Mark IIC to C+ Steps Required for Upgrade

[Jay Omega]

I think, I have done it.. The photos, layouts and schematics were very helpful. Thanks to all, who have shared their experience on this forum..

I did remove the board to inspect and repair some missing traces.. There was a resistor under the board, which didn't belong there.. Swapped the tantal caps with newer ones. Used cheap 1W metal film resistors and 2/3KV ceramic caps , which usually work fine.. Used Silonex NSL32SR3 LDR's.

The amp sounds actually very nice. I expected a much noisier amp.

There is a small problem though.. If the lead master is on lower settings, the sound level increases for a few seconds when I switch to the clean channel. The lead master pot measures 140K. I will install a larger value, but this will stay as a problem. This is probably normal?

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It may just be how the SR3 behaves, or maybe a bad LDR. It should switch to around 100k after 10 ms, but may take a while to get to 1.5M where the pot is effectively removed from the circuit. You could try an SR2, I don’t have that behavior in a clone I just built. Does it also take a while to fade from dirty to clean or is it just a volume thing?

 

[aiyiadam]

It may just be how the SR3 behaves, or maybe a bad LDR. It should switch to around 100k after 10 ms, but may take a while to get to 1.5M where the pot is effectively removed from the circuit. You could try an SR2, I don’t have that behavior in a clone I just built. Does it also take a while to fade from dirty to clean or is it just a volume thing?

It takes a while to fade from dirty to clean..

Yes.. Thank you.. Good point.. The LDR.. I think the resistance of this LDR (LDR4A) rises up too fast and to a very high level.. I chose those LDRs for the high dark resistance, but it is probably too much in this position..

And.. I have just paralleled this LDR with a 2M2 resistor and it seems to work fine now.

Nope.. I need a slower LDR like VTL5C4 here.. It is designed this way..

 

[Jay Omega]

It takes a while to fade from dirty to clean..

Yes.. Thank you.. Good point.. The LDR.. I think the resistance of this LDR (LDR4A) rises up too fast and to a very high level.. I chose those LDRs for the high dark resistance, but it is probably too much in this position..

And.. I have just paralleled this LDR with a 2M2 resistor and it seems to work fine now.

Nope.. I need a slower LDR like VTL5C4 here.. It is designed this way..

If I’m thinking correctly, you want a faster ON to OFF transition / decay time. You might be pumping too much current thru the LEDs, changing the memory behavior. If you’re looking at the sloclone schematic, measure the voltage across R77 (470 ohm) when the amp is in the lead mode with the treble pot pulled and let me know what it is. BTW, it looked like the resistor under the board was a 470 ohm that could have been R77

 

[aiyiadam]

The SLOcloneforums schematic is very nice , but I have used the attached schematic. On the schematic there is a VTL5C4 in this position, which is much slower than a VTL5C1.

I actually see the point of using a slow LDR here..This LDR is in series with the lead master pot. If signal is still leaking from the lead circuit, a fast LDR can amplify this signal. This is probably, what is happening here.

The LDR circuit is probably okay.. There is a 470 ohm resistor on the other side and I have soldered the missing 3K3 resistor. The voltages are 1,75V and 13,8V.

 

[Jay Omega]

The SLOcloneforums schematic is very nice , but I have used the attached schematic. On the schematic there is a VTL5C4 in this position, which is much slower than a VTL5C1.

I actually see the point of using a slow LDR here..This LDR is in series with the lead master pot. If signal is still leaking from the lead circuit, a fast LDR can amplify this signal. This is probably, what is happening here.

The LDR circuit seems to be okay.. There is a 470 ohm resistor on the other side and I have soldered the missing 3K3 resistor.

Unfortunately, that’s not quite how the circuit works. Boogie used a VTL5C4 here, and for the EQ reference, because of its lower on resistance compared to a VTL5C1. They limited the current for the LDRs thru R91 so that it protected the LDRs and switched relatively quickly. There is a trade off between on resistance and decay time.

Also, JRB32 did some good work but there are errors in the switching section of his schematics. It doesn’t appear that you’ve been affected by the errors though.