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Commercial Gainclone kit- building instructions

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#641
Problem solved maybe

My amp still overheats and isn't right but I think I figured out the problem. I looked back at the instructions and I believe I have input and output wires wrong.

I have a picture and the black and white wires are input output; you can see the connection to the terminals for the output. I think I should move them to the Cz Rz locations

(I hope the picture link works it's my first try)

It l
An externally hosted image should be here but it was not working when we last tested it.
 
#642
Don't move anything to Cz, Rz, those pads are not being used.

It should be obvious that IN means input and OUT stands for output 😉

One other thing; the nuts on mounting screws are two big, while power plains are protected by solder mask, it is not enough isolation and you are risking shorting the rails to the chassis: use either plastic isolation washers or screws with small heads.
 
#643
Grounding RCA Jacks

The RCAs need to be isolated from a chassis. You are using what looks like Cardas RCAs, they should come with plastic isolation washers.
Click to expand...

I saw this earlier and noticed that my RCA input jacks clearly aren't isolated from the chassis. Actually I have them screwing fairly tightly into the chassis and there is no isolation washer. Could this be my

Are there isolated and unisolated RCA Jacks?

Tom
 
#644
Re: Grounding RCA Jacks

tbrooke said:

Are there isolated and unisolated RCA Jacks?
Click to expand...

Tom-
The isolated type come with plastic washers and require a larger hole.
Alternatively you can mount the non-isolated style on a plastic/phenolic plate, so they are electrically isolated from the rest of the chassis.

Cheers
John
 

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#649
More Troubleshooting (newby)

I can't figure out what to do next

So far I have

1. measured voltages at V+ PG+ V- PG- etc its about 35 volts

2. Checked DC Offset on output terminals - it varies between
.005 V .050V I have tried it with a 8 ohm load and I used a preamp to vary the input and I have even played music which does come out but unamplified and distorted.

3. Removed the bolts that held amp boards down and may have caused a short

see picture:
http://picasaweb.google.com/tom.brooke/Amp?authkey=Gv1sRgCPaAyKmCqN6wdQ#5331279046144343858

4. Insulated RCA input jacks.

Amp still get very hot, no sound except for static

I ran continuity checks with my multimeter and it seems almost everything may be shorted to ground but then again maybe this is the way it is supposed to be - it is ground - maybe continuity isn't the way to check for shorts

So
1. Anything else to check and how?
Visually soldering looks ok

2. Could I have damaged the LM3875? How do I check?

3. Anyone in NC that can check it?

4. Should I order a new kit and start from scratch?

Tom
 
#650
One other thing to check is if you place resistors in right places. I recall someone had sililar problems and it was because of mixed up resistors. You can find some pics here: http://www.diyaudio.com/forums/showthread.php?postid=1508848#post1508848

R1 which is 220R looks slightly different now, but the other two 22K should be the same.

If the chip shows offset, that may indicate it's not damaged.

If you won't be able to make it work, you can ship it back to me and I will fix it 😉


cyxg said:
I finished my 3875 chip amp, sound very good!
Click to expand...

Congratulations, it looks really good 😉
 
#651
I've been directed to this thread and I'm looking forward to ordering all my parts and starting the build.

All the prices look reasonable and then I came to the transformer, which was a nice kick in the guts.

So I searched around for options in Australia and can only find 2x18V 300VA Toroidal Transformers and 2x25V 300VA Toroidal Transformers.

My plan is to use this amp with 6ohm speakers, which transformer should I go for?

Also what is the difference between this transformer and http://www.airlinktransformers.com/transformer/0300222-open-style-with-leads.asp]this one[/URL].
I ask because in one build I've seen (with the airlink t'former) there was two used and in another build only one partsexpress piece was used.


Thanks,
Matt!
 
#652
I don't know the ambient temperature inside your home, but 25Vac and 6ohm is going to require a big heatsink particularly if the house cooling goes on strike.

22Vac or 20Vac would be a better match.

Are you up to adding turns to the 18Vac transformer?
 
#653
Peter Daniel said:
One other thing to check is if you place resistors in right places. I recall someone had sililar problems and it was because of mixed up resistors. You can find some pics here: http://www.diyaudio.com/forums/showthread.php?postid=1508848#post1508848

R1 which is 220R looks slightly different now, but the other two 22K should be the same.

If the chip shows offset, that may indicate it's not damaged.

If you won't be able to make it work, you can ship it back to me and I will fix it 😉




Congratulations, it looks really good 😉
Click to expand...

I'll check - the Rf reister is the one that can be mounted on LM3875 pins Right?

- I'll do some more checking and I may contact you by email about sending it up for you to look at - but I'm going to try a few more thiung and see if I can solve it myself

Thanks
Tom
 
#655
Peter Daniel said:
One other thing to check is if you place resistors in right places. I recall someone had sililar problems and it was because of mixed up resistors. You can find some pics here: http://www.diyaudio.com/forums/showthread.php?postid=1508848#post1508848

R1 which is 220R looks slightly different now, but the other two 22K should be the same.

If the chip shows offset, that may indicate it's not damaged.

If you won't be able to make it work, you can ship it back to me and I will fix it 😉
Click to expand...


Went back and checked resisters and the 220 was ok but the 22K's (sg) were 246 R and 265 R - not sure where I got them from they must have been in the kit - might check and see if Radio Shack has 22k or else digikey.

I believe that might be the problem
 
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