Frontman 212R

[dlbraly]

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

[joecool85]

Was the switch you replaced the overdrive switch?  It seems that is the issue here.

[dlbraly]

S1 More Drive switch
Thanks

[dlbraly]

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

[Enzo]

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.

[dlbraly]

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

[phatt]

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.

[dlbraly]

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

[Enzo]

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.

[dlbraly]

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

[Enzo]

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.

[dlbraly]

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

[Enzo]

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.

[dlbraly]

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?

[Enzo]

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.