Australian HMV Rhapsody Radiogram. Replacing Amplifier - Questions!?

[lonnyholl]

Hi Joe thanks for your reply & queries

Yes C26 and C34 replaced.
The Germanium Transistors are clamped to the chassis, acting as the heatsink. none of the transistors are hot and all about the same temp to touch.

(See PCB Layout attachment PDF) - RT1 and RT2 are sitting on the PCB and although close to the Germanium Transistors wire connections are not in connection physically to the Germanium Transistors because they are actually clamped to the chassis, the transistors being connected to the PCB by wires.

Also attached a photo of actual board - taken before all Caps were replaced - you can see how the 4 Germanium Transistors are clamped to the chassis adjacent to the board

I havnt measured Amplifier ampage yet...will do that and report back.

Ive got all resistors needed to do a complete re-resistor.

Cheers

[lonnyholl]

Amperage to both Channels is 16mA with no signal and zero volume ...varies less than a mA with Volume to max.

[joebog1]

NICE scan and pics !!!.
Next step is to check "idling current ". Thats the current each output stage carries without sound at all, BUT after its reached operating temerature. Remember that this is a bit of aussie history and geranium , my bad , Germanium transistors do have a mind of their own.

The quietscent current is of major importance. The resistors on that board are, as I said, carbon composition. In their day they worked fine, but dont have grandpah status. They age!!, and can go wandering off on their own.
Its a little difficult to measure all of them, but most can be measured with a standard multimeter. You will need to check both ways ( red to one end, black to the other, then swap the meter leads around, black where red was, red where black was ) to get a fairly close idea.
The ones that "mostly " give trouble are high values, BUT where the current might be over spec, thay can overheat and go high in value as well. A few OP's here have reported that some go low, but I have never seen that myself.
In the interest of the age of the unit, and the fact I would say, never been cooked before, I would change only those that prove out of spec.

That said, again, If you find one or two that are out of spec, lift one end of the resistor from the PCB and measure it gain, WITHOUT your fingers holding the probes onto the resistor leads and measure it again. The resistors " I think " ( I dont know for sure )"
are made by IRH on Liverpool Road in Sydney, but based on the American resistors that look "almost " identical. They cook and go high, but as a sort of general rule were much better behaved than the American versions. ( Fender from the same era, are FULL of the horrible things ).

When checking quiescent current, DO IT VERY slowely!!!!!
REPEAT SLOWELY. Adjust by VERY small increments and wait till the transistor temperatures adapt to the new current setting.

Germanium has the property of thinking its in charge now and can over current VERY quickly.
HOW do I know? I have changed quite a few where I was fast and smart, rather than steady and slow.

The sound quality from this amp when running properly will be very close to valve sound!!. Germanium tends a bit to even harmonic distortion, rather than brash hard sand amp distortion, which "can be " very odd order.

I have looked in my transistors stash ( all brand NOS Mullards from Adelaide ) and can supply a few if you need ( or blow some ), but remember, transistors in origional packaging from this era are very rare.

Chat soon, and keep up the excellent work.

Joe

[lonnyholl]

Thanks very much for your assistance and info Joe!

Decided I'm going to change all resistors..(Ive done a couple and another 8 or so to go!) - because I've found with this board is that all resistor components have leads that are black and tarnished subsequently hard to get an accurate measure...so probably best if I change them out and eliminate any possibility. ..about half way there now and only doing the right channel.

Hopefully complete that task by tomorrow and post results.

Thankyou much also for the offer of transistors. Still hoping its something else at this time...will see if resistor swap out will make any difference...if not then I guess the geraniums! are more likely.

Cheers

[paulsherwin]

Joe's advice is all good, but this sounds like an earthing problem to me. Do you get the hum on the radio, or just when the gram is selected? Can you confirm that the hum level changes with the volume control?

[lonnyholl]

Hello Paul Thanks for your reply

The hum - I get it without either Gram or Radio selected or with Gram or Radio selected -
when at zero volume - it sounds like a steady mains/50Hz hum.

Increasing Volume does not increase the hum although when you get to near max volume there is a slightly louder hum that comes on...I had just put that down to the amplifier being at its peak.

I had thought it might be earthing related.. I grounded all pot Casings back to the common grounding point..I even tried a thick wire (one end at the star point) one end touching the chassis in different spots but it didn't seem to make any difference...unless the germanium Transistors are leaky to ground!? - they are metal-clamped to the chassis.

The radio is quite good - AM...seems performance is favourable over my portable radio in that location.

