Mullard 5-20 new chassis problems

[Ed_Dinning]

Hi Gents, resistance measurements can often be a problem on transformers. It is far better to feed a few volts AC into what looks like the secondary wires and then measure the ouputs on the primary wires.
When the primaries are all correctly connected in series you will have max volts out of the full winding (the 2 anode leads). The centre tap will be exactly 1/2 of this.

Ed

[PJL]

Hi Ed, only looking for incremental readings just to check the taps are in the right order. The transformer is interleaved and is wired in two halves.

I think this thread is complete once we have a message back from David that it's working OK!

[Alistair D]

Yes ED thats the way I'd do it. Be careful though. You could be talking 50 to 1 step up ratio on each of the primaries. Applying 6V to the secondary could result in something like 300V on the primary.

Al

[qualityten]

You guys are geniuses, and patient ones at that. The joined yellow wire was the problem, as well as the earlier crossed link between C10 and C11. It's working perfectly. I heard a little hiss, and then realised that was the tape of the original recording.

Just for completeness, the resistances measured out fine:
Blue to Brown V3 39.5R and V4 40R
Blue to Yellow V3 84R and V4 65.6R
Blue to red V3 190R and V4 158R
Then measure blue to blue. No continuity between blue and blue.

Dave, you asked about the secondary:
Black (to 0 ohms socket and also to ground) to other black 0 R
Black (also to ground) to brown 1.1R
Black (also to ground) to red 0 R
Other black to brown 0 R (these are joined and attached to 5ohm socket), with NFB.
Other black to red 1.2R
Brown to red 0 R
Red alone is attached to 15 ohm socket.

Peter and Dave (and other forum members) my very sincere thanks to you both for getting this amp back to working order. I am appreciate your patient, step by step help. And I've learned a lot more than if I'd just put it back as it was!

[GrimJosef]

Good to hear it's working ! A couple of general principles have been illustrated here. I may be stating the obvious but i) if something was working, then I did something to it and that caused it to stop working, the fault is almost certainly associated with what I did (even if I'm really confident that I did it properly) and ii) that, as David has said, learning by doing can be a very effective (if sometimes slow and painful !) way of making progress. As the physicist Niels Bohr said, 'An expert is a man who has made all the mistakes ...' .

And to answer magnetic's question (post #83) it was hot, humid and sometimes rainy in Shanghai. Beijing seems, if anything, to be hotter but drier, thank goodness .

Cheers,

GJ

[Amraduk]

Hello David,

Originally Posted by qualityten

The joined yellow wire was the problem,...

I suspected as much!

...as well as the earlier crossed link between C10 and C11. It's working perfectly. I heard a little hiss, and then realised that was the tape of the original recording.

I'm very pleased to heat that! It took a bit of time to get there (that's the trouble with fault finding by remote control!), but it worked out OK in the end.

Just for completeness, the resistances measured out fine:

Apart from the difference between the two brown to yellow values (45.5 Ohms/25.5 Ohms), they seem about right. Anyway, it's now working, which is the main thing.

Dave, you asked about the secondary:

It would seem that your meter, in common with most general purpose ones, doesn't have sufficient resolution to measure very low resistances. The only reading, if how I think the transformer is arranged (see attachment) is correct, is the zero ohms between the brown and read leads, which I would have expected to be about 3.3 ohms. Was the secondary winding completely disconnected (all wires not connected to any others) when you took the measurements?

I expect the transformer has been re-assembled by now, so I don't expect you to take it all apart again just to confirm those readings!

Peter and Dave (and other forum members) my very sincere thanks to you both for getting this amp back to working order. I am appreciate your patient, step by step help. And I've learned a lot more than if I'd just put it back as it was!

You're very welcome, and I'm pleased to hear that it has also been somewhat educational for you!

Regards,

Dave.

[qualityten]

Thanks Dave and GJ. This amp has proved a little more challenging for me to work with as it is a home construction with old transformers with extra taps (on both the power tx and output tx). My limited understanding of transformers led me to assume that two neatly joined and sleeved yellow wires had been connected.

Aging components were also a problem, when moved. E.g. one resistor looked and measured okay at first, but after being moved slightly increased its resistance greatly. After the repair, the brown secondary wire snapped off at the solder joint with metal fatigue, just in the move from one room to another!

