Hi all,

I'm new here and seeking some advice regarding the repair of the above portable record player. It was picked up by a friend who asked me to repair it. I have tinkered with some valve radios before and understand the operating principles of the valves for the most part.

Looks like this:
http://carnegie.australialisted.com/3163/art-antiques/antique-collectable-his-masters-voicehmv-record-player-turntable_18100843.html

My issue is that one of the output transformers appear to be blown, as one of the channels is dead. I have tested this though continuity tests across primary and secondary taps. Secondary side is fine but primary side is gone.The transformer part no is 9050322 (HMV proprietary I think, can find a data sheet for it alas) and there have been some other people on the forum with issues with the same type of transformer but no resolution was reached (that I could see).

The transformer itself is a two wire secondary, with a 3-wire, single tap primary. I believe this is an ultra linear type transformer, used to give feedback to the valves. The valves used are 6BM8 triode, pentrode valves.

I have pulled out the working transformer and characterised it using a scope and sig gen, and it has the following characteristics:

Winding ratio= 6:1

Primary side tap ratio=1.2:1

Can anybody advise me as to replacing the transformer? I believe it is unlikely that I can get one with the same winding ratio, so what would the effects of changing the ratio of the centre tap? Will this require returning of the circuit?

FYI I'm studying electrical engineering at uni so please be as technical as needed. Also I have access to signal gens, scopes etc.

Also can I post picture to the site of the circuit ? I am working on circuit diagram at the moment but if someone had one that would be great.

Thanks in advance.

Hi Israel.

You may struggle to find a transformer unless someone has a junked chassis somewhere.
Why not get it re-wound? I have used David Browne to rewind a choke and the price was very reasonable.

http://www.transformerrewind.com.au/

Getting this done saves a lot of heartache chasing up parts.

cheers
Ben

Is that a simple 2 x 6BM8 chassis, with solid state rectifier?

Thanks for the replies, dare I ask how much reasonable is? Not sure how much my friend is willing to spend on the job at present, wanted to go to him with rough numbers if possible. Also if I get one rewound it would probably be worth while to get both done to prevent future failures.

Not sure as to what a "standard 6BM8" chassis looks like. But the power supply is a mains transformer (not characterised) driving a selenium bridge rectifier manufactured by sentertcel. The filter cap is a weird 24+24mF electrolytic (no other supply filtering). Can't find much about it, but based on the pin out (has 3 pins, two to +ve, at different points in the circuit and one to ground) I'm assuming it's two 24mF caps in the one package.

Also I made an error with the characterisation of the output transformers, I had the scope DC coupled on initial test. The values are in fact:

5.6:1 across Secondary

4.8:1/0.8:1 on tap

Possibly a O1 chassis.

First thing is to get rid of the selenium rectifier. The HMV technical data sheets give instructions on replacing selenium rectifiers with silicon solid-state types for chassis' such as the O1, N1 & N2.

The model number should be inked somewhere on the chassis. Service Manuals are available for most HMV models.

Again: what is the valve line-up on that chassis -- that is, tell us exactly what valves are on it.

Valve line-up is often the best clue to what you have.

What do you mean by valve line up? I going to guess you mean the amp class and how the tubes are employed?

Valves used are two 6BM8 valves, one per channel.

Phono signal through a potentiometer (for volume) into class A triode amp with a pentode in some form of cathode follower current source configuration. Also 4 ohm speakers, for reference. Also has tone ( on pentode), balance controls.

Also regarding the technical data sheet I have not been able to find one for this record player. The O1 chassis mentioned appears to be a 5-valve superhet? Is this correct? Is there some better search key words I might use?

HMV service manuals often come in two parts - one for the chassis & the other a model sheet. That is because HMV often used the same chassis for a number of models. The Technical Data Sheets are published for updates & modifications.

What do you mean by valve line up?

As I said in post #6 "that is, exactly what valves are on it." Line-up is a commonly-used term meaning the valve contingent or complement of a radio or other vacuum tube device.

I agree with Mono that this sounds like an O1 chassis. If you unhide your email address using Control Panel I will send you the schematic.

I agree that rewinding the speaker transformer is probably the best option unless you can find a donor. The re-winder will determine the winding details from the existing transformer.

Also agree the the selenium rectifier should be replaced by a silicon bridge. Selenium rectifiers are unreliable at this age and when they blow they create a god-awful smell, and selenium is toxic.

Finally, I would replace all of the electrolytic capacitors and any wax/paper capacitors on principle as they are well past their use by dates.

If you do all of that, the amplifier can be expected to operate for many years to come (assuming that the other OP transformer holds up).

The Electrogram shown in the link is a model O1-8H. The other 2 are a O1-8A & O1-88. All use 2 variants of the same chassis.

The Technical Data Sheet No. 66 is for instructions on how to replace the selenium rectifier with silicon diodes for chassis types N1, N2, N3 & O1. Since there is less of a voltage drop with silicon diodes it suggests using a 110 ohm, 2 watt resistor in series between the B+ and the first filter cap.

Hey All,

Thanks for the help so far, the model is O1/8, just checked (looks like you were all spot on), missed it completely till now... I have also revealed my email, a schematic would be awesome.

Cheers

Schematic sent.

Thanks for the replies, dare I ask how much reasonable is?

$40

After pawing over the TDS and manuals, I find myself somewhat confused with the power supply configuration. Specifically the schematics show the output of the bridge rectifier to be at 245V in the unmodified circuit for the 01 chassis, but the input for the bridge is 222V off the power transformer secondary, so how can this be? Is this a error in the datasheets or is this a convention I am not aware of? From a technical point of view neither the peak voltage of the rectified signal before filtering nor the DC average voltage of the power supply ripple can be greater than the peak input voltage.

I have also simulated the circuit using spice as a sanity check, and confirmed that this is definitely the case. At no load the output voltage is 221VDC (with filtering) which makes perfect sense to me as the forward bias voltage drop of a 1N4007 diode (what I used for simulation as I intended to replace the selenium bridge with a suitable bridge IC) is 1V.

Does anyone know the reason for the discrepancy?

I think you have failed to appreciate the difference between DC & RMS. RMS is a practical value based on heating and is what is measured on the mains and the secondary.

You will note that on a graph DC appears in time & amplitude as a solid block. AC sine wave as akin to a series of triangles. The area of a triangle of the same height is far less than the DC block. So in order to get the same heating (energy) as is in the DC block, you need a taller triangle.

So the RMS voltage is set at the point of the triangle where it gives the same heating as DC and that just happens to be 1.414, or the square root of two. So when you rectify it you actually get the peak of the AC wave and off load (open circuit) close to twice EMF.

I have noted that issue on a Vacuum cleaner that's head motor is 240VDC max. yet on load, with 230VAC (Modules stamped voltage: European) fed by Variac, the motor is running at 265VDC and 295DC with 245VAC.... and one wonders why these have problems with the motor being stopped by the Poly switch?

Marc