Orion 225G4 problems

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Hi gang;

A customer brought in this HCCA 225G4 with "no audio"

Actually, it has very distorted audio. Left channel has a dc offset at the speaker terms of 2.5vdc.
Right channel has 1.5vdc.

Checking it out, it has full power supply......plus and minus rail voltage, + /- 12V and + / - 5V.

No shorted or leaky mofsets.
Didn't see any off hand reasons for the DC offset.

There's a pic micro,
Not sure if it's there to simulate an h bridge or what.

Any advice ?

Thanks in advance,

[Perry Babin]

If no one else has any suggestions, post a photo of the inside of the amp.

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Here's one.

Let me know if you need more.

[Perry Babin]

If you have ~0 ohms between the non-bridging speaker terminals, the RCA shield and the secondary center tap...

There are two 8 pin op-amps near the 'Orion' on the board. Post the DC voltage on all of the pins for each one (black probe on the secondary center tap).

IC nearer output transistors (circuit board designation?)
Pin 1:
Pin 2:
Pin 3:
Pin 4:
Pin 5:
Pin 6:
Pin 7:
Pin 8:

IC nearer 'Orion' (circuit board designation?)
Pin 1:
Pin 2:
Pin 3:
Pin 4:
Pin 5:
Pin 6:
Pin 7:
Pin 8:

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Perry;

here's the results
IC nearer output transistors (circuit board designation?)
Pin 1: 0
Pin 2: 0
Pin 3: 0
Pin 4: -13v
Pin 5: -300mv
Pin 6: -.3v
Pin 7: -.42
Pin 8: +13v

IC nearer 'Orion' (circuit board designation?)
Pin 1: 0
Pin 2: 0
Pin 3: 0
Pin 4: -13v
Pin 5: -.44v
Pin 6: -69mv
Pin 7: -.6v
Pin 8: +13v

[Perry Babin]

Pin 5 probably goes back to one leg of the muting transistor (possibly through a resistor). Another leg of the muting transistor is connected to the secondary center-tap (secondary ground). Short those 2 legs together. You may want to pull the muting transistor. It can sometimes make it difficult to determine which leg is ground (if you can't see the connection to the ground plane). Does the voltage on pin 5 go to 0.00v DC?

Does the DC on the output go to very near 0v?

If so, follow the signal line back beyond the muting transistor to an electrolytic capacitor. Does the other side of the electrolytic have excessive DC (more than a few millivolts)?

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Yes,

When both legs of the transistors are shorted, the pin 5's
go pretty close to 0. (aprox .4mv)


I found the electro caps right off of the muting transistor lines (10uf/50v)

Here's where the caps are sitting at
C49: + lead: -21vdc - lead: .6vdc
C50: + lead: -7.1vdc - lead: -2.1vdc

[Perry Babin]

I'm assuming that the DC on the speaker output also went to near 0v.

Are you sure you had -21v DC on the capacitor? It appeared that you had the correct regulated voltage. If the caps are in the signal line, they could not have -21v (not unless there is a short circuit somewhere on the board).

Is it possible that you were looking at a capacitor on the gate leg of the muting transistor? I'd expect to see something near -21v there.

Did you confirm that you had continuity between the sec CT, the shields and the non-bridging speaker terminals?

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Yes, I confirmed the continuity between the sec CT, the shields and the non-bridging speaker terminals.

The DC on the speaker outputs did go to near 0v. when I shorted the transistor leads

I do have -21v DC on the + lead of the one capacitor.
These caps are right on the transistor drain (also pin5 of chip)
not the gate.

-7v on the other cap (??!!)..That's puzzling to me not having a scheme.

I did test the caps too

[Perry Babin]

I don't have anything specific to that amp but I have a couple of things that may be helpful. Email me and I'll send them to you.

babin_perry@yahoo.com

You need to check the output from each op-amp to see where the DC is entering the signal line.