75 volts AC on chassis

Fritz

Member
2002-08-27 1:27 am
CA USA
My hafler DH 500 has 75 volts AC between the frame and neutral or ground. I discovered this when it gave me a couple shocks.

Any ideas what might cause this and why it doesnt fuse?

Ive checked for continuity between the frame and trans. secondaries and there is none. Seems to me the AC means it has to be before the bridge rectifier?

Any suggestions?

thanks,

Fritz
 
Did you try checking the caps?

Check to see if there is a cap (or pair of caps) on the AC line into the power switch -- -- could be dirty (or blown) in which case you could get some leakage, also check the filter caps -- (do you have a half-cycle of AC?) I just replaced one of the filter caps in a receiver -- but I would think you would blow fuses first. Lastly, check if there are any ceramic caps across the rectifiers --
 
DH500 PS

Check C406
 

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djk

R.I.P
2001-02-04 4:23 am
USA
So did you check for any resistance between the frame and transformer primary? If that's OK see below.

The fact that you got no resistance between the frame and the transformer secondary means you have a very early DH500. There is a 0R5 0W5 resistor on each circuit card between the speaker output ground and the input jack ground. Check for continuity between the black speaker binding posts the ground shell of the RCA jacks. This resistor acts like a fuse when the amplifier is hooked up to a piece with a hot chassis.
 

Fritz

Member
2002-08-27 1:27 am
CA USA
Great advice,

I checked c406 and it was ok but then saw that the next possible short was the fan and transformer, when I clipped the fan lead the voltage on the frame disappeared! Now where to find a hafler sort of fan.

However

Now when I flip the AC plug I get either 0 volts on frame used to be 75 or, when I flip the plug, 120 on frame relative to ground.

Now Ive isolated the transformer by disconnecting all three secondary wires and also removed one of the two primary wires.
When I apply current to the one remaining primary wire I get about 90 volts on the frame relative to ground. Is this a sick transformer?

Thanks

Fritz
 

Fritz

Member
2002-08-27 1:27 am
CA USA
DJK

I did have resistance between the secondaries and frame my diode check indicated there was no continuity. I checked reistance between the RCA neutral and neg speaker and I measure about 1500 ohms on both sides so i think that resistance is intact.

The resistance between all three secondary leads and frame is infinite. And the resistance between the primaries and frame is infinite.

Now I have removed all leads from the trasnformer and the frame voltage is gone. When I hook the black transformer primary to the 120 wall voltage 80 volts comes back on the frame.

Bad transformer?

Thanks,

Fritz
 

djk

R.I.P
2001-02-04 4:23 am
USA
"I did have resistance between the secondaries and frame my diode check indicated there was no continuity. I checked reistance between the RCA neutral and neg speaker and I measure about 1500 ohms on both sides so i think that resistance is intact."

Yeah, right. I say it is supposed to be 0R5 and you measure it as 1K5. What part of that don't you understand?

With no ground what kind of voltage will develop across the 10M input impedance of the VOM? Let's see, 10µA times 10M equals 100V.
 

Fritz

Member
2002-08-27 1:27 am
CA USA
Hey DJK,


Give me back my lunch money!


I didnt mean to offend you. Perhaps you interpretted me as a smart *** from my uninformed reply? This is not my field and wasnt a hobby until recently so I dont understand the abreviations.

"Yeah, right. I say it is supposed to be 0R5 and you measure it as 1K5. What part of that don't you understand?"

Well basically the part of the sentence where you state a resistance value. Until I saw OR5 in this context I thought you were calling out a part designation.

I still have no idea what VOM abreviates.


Fritz
 
"Now when I flip the AC plug I get either 0 volts on frame used to be 75 or, when I flip the plug, 120 on frame relative to ground."

Well Fritz....
This is a method to check if the polarity of your plug is correct! To be more specific, if the voltage across the frame and the ground is zero (or very close to zero) then you have connected the plug in the right polarity. If you get high voltage readings, then you 've got wrong polarity. The sound quality difference should be audible too!
In my opinion the only problem is the fan
 
The transformer is there to do what its name says: to transform the voltage from the mains to a voltage suitable for your amp. Of course it also isolates the hot lead. Don't worry about the voltage on the chassis. It is not dangerous. It is just a potential (i don't know if this is the right term) , with no current flow . Do you know that your TV set has a voltage of about 200V across the frame to the ground?
DON'T put your tonge on the secondary because in your amp the voltage is rather high......
 

djk

R.I.P
2001-02-04 4:23 am
USA
"Hey DJK,Give me back my lunch money!"

Ha! I deserved that!

"This is not my field and wasnt a hobby until recently so I dont understand the abreviations."

I like using the ones from the European magazines, they make more sense. Decimal points and such are omitted for clarity, they don't always print well. A 1,500 ohm resistor becomes 1K5, a 10,000,000 ohm resistor would be 10M, a 0.5 ohm resistor is 0R5. A 1/2 watt, 10 ohm resistor would be 10R/0W5.

Capacitor values are in picoFarads(10^-12), nanoFarads(10^-9), and microFarads(10^-6). Examples might be 100pF, 10nF, 1µF. Again, the idea is to do away with decimals and such. You could say 0.000100 micro Farads or 10,000 pico Farads, but 100pF and 10nF work better in print(µ is Alt+0181).

VOM is just volt ohm meter.

A typical hand held digital VOM has an input impedance of 10M so even a few µA of leakage current will give very high voltage readings. Ground fault devices usually are set in the 1~10mA range. 100mA can kill you.

I am still concerned that there may be something wrong with your Hafler DH500, it should measure less than 1R from the speaker ground to the input signal ground. The only place the chassis is connected to ground is at the input jacks. The speaker grounds are connected to the center tap for the transformer secondary. The chassis is thus connected to the power supply ground at this point.

To avoid ground loops the early Hafler amps all had resistors between the input grounds and the power supply ground. On a DH200 the resistor was a 2R2 0W5 on the left channel only(looking at the amp from the back side), on early DH500 there was a 0R5 0W5 in each channel.

Strange things happen when these open up. A DH500 can have a humm or buzz, then all the sudden the noise will quit, but the gain will go up 6~10dB and the sound might (or might not) be distorted.

Please check this.

I am glad you found the main problem, the stock fan used to be available at Radio Shack for ~$15. Better fans are available at higher prices, but you will probably need to change the fan speed resistors as well.
 

Fritz

Member
2002-08-27 1:27 am
CA USA
Hi DJK,

I am at a different address than my amp now but I will go through and check things this weekend. I suspect I may have some other problems you mention in addition to the pair of mismatched boards. One of the boards has no offset control? The boards and components are in general sort of tired looking and if I can find someone that is printing new ones Ill certainly replace them. Im not sure I want to go with the musical concepts boards as I could have 500 dollars in this amp if I start down that road. I am just going to use it as a sub amp once it is repaired.

Ill post my findings regarding the resistance once I have it back up and cooling.

Thanks,

Fritz