Identifying Part from Schematic

[budm]

Re: Identifying Part from Schematic

8 or 4 Ohms with 100mV? barely any concern, most speaker protection circuits are not trip until 2V.
Check the offset when first power applied and then test it again after an hour to see how much it drifted off.
0V is perfect.
The circuit being used has no Idling Bias Adj, no Offset ADJ, so it is a cheap way of building thing if they expect all the parts to be perfectly matched, they are using 5% resistors, not hand matched transistors, no real protection for speaker in case of DC dump to the speaker so do not expect too much, it is also sub woofer after all.
You can add offset trimpot circuit or if you wan to get real fancy you can then use Active Servo Offset circuits.
BTW if that C1 DC BLOCKING/COUPLING cap has any leakage resistance that will allow the DC to pass through from the other stage, that will also cause the DC to show at the Output. How equal are the power supplies +/- Voltage?

[mikay786]

Re: Identifying Part from Schematic

I though sub 50mv was good, 50-100mv was a concern and anything above 100mv was dangerous?

Yes there is no trim pot, I'll check dm later but is pretty low from memory. Definitely a leaky transistor so still best to sort out.

[budm]

Re: Identifying Part from Schematic

0.135V is really nothing to worry about, I guess there is no BALANCE/centering pot the set the 0V at the output, even with the trimpot, it can still drift off the center point.
What does your meter show when you touch the two probes together.

[mikay786]

Re: Identifying Part from Schematic

Found the problem with high DC offset

Changed over quite a few transistors as were fairly cheap including 5 x C1815, 1 x A1015, 2 x A92, which didn't fix it.

There were also 4 x C2240 which hadn't arrived yet so for the time being swapped them around and DC-Offset has dropped to 38mv. So just got to wait for them to arrive and will change the lot. The one that was causing issues was the one near the blown resistor so maybe got damaged or perhaps that's what caused the resistor to blow?

[mikay786]

Re: Identifying Part from Schematic

Got the amp all back together and powered up and yay seems to be working. Not connected it to the subwoofers yet.

I measured DC offset and its very high at 135mv, what can be causing this?

It had a full recap including main filter caps.

[mikay786]

Re: Identifying Part from Schematic

Guys is it ok to solder wires from transformer primary to snipped off cables from AC socket?

I've already done it but worrying that may not have been the best way and should have removed old wires and soldering the primary wires direct to the AC socket terminals. I did heat shrink the soldered cables and have wrapped it all up with insulation tape.

[mikay786]

Re: Identifying Part from Schematic

Other one had wires, 15mm should be fine.

On one it's connected from fuse to one of the ac socket pins and the other between ground and Earth on the chassis. Why would you need a cap between ground pin and chassis?

[budm]

Re: Identifying Part from Schematic

They are not bad but when I removed old transformer I cut the leads a bit too short so not enough room to solder back in.

So the one I linked to is fine then?

Yes. It has 15mm lead spacing like the original one, right?

[mikay786]

Re: Identifying Part from Schematic

They are not bad but when I removed old transformer I cut the leads a bit too short so not enough room to solder back in.

So the one I linked to is fine then?

[budm]

Re: Identifying Part from Schematic

It is X1 type safety film cap, 0.1uF 300VAC for using between Line and Neutral. Is it bad? You should have no problem getting one.

[mikay786]

Re: Identifying Part from Schematic

Quick question, is this a regular film cap or a suppression film cap?




Looking to buy this to replace - http://uk.farnell.com/epcos/b32912a3...rad/dp/2469104 which is a suppression one.

[mikay786]

Re: Identifying Part from Schematic

I told them what you said so they had me send in the old transformer and they reverse engineered it to original spec. I've got the replacement in a couple of days ago so will get it fitted as soon as I get time. Will need some help when reinstalling as the colours don't match on new transformer!

[budm]

Re: Identifying Part from Schematic

@55V there is not much room for error, especially if the line Voltage goes up, caps are only rated at 80VDC (55V *1.414 = about 78Vdc) so the circuit probably not designed for Voltage that high.
I would use 40V-0-40V.

[mikay786]

Re: Identifying Part from Schematic

360w RMS

Uses 2 x 12" Drivers, class A-B Amplifer

Will it being overpowered be harmful, or can we just class it as having room to spare?

[budm]

Re: Identifying Part from Schematic

https://www.badcaps.net/forum/showpo...56&postcount=9

I still have doubt that it should be 55V, and 55V*8A*2 will be much higher than 600VA.
BTW, what is the output rating of this sub woofer?

[mikay786]

Re: Identifying Part from Schematic

So just spoke to a few transformer places and one saying roughly around £150 for a rewind on a laminated unit.

The 2nd place I called (http://www.jmstransformers.co.uk/pages/rewinds.php) are saying they can build a new 600va one for around £90 and a rewind wouldn't be much cheaper than a new one. I'm ok to spend upto a £100 so that sounds ok to me?

He wants me to send over specs and photos

600VA Shielded Transformer

Dual Primary (4 cables (1 split with thermal fuse) (115-230v)

6 Output Cables
2 x 55ac (8A)
2 x 12vc (2A)
2 x GND

Is that ok for specs, or anything I'm missing that they would require?

[budm]

Re: Identifying Part from Schematic

Yes. But you should try to locate the original one first, I will not use two transformers in customer set or sell it using two transformers.

[mikay786]

Re: Identifying Part from Schematic

So will that be for two seperate transformers?

[budm]

Re: Identifying Part from Schematic

The smaller transformer 12-0-12 (24V center tap) 2 A will be fine.
The larger one 40-0-40, 7~8A will be OK. 40V will give DCV on the filter cap about 56VDC (40V * 1.414), in case if the line Voltage goes up by 20%, the Voltage on the cap will be about 67V (48V *1.414) , the filter cap is rated at 80V so you will still have some head room left.

[mikay786]

Re: Identifying Part from Schematic

I'll check for you tommorrow Paul.

What options do I have for this transformer, I'm willing to put £100 towards a transformer.

Is a repair feasible, professional rewind?