Australian HMV Rhapsody Radiogram. Replacing Amplifier - Questions!?

[lonnyholl]

Hello Forum
I usually tinker with Industrial Electrics/Electronics, and mostly I have skills to build replace and fault find.

- so audio amplifiers are not my thing however..and looking for some assistance - this unit is for a close family member and has the sentimental value as well... so I need to try and see what can be done to help keep this lovely period piece as more than furniture.

It is an Australian HMV Rhapsody Radiogram from 1969 - it uses a T8-49 Chasssis.

At the moment one channel is weak....the unit uses unobtainable Australian Manufactured Transistors from Fairchild.

I currently have the chassis on the bench, visually I cannot see any damage burning, bloating of caps, or bad connections..looks clean.

The unit comes has an AM Radio and a BSR A10 Turntable with a C1 Cartridge.

If I cannot get the weak channel going by changing its caps...then I will assume one or more of the unobtainable transistors are gone.

Then need a plan B - I am looking to see if its possible and what needs to be considered in replacing the amplifier with some relatively cheap amplifier module....as can be seen on the net - there are certainly lots to choose from these days.

I have a few questions with that in mind.

I do know the BSR C1 Cartridge outputs 43mV

I am not 100% sure but measuring radio audio signal output appears to be from 2-10 mV

Thats about the only specs I know -
My questions are - what would I need to do and consider in matching a replacement amplifier module to the existing inputs I have.

Two things Im not sure about is - would a replacement module be able to amplify existing signals directly.... considering the low outputs from both the BSR and Radio...(would I need a Preamplifier)...and what about matching impedance to the amplifier - what should be considered/required to make it work?

Lastly - Ideally I would like to retain the existing Volume, Tone, Balance Control Pot's....is this at all possible ?.....or do I need to find a Preamp Module which has existingVol/Tone/Bal Pots and replace existing Audio controls.

I thought members may have had these challenges before with using new amplifier modules due to unobtainable Components and hope to hear from Peoples experiences and thoughts.

Schematic as shown.

Many Thanks

[joebog1]

If you give me the transistor numbers I may possibly have some. If not then direct replacements.
As a rule Fairchild transistors are pretty "blow proof " although they are like any other good quality transistor, and failures do occur.

A decent PDF scan is all I really need. JPG files fall to bits when you enlarge them. PDF files dont and can be enlarged at will.

Joe, west of Cairns

[lonnyholl]

Originally Posted by joebog1

If you give me the transistor numbers I may possibly have some. If not then direct replacements.
As a rule Fairchild transistors are pretty "blow proof " although they are like any other good quality transistor, and failures do occur.

A decent PDF scan is all I really need. JPG files fall to bits when you enlarge them. PDF files dont and can be enlarged at will.

Joe, west of Cairns

Hello Joe
Thankyou for your post

You were spot on! I just finished changing the Electrolytic Caps today and found a couple of 'Ducon' Caps that were bulged at the bottom (see pic). Once they were replaced I had both channels working! Something to be said for the quality of these transistors going fine for so many years.!

For the record here are the transistors used in the Rhapsody.

SE1001
SE4010
AX1108
OC987
OC988

So far I have now changed all Electrolytic Capacitors...sound is improved.
So far I have left in place as-is all ceramic Caps and there are a couple called 'Styroseal' Caps? assume they are polyester type ?...is it worth changing the rest of the Caps do you think?...I see some 400V polyester caps on the speaker outputs.

There is still a lot of hum on this unit - not sure why its so bad.
Speakers are OK.

Ive noticed that initially the radio is quite sensitive but after a while the AM receiver seems to lose its sensitivity and that gets replaced with more noise...and then only the strongest station is audible... Im unsure why..

...Even so ...The main thing today is that it lives again !
Have a new Cartridge and Stylus now coming and then will see how the turntable is sounding as well.

For Reference I have uploaded the PDF Service Manual.

Cheers

[Diabolical Artificer]

The usual thing to do with any old amp/radiogram is to clean the switches and pots with electrical cleaner,same goes for any RCA/DIN IP connectors.

