Hi Folks,

I have just finished re capping an HMV N3 chassis along with having to replace both audio output transformers.
When I reassemble the unit and connect it to it's speakers, I get a loud whistle sound - almost like feedback from both channels - this occurs when the volume is at any level - even turned right down - if I switch the balance control all the way to left or right the noise stops, but as soon as I set it to bring both speakers up, the noise is back.

I'm not sure what the problem may be, everything else works fine. I was wondering if it maybe a polarity issue with the OP transformers?

Any assistance welcomed.

Cheers
Scott

I'm not familiar with that chassis, but if the design employs negative feedback from the speaker back into the amplifier, having the connections reversed on the transformer will change it to positive feedback, causing undesirable noises.

Reversing which side? The input or output? I'm guessing the output side

If you have the schematic, you'll notice the feedback wire from the secondary side of each output transformer. One side of the secondary is earthed. The feedback link is from the other side of the secondary.

Reversing which side? The input or output? I'm guessing the output side

Either side is ok, but it is customary to do the output side.

OP transformers with feedback (from secondary) are a common trap. Most will cause motor-boating when the phase is reversed. Many had a blue and a red primary lead and many circuits showed which went to B+ & to OP valve plate.

Take note of the secondary often one wire is earthed (Philips 132L) In this set, it will motorboat if the OP primary is reversed. In this set when I rescued it. The hacker could not get it right & left it disconnected, after changing the speaker transformer and ruining the speaker cone in the process with the drill.

A lot of work fixing it. 6" speakers are harder to find, but a NOS door speaker, intended for a Mitsubishi Magna, fitted.

Marc

Thanks Marc,

The transformers were both blown and have been rewound.
As with the Philips you mentioned, one side of the OP goes to earth. My guess is that in the rewinding process, the polarity may have been reversed - I'll disconnect one of the OP side of at the a transformer and see what happens before switching them around permanently.

Interestingly, If you disconnect one of the speakers entirely it feeds back considerably from the speaker that's still connected.

Cheers

One does not run the amp with an open circuit on the OPT secondary, whilst the fact that it can dissipate some load, with feedback, the risk is that the transformer will "ring" and that generates EHT at the primary.

The flashover on miniature tubes between pins may look good but, that's only if it does. How good is the insulation's dielectric strength, if it happens (seen it)?

Marc

The customary colour code of the transformer leads has it that red goes to B+ and blue to the output tube's plate. But there's nothing inherent in the transformer to dictate this, other than for factory workers to get feedback polarity right. You can reverse whichever is easier for you to unhook, most likely that's the secondary. Less messing with tube sockets and electrolytic caps.

Reverse the output wires from what you had.

Negative feedback is via the 2.2k resistor on the positive speaker terminal, that will stop the squealing.

Cheers

Thanks everyone for the assistance, reversing the polarity of the OP on one transformer solved the problem.
Cheers

Hi Scotty

Re your N3, I've just done one of these.

Mine is an N3-4Y and had green Ducon polyesters right through. There's no need to replace these, they are good.

Yours must have been older? Mine was dated November 1966.

The 6GW8s are close to the output transformers. The heat melts the wax. Same with yours?