Dave, I was a bit rushed with the secondary measurements. I use an old Select ADM 515 and Fluke 77III, both s/hand. I don't have a manual for the Fluke, so am still working out how best to use its manual ranges. Assembled, the resistance from black (0R) to the 5ohm socket is 1.1 ohms and from black to 15ohm socket 2.2 ohms. I am confident that I am using the same connections as before, but no other knowledgeable person has physically inspected the amp.

While my ideal would be to learn directly alongside someone, this sort of problem-based distance learning is effective, with the generous support of members of this forum. It is helpful to have to write down one's observations and measurements as carefully as possible and invaluable to have the advice of more knowledgeable people.

[PJL]

Hi, pleased to hear it's working again.

Resistance measurements on transformers do not mean anything, its the number of turns (turns ratio) that matter. Ed's post 101 tells you how to work out the turns ratio and therefore the appropriate speaker impedance to match to the output valves. I normally just connect the heater winding across the primary and measure the AC voltage on the secondaries.

I am sure the 5-20 output transformer turns ratio must be specified somewhere, if not it is easy enough to calculate using the n*n rule.

[Amraduk]

Hello David,

Originally Posted by qualityten

Thanks Dave and GJ.

No problem!

This amp has proved a little more challenging for me to work with as it is a home construction with old transformers with extra taps (on both the power tx and output tx).

That's fairly common.

My limited understanding of transformers led me to assume that two neatly joined and sleeved yellow wires had been connected.

That's quite understandable. It's all part of the learning experience and will probably be of help in the future.

Dave, I was a bit rushed with the secondary measurements. I use an old Select ADM 515 and Fluke 77III, both s/hand. I don't have a manual for the Fluke, so am still working out how best to use its manual ranges. Assembled, the resistance from black (0R) to the 5ohm socket is 1.1 ohms and from black to 15ohm socket 2.2 ohms. I am confident that I am using the same connections as before, but no other knowledgeable person has physically inspected the amp.

OK. those resistances are what I would expect, which confirms that the secondary is also made of two separate windings. When I said I would have expected 3.3 ohms between the brown and red leads, that should have been 2.3 ohms, which is more-or-less what you're getting!

While my ideal would be to learn directly alongside someone, this sort of problem-based distance learning is effective, with the generous support of members of this forum. It is helpful to have to write down one's observations and measurements as carefully as possible and invaluable to have the advice of more knowledgeable people.

In my experience, you learn much more about how something works by having to fix it than you do by throwing it together and having it work straight away!

Regards,

Dave.

[qualityten]

A quick follow up to this. After half an hour today the amp suddenly stopped playing! As the whole amp has now been gone over, I scratched my head, redoubled my commitment to learning to use a scope and wondered whether to try the patience of kind forum members again.

But it turns out just to have been a bad Sovtek ECC83 (obtained secondhand in the first place)! Luckily the transformer was already known to be sorted, otherwise this ECC83 might have been a further red herring. Well, it all sounds good now.

[Amraduk]

Hello David,

Originally Posted by qualityten

But it turns out just to have been a bad Sovtek ECC83 (obtained secondhand in the first place)!

I'm pleased to hear that you have managed to fix the fault! I have just sent you a PM about it but you won't need the information at present, though it might otherwise be useful!

Regards,

Dave.

[Diabolical Artificer]

Happy for you that its all worked out David. Its a crazy hobby with lots of sleepless head scratching moments,but well worth it. Sit back and enjoy your tipple with some groovy sounds. A

[qualityten]

Thanks Andy--I'll drink your hifi building success too .

[qualityten]

Mods, I'm happy for this thread to be closed now. Thank you.

[qualityten]

As this thread is still open it gives me an opportunity to thank those who helped me once again. I am delighted with the sound of my pair of 5-20s and grateful for all I've learned from forum members.

[Amraduk]

Hello David,

Originally Posted by qualityten

As this thread is still open it gives me an opportunity to thank those who helped me once again.

You're very welcome.

I am delighted with the sound of my pair of 5-20s and grateful for all I've learned from forum members.

I'm pleased to hear that the 5-20s are still working OK and giving you pleasure.

Regards,

Dave.