Dod R-870 flanger/doubler rack unit channel not working.

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guitarman89

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I have repaired several of these units. You most likely have a bad electrolytic capacitor. I always replace all the electrolytic caps. I also replace all the RC4558 Op Amps in the audio path with Burr Brown OPA2134PA Hi-Fi Op Amps. You have to be careful when de-soldering though. The PCB is low quality and it is very easy to break and/or lift traces. Once I complete the parts replacement I perform a point-to-point check with an ohm meter to verify all connections to the replaced components. These are good units once restored. My only complaint is that they went cheap on the power supply. This results in reduced headroom when using with line level signals. You have to be careful not to overdrive it.

I can provide repair service at a reasonable cost if you are interested
I have repaired several of these units. You most likely have a bad electrolytic capacitor. I always replace all the electrolytic caps. I also replace all the RC4558 Op Amps in the audio path with Burr Brown OPA2134PA Hi-Fi Op Amps. You have to be careful when de-soldering though. The PCB is low quality and it is very easy to break and/or lift traces. Once I complete the parts replacement I perform a point-to-point check with an ohm meter to verify all connections to the replaced components. These are good units once restored. My only complaint is that they went cheap on the power supply. This results in reduced headroom when using with line level signals. You have to be careful not to overdrive it.

I can provide repair service at a reasonable cost if you are interested.
I already replaced the op-amps with 5532s. There is low headroom when used with line level signals. Ill check the caps and continuity near resistors.

I have basic supplies multimeter, soldering iron, spare components.
 

guitarman89

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No, because if A gives a satisfactory wet signal, then the NE571 is working.
According to that schematic, the B output mix pot gives you Wet or Dry, whereas the A output is Wet or Inverted Dry.

If you have no Wet B output it can only be the 1.0uF capacitor, the 4.7kOhm resistor, or the 100kOhm Mix pot.
Thank you. I'll have to find the cap and resistor on the board somewhere.
 
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JohnRoberts

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I try not to debate people about what they hear on the WWW but the 4558 at 8nV rt Hz, and 1.7V/uSec can probably keep up with a dry guitar signal's rise time. Of course better on paper is better if cost is no object.

Decades ago I wrote a magazine column for my audio mythology series comparing +4dBu pro audio gear to -10dBV bedroom recording gear. The -10dBV gear fared a lot better than expected considering the significant cost savings.

JR
 

Ike Zimbel

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It has been my experience that you can absolutely hear differences in slew rate between op-amps, and that faster equates to better transient response. I would be very, very surprised if every member of this forum could not hear the difference between 1.7v/us and the 20v/us of the OPA2134.
 

Bo Deadly

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I already replaced the op-amps with 5532s.
Note that you cannot always just swap in any op amp with another. In this particular case the NE5532 is very comparable to the RC4558 so it's almost certainly fine but the NE5532 is actually a pretty fast amp. It's right on the edge where you should probably have bypass caps on the pins.

Other than the usual things like does it have the right supply range (what are the supply voltages in this unit?), other things to consider are input bias current which can cause a significant offset on the output. Bipolar amps have significant bias current whereas JFET inputs do not. So there can be instances where it is NOT ok to swap a JFET amp for a bipolar for example.

The quiescent supply current can be a problem. OPA2132 uses twice the power of an RC4558 and there are amps that use a lot more mA than that. If you replace a lot of amps, you can tax the supply which can easily translate into noise.

One of the more important differences in op amps is drive capability. Many op amps cannot drive a load of 1K or less to a high level. In this particular case the loads are light and BBD circuits can't handle a lot of signal so the signals in this unit are probably pretty low. And the build out resistors on the outputs are 1K which will limit load current.

Finally, you should really ask yourself if changing the amp will actually matter. If the noise floor of the amp is below the circuit (definitely by a lot in this case), there will be no noticeable change in noise. The slew rate of an RC4558 is pretty low so I'm not going to declare that it's impossible for there to be a problem with "transients". But generally op amps do not sound different despite what a lot of folks like to claim. If you change an amp and it sounds different, it's probably because of a problem with not being able to drive the circuit or supply problems are causing noise or bias currents are causing offsets which change the circuit in some way.
 

JohnRoberts

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TMI warning...

For some inside baseball about designing with BBDs, the untrimmed outputs of BBD source follower pairs can have significant clock frequency content. Designers routinely use anti-alias low pass filters in front of BBDs and anti-imaging low pass filters in the output. Consideration should be made when selecting an active LPF topology especially for that output filter.

I just checked the DOD schematic and it should be OK. The VCVS (aka Sallen and Key) active output low pass filter topology expects the op amp output to be a very low impedance at very high frequency for the LPF to shunt and attenuate clock frequency energy. I have seen active op amp filters following BBDs unhappy with this topology, but a redeeming factor in the DOD circuit is that they use a real passive pole between the output of the BBD and the input to the 2 pole active filter. This passive pole will effortlessly sink clock frequency content without complaint or artifacts. The real pole feeding a 2-pole active stage results in a composite 3 pole LPF that is stable when attenuating clock frequency content.

JR
 

Whoops

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I guess Guitarman89 wants the unit working, at least that's what was presented in the first post.
I think the best thing to do is to troubleshoot it, find the fault and fix it,
that's what I think should be done first and before any Op amp swapping talk.

In the end doesn't matter what high end Opamp you put in there if you realize that you have a dead SAD512,
I hope not, I hope the SAD512 is good, but troubleshooting is needed and the opamp swapping talk just diverges from the goal.
 

guitarman89

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Does this look blown to you all? I just replaced it. No result. Kind of looks blown at the bottom. It was originally 16v, only replacement i had was 50v. Could be a placebo, but it seems to sound better.
 

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guitarman89

Member
Joined
Aug 23, 2017
Messages
10
I have repaired several of these units. You most likely have a bad electrolytic capacitor. I always replace all the electrolytic caps. I also replace all the RC4558 Op Amps in the audio path with Burr Brown OPA2134PA Hi-Fi Op Amps. You have to be careful when de-soldering though. The PCB is low quality and it is very easy to break and/or lift traces. Once I complete the parts replacement I perform a point-to-point check with an ohm meter to verify all connections to the replaced components. These are good units once restored. My only complaint is that they went cheap on the power supply. This results in reduced headroom when using with line level signals. You have to be careful not to overdrive it.

I can provide repair service at a reasonable cost if you are interested.
Hey, im ready to take up your offer. Ill dm you.
 

Whoops

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Does this look blown to you all? I just replaced it. No result. Kind of looks blown at the bottom. It was originally 16v, only replacement i had was 50v. Could be a placebo, but it seems to sound better.

It doesn't look blown, normally they blown and top and not bottom, but in normal operation conditions they don't blow but Bulge on the top. Blowing normally happens when caps receive the reverse voltage polarity on an higher voltage than the rating.

Send the unit to "DanMcCullough", he already fixed those units and has the experience, he will take care of it

Regards
 

guitarman89

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Joined
Aug 23, 2017
Messages
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It doesn't look blown, normally they blown and top and not bottom, but in normal operation conditions they don't blow but Bulge on the top. Blowing normally happens when caps receive the reverse voltage polarity on an higher voltage than the rating.

Send the unit to "DanMcCullough", he already fixed those units and has the experience, he will take care of it

Regards
Is it possible to add a "bounce" knob, like an eventide instant flanger?
 
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