A little bit of conflicting info, but sounds at least we may have fixed radio side.
I meant for you to short to ground individually one at the time K and F.
C25 the main filter cap 2000UF has been replaced and is new?
Perhaps try another 1000μF in parallel and see if it improves the hum.
Next step would be an oscilloscope to check ripple on main rail.
But if shorting the inputs of the seperate channels did not kill the hum, you have power supply problem.
If it did kill it then the hum is being amplified from front end, probably open bottom end of one of the volume pots, or open circuit and shielding problems. Gram/Radio switch probably needs a spray of cleaner as well.
JJ

Hello Johnny

Thanks for your reply

Not sure what I did yesterday..but today if I short an input pin to ground the output is very quiet...only the slightest of hums.!

So probably not specifically the power supply as the major cause.?!

I have noted that one speaker produces slightly more hum than another ...swapped channels and confirmed...so was thinking that one Amp was doing this ..but it appears not.

So the hum must be a Pot issue?...one thing to note...when using the Gram input (connecting a radio headphone output) I noticed when volume is increased so does hum..this does not happen from the units radio input... noted then the Gram input is using its own Pot, as part of the 2 gang Volume Control Pot.

I heavily sprayed all pots and input switches..worked them while spraying...but no better than before. There is no scratchiness heard from the usage of the pots nor the switches

I also tried removing the 240v supply from the Switching Array (as its adjacent to signal switching ...and connected the Primary side of transformer directly to 240v Supply but that did not improve the hum.

Could this issue be isolated further ? could/should I by pass input switches and input jacks...and should and is it possible to bypass tone / balance even volume?

Checking with my phone decibel meter app...not sure how accurate..but hum is about 90 Db .. shorting to ground is about 70Db.

Thanks

I have seen a couple of these units recently and those had issues with the volume pots.
They are a special dual gang pot and taped towards the top.
It appears they go high or open on the bottom of the divider to deck.
This electrically sits everything up away from earth, and the high gain amplifier picks up stray noise.
Also note unlike most solid state equipment all this front end/volume area is high impedance circuitry similar to valve equipment.
250K potentiometers instead of the usual 10 to 50K, seen in most solid state amps.
Pay to check all high value resistors in this area also.
As stray pickup from faulty pot may be the problem.
Unfortunately if one channel is OK you will have to spend time and compare voltages and component values until you find the discrepancy.
Very difficult without the unit here in front of me.
An oscilloscope would sort it out quickly. JJ

Im having trouble getting the forum webpages up...anyone else?
lots of the time it takes ages to load and often it says 500 internal Server error.


Thanks Johnny

The Rhapsody Volume control from the service manual --> 2 meg 2 gang Curve B with a Tap at 900k.
Does the Curve B mean its an audio Linear Pot or an audio Log pot ?

If it turns out to be the pot causing the noise...then wondered if I could get away with a non tapped pot and bypassing/removing the tapped circuitry... the circuitry the tap is using - (assume) is this is a tone/loudness compensation circuit ?

My thoughts and plans tomorrow
I'm going to permanently bypass the Input Jacks...they are an old odd size anyhow..nothing I have fits...so will leave the jacks in place but remove the wiring to them. Another less point of possible signal affecting noise to worry about.
Then I will temp bypass the Audio Input Switches..then it only leaves the Pot as a potential source of noise trouble.

I do have a dusty Scope (have to see if it still works) - if I dont get any further I will get it out and try and see where the noise is at.

At the moment my thoughts are the volume pot.... If it turns out to be the culprit of the hum noise - Could I just bypass/remove the tapped circuitry and replace with a similar Curve B 1 meg 2 gang pot....be it a log or linear ?

Cheers

Im having trouble getting the forum webpages up...anyone else?
lots of the time it takes ages to load and often it says 500 internal Server error.

Common problem, been around for years. Just keep trying until you get through.

Yes, just don't use tap and experiment with fixed values. If it proves to be the potentiometer. Measure carefully.
Will be away for 6 weeks, all the best with the repair. JJ

Thanks Robbbert its working for me today ok atm

Thanks Johnny

All the best for a good trip! thanks much for the help!

Turns out the Pot only has 3 tags on each gang...no extra for a tap.
Assume then it is not a tapped pot ?
Lots of play in the shaft...

Printed on base of Pot --> '2m2 Log B2'

These pots are available WES Sydney, although matching the shaft/knob requirements may be a problem.
So, circuit not exactly correct?
2 Meg log dual gang. And not tapped.
Should be able to check with meter, but I think earthy end of one or both pots going high or open.
Intermittent carbon dust mixed with wd40 leaking to case etc. Track worn out.
This pot has probably taken a hiding over the years.
Would make the volume jump around as well.JJ

Thanks Johnny

Im not sure but suspecting perhaps the pot has been changed out - as there is a tag board near the pot with a couple of 0.022μF Caps (probably C36 and C35) that are not connected to anything but tags!.... should also be resistors according to circuit -

so the pots are currently wired with high side (signal input/gram/radio ), wiper to amplifier and low side tag to chassis/ground via a wire to the tone pot....no other caps or resistors connected to the pot at all.

Im going to look at grounding again around the pot and might actually ground it directly to the chassis as well..just to make sure.
Will be digging out the scope tomorrow as well.

or Maybe the pot is original as this is a later model with modifcations...not sure.

Will check out WES

Cheers

After a bit more playing and chasing my tail -

I am set that the amplifier Right channel is the source of the noise - if I swap left and right signal from the Volume pot - the noise/hum still remains on the Right Channel...if I swap speakers hum/noise still from Right Channel.

So Vol Pot not culprit.

Speaker Scope traces without signal and zero volume

One Channel is about 300 mV pk to pk
The Other around 200 mV pk to pk

Have submitted Pictures to Admin to add them to this post -

Im not sure what is 'normal' output as this is the first speaker output I have ever put a scope on (its the first time Ive used this scope in many years)! a Trio Analogue

Any suggestions what to test and where please go ahead.

I tried to scope the Ripple on the DC Supply...however it is very small and barely seen.with the leads and scope I have - ok as far as I can tell.
The DC Supply is lower than the Schematic suggests it should be at about 29V DC instead it is 26 V DC.

Any suggestions as to where and what to scope welcome.

Photos uploaded to Post 15.

Thanks Brad!

Ive been working on the Right Channel -

In summary the right channel still has about 10db mains hum more than left channel...when measured at the speakers & Volume is at zero and no signal input.
When the Input to the Amplifiers are grounded there is no hum.
If you swap the Left for Right Signal inputs the hum remains on the right channel
If you swap speakers the Hum remains coming from the right channel.

I have now replaced all capacitors in right channel - some resistors - no improvement.
I have upgraded the DC supply Smoothing Capacitance to 3000μF - slightest of reduction in hum to both channels - but right channel is still 10Db noisier.

i have grounded/bonded across each Potentiometer casing (soldered thick wire) to the star ground..no improvement.

I have updated the circuit diagram of the amplifier channels...the circuit is somewhat different from the service manual..namely reduced Voltage to preamp transistors...and overall the supplied DC Voltage is reduced from 29V to 26V..Will ask Brad to upload the before and after photos...

Speakers should be 27ohm - except one speaker is at 26ohms

Not getting any further forward is frustrating! I'm almost now to the point of trying a swapping of transistors from one channel to the other.

Update of HMW Rhapsody T8-49 Circuit Diagram

Go easy with the soldering iron on those germanium transistors Jinow. They are old, grumpy and not as tolerant as silicon semiconductors.

Maybe delay swapping them over till some more suggestions are posted?

Document uploaded to Post 28.