Amp Camp Pre+Headphone Amp - ACP+

[2 picoDumbs]

Is C109 (or is that R115) and R109 unintentionally solder bridged together?

Component numbering is all over the place

 

[dBel84]

the R116 is an R0 jumper

The open solder holes you see are for the heatsinks

 

[2 picoDumbs]

Measure resistance across gate and drain pins

 

[dBel84]

Is C109 (or is that R115) and R109 unintentionally solder bridged together?

Component numbering is all over the place

yes - component numbering on the board is not always apparent.

I think you are looking at R115 - R116 ( I had tried not to have them bridge, but they are linked anyway , so probably does not impact the circuit )

I have attached the schematic so that these part numbers make more sense

Resistance across gate and drain is as follows:

upper mosfet - V111 ~ both boards around 4K
lower mosfet - V112 ~ slowly increases ( starting at about 1K5 ) after 3 min 45K

 

[2 picoDumbs]

Measure resistance aross pin 1 and pin 2 of the opto

 

[xrk971]

Hi Dbel84,
Sorry to see you are having trouble with this board. I seem to to recall you said it used to work but the bias current was too high so it was running hot. Then you tried to change the output source resistors (per my recommendation) and that’s when it started smoking. Did it ever work? I can’t make out the colors on R117 but is that 6R8? Also what do you have on R105 it’s unlabeled? It should be 200R.

Make sure R116 is not shorted to the trace beneath the solder mask that travels beneath it. A 0R resistor is insulated across its body, whereas the wire jumper is not.

Here is a photo of my working amp. It seems to compare well with yours except for the two I indicated as not clear. I put your photo side by side to compare.

Here is the schematic for easy reference:

 

[dBel84]

Measure resistance aross pin 1 and pin 2 of the opto

113R

Hi Dbel84,
Sorry to see you are having trouble with this board. I seem to to recall you said it used to work but the bias current was too high so it was running hot. Then you tried to change the output source resistors (per my recommendation) and that’s when it started smoking. Did it ever work? I can’t make out the colors on R117 but is that 6R8? Also what do you have on R105 it’s unlabeled? It should be 200R.

Make sure R116 is not shorted to the trace beneath the solder mask that travels beneath it. A 0R resistor is insulated across its body, whereas the wire jumper is not.

I think it had trouble before which is why it was running so hot , it also pulled the voltage down further than I had anticipated which would make sense if it is was draining close to 2 amps.

R105 is indeed 200R, I changed R117 to 10R. I wonder if it is as simple as the wire jumper. The 0R resistors I bought were too long to fit an I could not use them which is why I substituted the wire.

I will correct this as a next try

Thanks ..dB

 

[xrk971]

2A is a short it sounds like.

Probably not this as it’s affecting both boards. An you check for solder bridges here?

 

[2 picoDumbs]

Measure resistance between pin 2 and 5

If that checks out consider changing R111 back to 1k5 (since you have now adjusted R115 and R117 to higher values)

The opto could be current starved. This could be your problem.

 

[2 picoDumbs]

One more thing to check.

Is C4 installed in the correct orientation?

 

[2 picoDumbs]

Measure resistance from drain of V112 and pin 5 of opto.

Just to be sure the previous measurement between pin 2 and pin 5 is the same.

 

[dBel84]

Pin 2 to 5 on opto is zero (that is the wire bridge), as is drain to 5.

C4 is correctly oriented.

No change in behaviour after ensuring wire bridge does not short and switching R111(on this board) to 1K5

Edit - I lied 4.3V, just a probe connection issue

Ignore below
I tried to check the gate source differential on V112 and I do not see anything before things start cooking. Wondering if the problem is the front end of the circuit

 

[2 picoDumbs]

Thinking

 

[2 picoDumbs]

Measure resistance across R107 and R108

If it's possible power it up then measure voltage across R107 and R108

 

[2 picoDumbs]

I am thinking you might have killed the mosfets the first time the resistors got really hot.

Try replacing them.

Either you have an opto control problem or a mosfet issue.

 

[2 picoDumbs]

consider changing R111 back to 1k5 (since you have now adjusted R115 and R117 to higher values)

The opto could be current starved. This could be your problem.

Try this first.

Then replace mosfets, if that doesn't fix it.

 

[2 picoDumbs]

If you can turn it on measure Voltage across gate and source of both V111 and V112

 

[dBel84]

If there is an equivalent "dumb" award to the Darwin series, please nominate me. I very nearly quietly disappeared and hid my shame. I was rifling through my parts bin to replace the mosfets and I kept finding p channel fets of all variations and the possibility of me using p channel fets instead of n channel slowly began gnawing at my conscience.

I don't think I have ever felt so silly, I knew it was going to be a epic fail on my part and now we know what.

With n channel fets in place it fires up as expected, I now have 100 MA of bias and heatsinks are sitting at a healthy 40deg.


Thank you all for offering to help and sacrificing valuable time for my poor eyesight and failure to make absolutely sure I had the correct parts.

Thanks Nelson for keeping me in mischief..dB

 

[2 picoDumbs]

No Worries.

Awesome.

Thanks for reporting back.

 

[dBel84]

Thank you again. Now onto finishing the project without more foolishness