Hum problem....

[Greg.]

Well, it is great that you have now made sure that your earthing/grounding is working well. If you still have the hum problem, I suggest you need to go back to post 14 from John and carry out that test and as he says, report back.

Also, clearly you have had the system running satisfactorily in the past. So have you changed anything from the time it worked well and then became a hum problem? Please spill the beans

[A Stuart]

Originally Posted by Chris

10R on both + and - speaker connectors

Suspect this has nothing whatsoever to do with the actual problem, but - If one side of the speakers is earthed, the other should surely read 10 ohms plus the DC resistance of the transformer (2 ohms or so??)

Alastair

[bob orbell]

So Chris, with your multi meter set at ohms with one clip on the earth post and the other to the chassis you have 0 ohms, , now leave the clip on the earth post and connect the other test lead to the speaker red terminal and you say you read 10 ohms move to the black post and you still have 10 ohms something is very wrong, or are we reading your post incorrectly ? Bob

[Chris]

Bob that is correct, 10R reading on both + - speaker terminal, the Negative terminal is not connected to earth at the terminal.

Chris.

[Richard]

To clarify the readings at the red and black speaker posts,

the black should be connected to Signal Ground 0V so you should get 10R from black post to chassis Mains Earth post

the red is the other end of the OPTX secondary(!) so you should get 10R + the secondary winding R.

The secondary R is very low, a fraction of an ohm. I just checked a similar OPTX at 0.3R with 2 decent meters so it may make a difference in red and black readings that is easily missed.

See the relative thickness of the secondary winding wire in the attached pic.

[A Stuart]

My memory stands corrected.
It is twenty years since I measured it.

Less than a factor or ten out . I should apply for a job advising the goverment.

[Chris]

Hi Richard, just to confirm that I have found .2/.3 ohm differences between both positive and negative side of the binding post as predicted. Wiring exactly as diagram.

Chris.

[Chris]

Just wanted to cover the safety issues of correct earthing before hunting down the Hum problem.

The other issue I wanted to cover is the earthing of the output transformers, is this is done purely for safety reasons? Can this be done with a wire with ring connector/ locking washer under the transformer bell with the other end wire being connected between locking nut on fastening bolt and chassis?

Chris

[Richard]

I think you're going the right way about this Chris, you'll become more familiar and also check the original build by doing it this way.

Earthing the OPTXs;
For safety connect any exposed metalwork to Mains Earth. So yes connect the top bells to the chassis via a wire or just one of the mounting bolts scraping off the paint if needed and checking with a meter. Isolate the other mounting bolt(s). The lams are covered in varnish and generally not earthed - rather they are isolated - you will see some builders wrap the lams with plastic tape as an added precaution.

Circuit Grounding 0V of the OPTX secondaries;
The instructions I have show the black posts connected back to the circuit tag strips by the feedback wire braids, then to the star earth point on the PSU tag strip. It's up to you but I would disconnect the fb braids at the black speaker post (leave the braids connected at their tag strip ends so they will still act as shielding for the fb wires), then run a wire from each black speaker post to the PSU earth point or preferably even to the negative tag of C2 which is the earthiest point. This would keep any output ground modulation away from the input section of the amplifier.

I only have experience of working on this amp once and ran out of time trying to stop it bursting into oscillation at 19W output. It sounded fine on music as this level was probably not reached but it showed every time on test despite moving wires around and swapping valves.

[Greg.]

Originally Posted by Richard

The lams are covered in varnish and generally not earthed - rather they are isolated - you will see some builders wrap the lams with plastic tape as an added precaution.

Yes, 50mm black tape is available from eBay, provides the additional precaution and actually improves the image of the amp for a long time. Although lams are coated in varnish, it tends not to be a uniform covering and rusting will develop in time, spoiling appearance. The other option I have done is to spray the Tx’s with satin Plasticoat paint which looks good, but may not offer the electrical isolation that tape does.