Vintage Japanese valve tape recorder needs Output Transformer. Help please

[jascha]

Hi Everyone,

A few days ago I decided to give one of my 5 identical machines a run just to keep me some company on a lonely morning and sadly after starting to play a tape and put the volume up a bit, it went totally silent. Of course very upsetting as I treasure these machines from my childhood. I had a look inside and first thing I did was to swap valves around with another machine. Valves were fine!

Cut the long story short I came to the conclusion that the Output transformer primary is an open circuit as it is measuring open circuit and on test one side of the primary coil measured 332V DC and the other side of the coil a mere 10V! Idid inject a signal from a signal generator and put the machine into record and the recording level flickered correctly (machines use a diode lamp) and I got a signal out of the monitor output (does not involve the amplification side of the circuit it seems).

Earlier today I decided to investigate the same area on an identical machine and the voltages at both ends of the primary coil are measuring 295V and 278V with respect to ground. The resistance of the primary coil measures 530 ohms and the secondary measures around 1 ohm.I therefore came to the conclusion that the OT has packed up on the broken machine and need a replacement. I have thought of having it rewound but that could be costly?
I have attached a few photos and a circuit. The machines use a 6AR5 as an output valve

The small OT is measuring around 4cmH X 4cmW (base measures 6cm including side mounting strips) and around 6 cm deep. Would anyone give me some idea please where to start looking for such a transformer and what to look for please as regards to ratios etc as I am lost. I did find one on ebay but not sure this would suit my needs:

http://www.ebay.co.uk/itm/2915467745...%3AMEBIDX%3AIT

Any help much appreciated please to get my machine all happy going again.
Many thanks

[paulsherwin]

That looks like a very simple transformer and any transformer from a scrap radio is likely to work if it will physically fit. The turns ratio may be suboptimal but that shouldn't have an audible effect.

A forum member will probably have something suitable in the junk box if you ask in the appropriate section: https://www.vintage-radio.net/forum/...splay.php?f=28

The commonest cause of an output transformer primary blowing is operating the amp without a speaker connected, though they can also fail through corrosion or simple fatigue.

[Refugee]

It is possible that there could be a dry joint between the inner end of the solder tag and one end of the primary.
Try measuring the primary with a meter or apply audio from a transistor amplifier and waggle the tabs one at a time and see if there is any sign of life.
If there is you can carefully dig the terminal out and fix it.

[jascha]

I have thought of the dry joint situation but to access the primary winding terminals is a headache! I just about managed to get to them with a meter probes however I guess it is worth dismantling the set up to get the transformer in hand and re soldering the terminals. The trouble is that it just stopped working suddenly after producing sound for a few seconds and as soon as I highered the volume a bit it went silent. Curious situation!

[paulsherwin]

You will need to access the transformer anyway to replace it, so you might as well do it now.

The fault is unlikely to be a dry joint as such but there may be a break in the primary wire close to the solder tag. You have nothing to lose in investigating the old transformer, as if you can't repair it you will need to rewind or replace it anyway.

[julie_m]

Another possible cause of output transformer failure would be for C14 to have become electrically leaky. This would allow DC to flow into the grid of the 6AR5; causing too much current to flow via its anode. If all the recorders are the same age, all the C14s will be in similar states of deterioration. Any 47 nF, 400 V capacitor will make a suitable replacement, and capacitors are so cheap -- especially compared to output transformers -- it's not worth risking notreplacing it in all your machines.

[ricard]

Looks like a nice machine, I can understand you want to get it working again.

[jascha]

Many thanks for your replies. @Julie_m: I have measured negligable DC voltage at the other end of C14 and also checked the other 2 signal capacitors connected to the other anodes of their respective valves and again there was hardly a trace of dc (measuring like 0.003v). Does that count like a leak please?

I have been advised to ask my questions in a different thread which I attach here and have also had some amazing responses and am so grateful to all of you and appreciate your help immensely:

https://www.vintage-radio.net/forum/...1663:thumbsup:

[Refugee]

Those voltages are good and the lack of heat damage on the faulty transformer fits in with that too.
It would not be the first transformer I have seen with a loose wire in it. I found a loose terminal in a transformer that had short circuited against the one next to it while doing photos to send with a request for rewinding. I was able to fix it there and then.

[Refugee]

I have just repaired an audio oscillator coil on a signal generator that needed a termination dug out for repair.
Post #12 in this thread about half way down.
https://www.vintage-radio.net/forum/...d.php?t=118937
The type of tags on your transformer are more prone to dry joints than mine as they are big and long.

[jascha]

Just an update to say thank you to you all for your replies and sharing your knowledge and especially thank you to Restoration73 for supplying me with a replacement tx that is all working well. I attach photos