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EH Polyphase transformer dead

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Re: EH Polyphase transformer dead

Postby ef37a » Wed Jan 09, 2019 10:57 am

Adam Inglis wrote:Should I increase the resistance to the "ON" LED while I'm at it?
What do you suspect the collector voltage was with the original transformer?

Like Will I am not happy either with running old kit at higher than original voltages. 14.8 I would live with but anything approaching 16V is possible trouble.

Easiest solution all round is to try some resistance post the bridge, before the series transistor collector. Has the added advantage in reducing transistor dissipation a bit and you could also hang 100mfd after it for a bit of extra smoothing.

If you can do it safely always good to check the mains voltage? Circuits should be abe to cope with top tolerance, 253V in UK.

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Re: EH Polyphase transformer dead

Postby Folderol » Wed Jan 09, 2019 11:37 am

Adam Inglis wrote:Should I increase the resistance to the "ON" LED while I'm at it?
What do you suspect the collector voltage was with the original transformer?
No need for the power led resistor to change, because of the total voltage drop the current will change very little here.

I would expect the original transformer was 15-0-15V which would have given about 20V DC on the smoothing cap. After the unit has been running for a while, check the temperature of the power transistor. If it's just 'comfortably' warm it's fine. If is seriously hot - too hot to touch, then it will need more protection. Is it on a heatsink, or just sitting on the PCB?

Dave's idea of a series resistor here would help, but it starts to get a bit more complicated as you then need to know the current in order to to work out the needed resistance, and the power rating. If that turns out to be more than a couple of watts you also need to think about where the heat is going.
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Re: EH Polyphase transformer dead

Postby Adam Inglis » Wed Jan 09, 2019 1:12 pm

Folderol wrote:Dave's idea of a series resistor here would help, but it starts to get a bit more complicated as you then need to know the current in order to to work out the needed resistance, and the power rating. If that turns out to be more than a couple of watts you also need to think about where the heat is going.

When I drove the unit from my bench supply, it never pulled much more than 50 mA, so power consumption would be around 0.75 watts, no?
What does that suggest the series resistor value should be?
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Re: EH Polyphase transformer dead

Postby ef37a » Wed Jan 09, 2019 1:33 pm

Now we all have superbly accurate DMMs just put 10R in circuit and measure the volt drop.

Mr Ohm then gives you the current and you can easily work out R for the volt or so drop you need.

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Re: EH Polyphase transformer dead

Postby Folderol » Wed Jan 09, 2019 2:24 pm

Adam Inglis wrote:
Folderol wrote:Dave's idea of a series resistor here would help, but it starts to get a bit more complicated as you then need to know the current in order to to work out the needed resistance, and the power rating. If that turns out to be more than a couple of watts you also need to think about where the heat is going.

When I drove the unit from my bench supply, it never pulled much more than 50 mA, so power consumption would be around 0.75 watts, no?
What does that suggest the series resistor value should be?
At that kind of current forget it!
Just do the one feeding the zener and diode. That in itself will almost certainly drop the final voltage slightly as they will not be working so hard. In fact that resistor could probably go up to 1k
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Re: EH Polyphase transformer dead

Postby Adam Inglis » Thu Jan 10, 2019 5:05 am

OK, replacing the 510R with a 1k, I now get 15.1v at the zener, 15.8 at the base, and 15.2 at the emitter.

Anyone would think you guys knew what you were talking about, it's uncanny! ;-)

And now, a new riddle has presented itself.

If you look at the lower right of the schematic, above the power-mute circuit, you'll see a circuit comprising a 1458 dual op-amp, with a pot labelled "Mod Rate". This is the separate dedicated LFO for the Envelope circuit (It works like this: you pick a note and the phase changes with the envelope, as the input amplitude drops from the initial attack, the LFO CV becomes dominant over the Envelope CV and you hear the oscillation, very nice effect).
Anyway, when I first turn it on, this LFO refuses to oscillate!
I tried bridging the cap with similar values, I replaced the op-amp, no avail. Then I accidentally briefly turned up the signal from the function generator, and the thing sprang to life!
I have been able to reproduce this behaviour each time. I haven't tried it with normal guitar signals yet.
Any clues?
You'll notice, and this has me scratching my head also, the power supply for that particular op-amp comes off a voltage divider made from a 330R and a 2k7, giving it a supply voltage of around 13 volts. Why would they do this? It's a bog standard dual op-amp, not a great deal different from the 4458s they use elsewhere on the board which are running off the 15 volt line.
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Re: EH Polyphase transformer dead

Postby Adam Inglis » Thu Jan 10, 2019 8:21 am

Update: Unfortunately, it seems guitar level signals are not enough to jump-start this LFO, so I'm gonna have to find a fix!
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Re: EH Polyphase transformer dead

Postby ef37a » Thu Jan 10, 2019 11:20 am

My first guess would be to replace the 0.33 cap with a low loss polystyrene or Teflon component. I have had gitamp tremelo circuits that were similarly reluctant to start and a cap in the cathode circuit sorted them.
The reduced supply could be so that the oscillator clips before the next stage?

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Re: EH Polyphase transformer dead

Postby Adam Inglis » Thu Jan 10, 2019 12:16 pm

That .33 uF cap is weird - large pale grey block, huge compared to the other components. No markings. Not really sure what type it is. I'll try a few different types and sizes.
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Re: EH Polyphase transformer dead

Postby ef37a » Thu Jan 10, 2019 12:20 pm

Adam Inglis wrote:That .33 uF cap is weird - large pale grey block, huge compared to the other components. No markings. Not really sure what type it is. I'll try a few different types and sizes.

I suggested those types because they have the lowest dielectric losses. Does not usually matter in audio circuits but oscillators are fussy buggers!

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Re: EH Polyphase transformer dead

Postby Folderol » Thu Jan 10, 2019 4:49 pm

Many modern caps are far smaller than ones even a few years old, so don't be surprised by the size of a replacement.

Apart from the LFO are you getting correct Envelope behaviour?

Another possible miscreant could be the 1u cap above the envelope sensitivity control. If that's gone leaky it could screw up the DC levels. Again, a small 'dry' type might be better.

Also check the 220u one alongside the {indecipherable} Trim control. There are some seriously screwy DC paths in this circuit, so anything that could upset it is worth replacing.
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Re: EH Polyphase transformer dead

Postby Adam Inglis » Fri Jan 11, 2019 5:55 am

Thanks for drawing my attention to that trim control.
Someone on a stomp box forum explained this area as setting the bias voltage for the LM324 chip, which is responsible for the envelope follower CV.
You can see this clearer on a re-drawn schematic (from Morocotopo on DIYstompboxes) with my notes added,
here
EH was very keen for this voltage to remain fairly steady - note the thermistor in parallel with the trim.
When I finalised the new power supply, I duly adjusted the trim as suggested by the schematic, to read 6.7 volts. Today, I checked it again. It was down at 6.5. As soon as I trimmed it over 6.7 the LFO came on. So as it turns out, that level is critical to the operation of the LFO. A tenth of a volt less, and the LFO stops oscillating! You can actually hear this if you have a signal going in when you turn the power on. The voltage takes about 15 seconds or more to come up to level. As it does so you hear the LFO come on. I assume that the need for a reduced supply voltage just for that chip is somehow linked to all this...!
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Re: EH Polyphase transformer dead

Postby Folderol » Fri Jan 11, 2019 6:47 pm

It is indeed a very strange circuit. I rather suspect something in there is temperature sensitive, and that's (at least partly) the reason for the thermistor.
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