Morley Tel-ray transformer calculation

[VintageGear]

Hello all, hope you had a great X-mas. I have a quick question I'd love to verify with you. In my vintage Tel-ray Morley Autowah pwa pedal the transformer burned out and there is no label on it. Am I correct that in order to get +55v , the transformer must have been a 20V VAC (2 *  20 * sqrt(2) )?
I am a bit hesitant to throw in a replacement and burn out my pedal if I get my calculations wrong.


(click for large)

Cheers!!

[iainpunk]

well, if there should be 55V over the 20k (2x10k) there should be 59V over the 21.5k total.
( 59V / sqrt2 ) / 2 = 22-ish volts, so a 20V secondary should be correct.
that is a voltage doubling rectifier right?

cheers, Iain

[willienillie]

that is a voltage doubling rectifier right?

Yes.

[iainpunk]

that is a voltage doubling rectifier right?

Yes.

in that case, your 20V secondary should be correct

cheers, Iain

[Rob Strand]

Not that it matters but the correct working is something like,

We don't know the current going out of the 25V and off-hand the 55V has no load (true?).
In order to ignore the 25V load current we use,

I = Current from 55V = (55 - 25) / 10k  = 3mA

Then the drop across the 1.5k resistor is, 1.5k * I = 1.5k*3mA = 4.5V.

So the input side is 55 + 4.5 = 59.5V.

Since it's a doubler one half of the supply is 59.5V/2 = 29.75V.
There is one diode drop for the half supply, so Vin_peak = 29.75 + 0.7 = 30.45V
Then the *loaded* rms voltage is 30.45 /sqrt(2) = 21.5V rms.

Transformer voltage labels are  under full load and with a resistive load.  That means under light load the output voltage will be higher than the label.    How much depends on the transformer regulation which can be anything from 10% to 25% for small transformers.

If we allow 10% then the required transformer voltage rating would be 21.5/1.1 = 19.5V.  For 25% regulation that drops to about 17V (that would be the lowest rating).

We haven't allowed for DC regulation and ripple which means the DC supply will be lower than predicted and pushes the required  transformer voltage up a bit.

Despite all the fine details a 20V transformer is looking like a reasonable choice.

EDIT
FWIW,  different voltages on the schematic.    It shows a *big* region of uncertainty to the calculations, perhaps due to tx regulation!
https://www.diystompboxes.com/smfforum/index.php?topic=44123.0

It could be a 22V tx with poor regulation and the supply drops with load because the tx is small.

EDIT:

https://www.morleyproducts.com/faq/

"QUESTION:
Do you have specifications for Tel-Ray products?

ANSWER:
Here is what we have available:
Transformer: Stancor #P8600 28V Center Tap 85mA
Bulb: Lamptronix 815-455-0400     P/N LTX 387-028 AS25       Specs: 387 28V 40mA
Oil: Union Carbide UCON LB 65 about $100 per gallon (we do not know of a source we can refer you to)
Fill container 1/3 full     Dielectric Constant 2.8 to 3.7 Strength 7000 v/mil
Transistors: MPS-A06   MPS-A12   2N5087"

They state a 28V transformer.    I've got some doubts that's a 28V rated winding (which would be higher than 28V under light loads).  It could be 28V *measured* but it's not clear if that's with no load or in circuit.   The circuit voltages suggest the AC voltage drops somewhat from 28V.     The transformer might be a 22V rating that's in the ball-park of the calcs.

[PRR]

The FAQ with oil-specs is for the drum-delay, not the Auto-Wah.

We could look at the *whole* schematic for clues. (The 55V IS used for something.)

[Rob Strand]

We could look at the *whole* schematic for clues. (The 55V IS used for something.)

So it is. 

So if the collector of that MPSA06 is at 22.5V.  That's another (55-22.5) / 10k = 3.25mA down the 1.5k, 1.5k*(3.25mA + 3mA) = 9.4V.
So add 55V + 9.4V = 64.4V = 2*32.2V.  Which raises the loaded AC voltage estimate from 21.5V rms to 23.3V rms (8% higher).

Still have to factor in regulation, ripple etc.


----------------------------------------------------------------------------------------------------------------------
EDIT:
Can't say if this is the same transformer but in this thread,
https://www.tapatalk.com/groups/telrayoilcanaddicts/morley-pfl-transformers-t702.html

They quote
"1. ~ 30vac secondary non-tapped ~ 100-200ma - bulb supply. Stancor #P-8395 (24vac)"

On one hand 30Vac and on the other 24Vac.

