Solid State Guitar Amp Forum | DIY Guitar Amplifiers

Solid State Amplifiers => Amplifier Discussion => Topic started by: dlbraly on May 22, 2020, 08:24:09 PM

Title: Frontman 212R
Post by: dlbraly on May 22, 2020, 08:24:09 PM
Hello,
Novice here.

I have a Frontman 212R I bought 2 years ago in hopes of repairing. I replaced a bad switch back then that would not stay in when pressed.
Just getting back around to it again.

It has had a loud hum since i got it. (I will read the post on hum, that is a TON of info)

I do not have a tone generator, so I tested only the test points the schematic (attached) said I did not need to use one. Test point 1-4 and Test points 20-27

Test points were normal except:
TP 24 normal channel was only .93 VDC More drive was - .95 VDC (it should be -2.4 and -9.3)
TP 26 NormCH -1.9, DriveCh -2.0, and more drive -7.2 VDC (it should be -11.8, -5.14, -4.82)
TP 27 NormCh -15.6, Drive 0, More Drive -15.6 VDC (it should be 0, 0, -16)

Any help and advice would be greatly appreciated.

Thank You
Title: Re: Frontman 212R
Post by: joecool85 on May 22, 2020, 09:19:36 PM
Was the switch you replaced the overdrive switch?  It seems that is the issue here.
Title: Re: Frontman 212R
Post by: dlbraly on May 22, 2020, 10:25:09 PM
S1 More Drive switch
Thanks
Title: Re: Frontman 212R
Post by: dlbraly on May 22, 2020, 11:08:47 PM
yeah, Maybe I put that switch in wrong, or it is not the correct switch for this.
I was thinking I made sure it acted the same as what I took out.


Test Point - Actual Voltage - Supposed to be
TP1          - 29VAC           - 32 VAC
TP2          - 29VAC           - 32 VAC
TP3          - 38 VDC          - 41 VDC
TP4 -        -31 VDC          --41 VDC

T20 pannelSwitch NormCH - .67 VDC - .75 VDC
T20 PannelSwitch DriveCH - 2.14 VDC - 2.3 VDC

T21 NormCH - 1.2 VDC      - 1.2 VDC
T21 DriveCH - 1.2 VDC       - 1.2 VDC

T22 NormCH - 15.32 VDC    - 16 VDC
T22 DriveCh - -13.73 VDC - -16 VDC

T23 NormCH - -15.55 -       -16 VDC
T23 DriveCh - -0.2 VDC      - 0 VDC

T24 PannelSwitch S1 Drive - 0.93 VDC - 2.4 VDC
T24 MoreDrive - -0.95 - -9.3 VDC

T25 NormCH - -15.2 VDC   --16 VDC
T25 NormCh - 15.2 VDC     - 16 VDC

T26 NormCH - -1.9 VDC      - -11.8 VDC
T26 DriveCH - -2.9 VDC       - -5.14 VDC
T26 MoreDrive - -7.2 VDC      - -4.82 VDC

T27 NormCH - -15.6 VDC      - 0 VDC
T27 DriveCH - 0 VDC            - 0 VDC
T27 MoreDriveCH - -15.6 VDC - -16 VDC
Title: Re: Frontman 212R
Post by: Enzo on May 22, 2020, 11:18:11 PM
OK, the hum.  Is it LOUD hum?  No controls affect it?  Does the speaker cone move one direction and stay there when you turn the amp on?  If so TURN IT OFF,  You have DC on your speaker, which will destroy it.

Or is it just hum, but you can still hear the music?

No circuit works right without good power supply.  This amp uses 47v rails for the power amp and 16v rails for the preamp and low voltage circuits.  So.  Do you have +47v and -47v, with just a tiny amount of ripple?  And do you have +16v and -15v with zero ripple?

Your test points seem confused.  TP27 doesn't seem like it could be reversed unless TP25 was too.
Title: Re: Frontman 212R
Post by: dlbraly on May 23, 2020, 09:51:58 AM
Hum:
It is a loud hum, Speakers vibrate, but go in one direction from rest. No output with an input.
Can I test the circuit with the speakers unplugged? Maybe with a 4 Ohm resister or something?

