Folsom's great little 7297 Chip Amp

Folsom · « **Reply #640 on:** 16 Oct 2016, 06:37 pm »

Do you have the shoulder washer correctly oriented? You only need the one washer, that prevents contact from the bolt to the tab on the chip. The Mica should insulate the rest of the chip from the heatsink.

It sounds like something is touching, so you may need take a closer look, or nudge the chip.

slefley · « **Reply #641 on:** 16 Oct 2016, 07:53 pm »

I have one each new/unused circuit board for the AMP and PSU that I will not be using, along with all of the documentation that Folsom provided. I'll sell them for the same price I paid: $24 Amp PCB, $21 PSU PCB and $8 Shipping priority USA ($55 total thru PayPal).

lacro · « **Reply #642 on:** 16 Oct 2016, 08:10 pm »

Quote from: KR500 on 16 Oct 2016, 04:37 pm

Folsom , I have an insulating washer on the bolt going through the chip, but thought I had to have another insulating washer on the backside of the chip ? . i.e. one on each side of the chip .
Is there just the one washer ? Just want to make sure before plugging in the unit.I made sure not to over tighten the bolt into the heat sink.
I put the DMM probe on the chip and one on the heat sink and get all zeroes at the resistance settings until I set the resistance dial down to the lowest 200 ohm setting, then I get 00.2 on the readout , same 00.2 with probes on the bolt head and heat sink
Continuity check with the probes on chip and heat sink gets a zero readout
thanks

That just doesn't make any sense; it looks like you have the washer (only one is needed) inserted in the chip's hole. If the washers shoulder (wider part) is oriented toward the amp which it looks like it is, there is no way you can have continuity between the screw and the chip unless the mica is not positioned correctly, thus allowing the chip to contact the heat sink. Are you using electrically non-conductive thermal grease?

Folsom · « **Reply #643 on:** 16 Oct 2016, 08:45 pm »

Sometimes if the bolt is at a little bit of an angle or oritented too far in one direction it can make contact with the tab.

You can also use a drill bit by hand to reduce the tab's edge that it toward the heatsink. You don't want to go all the way through since it would cause issues with the shoulder washer, but a little bit can be enough to help you not make contact with the bolt.

Another option is to drill a bigger hole into the heatsink about 2mm, not enough to ruin the threading for the bolt, and use a shoulder washer that sticks out the other-side a little bit so it's very impossible to make accidental contact.

KR500 · « **Reply #644 on:** 16 Oct 2016, 09:25 pm »

I slid the washer on to the bolt wider surface first before inserting bolt into the chip tab hole . Just now I loosened the bolt and moved the chip around but am still getting zero readings on the meter continuity test.
When I initially soldered the chip to the pcb and attached the pcb with chip to the heat sink I got good DMM readings. I had slid washers on both sides of the chip at that tryout for fitting to the sink . Maybe the chip is too tight to the sink now ?
I'll take the PCB off and reattach and see what meter readings are.
Thanks for your advice

zek · « **Reply #645 on:** 16 Oct 2016, 09:54 pm »

Is your washer looks like this

KR500 · « **Reply #646 on:** 16 Oct 2016, 10:14 pm »

yes , the insulating shoulder washer at mouser p/n 532-7721-7PPS

Folsom · « **Reply #647 on:** 16 Oct 2016, 11:27 pm »

Everything looks good, maybe I just don't understand what your readings are? You should get no reading at all, ideally.

lacro · « **Reply #648 on:** 17 Oct 2016, 02:44 pm »

Quote from: KR500 on 16 Oct 2016, 09:25 pm

I slid the washer on to the bolt wider surface first before inserting bolt into the chip tab hole . Just now I loosened the bolt and moved the chip around but am still getting zero readings on the meter continuity test.
When I initially soldered the chip to the pcb and attached the pcb with chip to the heat sink I got good DMM readings. I had slid washers on both sides of the chip at that tryout for fitting to the sink . Maybe the chip is too tight to the sink now ?
I'll take the PCB off and reattach and see what meter readings are.
Thanks for your advice

From the shoulder washer spec sheet, the recommended tightening torque for the screw is 4 inch-pounds which is not much. If you over tightened the screw, maybe the washer is broken? Do you have more shoulder washers? I just drilled/tapped mine yesterday, and doing a dry fit without thermal grease I am not getting continuity between chip and heat sink. The washer and mica are doing their job. I am using an M3 screw, what size are you using?

