speakers unplugged to go back and retest the test points?
OR
I assume I need a load since to complete the circuit. Do I just use a 4 ohm resister?

After the Rectifier - That is the big diodes right?
Does that mean put my ground lead of my DVM at TP4 or TP3 to measure DC Voltage? I assume TP4.

Thank You

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