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EH Polyphase transformer dead

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Re: EH Polyphase transformer dead

Postby Folderol » Sun Jan 06, 2019 10:14 am

Hmmm, a few odd things here.

That 20V is lower than I'd expect. 18 * 1.414 is nearly 26. Subtract 1.5V for rectifier drop should still leave over 24V, unless it's a very inefficient transformer.

The CMOS chips are rated at 18V so should be OK, but I'm not too happy about the rail being that high. It is worth checking the Zener and series diode. I'm guessing the zener is supposed to be a 15V one, and the diode compensates for the power transistor base-emitter drop. As a quick check, you should get between 0.4 and 0.6 across that diode. If it's much higher, the diode is O/C. If it's zero the zener is O/C. Either would account for the high output voltage.
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Re: EH Polyphase transformer dead

Postby Adam Inglis » Tue Jan 08, 2019 12:46 pm

Thanks Will, spot on ;-)

the secondaries were measured on an AC meter - there's just under 26 volts at the collector as you predicted, on a DC meter.

The diodes are swapped in series position on my unit, with the 4148 between the zener and the base, but that probably doesn't matter here. The usual 0.7 volt drop was measured on the signal diode, the zener had 16.5 volts or so at the cathode, so I swapped that for one in my box that was labelled "15v" (but perhaps I missed the decimal). I then measured 15.5 at the top of the zener, 16.2 at the base of the tranny, and the emitter shows 15.6 or so.

All appears to be working, and it looks like I can snip off the pins and the traff may just fit after all (a PCB mount type transformer was the smallest physically I could find). Just gotta make sure there's no chance of 240 volts appearing on the chassis ;-)

Your assistance has been invaluable, many thanks (and thanks Dave!)
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Re: EH Polyphase transformer dead

Postby Hugh Robjohns » Tue Jan 08, 2019 5:00 pm

:thumbup: :clap: 8-)
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Re: EH Polyphase transformer dead

Postby Folderol » Tue Jan 08, 2019 5:41 pm

Very pleased to hear this.
There is one final check I'd suggest, and that's the 510 ohm resistor feeding the diode and zener. As you now have about twice the voltage across it than would have been specified, it might be worth bumping that up to say 680 or even 820.
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Re: EH Polyphase transformer dead

Postby Adam Inglis » Wed Jan 09, 2019 2:50 am

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?
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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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