Earthquake cinenova7 worth repairing?

[ollieimpossible]

We had to pull this Earthquake Cinenova7 out of one of our clients houses because the amp boards kept on failing. It seems to be the same problem on every board. It looks to be that the transistors at location q111 and q112 have burned out. Earthquake won’t service this amp anymore so I’m wondering if it’s worth repairing. The only weird thing I have found is the voltage on the secondary is a bit higher than the rated voltage written on the transformer(44v vs 39v). Our mains voltage out here is around 120 if that makes much difference.
Any help/education would be greatly appreciated as I’m new to amplifier tech.

 

[ubergeeknz]

So every board has the same transistors blown?

If so, I’d be looking at the power supply first

 

[wiseoldtech]

Sadly, that's the way they build the crap these days.

 

[carlthess40]

Do you want to do a [emoji91] sale on it? Lol

 

[ollieimpossible]

I don’t have a lot of understanding on how it works so I may be way off on this. The amp has a dual torodial power supply with 4 secondary’s off of each so that is 8 secondary lines powering each amplifier board separately. I measured 44v off of the secondary but the rating written on the transformer says it should be at 39v. As far as I understand if the rectifier portion of the amp is getting too much voltage it will pass on that overage into the rest of the board. I don’t have any specs on the boards themselves so I don’t know the voltage tolerances or if that’s even the place I should be looking.

 

[Douglas Blake]

I don’t have a lot of understanding on how it works so I may be way off on this. The amp has a dual torodial power supply with 4 secondary’s off of each so that is 8 secondary lines powering each amplifier board separately. I measured 44v off of the secondary but the rating written on the transformer says it should be at 39v. As far as I understand if the rectifier portion of the amp is getting too much voltage it will pass on that overage into the rest of the board. I don’t have any specs on the boards themselves so I don’t know the voltage tolerances or if that’s even the place I should be looking.

44v RMS from the secondary should give you about 62 volts DC.
39v RMS would give you 55.

With a center tap that's 7 volts on each rail for a total of 14 volts on the bulk lines... Yep, that could be damaging.

If this is a recurrent issue with boards going dead on you, I'd say you got a real nice box of rescueable parts there....

 

[Mark Tillotson]

There seems to be a design error on those boards - I suspect Q111/Q112 are cascodes for the VAS/CCS circuit, and although a larger package SMT device was chosen, it has no extra copper pour for a heatsink - they've used all the spare board area for the high current traces on the output transistors, and let the VAS devices cook.

The higher voltages will not have helped, but they would have run too hot even so.


Some stick-on heatsinks on top of the offending transistors may help a bit, adding some cancelling secondary windings to reduce PS voltage is another option.

 

[ollieimpossible]

There seems to be a design error on those boards - I suspect Q111/Q112 are cascodes for the VAS/CCS circuit, and although a larger package SMT device was chosen, it has no extra copper pour for a heatsink - they've used all the spare board area for the high current traces on the output transistors, and let the VAS devices cook.

The higher voltages will not have helped, but they would have run too hot even so.


Some stick-on heatsinks on top of the offending transistors may help a bit, adding some cancelling secondary windings to reduce PS voltage is another option.

Since I have this amp with zero investment would it be worth toying around with? I have no problem replacing those transistors and reducing the incoming voltage. I'm wondering if there may be more problems on the board that I cant see.

 

[Mark Tillotson]

Looks worth trying. Another idea: you could also pick a small chipset fan (they are close to silent esp. if run at lower voltage) to help cool the relevant part(s) of the pcb.

 

[Phloodpants]

39V rating on the transformer is going to be with some load applied. 44V with no load seems normal.