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Need Help - Tech 21 Power Engine 60 Hum

Started by Jasz2, September 25, 2016, 02:49:09 PM

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Jasz2

Quote from: phatt on October 07, 2016, 03:47:18 AM
I've drawn up a generic Split rail power supply for you which is the most likely setup.
The Y/B/Y connector block is the secondary ACV from the transformer.
The Full wave rectifier converts the AC into DC voltages which power the circuit.
The 2 Big Electro cans filter the DC voltage ripple and that is the highest DC voltage which powers the main Chip. The voltages are then reduced via those 2 large 1k resistors and the Zener diodes regulate that to the required low voltage to run the preamps.
With probe in that black connector block just set your meter to DCVolts and the other probe to both ends of those 1k resistors. One side should be the power chip higher voltage and the other end should be the lower preamp voltages.

If you can't understand what is being explained then it might be wise to send it to a repair shop. :tu:
Phil.

I'm getting 40 VDC at the rail
The resistor nearest the input reads 41.8 and 15.21 VDC
The other resistor reads 41.8 and 15.5VDC
When I check the resistance from the black wire to the case, I get a reading of zero.

phatt

Good effort.  :tu:
Then all the voltages are fine and common is grounded to case. 
So now the hard part is finding the source of the hum.

Before you dig further,, Q How bad is this hum?
Remember that a lot of gear does hum at least a little bit.
Keep in mind that if you are using amps in a bedroom and the volume is low *Normal hum* can be annoying but at gig levels you never hear the low hum.
So with the volume up at least halfway with music playing can you hear that hum?
Phil.

Jasz2

Quote from: phatt on October 07, 2016, 08:21:36 AM
Good effort.  :tu:
Then all the voltages are fine and common is grounded to case. 
So now the hard part is finding the source of the hum.

Before you dig further,, Q How bad is this hum?
Remember that a lot of gear does hum at least a little bit.
Keep in mind that if you are using amps in a bedroom and the volume is low *Normal hum* can be annoying but at gig levels you never hear the low hum.
So with the volume up at least halfway with music playing can you hear that hum?
Phil.

The hum is BAD, it is not a low hum. I've have many amps over the years, and have never had an issue like this. I could never dream of recording with this thing. It can be heard at low levels and increases as the volume is turned up.

Jim

phatt

#18
If the Volume control can turn down the hum then that narrows it down to preamp as the volume is usually the only link between preamp and poweramp. :tu:

See C3 & C4 in my drawing, Now see if you can establish the location and Value of C3 & C4 on your circuit then tell us what value they are.
C3 & C4 are the filters for the preamp and they might need to be increased in value.
Follow the tracks from those big resistors to find  C3/C4.  Looks like C3 C4 are the black ones on the edge near the Zener diodes which are also connected to those tracks.
Zener diodes will be those little red parts.

Phil.

Jasz2

Quote from: phatt on October 08, 2016, 05:41:37 AM
If the Volume control can turn down the hum then that narrows it down to preamp as the volume is usually the only link between preamp and poweramp. :tu:

See C3 & C4 in my drawing, Now see if you can establish the location and Value of C3 & C4 on your circuit then tell us what value they are.
C3 & C4 are the filters for the preamp and they might need to be increased in value.
Follow the tracks from those big resistors to find  C3/C4.  Looks like C3 C4 are the black ones on the edge near the Zener diodes which are also connected to those tracks.
Zener diodes will be those little red parts.

Phil.

The value on those is 16V, 220uf

Jim

phatt

Hum,,, 220uF seems a bit low.  Replacement might be a little tricky and potentially destructive unless you are familiar with surface mounted components and Lead free solder.
You can raise that value without upsetting the PCB by simply adding more capacitors, see pic;

Now of course I'm just guessing the polarity and position of the parts so it's up to you to make certain which is which. just remember that the positive terminal of the capacitor goes to the pos rail and negative terminal goes to neg rail.
The junction in the middle of the two added caps will of course go to common. A couple of 470uF caps will raise the value to 690uF which should be enough. This saves you having to mess with the PCB and can be easy removed if it does not work. :tu:

One thing that does bother me is the 16Volt rating of the caps which is very close to the working voltage. Only .5 volt headroom. ouch!! :o
Others here will know far more about the limits but a rule of thumb, caps must be higher than the working voltage by 10~20%

in this case 15.5 + 10% = 17.2. So those caps are working on the limit. :-X
If you feel confident I'd replace the originals with 680uF 25 Volt caps.
Even 1,000uF will be ok if they fit.

Mass production procedures often use parts that are just on the limit, just long enough to get past the warranty time. :grr
Phil.