Failing that you amp shouldn't be too hard to fix. After a good visual check on both sides of the board try putting a sinewave in then follow it from start to finish. A scope helps here but there are other methods.

Lastly fixing the existing amp will be easier in the long run than fitting a power amp module. Audio amp transistors are usually nothing special finding a replacement if any are dead is pretty easy.

Andy.

[Robert Gribnau]

Are you sure about the output voltage of the BSR C1? On the web I see output voltages ranging from 110 mV to 430 mV. See:

https://worldradiohistory.com/hd2/ID...-Page-0013.pdf
https://www.adelcom.net/BSR%20Cartridges.htm
https://thevoiceofmusic.com/catalog/...ld&Categories=

The output of only 2-10 mV of the radio section seems too low to me. Looking at the schematic I would think the output voltage of the radio section has to be in the range of the output voltage of the phono cartridge.

Which of the two channels is working now? What happens when you temporarily connect points "F" and "K" to each other? By doing so both channels of the amplifier section will see the same input voltage.

With respect to the transistors, this might be helpfull:

SE1001: https://www.radiomuseum.org/tubes/tube_se1001.html
SE4010: https://www.radiomuseum.org/tubes/tube_se4010.html
AX1105: https://www.radiomuseum.org/tubes/tube_ax1105.html
OC987: https://www.radiomuseum.org/tubes/tube_oc987.html
OC988: https://www.radiomuseum.org/tubes/tube_oc988.html

[paulsherwin]

As Joe says, obscure transistor types can usually be substituted.

The OC987/OC988 appear to be broadly the same as the AC187/AC188. As we know, these are starting to develop internal tin whisker faults. Perhaps that's the problem here?

[stevehertz]

I'd definitely fix it, don't fit another amp. Stuff from 1969 is eminently restorable, we on here fix 1930s stuff containing obsolete components on a daily basis. Virtually any component is replaceable one way or another, be it new old stock, used but tested good, or a work around solution. Best of luck.

[lonnyholl]

Hello Everyone and thankyou for all responders
My posts are being moderated as Im new..so I can t respond in realtime at the moment so to speak.

Ive now got two Channels that output full volume ...the Ducon Caps in each Channel were gone!

All Electrolytic Caps have been replaced. Other Caps remain original at the moment - Polyester and Styroseal caps and Ceramic.

After some more time looking at this - I can hear there is quite a lot more hum in the Right Channel than the left - and when turning the Vol Pot to near zero - sometimes the Right Speaker goes into a low bass oscillation? Hard to describe ..but you can see the speaker Vibrating. So still have some issue to work on.

[lonnyholl]

Originally Posted by Robert Gribnau

Are you sure about the output voltage of the BSR C1? On the web I see output voltages ranging from 110 mV to 430 mV.

The output of only 2-10 mV of the radio section seems too low to me. Looking at the schematic I would think the output voltage of the radio section has to be in the range of the output voltage of the phono cartridge.

Which of the two channels is working now? What happens when you temporarily connect points "F" and "K" to each other? By doing so both channels of the amplifier section will see the same input voltage.

Hello Robert
Thankyou for your reply, you are correct with the Cartridge output...I must have misread 430mV for 43mV !

As you say the radio output signal Voltage must be similar to the Cartridge/Turntable output..I attempted to measure the radio output with a DVM.. may have not been at all accurate. Thankyou for making me think more about it...Both the Radio and Turntable connect directly to the exact same Amplifier circuitry..so they must have similar outputs.

Currently trying to work out why I have bad hum on the right channel.

[joebog1]

Styroseal caps ( probably with a pink label ) will last forever!!!. I have NEVER changed one yet!!.
Believe it or not, they are also made by Ducon. They are polystyrene capacitors. I would use them exclusivly if I could get them. The yellow polyester caps will be "mustard " caps and unless they crack around the end connections, they are also excellent.
So, DONT change the styroseal caps.

A couple from my collection of Styroseal's, These were all willed to me by an old master engineer that taught me before I even started high school. The largest, .47 uF at 600 volts would have cost about $20 back in 1960's. Half a weeks wages !!!!!.


Joe

[lonnyholl]

Thanks for the info Joe...good to know some good caps in this unit!

