Hi All,
I am going to return to work on this amp again now that the WEM is fully operational.
If the output transformer has failed, can I still check the output at the valves to start with without creating any further damage?
From a completely different perspective, I did try to change the speaker grill covering with some old radio speaker cloth as the original vinyl adhesive type had split and was dirty. As always, I couldn't get the new cloth to line up correctly and it looks terrible!! If I recover this again, can anyone recommend a suitable grill cloth thats relatively easy to put on and doesn't create any issues with the the pattern/lining up etc?
Update....I've tested the speaker and its working perfectly.
I'm beginning to think that it's the output transformer. It looks like the only way I can really test it is to take it out of the chassis as it has a mustard cap that I installed (replaced. an old black 0.01uF Hunts...ugh!) across the primary (black and brown leads) so I am not sure that I can get accurate resistance readings whilst its still wired in. I did undertake a couple of quick resistance readings (I'm sure it wont be accurate or correct) and it was about 4M R across the primary connectors! From secondary to ground it was something like 400R. In both cases this was with no power on.
I have also attached a schematic that was kindly put together by Lawerence (who was extremely helpful) on another well known other forum. I wont dwell on this but to say my posts where not well received by some members so I didn't want to pursue it any further and cause issues!
As always any guidance or help is most gratefully received.
Best Regards
Rob
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It looks like the speaker is connected across the primary winding.
There is also a loose blue wire going towards the speaker.
Hi,
I'd get the speaker leads to test the speaker. I had to get these to take the chassis out again.
I'd agree with Refugee.
Certainly the colours connected would relate to the speaker being wired to the primary. An elemtery mistake and one that could have caused a nasty DC shock. Oh dear.
(20-06-2021, 07:00 PM)Bushbaby Wrote: [ -> ]Update....I've tested the speaker and its working perfectly.
I'm beginning to think that it's the output transformer. It looks like the only way I can really test it is to take it out of the chassis as it has a mustard cap that I installed (replaced. an old black 0.01uF Hunts...ugh!) across the primary (black and brown leads) so I am not sure that I can get accurate resistance readings whilst its still wired in. I did undertake a couple of quick resistance readings (I'm sure it wont be accurate or correct) and it was about 4M R across the primary connectors! From secondary to ground it was something like 400R. In both cases this was with no power on.
I have also attached a schematic that was kindly put together by Lawerence (who was extremely helpful) on another well known other forum. I wont dwell on this but to say my posts where not well received by some members so I didn't want to pursue it any further and cause issues!
As always any guidance or help is most gratefully received.
Best Regards
Rob
Hi.
Unfortunately on the diagram of the main amp there is three caps with no values and the coupling cap to the EL84 Grid 1 is incorrect at 500pf.
Incidentally re the speaker connections look at the photo on Post 1 you'll see you have transposed the speaker leads on the output transformer.
OMG! Have I made a HUGE error here! I've attached a pic of the original OPT transformer wiring and it looks like I have wired the speaker to the PRIMARY leads!!! Is this correct and if so have I damaged anything?? The speaker does seen to be fine but what about the OPT??
Oh dear!
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(20-06-2021, 07:00 PM)Bushbaby Wrote: [ -> ]... it has a mustard cap that I installed (replaced. an old black 0.01uF Hunts...ugh!) across the primary (black and brown leads) so I am not sure that I can get accurate resistance readings whilst its still wired in.
Hi Rob,
Good as Mullard 'mustard' capacitors are, the one you fitted is only rated at 400V and is not suitable for use in that position.... it needs a capacitor rated at 1000V.
Also, there would be no problem measuring the resistance across the transformer primary with the capacitor in circuit, as capacitors don't pass DC (from your meter)
unless they're faulty - and even an old leaky capacitor in that position wouldn't affect the reading much (if at all) because the winding resistance should be significantly less, probably less than 1000 ohms.
Andy
Hi Rob.
OK so you have realised your mistake, like I pointed out the issue is of course the HT would then be on the speaker terminals and possibly on the speaker framework and magnet depending if its connected to one of the speaker terminals. The problem here to me is the shock hazard, its DC at perhaps 250 to 300 volts DC can with some people cause you to latch on and not throw you off which is more likely on AC.
The advice I give to novices is check check and check again everything you do.
So my question is. What damage do you think it may have done? Hopefully no one will prompt you I just would like you to work this one out
(21-06-2021, 06:16 AM)radioman Wrote: [ -> ]Good as Mullard 'mustard' capacitors are, the one you fitted is only rated at 400V and is not suitable for use in that position.... it needs a capacitor rated at 1000V.
While it was common practice to fit 1000V parts in this position back then this was recognition ofthe relatively low relaibility of older capacitors. Lots of the standard paper caps back then were only rated at 350V which really isn't enough.
In an ordinary radio or amp with HT under 300V I wouldn't worry about using modern 630V parts. Modern (which includes "mustard") 400V caps are likely to survive but not recommended.
If you disconnect the speaker and turn up the volume both the capacitor and output transformer will be at risk of overvoltage.
PS: At least in that design they fitted the cap across the transformer primary so if it goes short circuit the output tranformer doesn't suffer. The valve, rectifier and mains TX will all be unhappy. A lot of designs had the cap between the anode and 0V. Not a good idea as a short can destroy the output transformer, even though the output valve will be safe. I'd rather replace a valve than a transformer.