Officially it's a 24Vac 200mA transformer,
https://www.alliedelec.com/product/stancor/p-8395/70213331/
http://www.stancor.com/jsp/products.html
http://www.stancor.com/pdfs/Catalog_2006/Pg_002_4.pdf

So the 30Vac *might* be the unloaded voltage.   No regulation specs in PDF.
----------------------------------------------------------------------------------------------------------------------

So is the 28.2Vac from the old thread the unloaded voltage and we are talking a 24Vac rated transformer?

[PRR]

I assume we see the two Zeners. Not specified, and drawn funny, but clearly add-up to 25V (or so). Combined with R5 10k, there is no way you can blow this up until like 100V at "+55V". (100V-25V is 75V, in 10k is 7.5mA, if all in the Zeners is 1/10th Watt, Zeners don't come smaller.)

The 55V is unexpected. I guess Q2 was not giving the voltage gain, they were not hep to making Q1 Q2 a high-gain compound, '741 still cost bucks, the 55V was there (if for no very good purpose), so they jacked Q2 Vcc.

[Rob Strand]

Is the lamp a power indicator or is it part of the LDR control?   On one of those Morley units if the lamp fails the pedal stops.

[VintageGear]

Wow thanks everyone for their input on a seemingly simple question! Much appreciated!

Is the lamp a power indicator or is it part of the LDR control? 

It's part of the LDR control; there are LEDs for status on the switches.
Looking up the partnumber 387 I get: LAMP, INCANDESCENT, MIDGET FLANGE, 28V

The 55V is unexpected. I guess Q2 was not giving the voltage gain, they were not hep to making Q1 Q2 a high-gain compound, '741 still cost bucks, the 55V was there (if for no very good purpose), so they jacked Q2 Vcc.

Hah, interesting hypothesis!

Transformer: Stancor #P8600 28V Center Tap 85mA
[...]
They state a 28V transformer. I've got some doubts that's a 28V rated winding (which would be higher than 28V under light loads).  It could be 28V *measured* but it's not clear if that's with no load or in circuit. The circuit voltages suggest the AC voltage drops somewhat from 28V. The transformer might be a 22V rating that's in the ball-park of the calcs.

Exactly, I read that as well, but it didn't make sense so I decided to calculate it. But everybody's calculations are next level, heheh.

So.. summarizing... we got a 28V lamp.. 28V transformer on a dodgy Q&A.. 22v on calculations..
Let's go 24v AC modern-spec transformer and let's see what happens?! :-) (Yes yes, I'll let you know)

[Rob Strand]

It's part of the LDR control; there are LEDs for status on the switches.
Looking up the partnumber 387 I get: LAMP, INCANDESCENT, MIDGET FLANGE, 28V

Ah OK.  I was thinking two things:  the resistor in series with the lamp might put an *upper limit* on the expected DC voltage.  There's no guarantee they are operating the lamp at full current.    The other thing was if the supply voltage is too far off it will mess with the lamp voltage ad brightness and the pedal will need adjusting to compensate.

So lamp #387, 28V @ 40mA,   40mA through the 68 ohm lamp resistor is 2.72V, so an upper limit for the DC voltage is 28V + 2.7V = 30.7V  which points to about 22Vac rms when loaded as per the circuit.

The refined estimate in reply #6, based on the schematic and circuit currents,  gave  32.2V DC  and 23.3Vac rms.

So it looks like the lamp is the deciding factor.

The tricky part is getting 22Vac rms *loaded*.   The complicating factors are factoring in DC regulation, which is always worse than AC regulation.   However, a replacement transformer is likely to have a rated VA somewhat above the running VA so the loaded voltage will be higher than rated voltage.   

So perhaps a 22Vac rms transformer is a safer bet.

*IF* we suppose the 28.2Vac rms that was measured in the earlier thread is under no load.   28.2/22 = 1.28 ~ 28% regulation which is on the upper limit for regulation on a 22V transformer.  Whereas 28.2/24 = 1.18 ~ +18% regulation.   So from that a 24V ac transformer looks more likely.

It's a tough call.   I seriously doubt the difference would affect the circuit.   The lamp is the thing that needs care.  If you got a 24V transformer and the lamp voltage is too high you could add a resistor in series with the transformer winding to knock the voltage down - no real problem doing that.

[VintageGear]

Let's go 24v AC modern-spec transformer and let's see what happens?! :-) (Yes yes, I'll let you know)

It's alive and working great!  100 points to all.

[iainpunk]

nice to hear it works!
can't wait to hear a tone demo! ([hint hint])

cheers, Iain

[Rob Strand]

It's alive and working great!  100 points to all.

Excellent.  Another one saved  .