I took a picture of some more caps -not sure if they are styroseal...can you recoginise them ?

Thanks

[joebog1]

Early polyesters. Yes, good caps. I will remember shortly what brand they are. It was in the late 60's, early 70's that many companies in Au were taken over, or refinanced. Another excellent company that springs to mind is Rola speakers.

Joe

[lonnyholl]

Hello Joe
I think it is amazing and great to see the components on this set have been manufactured in Australia... and sad to think that Australia has lost most of its manufacturing - we are so overly reliant on other countries these days for pretty much everything.

Joe this set has a pair of 27ohm Magnavox Speakers also Australian I think!


Putting out a request:
for anyone that has a spare plastic control knob they might be willing to part with - the HMV Rhapsody is missing one knob - the original Knobs are plastic about an inch/25mm diameter and have a knurled Edge - they fit a D shaft and the D shaft socket protrudes into the Cabinet about a 1/4 of an inch....hope by looking at the photos you can see what I mean - I have not found any new style knobs that have the protruding shaft.

Thanks

[joebog1]

Magnavox are in fact Rola after Rola was taken over/refinanced.
They will be fine, but I "think " will be ceramic magnets not alnico.
27 ohms ?? First time I have heard that. All I have seen from that era were 15 ohms with 8 ohms coming along quickly after to fit solid state sand amps.

Joe

[Robert Gribnau]

In this advert from 1976 some Magnavox loudspeakers with 27 Ohm (and even 33 Ohm) impedance are listed:

https://worldradiohistory.com/hd2/ID...-Page-0123.pdf

[lonnyholl]

Hello Joe

Yes indeed it says on the back of the speakers 'Magnaflux !'

27ohms...Id never heard of it...was surprised I measured them to make sure and indeed it is correct!

A couple of pics below.

[lonnyholl]

Originally Posted by Robert Gribnau

In this advert from 1976 some Magnavox loudspeakers with 27 Ohm (and even 33 Ohm) impedance are listed:

https://worldradiohistory.com/hd2/ID...-Page-0123.pdf

Thanks for posting that Robert interesting.
Must have been the follow on models to the speakers I have (1969)

[joebog1]

Yes, I have seen 33 ohm speakers, also 800 ohm speakers, its a first for me with 27 ohms.
The Magnaflux speakers are alnico magnets and they will sound delicious.

Joe

[lonnyholl]

Been working on the chassis on off for a while now - radio is working well

Problem I have is large amount of Mains Hum from the Right Channel.
Right Channel measures 70Db of hum with volume at zero and no signal input.
This is about 12-14Db more than what is measured and heard from Left Channel.


In summary what has been and tried so far
Ive changed all caps on the amplifier channel - no improvement
Some resistors - no improvement.

If I swap speakers the hum still comes out of Right Channel
If I swap signal left for right signal inputs - still hum comes out of right channel

If I ground the amplifier inputs - no hum

Have Grounded/Bonded all Pot casings to star ground. - no improvement

Increased smoothing capacitance to 3000uF - slight improvement in overall hum..but still the right Channel is far worse than the right.

At the moment I'm looking at swapping the Transistors from Left Channel to Right....to see if there is any improvement...however given these transistors are so hard to come by ( I cant find any equivalents in Australia) then I am somewhat reticent to do that.

Any thoughts appreciated.

I have uploaded two Schematics - one is the original form the SM and one is more the actual circuit - note the differences around the Volume pot and also the amplifier circuit and DC supply Voltage.

[joebog1]

When you say " Have changed all capcitors" have you changed the output caps ? C26, C34 ?.
Have you checked standing current ( there are two links on the CCT diagram that show the links. You fit a milliamp meter in there and measure the current. It says 17 mA total, so you should read around 8.5 mA per output section. Does one amplifier heatsink section ( maybe just a lump of alloy plate ) get warmer than the other?.

Old Germnaium transistors DO get leaky which can exhibit itself as hum.

Can you post a pic of the PCB, because by the "sound " of it you may have carbon composition resistors which are notorious for going high in value.

Are RT1 and RT2 physically connected to the heatsink ?. They are little round discs of carbon, probably solid yellow with a tiny purple blob. ?

Joe