Pointer on fixing a Rockford Fosgate amp

[DigitalJunkie]

Hey guys,I've got a sick RF punch amp here. It's hard to make out the complete model number,because the sticker is scratched up a bit..it looks like
"X1000" possibly? It's a mono sub amp. It looks like it says 500x1 at 4 ohms,and 1000x1 at 2 ohms. Anyways,the PCB and circuit looks pretty similar(identical) to the BD1000a1,1000BD,etc. that are all listed here:
http://194.204.29.93/rockford/techlib/html/amps/bd.html
The PCB looks identical to this pic:
http://ampguts.realmofexcursion.com/...osgate_1001bd/
except it doesn't have the phase reversal switch,or the BD sync jack.

The symptoms are a short chirp/squeal from the power transformer on power-up,and the protect LED on. The various rail voltages seem to be about right,except for the HV output,I'm seeing ~105Vdc here(!100V rated caps!) and it appears that it should be ~70Vdc. any thoughts why it's so high?
Idle current is about 1.25A at about 12.25V,that seems fairly reasonable I suppose..There is no DC present on the output when it's turned on..Nothing seems to get warm,but I haven't left it on for long during investigation,because of the over-voltage situation.
I haven't discovered anything else yet,just did a quick checkout with the DMM..
I'll dig deeper with the scope and see whats up later on.
Got any tips? Thanks!

[midnightg35x]

Have you checked for shorted power supply or output FET's? Those are the most common failure points.

As for the higher HV voltage, what are you using as your reference? Are you using the ground cable on the power supply side or the negative speaker terminal? Try using the opposite of what you looked at initially and see if the reading changes. On the HV side of the transformer, I would guess you should use the negative speaker terminal.

[Perry Babin]

As was suggested, check for shorted output transistors.

In the BD amps, you have to measure the rail voltage across the red/black twisted pair.

When working on these amps, you MUST discharge the rail caps EVERY time you power the amp down. They will remain charged for a week or more if not discharged. Discharge them through a resistor (I use my dummy load). If you discharge them without any resistance, it damages the foil on the board.

If there are no shorted outputs, post the DC voltage on each pin of U21. Place the black meter probe on chassis ground. Place the red meter probe on the point where you need to measure the voltage.

U21
Pin 1:
Pin 2:
Pin 3:
Pin 4:
Pin 5:
Pin 6:
Pin 7:
Pin 8:

[DigitalJunkie]

I'm gonna go dig into it some more in a minute,but I did a quick check of the FET's and gate resistors,and everything seems OK,no shorted FET's,and no open resistors.. I should pull them off and put each one in the tester to be 100% sure.

The 105V is measured across the leads on the caps.(100V 2700uf each.)
For now I have soldered a bleeder resistor on the bottom of the board,while I work on it.

I'll do some more testing of the FET's,and post U21 voltages later.

Thanks guys!

[Perry Babin]

Don't remove the FETs yet.

In addition to the voltages on U21, post the voltages on pins 1, 2 and 18 of the SG3526.

[DigitalJunkie]

note:I had previously checked R26 (measured 3.91K) and R27 (measured 10.05K) and both were good.

SG3526:
Pin1=5.01V
Pin2=4.43V
Pin18=5.01V

U21
1- -8.1
2- +7.8
3- -8.7
4- -9.5
5- +5.0
6- -6.8
7- +8.3
8- +9.7

Some other things I checked,
The rectifiers are good,(D1-D4),(D5-D8),(D9-D12).
D13 and D14 are also good.
R46 is good (10ohms)
R43 is good (18.2K)
R28 is good (10.0K)

BTW,FYI,I'm going off of this schematic: http://194.204.29.93/rockford/techli..._schematic.pdf

Thanks!

[Geek]

Heya Pat,

How about a lame soft start cap, C3 on pin 4 of U14?

Pop in a tantalum there if you have one (for low leakage).

Cheers!

[DigitalJunkie]

Poking around the Regulators Q15 and Q16,that feed U21,U18,etc.
R88 measures 532ohms. R89 measures 777ohms.
The schematic says they should be 820ohms,but the output voltages are close enough to those specified.

The raw 'LV' supply is +/-15.8V
Q1 output is 14.28V
Q3 output is 13.35V The schematic says these should be +/-10V,but the resistors are in spec. R16 measures,1.21K and R11 measures 121ohms. R29 measures 2.41K and R23 measure 238ohms.

Minor discrepancy I guess..

[Perry Babin]

U21 is defective.

When replaced, if there are other problems, the amp may draw excessive current. Insert a 15 amp fuse in the B+ line before applying power.

The amp is regulated by the +LV line feeding pin 2 of the SG3526. When the voltage on that pin reaches the voltage on pin 1, the amp reduces the pulse width to regulate the output. Check R26 and R27 to confirm that they're within tolerance.

[DigitalJunkie]

Originally Posted by Geek

Heya Pat,

How about a lame soft start cap, C3 on pin 4 of U14?

Pop in a tantalum there if you have one (for low leakage).

Cheers!

Hmm,that might cause the squeal I suppose,I'll check it out!
I just noticed that it seems to be the main 2700uf 100V caps charging.If I turn it off,and back on shortly after,it doesn't squeal..but If I let the bleeder resistor drain the caps down,it squeals until they get charged up.
It sounds like it's coming from the transformer area.