Rail +-47:
Is the rail the output of the transformer? When I measure the secondary of the transformer i get 64 VAC (red wire to red). The center tap gets 32 VAC (either red wire to yellow).

Confused Testing - I am a Novice. I'm most likely doing something wrong. I was using Chassis ground. Should I be using something else? Do I use a leg of the secondary?

Also, I just noticed I had all the pots turned all the way to the left - I'm suppose to have them at 50% right?

Thank You for your advice
Title: Re: Frontman 212R
Post by: phatt on May 23, 2020, 10:12:13 AM
Disconnect speaker then set meter to DCV then read voltage at the speaker output terminals/leads of amp.
It should be very close to Zero Volts.
If you have large voltage like 40Volts, pos or neg then power stage is faulty.

Terminals of the transformer are ACvolts,, you need to read the DCV after rectifier,, often easy to do at the main filtercaps.
Front panel knobs is irrelevant if the power stage has blown.
Phil.
Title: Re: Frontman 212R
Post by: dlbraly 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
Title: Re: Frontman 212R
Post by: Enzo on May 23, 2020, 12:15:49 PM
UNPLUG THE SPEAKER NOW, you will destroy the speaker with the DC.  And no dummy load until we fix the amp.  It does not need a load.

I find the easiest place to check the main rails is at the collectors of the output transistors.  The filter caps are radial cans and so you can't get at the leads without dismounting the board.  And if you do that, you lose the heat sink.  I do not recommend power up without heat sink.  Another convenient test point would be the dropping resistors for the 16v zeners.

Rail is just industry slang for a power supply voltage shares by multiple points in a circuit.  it does not describe something physical.

Always read the notes on a schematic.  Look lower right, read note 6.  All voltages are read from ground.  SO TP3 and TP4 are two separate readings to ground, not one to the other.
Title: Re: Frontman 212R
Post by: dlbraly 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
Title: Re: Frontman 212R
Post by: Enzo on May 23, 2020, 03:54:28 PM
Dropping resistors are in power supply,  R144, 145.   200 ohm 5 watt.  Good for TP3,4,5,6.   They drop the 47v down to the 16v.

Yes, those are outputs and center is collector.  Always careful not to slip and short legs together.
Title: Re: Frontman 212R
Post by: dlbraly 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
Title: Re: Frontman 212R
Post by: Enzo on May 23, 2020, 06:42:20 PM
When, that's a lot to read.

TP21,22,23,24 look fine.  We are not concerned with a difference between 1.19 and 1.23 or something.

I know you are trying to be thorough, but it helps to stick to the specifics.  In other words say TP27, there are only two states.  Drive/normal or more drive.  it is confusing to list three states.  It toggles between zero and -16.  SO it looks OK to me.

HOWEVER.   Did we not determine you have large DC on the output?  Channel switching should be the least of our concerns.  Fix that power amp before bothering with details.

It is like our car motor has a boken valve and a bad water pump and we are worrying about the turn signals.
Title: Re: Frontman 212R
Post by: dlbraly 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?
Title: Re: Frontman 212R
Post by: Enzo on May 24, 2020, 01:59:08 AM
I doubt the outputs are shorted, you are not blowing fuses.  But do check.  If you can get your meter on a component lead from the top, there is no need to go underneath.   I would not power the board out of the chassis anyway due to lack of heat sink.

Y'know, I don't know from in and out.  The switches have function.  MORE DRIVE is either on or off, watch the lights.  I assume MORE is button pushed, but...

The MORE button might make some voltage changes, but in terms of hte amp it ONLY has effect when you are in drive channel.
Title: Re: Frontman 212R
Post by: dlbraly 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?
Title: Re: Frontman 212R
Post by: Enzo 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.
Title: Re: Frontman 212R
Post by: dlbraly 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.
Title: Re: Frontman 212R
Post by: dlbraly 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?


Title: Re: Frontman 212R
Post by: dlbraly 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.
Title: Re: Frontman 212R
Post by: Enzo 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.
Title: Re: Frontman 212R
Post by: dlbraly 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.
Title: Re: Frontman 212R
Post by: Enzo 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?
Title: Re: Frontman 212R
Post by: dlbraly 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.


Title: Re: Frontman 212R
Post by: Enzo 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.