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

[guitarman89]

Hello, long time lurker here, first time posting.
The B channel on this DOD r-870 BBD Flanger/doubler only seems to have a little effect on it when I turn it completely to wet. From what I read, the B channel is supposed to have the inverted version of the wet signal along with the dry signal, while the A channel has the wet signal in opposite polarity as well as the dry signal.

the B channel mix knob is third from the left. I havent used my soldering iron in ages, is it possible that one of those internal trim pots just need biasing?
Thanks again in advance.

 

[pucho812]

do you have a schematic?

As always whenever working on a unit, it's a good idea to first check and confirm the PSU is working and outputting the voltages it is supposed to

 

[JohnRoberts]

The BBD ASRs are a weak link, if you have two channels you can carefully swap them and see if the problem follows a bad part. I think I see a matsushita logo (panasonic) but can't read PNs.


JR

 

[Youngwhisk]

Schematic etc is here:
DOD Flanger/ Doubler r870 volume drop issue

 

[mjrippe]

MN3005 *and* SAD512 BBDs, never seen them mixed before!

 

[Bo Deadly]

Could be a bad solder joint. Use a meter and schem to verify that you have expected resistance between different points.

 

[Whoops]

I had a DOD flanger stompbox with a dead SAD512, was never able to get a replacement. So it was trashed
For the MN3005 I think there’s replacements available.

and replying to your question, No, Im
pretty sure your problem has nothing to do with the position of the trim pots, if no one messed around with them there’s no reason for them to be the reason for the fault, so you should also not to mess with them.
The problem is elsewhere in the circuit, Lytic capacitors and the BBD chips would be my first suspects

 

[JohnRoberts]

the panasonic MN3005 is probably for chorus length double tracking delays, the sad512 for shorter flanging delays.

The reticon (sad) parts are less reliable than the panasonic, and long since obsolete (unobtanium). I think a Chinese IC maker is now selling MNxxxx similar BBDs.

JR

 

[guitarman89]

Thanks everyone. So you all recommend finding replacements for those chips?

 

[Walrus]

The A and B outputs are fed from the same output of the NE571, so I can't se how the BBD devices could be suspect.
As Bo said, check resistance around the switches and output controls. And check the 1.0uF caps before the 4k7 resistors.

 

[guitarman89]

Okay, should I also try replacing the ne571?

 

[Walrus]

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.

 

[Whoops]

Thanks everyone. So you all recommend finding replacements for those chips?

Don’t replace anything for the sake of it.
The unit needs someone that is experienced in repair work to troubleshoot it.

Take it to a local repair tech

 

[JohnRoberts]

Indeed with impossible to source key components just swapping may not be an option.

The SAD512"D" has a driver built into the chip so will be even more difficult to cobble together a substitute. You would need a short BBD "and" a clock driver. The MN3005 is using both sections of a CD4013 flip flop in parallel as a clock driver.

What troubleshooting tools do you have? With a scope you could probe around to see where the signal is or isn't. Since the SAD512D is unobtanium, if that component is busted the unit is probably a parts donor without interesting parts. If you don't have a scope you could try a crude audio probe... say something like a 0.1uF cap feeding a headphone amp. I would just probe to see if audio is present or not.

It could be as cheap and simple as a dried out cap, but why guess if you can troubleshoot objectively.

JR

 

[Whoops]

why guess if you can troubleshoot objectively.

JR

 

[DanMcCullough]

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.

 

[JohnRoberts]

I wouldn't expect a huge improvement from upgrading the 4558s with more modern chips. The noise floor for the wet path will be dominated by the BBD lines, even using the 2:1 companding noise reduction. It is an effect after all.

If the electrolytic caps are typical failure suspects perhaps swap them out in the audio path you are experiencing low output from.

JR

 

[Whoops]

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.

Best thing would be to Guitarman89 to send it to you to be repaired

 

[DanMcCullough]

My reason for swapping the app Amps is not to to improve signal to noise. I agree that the noise floor is dominated by the BBD lines. I install the Burr Browns for the transient response. If you use this device with guitar as an inserted device chain the dry signal is adversely affected by the 4558 op amps. The Burr Brown preserves the guitar attack which is very important to preserving tone. This does not help the wet path since the BBDs are the limiting factor.

 

[Bo Deadly]

I also replace all the RC4558 Op Amps in the audio path with Burr Brown OPA2134PA Hi-Fi Op Amps.

OPA2132 are high speed amps that necessitate bypass caps at the amp power pins (which the board does not have). And OPA2132 are a waste of money even when they were not $8.