Heathkit w7-M Power Supply Sidebands

[jhstewart9]

A friend brought his Heathkit W7-M a while back for a look at D%, Etc. While we had the Spec'A hooked up I figured, why not a look at the PS sidebands on the signal. Refer to the attached.


I was surprised at the size of the 120 Hz, Etc sidebands on the One KHz test tone. The PS is a SS FWDoubler running into a pair of 150 microF Caps. No choke, but they have used a simple trick cct we resorted too at times, another 100 microF across the result.


The PT utilization is good in a FWD, so no problem there. But the other surprise was a rather large 60 Hz (power frequency), component greater than the 120 Hz. One would think symmetry would hold the 60 Hz down. Must be a problem with one cap in the doubler I think. But we had to cut it short, so don't know.


This all done using a PicoScope ADC-216 & Differential Probe set to x1000.



Information only.

 

[trobbins]

As there are no 60Hz sidebands to tone, then the 60Hz may well be test setup.

The sidebands are one of the 'nice' aspects of hum - subtle vibrato! If their level is sufficiently below signal harmonic distortion then that was sort of acceptable for a commercial offering. The sidebands are certainly not unexpected.

 

[infinia]

60Hz is normally grounding / set up issue, The -40dB baseband spurs point towards the test setup.
but the doubler is half wave operation (depends on the topology and take off points ) so your hunch maybe right.

 

[jhstewart9]

I avoided ground loops in the test setup by using a diff probe. Always something to be aware of.

FWDoublers are seen by both source & load as FW. But half-wave doublers are seen as full wave to the source & half-wave at the load. This one is a FWD.

The PS sidebands in this one are easily the largest I've seen in a common hifi amp. A small choke of say 2H would provide a substantial improvement. But I guess the bean counters were busy doing there job.

Time permitting I will ask the proud owner to bring it back for another look. But unlikely before Winter.

 

[DF96]

The sideband result is not particularly surprising. To get a very rough estimate of the likely sideband level just add 20-30dB to the hum specification. This assumes a PP circuit, with simple PSU, so much of the hum comes from the output stage. The PP OPT will tend to cancel supply rail ripple, but it can't cancel second-order IM between supply ripple and signal. Feedback will reduce both hum and IM by the same ratio.

With a very low ripple PSU you get much less IM, but hum stays as it will still get in earlier in the circuit.