Ok here goes, I have to replace the plug (and possibly the power transformer) as the old cord frayed right by the plug and shorted on me. It was a very brief spark as I had my finger on a switch for the power receptacle and my ups also broke the connection so the transformer may have survived.
But now I need to decide if a polarized 2 prong or a iec 3 prong should be the replacement, I asked this question years ago on another forum and got dozens of conflicting answers, though at the time I didn't have a schematic for this amp now I do. So I will attach it.
Would some kind soul take a look and tell me if an iec is suitable for this amp?
I know how to install either but I was told that grounding some vintage amp chassis could cause problems with ground loops. So that's why I'm asking, and one last thing I did add a y2 safety cap to this amp from the aux socket to chassis because it was suggested as a modification years ago.
Thanks for any and all help with this.
But now I need to decide if a polarized 2 prong or a iec 3 prong should be the replacement, I asked this question years ago on another forum and got dozens of conflicting answers, though at the time I didn't have a schematic for this amp now I do. So I will attach it.
Would some kind soul take a look and tell me if an iec is suitable for this amp?
I know how to install either but I was told that grounding some vintage amp chassis could cause problems with ground loops. So that's why I'm asking, and one last thing I did add a y2 safety cap to this amp from the aux socket to chassis because it was suggested as a modification years ago.
Thanks for any and all help with this.
There's a good reason that's called a "death cap". Ditch it.
The power transformer is probably ok, from your description.
Also add an AC line fuse on the primary winding before going any farther.
Try a 3AG type 3A slow blow.
Looks like the "ground bus" is only connected to the chassis near the inputs.
A three pin IEC cord, with the third "safety" pin connected directly to the chassis,
would be best for safety since there are lethal voltages inside.
The power transformer is probably ok, from your description.
Also add an AC line fuse on the primary winding before going any farther.
Try a 3AG type 3A slow blow.
Looks like the "ground bus" is only connected to the chassis near the inputs.
A three pin IEC cord, with the third "safety" pin connected directly to the chassis,
would be best for safety since there are lethal voltages inside.
Thanks for the reply. While i wait for some new aux sockets for the back I'm going to do most of the work using these connectors so i can test the amp then I'll solder everything in permanently. I also removed the death cap though the image still shows it.
Attachments
Last edited:
The IEC safety ground should be a direct, wired connection between the IEC connector ground pin and a solder lug
that is properly bolted to the chassis. This must be the most secure and fail-safe connection in the chassis.
An example: https://electronics.stackexchange.com/questions/422550/proper-chassis-grounding
However, an AC line fuse (properly sized for the equipment) is also necessary.
that is properly bolted to the chassis. This must be the most secure and fail-safe connection in the chassis.
An example: https://electronics.stackexchange.com/questions/422550/proper-chassis-grounding
However, an AC line fuse (properly sized for the equipment) is also necessary.
I did not use an IEC connector as my chassis is to thin to fit one even without a fuse integrated. So I wired a IEC compatible cord directly into the chassis with a rubber gromitt and stress relief. The ground is soldered to a terminal strip which is sucured to the chassis by a nut and bolt from the transformer mount. I placed an inline fuse from the hot wire in to another terminal which then goes to the switch, transformer etc. The neutral is attached to the aux sockets on the rear of the amp which is used as a terminal point then to the transformer. I will remove the temporary connectors when I get the new polarized chassis mount sockets. I figured I might as well replaced the old non polarized ones so they will be compliant if I ever sell the amp.
Already there, look at the first pic and follow the red wires up to that top of the pic and you'll see the inline fuse with a 3a slow blow fuse.
I gotta say this is a sweet little amp, ahead of its time i believe. 20 watts per channel with outputs for 4, 8, 16, & 32 ohms with a binuaral rca output you can use for a subwoofer.
That is true. I would say stick with the 2-prong plug if the amp came with 2-prong plug originally.
I know someone would say it is not safe. I would suggest replacing your electrical breaker to GFCI type breaker if they are not GFCI type, and keeping the 2-prong plug.
Thanks for the reply! Unfortunately I already replaced it with a 3 prong plug and the amp is working just fine, no noise or hum. I guess I got lucky.
Use a 3 prong. Ask yourself this question... "Would I touch this chassis while standing on a damp concrete basement floor with bare wet feet?" Growing up with radios, guitar amps, stereos, hi-fi's, all with two prong plugs in our Chicago basement, I remember well all the shocks touching things barefooted. The worst was at 12 years old, I had a kids Stratocaster electric guitar plugged into a cheap Amp, my sister had a microphone plugged into the other channel to sing. Barefooted, she got a shock from the mic, I nearly flew across the room touching the guitar strings!
This is not a concern if you have GFCI breaker.
For me, ground loop is inevitable. If it came with 2 prong, stick with it.
Don't do that. This will make your equipment illegal, at least questionable. Just don't "upgrade" it into 3-prong at the first place.
Looking at the schematic that fat ground bus looks like it is not attached to the chassis, but you'd have to verify that in the actual construction. So the above ground loop breaker might be good to add, ground that fat ground bus to the ground loop breaker not directly to the chassis. Then check that the input/output jacks are all floating from the chassis. If not then replacing all the jacks with new jacks isolated with nylon washers would be a good change too, really bring it all up to date! So the circuit ground and the safety ground are separated by the above loop breaker, anyplace they grounded the circuit to the chassis should be to the circuit ground. Only exposed chassis is safety grounded, bell housings, pot shafts, etc. Anything having to do with the actual circuit then grounds to the lifted circuit ground, (that might only be the input jacks if you're lucky). If it's a sweet amp all worth the work. But if it doesn't hum using the outlet it's plugged into now, at least you have a safety ground if you do nothing more.
In solid state world, there is a known topology that can reject most of the common mode noise. It is still single ended interface (not balanced), but it offers some degrees of common mode rejection. I have written up here. https://www.diyaudio.com/community/threads/hum-rejection-topology.424261/post-7937854
For me, this is a must done for any amp with 3-prong plug.
There is no need to do obtrusive "ground lifting".
At one time, I wanted to convert my Dyanco ST-70 with grounded cord. Considering the efforts/time involved, I still haven't done that.
Again, the National Electrical Code requires GFCI protection almost everywhere for a new building. In most case, you are protected even there is no ground conductor. For old building without GFCI breaker installed, I suggest you upgrade to GFCI breaker or install a GFCI outlet.
For me, this is a must done for any amp with 3-prong plug.
There is no need to do obtrusive "ground lifting".
At one time, I wanted to convert my Dyanco ST-70 with grounded cord. Considering the efforts/time involved, I still haven't done that.
Again, the National Electrical Code requires GFCI protection almost everywhere for a new building. In most case, you are protected even there is no ground conductor. For old building without GFCI breaker installed, I suggest you upgrade to GFCI breaker or install a GFCI outlet.
I knew this was going to happen, as every thread on any forum on this topic that I've read turns into an argument w/ pros and cons but never a consensus. Sorry folks I didn't want this to happen. I wanted someone with knowledge and experience to read the schematic and tell me it is not a hot chassis type amp which "SHOULD" be ok to ground. No promises or assurances. I already made the decision to use 3 prong cord and test for hum if it was there I would change it to 2 prong polarized.
No hum so it stays 3 prong. Thanks everyone for your opinions and replies. I respect all answers but ultimately I have to be comfortable with any modifications i make. With a little man running around the house I'd rather go with safety.
No hum so it stays 3 prong. Thanks everyone for your opinions and replies. I respect all answers but ultimately I have to be comfortable with any modifications i make. With a little man running around the house I'd rather go with safety.