MR-78 Hot Chassis

[kman]

I have had this problem for a while with my Mcintosh MR-78 tuner. It has the older type of unpolarized AC plug with no third prong for ground. One of the 2 possible AC plug orientations puts over 90 volts RMS AC from the chassis to outlet ground. You can actually feel it if you touch the chassis!

With the other plug orientation there is only about 1 volt AC on the chassis. Needless to say I always make sure the plug is oriented for lowest chassis voltage.

The tuner seems to work fine with either AC plug orientation. I have taken measurements of various sections of the DC power supply and these are within spec according to the service manual. I temporarily removed two 0.005 uF caps that were wired from either side of the AC power cord to ground as a check and this did not help. There is a 2.2 Megohm resistor from one side of the AC cord to ground which measures ok. All measurements done with a Fluke 175 DVM.

Anyone have an idea of what is going on here? Should I just orient the AC plug for lowest voltage and forget about it?

[eb2]

I am electronically moronic, but I do know what this is. Your idea of orienting the plug and forgetting it is probably best. I have an older Pilot tuner and amp set that I got a while back. I was having them benched by a local tube guy, and asked him to put on new grounded plugs. He put one on the amp, but explained that it could not be done on the tuner. The design of the tuner incorporated the chassis into the circuit in such a way that hooking a grounded plug to it was impossible. It had caps to block any shock, but it sounds like the same deal.

[leadbelly]

Originally posted by kman
Should I just orient the AC plug for lowest voltage and forget about it?

No no no. If you are unwilling or unable to do the more intense job of converting to a grounded chassis, you should at least install a polarized 2 prong plug, the kind where 1 prong (neutral) is larger than the other in the orientation that is resulting in your "1 VAC" case.

[kman]

Originally posted by leadbelly


No no no. If you are unwilling or unable to do the more intense job of converting to a grounded chassis, you should at least install a polarized 2 prong plug, the kind where 1 prong (neutral) is larger than the other in the orientation that is resulting in your "1 VAC" case.

I can install a polarized plug but my main concern is whether there is a fault condition existing in the unit because of the 1 side being so much "hotter" than the other. Let me note here that I removed the 2.2 Meg resistor as a check and now the hot side measures around 75v RMS to ground whereas the less hot side measures about 11v RMS.

[leadbelly]

Originally posted by kman
I can install a polarized plug but my main concern is whether there is a fault condition existing in the unit because of the 1 side being so much "hotter" than the other.

Hard to say without a schematic. Do you have one? Depending on the vintage a hot chassis is exactly how some devices were designed and built.

[kman]

Originally posted by leadbelly


Hard to say without a schematic. Do you have one? Depending on the vintage a hot chassis is exactly how some devices were designed and built.

Hi, yes I do have the schematic but only in paper form, not pdf. There are 2 0.005 uF caps from either side of the AC cord to chassis ground in the power transformer primary circuit along with a combo power switch/volume control and line fuse. There is also a 2.2 meg resistor from 1 side of the AC cord to chassis ground. I removed the 2 caps and resistor just as a troubleshooting check. There are the usual low voltage DC power supplies connected to the transformer secondary of course.

[leadbelly]

Originally posted by kman
I removed the 2 caps and resistor just as a troubleshooting check.

What is the resistance between the chassis and the individual prongs with these components removed?

Also, if you have the schematic, are these 3 the only chassis connections shown? Is there anything like a chassis ground symbol anywhere else on the primary side?

Also, is there anything on the circuit side that looks like it is connected to the primaries, like tube heaters?

[kman]

Originally posted by leadbelly


What is the resistance between the chassis and the individual prongs with these components removed?

Also, if you have the schematic, are these 3 the only chassis connections shown? Is there anything like a chassis ground symbol anywhere else on the primary side?

Also, is there anything on the circuit side that looks like it is connected to the primaries, like tube heaters?

Resistance between both prongs and the chassis now reads infinite although this is a bit of pain to measure because of the difficulty in making good connections.

No other connections to ground on the primary.

I have sent you a PM.

[leadbelly]

OK, I have looked at the schematic, so if I understand the information provided, (1) the 90 VAC issue was with the caps and resistor installed, and (2) with those components removed, you are not reading a short to chassis on the primary side.

I think that there is no component failure at all, you were just measuring a high impedance charge on the chassis through the caps. You can do some more testing if you want to prove this (like reinstalling the components and rigging up a drain resistor to ground), but what I would personally do now is move straight away to trying to install a 3 prong cord without reinstalling the caps and resistor. Try installing a fused connection between the chassis and ground. If it doesn't blowor blow the unit's fuse, rewire for a 3 prong cord.

[kman]

Originally posted by leadbelly
OK, I have looked at the schematic, so if I understand the information provided, (1) the 90 VAC issue was with the caps and resistor installed, and (2) with those components removed, you are not reading a short to chassis on the primary side.

I think that there is no component failure at all, you were just measuring a high impedance charge on the chassis through the caps. You can do some more testing if you want to prove this (like reinstalling the components and rigging up a drain resistor to ground), but what I would personally do now is move straight away to trying to install a 3 prong cord without reinstalling the caps and resistor. Try installing a fused connection between the chassis and ground. If it doesn't blowor blow the unit's fuse, rewire for a 3 prong cord.

Can do, what value for the chassis to ground fuse would you suggest?