Need help, finished Crack, resistance readings not aligned with manual[resolved]

[rivendell86]

Hi everyone (anyone), this is my first DIY electronics build and first time soldering. The initial glow test passed. Finished the rest today, turned to the resistance check page and now I'm at a loss. If anyone can please help, I would be so grateful.

With black lead on terminal 12U (as instructed) I only seem to get resistance readings on terminal 12, 14, and 20 using the the DMM red lead. All other terminals seem to emit no signal to the DMM.

Let me know if you'd need to see more photos, or want photos of specific terminals for more intel.

Thank you, very much appreciated.

-Max

[rivendell86]

Forgot to mention my readings are 0.1 to 0.3 on said terminals, in case that is significant.

[Deke609]

Hi Max - I suspect your troubles are really a function of your measurement apparatus.

That alligator clip looks way too big to me - I'd be afraid of it contacting an adjacent terminal or wire. If I had to use it, I'd use it to connect the black lead to a safer ground spot: the ground lug bolted to the chassis near the iec (power plug socket) -- NOT the ground terminal on the IEC itself, - that's too close to live terminals when you do volatge tests. I'm pretty sure that 12U is connected (ultimately) to chassis ground (and so is the ground lug near the IEC). You can test this by measuring resistance between the ground lug and 12U: if you get 0 or 0.1/0.2 ohms or so, 12U and the solder lug are connected and you can use the solder lug as a (safer/easier) stand in for 12U.

YOu might also want to test the senstivity/accuracy of your meter and leads by testing the resistance of a short piece of scrap wire: just strip the ends a bit, put one lead on each end and see what you get: that is your "0 ohms" reading.

I've tried to use a similar type alligator clip in the past the same way you have, and it didn't work so well. I've found that those test lead points require a minimum amount of pressure to make a good contact (not a lot, but some), and just letting the tip sit in the barrel of the alligator clip didn;t really work for me.  You could try crimping the barrel a bit to get a tighter fit. 

But I avoid using those long pointed tips for testing wherever I can. I mostly use leads with spring loaded grabber hooks that snap around the target terminal or wire, allowing me to keeps my hands away from live wires.  The next best thing (IMO) is the alligator lead attachments specifically made for your test leads - the slip tightly over the points and are well covered with insulating plastic to prevent accidental contacts. And if I absolutely have to use points (e.g., on some pcb spots), I use te plastic covers that usually come with point leads that leave only a few mm of metal tip exposed. 

By the way: for a first time diy soldering project - the amp looks really good. I toasted a lot more wire insulation in my first few goes. 

cheers, Derek

[Paul Birkeland]

Well, you have some ground indications back in the power supply, which is great.  I'm also happy that you didn't turn it on

There are some black wires that meet at the hedaphone jack, what's the resistance there?  If it's not 0 (or close to it, 0.1-0.3 may as well be 0 in this context), then I would try tugging on each of those black wires to see if perhaps one is not soldered. 

Terminal 12 on the power transformer looks like it might not have been soldered, but that would be completely unrelated to what brought you here.

-PB

[rivendell86]

Thanks Derek, and PB, for your kind words, and quick and thorough responses.   

Derek, ok, I will look into obtaining smaller clips and sheathing the clips again. You're right, I did have to 'crimp the barrel' a bit to get the clips to stay on the leads. I removed the plastic sheaths because it made them even more obtrusive and hard to use, but I'll be safer going forward, especially when plugged in.

I have two DMM's that I've been using to double check things. (Had to order the second one because I blew out a fuse? in the DMM testing voltage on the first check; been a long time since I've used a DMM and kind of just spun that dial around, heard a pop and screen is blank when I test voltage) Anyway, resistance seems to work on both. On your suggested single scrap wire test, one DMM reads 0.1, the other reads 0.0. When testing the terminals, I'm getting the same results,  from both, as stated below, apart from that decimal difference from the respective DMM's.

PB, I resoldered the terminals you pointed out; they were done but indeed hard to see from the picture angle, and maybe undersoldered in general, so went ahead for assurance and clarity in future pics. That lower terminal on the jack where the black wires meet then measured 0.1 (and 0.0).


The following measurements were taken with the black lead alligator clipped to the ground lug near the IEC...

Terminals that should read 0 ohms according to manual:
3,6,8,10,12,14,20, and 22
They all read 0.1 (and 0.0)

Terminals that should read *  or 2.9Kohms according the manual:
They don't return a reading (zero)

RCA ground tabs read 0.1
RCA center pins don't return a reading (zero)

Let me know if anything else come to mind to try, or if additional information or pics would be helpful to continue assessing the issue here.

Thank you,

Max

[Deke609]

Zero or "OL", which stands for Over Limit?

[rivendell86]

Yes, OL, my mistake, thank you. OL is also the default when the black alligator is on the ground lug, and red is not connected to anything.

[Paul Birkeland]

Before you didn't have continuity between terminal 12 and terminal 3, but now it seems that you do.  Either the resoldering of the black wires on the headphone jack worked, or you haven't found the loose wire yet.  I'd go through and tug on each black wire with your needlenose pliers on each end to be sure they are well connected before powering up the amp for the first time.

[Deke609]

Max - you're most likely already doing this, but just in case you're not: I'd give the meter a few seconds to reach a reading. I'm not sure how an autoranging meter work, but I suspect that it needs to cycle through its internal settings one at a time - so it might take a second or two or more to read higher values (assuming it starts checking for low resistances - no idea if they actually do).

cheers, Derek

[rivendell86]

Well...

looks like somewhere along the way of rechecking and resoldering points in the last hour, and taking a bit more time with placement of DMM leads, everything now looks good and resistances measurements are all in line with the manual figures.

Thanks again you guys, for getting me back on track.

Cheers to this great community,

Max

[Deluk]

"kind of just spun that dial around, heard a pop and screen is blank when I test voltage"
Maybe you did or maybe you didn't, but don't change the DMM dial when the probes are connected.