Definitive Technology Prosub 1000 Repair

[csalmon]

Hi looking for some repair advice from any of the Definitive Tech experts here. Was watching TV this morning and started hearing some strange noises coming from my Definitive Prosub 1000. It was kind of garbled static sound that seemed to follow the voices in the audio and also an underlying hissing sound. I tried unplugging and replugging and it was still there. Tried turning the crossover and when it was at 0 I could only hear the hiss but as I turned it up the garbled voice stuff became louder. I turned the gain to zero and the hiss was still there. I unplugged the RCA cable and the hiss was still there. BTW I am only using the LFE input, not using any of the speaker level stuff.

My assumption is a bad amp which seems to happen a lot with these but the amps are no longer available. At this point I either try to repair or throw it out. I have done a lot of small electronics repair in my past but am not sure where to start with this one (no service manual). I pulled the amp and inspected, not seeing any evidence of a ruptured electrolytic cap but that doesn't mean there isn't one that is bad.

Any help is greatly appreciated, I love the sub and hate to have to get rid of it.
Thanks

 

[ramondo]

Try injection of audio at input of stk4050 (if you have this) Close schematic I hope.

 

[Nigel Goodwin]

As he's getting audio through anyway, injecting probably isn't a good choice?.

Assuming that diagram is correct?, shorting out R86 or removing C37, would remove audio input to the power IC - thus identifying if the problem is output stage, or pre-amp stage - and requires no test equipment at all. However, it's would essential to confirm the circuit is correct, particularly the circuit references.

It's fairly bad news if it does use an STK chip though, as they are quite unreliable, and many (most?) of them are long obsolete.

It could be worth resoldering all the pins of the STK, as dry joints on them are quite common.

 

[ramondo]

As he's getting audio through anyway, injecting probably isn't a good choice?.

Assuming that diagram is correct?, shorting out R86 or removing C37, would remove audio input to the power IC - thus identifying if the problem is output stage, or pre-amp stage - and requires no test equipment at all. However, it's would essential to confirm the circuit is correct, particularly the circuit references.

It's fairly bad news if it does use an STK chip though, as they are quite unreliable, and many (most?) of them are long obsolete.

It could be worth resoldering all the pins of the STK, as dry joints on them are quite common.

I actually meant to say inject a signal into the output stage just to narrow down the scope of the investigation. "Assumed' the OP had some repair skills.

Also this schematic may not be anywhere similar to the DT 1000 unit. Something is better than nothing though.

Chinese STK replacements are NOT reliable.

 

[csalmon]

Thanks for the quick response and advice. I have requested a repair manual/schematic from Definitive and they have forwarded it to their engineering group for response so hopefully that will help. I plan on pulling the amp again tomorrow morning on my bench and will investigate if there is a STK or not and will try pulling the equivalent C37 if it is there. Will report back what I find. Thanks!

 

[csalmon]

Opened up the sub again to have another look at it. Snapped a couple general pics of the amp. Not seeing any kind of STK chip but there are a couple heatsinked FETs and some large capacitors. I am still waiting to hear back from Definitive about the service book or schematic, they opened a case for me about it. It is going to take more disassembly just to get to where I can even see anything in there.

 

[Nigel Goodwin]

The same still applies, an STK is just a hybrid thick film circuit, essentially individual components mounted on a ceramic PCB - so there will still be a capacitor feeding the input of the power amp, but without a schematic it's impossible to say which one it might be, without actually having the amplifier in your hands, and not that easy them.

 

[ramondo]

If by chance DT tech support does provide a schematic or service manual,, would you be kind enough to upload it here.

Thank You.

 

[csalmon]

Yes of course!

 

[Nigel Goodwin]

If by chance DT tech support does provide a schematic or service manual,, would you be kind enough to upload it here.

Good idea!

 

[unclejed613]

when i worked in a service center repairing these, the schematics were not available from DefTech. the only way to repair a bad amp was to replace the whole back plate assembly. the actual amplifier is the vertical circuit board next to the heat sink, and the output MOSFETS are on the heatsink. it is a class-D amp. the problem you are describing was very common, and it's something getting flaky in the modulator.

 

[csalmon]

Thank you so much for the response. Explains why there are so many of these on eBAY being sold as "damaged, for parts only". I had gotten the boards all disassembled and was figuring that vertical board was the amp (wow they use a lot of glue on everything in there, what a pain). There were two small capacitors and inductor clustered together on the amp board that look questionable but I need to find time to remove those and test them. Although based on what you said the problem may be more systemic than just bad amp board parts.

I never did get a schematic so that now makes sense too, best they did was offer me 20% off a new one, not a great deal really.

 

[unclejed613]

wow they use a lot of glue on everything in there, what a pain

that's because it's inside a subwoofer where everything shakes around

 

[bramert]

FYI...I have purchased two "new in box" Prosub 1000 amps off eBay...both DOA. They power up, but no signal. Be wary.

 

[crutschow]

You might also consider a new plate subwoofer amp, such as from here.