LOFT Model 450 Delay Line/Flanger issue -not delaying...

GroupDIY Audio Forum

Help Support GroupDIY Audio Forum:

This site may earn a commission from merchant affiliate links, including eBay, Amazon, and others.

ritz

Active member
Joined
Jun 11, 2009
Messages
39
Location
Australia
I have a Loft Model 450 delay/flanger (single rack unit) that I picked up recently that is a little sick. It works in flange mode, but not in delay mode... I get dry signal passing, just no delays. It's probably a long shot, but I'm wondering if anyone would have a schematic available for this thing?

From poking around inside, it seems a few components have been removed. It looks like around four components have been snipped out from around the NE572 compandor IC (from their lead spacings, my guess is two resistors and two caps).

Also there's some funkyness going on with the signal mix knob (normally mixes the ratio of dry signal to flanged/delayed signal in the output) which I think is related to the above missing components... In delay mode, it seems like it's "mixing" properly between the dry and wet signal: turn it all the way dry and you get nice, loud, dry signal and as you turn towards wet it gets gradually quieter until you reach 100% wet where there's silence (the delay isn't working, remember, so no wet signal is actally being passed). In flange mode it's another story. The flanged signal does diminish as you turn the mix towards dry and does increase as you turn it toward wet (so far so good). The dry signal, however, is always present, even when turned to 100% wet. It seems in flange mode, the signal mix is just turning the flange level up and down, but leaving the dry signal constantly on.

It would appear to me that the issue is with how the signals are mixing in the output section (no delay reaching the output, dry signal always on in flange mode) and I'm guessing the removed components around the compander are the culprit. The problem is, as these components were removed before I got the unit, I have no way of telling what's supposed to go there. Obviously a schematic would be ideal, but I guess that'll be hard to come by.

Any other ideas? It really is a beautiful unit; very nicely made inside, nice PCB work and good components, and it really does produce some fantastic flanging sounds. I'd love to get the delay side up and running too...


 
ritz said:
I have a Loft Model 450 delay/flanger (single rack unit) that I picked up recently that is a little sick. It works in flange mode, but not in delay mode... I get dry signal passing, just no delays. It's probably a long shot, but I'm wondering if anyone would have a schematic available for this thing?
Sorry, while I designed it some 3 decades ago I don't have schematics and my recollection of circuit specifics will be limited.
From poking around inside, it seems a few components have been removed. It looks like around four components have been snipped out from around the NE572 compandor IC (from their lead spacings, my guess is two resistors and two caps).
I don't recall what that could be... There was a lot going on with the companding NR, so it may have needed to be removed, or not. I don't know.

If you have an oscilloscope you can trouble shoot.  even without a scope you can do some rough troubleshooting with an audible signal sniffer. A small cap used as a probe to feed the input of a small headphone amp... Probe around to see where the audio signal stops.  Admittedly not easy without a schematic.. But general signal flow you will be interested in is from compressor section in the  compander to delay chips, and back again.
Also there's some funkyness going on with the signal mix knob (normally mixes the ratio of dry signal to flanged/delayed signal in the output) which I think is related to the above missing components... In delay mode, it seems like it's "mixing" properly between the dry and wet signal: turn it all the way dry and you get nice, loud, dry signal and as you turn towards wet it gets gradually quieter until you reach 100% wet where there's silence (the delay isn't working, remember, so no wet signal is actally being passed). In flange mode it's another story. The flanged signal does diminish as you turn the mix towards dry and does increase as you turn it toward wet (so far so good). The dry signal, however, is always present, even when turned to 100% wet. It seems in flange mode, the signal mix is just turning the flange level up and down, but leaving the dry signal constantly on.
The flange delay and "delay" delay path uses different BBD chips. Clearly the delay path is losing the signal.

In flange mode the deepest depth of flange is 50% dry + 50% delay, so dry will always be present.
It would appear to me that the issue is with how the signals are mixing in the output section (no delay reaching the output, dry signal always on in flange mode) and I'm guessing the removed components around the compander are the culprit. The problem is, as these components were removed before I got the unit, I have no way of telling what's supposed to go there. Obviously a schematic would be ideal, but I guess that'll be hard to come by.
Actually most likely problem is not the post delay mix, but delay it self.

The BBD delay chips are not the most robust so they may have failed in 30 years. Also the delay chips require proper DC bias and clocking to pass audio signal.

While my recollection of specifics is vague, the flange delay path is probably also passing through the 572 so my first suspicion is the long delay chips or associated circuitry.  IIRC SAD4096 Reticon BBD

Another possibility is the anti-alias/anti-image filters around the BBD delay that switch different frequency roll-offs for different delay times, but if all delays are silent, back to BBD or clocks. 
Any other ideas? It really is a beautiful unit; very nicely made inside, nice PCB work and good components, and it really does produce some fantastic flanging sounds. I'd love to get the delay side up and running too...

In my judgement the flanger is by far the more useful effect (I have learned that a few folks were asking for a digital plug-in).

Analog delay using BBD technology is not very clean, or wide bandwidth.  The best non-flange effect (IMO) from the 450 involved using two of them patched together to generate a complex chorus effect, where one 450 was sweeping up while the other down. this gave a three voice chorus, and was also highly regarded by users.

At this point, I would enjoy the flanger, since replacement delay chips are probably hard to source, and the delay when working is not as good as a modern cheap digital delay. 

Enjoy...

JR
 

Latest posts

Back
Top