Jasz2

Quote from: phatt on October 09, 2016, 07:33:10 AM
Hum,,, 220uF seems a bit low.  Replacement might be a little tricky and potentially destructive unless you are familiar with surface mounted components and Lead free solder.
You can raise that value without upsetting the PCB by simply adding more capacitors, see pic;

Now of course I'm just guessing the polarity and position of the parts so it's up to you to make certain which is which. just remember that the positive terminal of the capacitor goes to the pos rail and negative terminal goes to neg rail.
The junction in the middle of the two added caps will of course go to common. A couple of 470uF caps will raise the value to 690uF which should be enough. This saves you having to mess with the PCB and can be easy removed if it does not work. :tu:

One thing that does bother me is the 16Volt rating of the caps which is very close to the working voltage. Only .5 volt headroom. ouch!! :o
Others here will know far more about the limits but a rule of thumb, caps must be higher than the working voltage by 10~20%

in this case 15.5 + 10% = 17.2. So those caps are working on the limit. :-X
If you feel confident I'd replace the originals with 680uF 25 Volt caps.
Even 1,000uF will be ok if they fit.

Mass production procedures often use parts that are just on the limit, just long enough to get past the warranty time. :grr
Phil.

Any recommendations on which brand capacitors I should purchase?

Jim

phatt

Brand is kinda irrelevant as it's just an amplifier.
Any reputable electronics parts supply store should have what you need.
I recycle a lot of electronic land fill So I don't need to buy Electros very often. ;)
Phil.

Jasz2

Quote from: phatt on October 09, 2016, 08:55:37 AM
Brand is kinda irrelevant as it's just an amplifier.
Any reputable electronics parts supply store should have what you need.
I recycle a lot of electronic land fill So I don't need to buy Electros very often. ;)
Phil.

Just wanted to make sure...

Thanks Phil!

Jasz2

Quote from: Jasz2 on October 09, 2016, 08:48:32 AM
Quote from: phatt on October 09, 2016, 07:33:10 AM
Hum,,, 220uF seems a bit low.  Replacement might be a little tricky and potentially destructive unless you are familiar with surface mounted components and Lead free solder.
You can raise that value without upsetting the PCB by simply adding more capacitors, see pic;

Now of course I'm just guessing the polarity and position of the parts so it's up to you to make certain which is which. just remember that the positive terminal of the capacitor goes to the pos rail and negative terminal goes to neg rail.
The junction in the middle of the two added caps will of course go to common. A couple of 470uF caps will raise the value to 690uF which should be enough. This saves you having to mess with the PCB and can be easy removed if it does not work. :tu:

One thing that does bother me is the 16Volt rating of the caps which is very close to the working voltage. Only .5 volt headroom. ouch!! :o
Others here will know far more about the limits but a rule of thumb, caps must be higher than the working voltage by 10~20%

in this case 15.5 + 10% = 17.2. So those caps are working on the limit. :-X
If you feel confident I'd replace the originals with 680uF 25 Volt caps.
Even 1,000uF will be ok if they fit.

Mass production procedures often use parts that are just on the limit, just long enough to get past the warranty time. :grr
Phil.

Hi Phil,

I replaced the original caps with 680uF 25 Volt caps, and there is absolutely no change to the hum at all.

Jim

phatt

#25
Oh dratz, :grr
Alright then things to try;
(I probably should have mentioned this before) :-[
Disconnect the ground wire, the Green one that bolts to the chassis from the PCB.
It connects the circuit common to the chassis. *Not* the one that comes from the Mains plug!!

Test by touching that tab on and off from the chassis if the hum is reduced when it's disconnected then the circuit is already grounded some other place causing a ground loop.

Some cannon sockets have pin1 internally connected to the mounting hole, inserting the screw then bridges pin 1 to the chassis. You have to pull them apart to see it. A simple test with meter can confirm that.

Leaves one other possible problem; that transformer is obviously for the balanced input BUT it has no shielding :o and could be an issue. Pro PA mixers would NEVER dream of using open frame xformers for balanced input because they are highly prone to picking up noise. :trouble :trouble :trouble

I'm thinking you might be able to bridge the leads on that transformer and see if that kills the hum. You may need to bridge the secondary side before it kills the hum. If it does then you found the problem and I can't think of any way to fix that except a complete replacement with a proper shielded unit.
the Jensen transformer site used to have a whole page of very useful PDF's with in depth info on transformers and the ways to use them.
Phil.

HutchAmpMan

I've got one of these in the shop right now.  There is a high frequency oscillation that is being fed to the power amp (causing it to heat up with no input and results in the buzz you hear. Pulled the PA (LM3876T) and the oscillation is present on the input to the chip, so back in the signal path from that.

What's irritating is the board mounted SMD chips don't have valid part numbers on them so it's going to be a reverse engineering job if the company won't cough up schematics and parts specs.

I'll have to ask the owner if he wants to go ahead with an expensive repair on an $350 amp or just leave it as an extension speaker (which he's already done)

phatt

Welcome Hutch,
IIRC, there is an FX loop or Preout/Pwramp input.
Send a signal to return only which should disconnect the Preamp from power amp and see if that resolves the Buzz and over heating.
I know they run those LM chips at the voltage limit and they can fail if driven hard enough, especially if driving 4 Ohms.

Phil.