The actual circuit is a bit different than the original schematic and I wondered if there was anything peculiar with the updated circuitry.. I noted the reduction in Voltage supplied (29vDC to now 26vDC) and the Preamp seems to have a Dropper resistor now to supply it with less at 25V..not sure why they did that ?! ... I had wondered if they cured one problem with their modifcations and maybe created another with the hum!...but I'm not knowledgeable with Amplifiers to tell if anything they have changed could cause an issue..or if there is anything wrong with the design. I'm nearly finished up changing all resistors on the right channel - will be tomorrow for sure...found so far one resistor that was about 20% over Value.

[paulsherwin]

But you said the hum disappears when you ground the input. That's effectively what the volume controls do when at minimum. There is some fiddly mono/stereo switching just before the volume controls which you should investigate. I assume you've cleaned all the contacts thoroughly?

[lonnyholl]

Originally Posted by paulsherwin

But you said the hum disappears when you ground the input. That's effectively what the volume controls do when at minimum. There is some fiddly mono/stereo switching just before the volume controls which you should investigate. I assume you've cleaned all the contacts thoroughly?

Thanks Paul!

With the fiddly/complicated switching -

I removed the mains 240v wiring from that switch array so that power cabling and switching is located physically away from the signal cables - & so now no 240V next to the switching of the audio signals.

Also have removed the mono jack input plugs (seen in the original schematic) to remove that as a possible issue...the input audio path is now simpler..but still being switched atm in that fiddly way.

I have attempted to clean the signal switches, using liberal amounts of switch cleaner, they appear to work ok...however...

..as you suggest it could point to somekind of pickup on the input side... so what I can do easy enough (after re-resistoring the Right Channel tomorrow) is bypass these audio signal's switches.. perhaps then that just leaves the Volume Pot as a potential issue as far as input goes.

The odd thing still to me atm that swapping input signal cables to the amplifier still sees the Hum from the right channel.

Will also retest grounding of input if Hum after all the steps has not gone...will make sure that is still the case.

[lonnyholl]

After re-resistoring the right channel...a good surprise !

Right channel is now quiet at zero volume...and now measures 6dB less than the Left. (although my ears cant hear that difference)
Was not expecting such difference. Thanks Joe!


Crude Db measurements taken at close to Speaker with Vol at Zero

Before Re-resistoring Right Channel
Right Channel 72 dB
Left Channel 56 dB

After
Right Channel 46 dB!

Feel I should Re-Resistorise the Left channel now as well!...however I might just limit to changing the couple of resistors that I found high in the Right Channel.

Given the improvement seen, the radio would likely benefit as well from a full recomponent - a lot of Ceramics and old carbon resistors in there..however I know it will not be used for the Radio so wont go any further and have enough still to do.

Got to get the unit back together! - need to add a DPDT switch and Aux Input Jack Socket to allow an external Logitech Aux/Blutooth Module to plug in (will simply hide the module on the back of the radiogram with Velcro) ...the DPDT switching between Gram and Aux - already tested crudely, sounds great!...This Germanium Amp has rather a Valve like sound - as Joe said.

Soon I will check/test repair maintain the old BSR Turntable and fit a new Cartridge and Stylus. - and then put it all back together.

Will fit a Mains Switched/Fused IEC Plug Socket...this will give a little more safety with a fused primary but also allows me to run the internal mains cable away from the old Metal Latched switch array and signal wires.

I've also got to try and mould a replacement plastic knob! I have seen a couple of videos that shows what can be achieved with epoxy - so will give that a go...the existing knobs will have to be sprayed gold metallic to bring them back.

Glad the issue was not Transistor.

Thanks to all who have responded!

[joebog1]

Those "old ceramics " will most probably check OK, they dont fail very often. Those carbon composition resistors are a right PITA if you want Hi-Fi. Actually, those ceramics will be pretty hard to change in any case, as although they were listed as +/- 20% they can be much more than that. If you change them willy nilly you will detune the radio and it will need a full alignment thereafter. THATS a horrible job with those little coils and screwcores.

Good job.

Joe

[lonnyholl]

Originally Posted by joebog1

Those "old ceramics " will most probably check OK, they dont fail very often. Those carbon composition resistors are a right PITA if you want Hi-Fi. Actually, those ceramics will be pretty hard to change in any case, as although they were listed as +/- 20% they can be much more than that. If you change them willy nilly you will detune the radio and it will need a full alignment thereafter. THATS a horrible job with those little coils and screwcores.

Good job.

Joe

Thanks for your reply Joe and thankyou for your help with the Rhapsody!
and for your thoughts on the radio - I will leave the radio be. It still performs reasonably well as is.
Yes the cores look fragile - I would be anxious to the possibility of breaking them.
'if it aint broke dont fix it'.