KR500 · « **Reply #649 on:** 17 Oct 2016, 05:38 pm »

Yesterday I removed the pcb/mica from the chip and inspected the washer , all parts look good with washer intact and chip aligned to sink.
I cleaned the sink / the mica then re-installed. I've been careful not to over torque the bolt/washer when installing .
Using a 4-40 1/2 inch bolt and Radio Shack Silicone Heat Sink grease.
Setting the DMM to continuity test with probes touching nothing results in a reading of "1" open circuit .
As soon as I touch one probe to sink and one to the metal part of the chip I get a reading of 000 . No decimal point.
Same 000 reading when touching the sink and chip bolt head with the probes
If I set the resistance to the lowest 200 Ohm setting on the meter I get a reading of 00.3 on the display when touching the probes to sink and chip tab.
Higher resistance settings display 000
I touched the probe to heat sink and then each chip leg resulting in a reading of 000 on outer legs and positive numbers on inner legs of the chip.
New battery in the DMM ( Sears Roebuck model , not the cheapest one )
Maybe Folsom is correct about the readings being a problem here ?
thanks for your continued help. I want to plug this amp in but don't want to fry the chip.

BRN · « **Reply #650 on:** 17 Oct 2016, 07:23 pm »

Does you DMM have a continuity tester? If so I would use it to check and see if the chip is isolated from the heat sink.

KR500 · « **Reply #651 on:** 17 Oct 2016, 07:24 pm »

I removed the bolt and moved the pcb/chip a quarter inch from the heat sink so it is not touching, and still get a 000 reading with probes on the chip tab and heat sink in the DMM continuity test setting
Visual inspection of my pcb solder joints don't show any issues that I can see

Folsom · « **Reply #652 on:** 17 Oct 2016, 07:30 pm »

It's the 200ohm setting that bothers me. If you have .3ohm you've got continuity. Does it read .3ohms on that setting when they're physically separated?

If you turned it on you'd just have RF, nothing would be hurt, so long as it was mounted to the heatsink.

The most likely thing is the bolt simply is touching the edge of the chip as it leaves the hole. That's why I suggested remove a little material from the heatsink side of the tab, right at the hole. I wouldn't use a drill though unless you spin it backwards. You can even use a razor blade to scrape some material away.

KR500 · « **Reply #653 on:** 17 Oct 2016, 07:53 pm »

when the chip and sink are separated the 200 ohm setting readout goes up to 00.7

Folsom · « **Reply #654 on:** 17 Oct 2016, 08:48 pm »

I suggest you put it back together. Turn the amp on. If you hear a radio station the meter was accurate, if not, you're all good.

BRN · « **Reply #655 on:** 17 Oct 2016, 09:04 pm »

You could try and cover the screw close to the head with just enough electrical tape to keep the screw from touching the chip.

Folsom · « **Reply #656 on:** 17 Oct 2016, 09:09 pm »

My experience with trying stuff like that is akin to stubbing one's toe.

BRN · « **Reply #657 on:** 17 Oct 2016, 09:52 pm »

I agree with that.

Could it be the thermal paste is conductive?

Folsom · « **Reply #658 on:** 17 Oct 2016, 09:59 pm »

I think the meter is stupid, so to speak. Sometimes they just don't understand what they're doing.

You wouldn't hurt the chip at all without the mica or washer, you'd just be listening to the local radio station quietly between tracks.

KR500 · « **Reply #659 on:** 17 Oct 2016, 10:33 pm »

Thanks guys
Tomorrow a.m. I'll hook it back up .....I'm also thinking the Craftsman meter is muy stupido .
Looked over my solder connections again with a lighted magnifying visor from HF and though they are not things of beauty, they are also not a mud fence . In other words , looked okay, didn't see any runs drips or errors.