PL 700 II Clair Bros Rising from the Ashes

[WOPL Sniffer]

If i'm troubleshoting an amp problem, Star ground is my reference.

 

[laatsch55]

Peter, one test lead on the wall receptacle ground then test hot and neutral on the wall receptacle, what do you see on the hot and neutral?

 

[Peter S]

Joe;
I will measure the 700B now, with inputs disconnects.

 

[Peter S]

Hi Lee;
Just did that an hour ago; Ground to Hot 121 VAC Ground to Neutral; 650 mV (line lose in the neutral leg) (but almost zero with the desk lamps and laptop unplugged)

 

[laatsch55]

Ok...

 

[WOPL Sniffer]

Perry;
The secondary centre tap is 60 VAC above earth ground. The wall outlet ground is my simplest connection to earth ground and only a temporary measurement. I very much appreciate all advice but please follow the thread closely. The chassis has an undeniable spurious voltage on it.
What would you use as a reference point?

Peter, I followed the thread from the beginning to the end and if I followed your crazy journey correctly.......... This amp never shocked you until you decided to upgrade it. So, I think you are chasing an amp problem... Not a wall power problem. Your other amp didn't shock you either??? So, looking at the pictures you have posted, I see a lot of solder blobs, and some cold solder Joints on the standoff Pems.... You'll be farther ahead by stripping this amp back down, an inspecting all the solder joints under magnificatin because I think you are chasing your tail but don't worry about me jumping back in.... It won't happen. Poor soldering will cause MANY issues and can it cause what you are experiencing??? YEP

Solder splashes, cold joints, blobs..... YEP

 

[Gepetto]

Hi Peter
Are you using the original 2N3403 transistors for bias or did you replace them with the provided 2N5088 transistors?

 

[Peter S]

New bias transistors installed. The old PL700 measures about 600 mV AC to earth ground power on, about 1.3 VAC with the amp switched off.

With input RCA's connected, zero volts (speaker negative to earth ground)

Perry......again; Yes it is probably something I have done incorrectly, but the transformer has been completely isolated from the amp. The back planes could WELDED to the chassis and not have an influence on this issue. Could you indicate which entry in this thread shows cold solder joints?

 

[Gepetto]

New bias transistors installed. The old PL700 measures about 600 mV AC to earth ground power on, about 1.3 VAC with the amp switched off.

With input RCA's connected, zero volts (speaker negative to earth ground)

Perry......again; Yes it is probably something I have done incorrectly, but the transformer has been completely isolated from the amp. The back planes could WELDED to the chassis and not have an influence on this issue. Could you indicate which entry in this thread shows cold solder joints?

Have you physically moved the transformer off of the steel chassis to see if the AC potential dissipates with distance from the steel chassis?

 

[Peter S]

Joe, I didn't want to hear that but...I will do it. A project for tomorrow. This onion is running out of layers! Just checked again; Chassis voltage drops from 60 to 4 VAC with a 1 meg shunt. 46 mV with a 10 k shunt. I will lower (and even completely remove the power trans) but the biggest leakage leakage was to the centre tap.

Note to Perry; If my journey was not crazy, I would have lost interest long ago.
Nevertheless; I may have to strip the amp down completely---Dang--all those outputs! OK, Sil-pad would have been nice! You get one more "I told you so"....but...at the moment, the only thing connected to the secondary is the bridge, no filters and no snubbers.

 

[Peter S]

Here's the transformer separated from the chassis by 1/2". 62 VAC measured on centre tap buss to wall outlet ground (which is more or less, the neutral side of the primary coil)
Still 30 VAC on the chassis

 

[Peter S]

Transformer lowered further; 62 VAC on centre tap, 28 VAC on the chassis.

 

[Peter S]

This particular amp has sanding marks on rack mount tabs. This could be evidence of past hum issue.
I have tested a small 24 VAC Hammond transformer for primary to secondary isolation. There was 14.6 VAC on the secondary to ground, dropping to .022 V when shunted by 10k.
(The PL transformer was 62 VAC, Centre tap to ground, dropping to 0.62 with 10k shunt. Is this tolerable?
Has anyone put a DVM between the primary and secondary of a PL700 transformer that has been disconnected from the amp?

 

[Peter S]

I am at a stand-still at the moment. I would like to put the amp back together and experiment with grounding the chassis, but before that, I would like to feel a little more confident that the transformer is healthy. Douglas Self claims that the lines ground should be tied to amplifier ground at the input ground. Sounds like the original PL, Input ground tied to output ground and Centre tap bus, but obviously no mains ground. Grounding the input shell to the output ground is not going to be a consideration with the White Oak upgrade. What about connecting the IEC ground pin to the amp's centre bus...or the 6-32 stud in the middle of L chan (single point chassis connection?

 

[BlueCrab]

Just to be sure I understand. This is your situation:

 

[Peter S]

Exactly! And if the meter is shunted by 1 Meg, it drops to 43 VAC. Other reliable sources tell me that this (42 uA) is not unsafe. It would have to be 100 times greater to be a safety issue. Nevertheless, it is a huge hum issue

 

[BlueCrab]

Do you have any idea what the impedance of your meter is on the AC scale? Agree that 42 uA is well below what would trip a GFCI circuit. With a high input impedance meter it's possible the voltage you see is due to stray capacitance/inductance of the transformer.

 

[Peter S]

Hi BlueCrab:
Just downloaded the manual for the Fluke 75. Can't believe I've had it for 39 years!

Greater then 10 Meg, less than 50 pf.

With the amp powered up and nothing connected, I could feel a very slight 'tingling' when touching the heat sink fins. As mentioned; drops to 43 V with 1 Meg shunt, .64 VAC with 10 k shunt

 

[BlueCrab]

So that all makes sense if you're getting a bit of leakage current due to the inherent capacitance/inductance between the primary and secondary in the transformer. Making the same measurement with one of those old 1000 ohm/volt VOMs, and the voltage would shrink to nearly zero. This also explains why you get nearly the same measurements with the second transformer.

 

[BlueCrab]

Question is: Is this the source of your hum. Sounds to me that your signal source and the amp are on two different ground planes. Connecting a jumper from the amp chassis to 120vac outlet ground is putting them on the same ground plane (or nearly so). The coax from the signal generator should be providing that return. Verify that it's (the coax) ok. And verify that the amp's ground (I'm sure you did this) is connected to the RCA phono jack ground (which should be isolated from the chassis to prevent a ground loop). See if the signal generator's output is floating with respect to the outlet. Sub a direct short on the RCA inputs to see if you still get the hum.