Hi all,

Thanks again to Dan for assisting with the Circuit for my HMV.

We have diagnosed that both audio output transformers require replacement - Can anyone assist??

Chassis is HMV N1-4X
Transformer types are 9050322 output transformers.

Look forward to hearing from someone.
Cheers
Scott

As many ar not privy to the circuit. It may be an idea to give an indication of the output Valve and voltages.

A universal type may do? Unless... It'a "ultra linear"

I would look at the thing very carefuly. Other than corossion. I find it unusual that both transformers have let go.

Hi there.

I would check those transformers again. They look to be very unique ones for that set. There is a tap off the primary side so it may be a SE Ultra Linear set-up.

the op valve is a 6M5

HMV had a bit of a habit of using ultra linear amps. I have one of their record players 2 x 6BM8 and that is SE Ultra linear. It may be the same transformer spec.

HMV Ultra Linear OP transformers have three wires on the primary and a standard secondary.

Hey guys, Yes, they are both the ultra linear type. 3 wires on the primary and the usual 2 on the secondary. I'm really not too sure how to go about finding replacements.
any suggestions are welcomed and appreciated.

If you have the specs you could try EVATCO in QLD, but get ready to spend some serious dollars.

Cheers. Though, don't really want to spend a fortune on it as I still have to repair the case & turntable on it

Hey guys, thanks for your help thus far. I'm wondering where I could find some replacement universal audio output transformers. Speakers are 4 ohm and output tubes are 6M5.

Cheers

EVATCO has a range of universal audio transformers for various wattages. Specs for each are on their site.

Thanks so much for that, at the risk of sounding ignorant, could someone please clarfiy the difference in a 'single ended' and 'push pull' transformer. I am not sure which type would be used in my HMV stereo gram.
Thanks

A push-pull output circuit requires a centre-tapped output transformer that is connected to a pair of matched output valves, whereas "single-ended" configuration is for a single output valve where the centre tap is not used. The transformers that EVATCO sell can be used in either configuration.

What sort of transformer primary is shown in your circuit diagram -- centre-tapped or not?

Looking at the diagram, it does not appear to be centre tapped. There is one output valve per channel 6M5 - though both transformers do have 3 taps on the primary side - diagram shows Transformer taps to pin 1 and pin 7 of 6M5 and the centre tap on each transformer is connected to 240V from the bridge rectifier.

I had a similar unit where the 3 primary tapped transformers were replaced with standard 2 tapped primary types, a slight modification done to the rectifier and all worked fine.

Hope this makes sense
Scott

I'm not sure that you and I are using the term "tap" in the same way. For me a tap is any wire other than the two ends of the primary winding. So, if there are three wires on the primary side, then I am calling that a primary with one tapping.

If that one tapping is a centre-tap, then it is placed at the half way point of the primary windings; that is, 50% of the windings are above it, and 50% are below it, so to speak.

If my valve data is correct then pin 1 of the 6M5 is the screen grid (with pin 7 being the anode), so your circuit seems to have a single-ended ultra-linear output stage in each channel.

Now, if that is correct, the question is: for that particular circuit, what percentage of the primary windings did HMV use to create the required screen voltage? It may be 50% or it may be less.

What exactly is the fault situation with that unit? Have both output transformers failed?

If not, then using the "good" one you could apply a small ac voltage (say 1 volt) to the secondary (i.e. speaker side) and measure the resulting primary voltages between tap and each end. Comparison of the voltages measured should give you the tapping ratio. If you're lucky they may be equal (i.e. 50/50, i.e. centre-tapped) in which case you can use a universal transformer with a suitable primary impedance.

If you do the above test, be careful of the resulting potentially high voltages present on the primary and keep the input test voltage quite low so that you don't cook the secondary.

Those are my thoughts on the issue. Somebody else may have some better ideas.

Hi there, thanks for the reply and info.
Both transformers have failed. From what you describe above they are both single ended ultra linear. As per above there are 3 wires on the primary side so it is a single tap (thanks for clarifying, I'm kind of new to valve technology). The circuit shows the songle tap as not centre but more towards the side which is connected to Pin 7 on 6M5.
Just to clarify, 6M5 pin 7 (voltage of 240V) goes directly to one side of the primary side of the transformer and pin 1 of 6M5 (230V) goes to the other primary side via a 2.7K resistor. The single tap goes to the bridge rectifier (240V)
The voltages I have shown are as per the diagram. Neither transformer reads volts (apart from the centre tap)
Scott

I lost track of this thread. (It would help greatly if this forum software would index posts by poster.)

I would like to take a look at the circuit. Can you email it to me? Send me a reply via my profile page.