28th Aug 2011, 8:32 pm | #101 |
Dekatron
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Re: Mullard 5-20 new chassis problems
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 |
28th Aug 2011, 9:25 pm | #102 |
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Re: Mullard 5-20 new chassis problems
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! |
28th Aug 2011, 9:27 pm | #103 |
Nonode
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Re: Mullard 5-20 new chassis problems
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
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28th Aug 2011, 10:09 pm | #104 |
Octode
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Re: Mullard 5-20 new chassis problems
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! |
29th Aug 2011, 12:48 am | #105 |
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Re: Mullard 5-20 new chassis problems
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 |
29th Aug 2011, 3:01 am | #106 | ||||
Hexode
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Re: Mullard 5-20 new chassis problems
Hello David,
I suspected as much! Quote:
Quote:
Quote:
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! Quote:
Regards, Dave. Last edited by Amraduk; 29th Aug 2011 at 3:08 am. |
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29th Aug 2011, 10:51 am | #107 |
Octode
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Re: Mullard 5-20 new chassis problems
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. |
29th Aug 2011, 11:39 am | #108 |
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Re: Mullard 5-20 new chassis problems
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. |
29th Aug 2011, 12:01 pm | #109 | ||||
Hexode
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Re: Mullard 5-20 new chassis problems
Hello David,
No problem! Quote:
Quote:
Quote:
Quote:
Regards, Dave. |
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29th Aug 2011, 2:38 pm | #110 |
Octode
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Re: Mullard 5-20 new chassis problems
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. |
29th Aug 2011, 3:33 pm | #111 | |
Hexode
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Re: Mullard 5-20 new chassis problems
Hello David,
Quote:
Regards, Dave. |
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29th Aug 2011, 3:36 pm | #112 |
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Re: Mullard 5-20 new chassis problems
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
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29th Aug 2011, 5:42 pm | #113 |
Octode
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Re: Mullard 5-20 new chassis problems
Thanks Andy--I'll drink your hifi building success too .
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30th Aug 2011, 8:03 pm | #114 |
Octode
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Re: Mullard 5-20 new chassis problems
Mods, I'm happy for this thread to be closed now. Thank you.
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16th Sep 2011, 11:00 pm | #115 |
Octode
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Re: Mullard 5-20 new chassis problems
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.
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16th Sep 2011, 11:12 pm | #116 | ||
Hexode
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Re: Mullard 5-20 new chassis problems
Hello David,
Quote:
Quote:
Regards, Dave. |
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