Phono 3 noise issues

[bob orbell]

Components and values are mostly the same in II and III, the component numbering is different and the III has a fourth time constant of 3.18uS, I have made a post, RIAA Equalization, worth reading, as I think it is wrong and have removed it from my Phono others also agree, your board does not have this time constant and I advise you to leave as it is. One thing I looked at on your pictures is the 3 watt resistor connected to the incoming HT before the stand alone capacitor, I can not see if it is 1000 or 10000 ohms (brown black red) or (brown black orange ) if you are just using the Phono and no Pre, it needs to be 10000 ohms, this is not your problem but check it. Bob

[Damien]

Thanks Bob.

It's Brown Black Orange.

I'll get it on the bench this evening & take some measurements first.

Regards

Damien

[Damien]

Hi Bob.

I've re soldered all components cables & generally tidied up.

I took some measurements before I started & the PSU with no load was showing 350V at the Pre HT but only 64V at the Phono HT.

Everything else checked out OK.


On the Phono amp only the heater voltage at 6.1V corresponded to the test information in the build manual.

I'm seeing 40V at the input capacitor, 9V at pin 6 of each input valve & 3.25V on pin 1 of the input valves.

Pin 3 & 8 of all 3 valves all reading 1.2V.

I'm wondering if my Fluke meter being auto ranging is confused as the above don't seem to make any sense.

It works fine as it did previously but no reduction on the noise front.

Regards

Damien

[bob orbell]

Something wrong here Damion, both Pre and Phono should have about the same voltage unloaded, loaded it should drop to 250v + - 10 to 15 %, the heater voltage is elevated, at the heater pins it should be 6.3 + - 10 %, I need to get my test papers out to tell you how to measure the elevated heater voltages, this is about 45 volts. Do you have the build papers yet, and are they for the II or III. Bob

[bob orbell]

Looking at the picture 1 of PSU, it looks as if the outlet socket for phono is incorrectly configured, the pre may be OK, a better picture of the sockets from the inside may help, you can plug your phono into the pre outlet and see if that is better, let us know. Test points with black meter lead to earth post, all unloaded, PCB phono out pad pos. lead 345, pre o pad 345, heater - pads 44, heater + pads 50. Bob

[Damien]

Hi Bob.

Greg has kindly sent me the documentation which matches the units I have.

Am I right you are referring to issue 2 PCB? this is what I have.

As you say my main concern was the differences between the main voltage on the pre & phono.

I can certainly try the amp connected to the Pre output & let you know.

I'll check the wiring of the connector, but I suspect the connector had rotated in the housing when I took the original picture as I tightened it up last night.

I'll borrow a meter from work today & re check all the voltages against the test data unloaded then the circuit diagram loaded.

It wasn't obvious where to check the heater - & + pads but looking at the diagram I guess I can just check these at the regulator output & nose of D6 respectively?


Regards

Damien

[Damien]

Hi Bob

As I suspected my meter was giving false information.

All the measurements check out against the build data for both the PSU & Phono stage.

I've tidied the units up & made sure there are no joint issues now but no change.

I've taken it to work today & we've had a scope on it.

The issue seems to be with the amp rails wandering in line with supply variations which are then being amplified through the phono stage.

I'm hoping to get chance to have another look tomorrow but is this a known issue?

Regards

Damien

[bob orbell]

The only issues that have been noted is motorboating (low frequency oscillation) did you try connecting to the Pre output ? and do you have 250 volts DC at the Phono ? Bob

[Damien]

Hi Bob.

I'm getting low frequency oscillation as well as hiss.

Yes I have 250VDC at the phono now & all the voltages
on both units are in spec.

The noise is coming from the PSU & generated even when the phono is not connected.

Is there a recommended fix for the motorboating? I guess this is what was causing the rather unnerving movement in my speaker cones.

Regards

Damien

[bob orbell]

Damien, you will see that your PSU board has provision for two more capacitors, the original PSU used all four, nothing in the Phono except for the two 10 uF electrolytic's, low frequency instability became a problem for some and the design was changed to what you have, try renewing all three 100uF capacitors, or adding one extra at C4, my chum had just this problem, we never did fix it completely until I built him a fully regulated PSU, it stopped the problem dead. I have some good used 100 and 220uF 450v capacitors if you would like to try them, PM me, there will be no cost to you. Bob