V72 Input Transformer Problem

[mhuss]

One of a pair of V72s I'm working on - is not.  It turns out the input transformer is acting odd. 1-10 (1-20?) IT, so the output voltage is supposed be a lot higher than the input, and on the working V72 it is. However, in the bad one the output voltage is a *lot* lower and severely phase lagged, even at 2000 hz (see picture, the larger sine is the input, at a 10x lower setting than the output). Each half of the primary measures ~46 ohms DCR and the secondary measures about 4.3 kohms DCR on both units. For all measurements nothing is connected except a 425k terminating resistor across the secondary.

Any ideas what could have gone wrong with it? I could swear I had this working before.

--mark

 

[ruffrecords]

Is it a single secondary or are there two wired in series or parallel? If the latter than perhaps you have them wired wrong or one winding has been connected wrongly by the manufacturer.

Cheers

Ian

 

[nielsk]

I have encountered a bad input transformer in these type modules before, it does occur.
Test with secondary dis-connected.
I was able to open the can and repair, but it was not easy.

 

[mhuss]

There are four input terminals and two output terminals. The two primary windings have no interconnection. There is no way to tell without prying the can apart how many wires are connected to each terminal, but the schematic indicates a dual primary, normally connected in series via a DC blocking cap. There is a single secondary.

The other V72 I have, the signal coming out is clean and clear.

--mark

 

[tubologic]

First check if the 1µF DC blocking/LF cut capacitor in series with the two half primaries is not open. Better to defeat this capacitor by shorting the two corresponding contacts on the tuchel connector. If this doesn't solve the problem there's nothing left but the transformer. These usually fails and develop insulation problems when the V72 has been modified for 48V phantom power, or exposed to excessive moisture. Rewinding or replacement is the only cure. Good to check the square wave response (with the secondary loaded by the original resistor) defective transformers will show a very distorted ,almost unrecognizable waveform.

 

[mhuss]

With a fairly loaded down 700 Hz square wave input, and the output loaded with the original resistor, the output is very short sharp pulses with some ringing. Doesn't look good.

Anyone know of a good rewinder, preferably in the U.S.?

--mark

 

[PRR]

> measures ~46 ohms DCR ...secondary ... 4.3 kohms DCR
> output is very short sharp pulses with some ringing

It isn't inducting.

Either the iron escaped or it has a shorted turn. (One short turn won't show on a DCR test.)

> in series via a DC blocking cap.

That's a complication, but I assume you have checked or shorted-out that cap. (Not needed in a DC-free test; only real need is in telco or Simplex powering.)

 

[SSLtech]

With a fairly loaded down 700 Hz square wave input, and the output loaded with the original resistor, the output is very short sharp pulses with some ringing. Doesn't look good.

Anyone know of a good rewinder, preferably in the U.S.?

--mark

I vaguely think I've heard that these transformers might be prone to insulation breakdown and sparking from large voltage shocks such as phantom power being switched on or off... or charged 48V caps suddenly being discharged into them... patching via Bantam/TRS cnnectors with 48V engaged, or microphone cable conductor-to-ground intermittent shorts, as conceivable causes.

Either way, I would ask Oliver Archut at AMI TAB/Funkenwerk. He knows more -FAR more- than I could hope to, and I think he might even own the machines that these things were originally wound on.

http://www.tab-funkenwerk.com/id53.html

 

[SSLtech]

A bit of hunting around led me to this paragraph from the Tab-Funkenwerk site:

Today the V72 is still in demand, but most units are now at least 35 years old and need to be rebuilt or refurbished.  The electrolytic caps of the power supply are dried out and need to be replaced.  The silk-wire of the unit falls apart to the touch and in most cases this winds up shorting out the input and output transformers.  The units also suffered when given phantom power.  If a standard on/off phantom power supply was used, it would send an arc of electricity up to 2000V across the input transformer that would slowly burn the transformer’s insulation, thus destroying it.  Most historic V72 units purchased second hand need to be completely refurbished (new caps, wiring, transformers and tubes) in order to operate at their designed specification.

Naturally they're soliciting work, and many of us here can do much of the work ourselves (replacing dried up caps, measuring for spec etc.) but I note that it includes a little more info about the input transformer and insulation breakdown etc.

Talk to Oliver. -He's an extremely knowledgeable fellow on the subject, and he can make it all right.

If you have two units (did I misunderstand?) then you might be able to confirm a transformer in poor health by simple circuit-substitution and seeing if the fault follows the component... but DO bear in mind the fact that if the insulation has deteriorated with age, even simple handling might induce a fault, so -in this instance- do this with great caution.  -If you DO have a bad transformer though, the next step is to contact Oliver.

 

[mhuss]

Thanks for the replies.

Yes, the DCR is the same on both, so I suspect a shorted winding. I could swear I had this working at one point, so stressed/intermittent insulation seems a likely culprit. I'll confirm by jumpering in the other unit's IT, but I'm pretty sure a call to Oliver will be in order.

--m

 

[jensenmann]

Sowter sells replacement iron which is made with the data CJ got from hacking my V72s (which he still didn´t send back... 8 years now???). This should be at least an alternative to look at. So the death of my V72 at least made sense.... for all the others, unfortunately not for me.
http://www.sowter.co.uk/vintage-audio-transformers.php

 

[KianMcEvoy]

Does anyone here own a working V72?

For my dissertation i'm looking into building a Digital clone of a V72 that models it's analogue counter-part component by component, designed from the circuit diagram and the tube specs available.

I was wondering if anyone could help me? In order to compare my model with the original I need to run a short set of test tones through a V72 (around 2 mins of audio) and record the output.

This comparison between the original and the model I make is the cornerstone of my dissertation and it's proving tricky getting access to one, if anyone is able to help me with this it would be hugely appreciated and i'm happy to reimburse anyone for any time/expense incurred.

If you or anyone you know can help, please get in touch.

Many Thanks,
Kian

email: [email protected]