Alpine MRV-F300S

[Dr Zeus]

Got this amp in for repair. I noticed some connection problems under the PCB for the RCA and fixed. Upon power applicaiton, the amp appears a slight attept at powering up but fails and powers off no more than 1/4 second later. Measuring with the scope I see the amp pulse a +10v wave at the fets. I can also observe a very brief ~+-60vDC on the center legs of the rectifiers. I'm guessing this amp is tethering in/out of protection as several pins of the TL494 pulse every ~4 seconds

 

[Perry Babin]

Do you see a pulse of DC on the bridging speaker terminals of any channel as the amp goes in/out of protection?

 

[Dr Zeus]

There is something wrong with chanel 2 (This is a 4 channel amp) where I can observe a +-20v severely clipped sine wave with no signal input to the amplifier. The outputs for channel 2 are Q211 and Q212 and they are not measuring shorted.

 

[Dr Zeus]

i just found C205, a yellow mylar cap near some of the opamps with a bad solder joint which I just fixed. Now I cannot get channel 2 to pulse anything on the scope more than just insignificant DC. All channels have +-.2vDC of pulsing activity on them. Amp still power cycles like before.

 

[Dr Zeus]

TL494

Pin 1: 0
Pin 2: 0.882
Pin 3: 0.090
Pin 4: 4.97 (Drops to ~2.5-3v every ~4 seconds)
Pin 5: 1.684
Pin 6: 3.738
Pin 7: 0
Pin 8: 13.74
Pin 9: ~0 (Pulses to .5v)
Pin 10: ~0 (Pulses to .5v)
Pin 11: 13.74
Pin 12: 13.74
Pin 13: 5.05
Pin 14: 5.05
Pin 15: 5.05
Pin 16: 0.001

 

[Dr Zeus]

When pulsing, the center leg of Q509 (a 5KF10P20FR) hits -40vDC momentarilly. On the other hand, Q508 (a 5JF10P20F) only hits about 12vDC before rocketting back to 0v.

 

[Dr Zeus]

Do you see a pulse of DC on the bridging speaker terminals of any channel as the amp goes in/out of protection?

Referencing amp general ground pulse of -15vDC is on all speaker terminals. Referencing RCA shields I see no more than about -0.150vDC on speaker terminals. Referencing non-bridging speaker terminals or RCA shields I see +-20vDC on the center legs of the main rectifiers; D508 D509. Also I meant to say D508/D509 in my previous post, not Q.

 

[Perry Babin]

Can you post a photo of the inside of the amp. I have no information on this amp.

In general, there is an isolation between the primary and secondary grounds. When the secondary ground is pulled away from the primary ground, the problem is often a protection circuit component. Typically it's a PNP transistor with it's emitter connected to the positive rail (or sometimes to 5v) and it's collector connected to something near the power supply IC. You may have a problem like this.

 

[Dr Zeus]

I like the "army" of opamps.

 

[Dr Zeus]

Heres some pictures of the back of the PCB where identifications are easier to read.

 

[Perry Babin]

Try pulling Q512. Be careful. The amp could draw excessive current. If it doesn't, check all channels to see if any has a problem.

Post what you find.

 

[Dr Zeus]

With Q512 removed the amp powers and does not draw excessivly. I notice +-26volts at the rail. Voltage is still offset referencing CT and general ground at +20vDC.

The amp is able to produce clean audio with a test speaker.

Q512 is measuring fine off the PCB.

 

[Dr Zeus]

Interesting to see the power supply is no longer operating regulated. Square wave coming off the drain, throgh the transformer, and to the rectifiers.

 

[Perry Babin]

What happens if you ground the secondary CT?

Be careful...

 

[Dr Zeus]

I thought sparks would fly, but they didnt Instead, the amp just seems to accept grounding CT to commong ground. Rail voltage referencing amp ground in that state is +-26vDC.

This amp has a VR501 pot which doesnt appear to do anything.

 

[Dr Zeus]

In general, there is an isolation between the primary and secondary grounds. When the secondary ground is pulled away from the primary ground, the problem is often a protection circuit component. Typically it's a PNP transistor with it's emitter connected to the positive rail (or sometimes to 5v) and it's collector connected to something near the power supply IC. You may have a problem like this.

I found Q510 to be one of the only transistors connected directly to the positive rail, and its an A1266 -PNP. The odd thing about it is, under power all four legs measure +6vDC which is the same as positive rail voltage. removed, the rail voltage offset shifts by +6v but it not enough. The A1266 off the PCB measures just fine so I put it back on.