Tel-Ray Morley power wah schematic

[jimitrader]

Morley Power Wah - should i replace the old capacitors?I am going through and cleaning it up it has been sitting unused for years and years.I have contacted Morley to try and get correct voltages with no luck.
I did find an old schematic with some Volt requirements here:after testing the volts at the 2 test points
+25v and +55v on schematic

I get this:
27.9v & 63.7v

caps in schematic are:

   
• 470 uf - actual read out is = 365 uf
• 50 uf x3                          =65/65/51 uf

   
• 10 nf x2                          = 10 nf x2
• 2.2 nf                              = 2.2 nf and 170 nf
• 1 uf x2                            =1.8 uf and 3.10 uf
• 5 mf -                             = 69 nf       

so as you can see the caps are kinda off a bit.
the voltages are close...it sounds ok ..but I am wondering if I should do a cap job to try an get volts closer to schematic?anyone have any thoughts?

as usual don't fix if not broken lol...
I want to get some other viewpoints. Since i am still massively learning.

[PRR]

> don't fix if not broken

May be the best policy.

[Rob Strand]

The voltages on the transistors might be more useful to detect a fault.
The Q1 base voltage will be lower than expected due to DMM loading.

If we assume the 63.7V  voltage then you can analyse the Q1 circuit to show the 27.9V is quite close to the expected voltage on that second supply.

If we assume 55V (is it 56V?)  then we end up with about 24.5V.  Pretty much the 25V on the schematic.

So 27.9V is there because the voltage is 63.7V and not 55V.

It's impossible to workvout what the 63.7V/55V should be.   It is possible to check Q2 is OK based on the measured voltages.

FWIW, there was a thread within the last two years on this forum where someone replaced the transformer.  That linked to an older thread where someone took a measurements of the transformer voltage.   However, there were some things which were unclear about measurement details.   

[jimitrader]

The voltages on the transistors might be more useful to detect a fault.
The Q1 base voltage will be lower than expected due to DMM loading.

If we assume the 63.7V  voltage then you can analyse the Q1 circuit to show the 27.9V is quite close to the expected voltage on that second supply.

If we assume 55V (is it 56V?)  then we end up with about 24.5V.  Pretty much the 25V on the schematic.

So 27.9V is there because the voltage is 63.7V and not 55V.

It's impossible to workvout what the 63.7V/55V should be.   It is possible to check Q2 is OK based on the measured voltages.

FWIW, there was a thread within the last two years on this forum where someone replaced the transformer.  That linked to an older thread where someone took a measurements of the transformer voltage.   However, there were some things which were unclear about measurement details.   

thanks Rob

[danfrank]

What year approximately was the power wah made? I fixed a Pro Flanger from the late 70s several months back and it used two 28vct transformers, one for the 387 bulb and one for the rest of the circuit. Morley liked using the same parts even if they had to double up in the same pedal. So your voltages look right in your circuit. Morley ran the 387 bulbs on DC and usually through a dropping resistor so the bulb sees 24-25 volts.
Also, I had to replace all the electrolytics in mine because they were bad from age. All the other types of caps were ok.

[jimitrader]

Quote from: danfrank on Today at 09:43:33 AM<blockquote>What year approximately was the power wah made? I fixed a Pro Flanger from the late 70s several months back and it used two 28vct transformers, one for the 387 bulb and one for the rest of the circuit. Morley liked using the same parts even if they had to double up in the same pedal. So your voltages look right in your circuit. Morley ran the 387 bulbs on DC and usually through a dropping resistor so the bulb sees 24-25 volts.
Also, I had to replace all the electrolytics in mine because they were bad from age. All the other types of caps were ok.

I will check out the transformer codes...and report back..thanks Dan

[jimitrader]

I could not find much info on the pedal. It does appear to be made in the 1970's

[Rob Strand]

Here's the thread I was thinking of,

https://www.diystompboxes.com/smfforum/index.php?topic=126076.0

So reading over the thread the uncertainty was if the 28V was no load, or it was a 28V transformer and the transformer regulation would mean the voltage would rise under no-loads or light loads.   Reply #6 shows all the uncertainty points.

If we allow 15% regulation light loads on a 24Vrms transformer gives 27.6Vrms.  15% to 20% regulation is a reasonable estimate for a small transformer.

So for the doubler,  24*1.15*1.4* 2 = 78V.

[Rob Strand]

Here some voltage and current estimates,

                         sch        meas
VDC2                56        63.7
VDC1                25        27.9
               
Q1 (high gain Darlington)               
VE1                 6.77        7.70
IC1 [mA]          0.68        0.77
               
Q2 (hFE = 150)               
VC2                 28.1        30.3
IC2 [mA]          2.79        3.34
               
VDC (@ caps) 64.95        74.05


The measured voltages are in the second column and it implies a 24V AC transformer which has about 15% regulation, which makes the no load AC voltage higher than 24V.