DOD G7, different versions

[Reidk]

Hey all. A voice from the old Ampage days here. (Search the archive for "Reid")

I'm trying to fix a DOD G7 rack unit ("Guitar FX Processor/Preamp"). I have a schematic, but it seems to be for a very different version. The schematic says "05-6016 Rev A0". The board I have says "05-6216 Rev A". There seem to be quite a few differences (not the least of which is that the schematic shows a -5V supply that does not exist in my hardware). My unit also has LM353 opamps, while the schem calls out NJM4560s. Lots of other diffs, too, although it seems to be the same product.

I got the "wrong" schematic from DOD support, and I'm waiting to hear back from them if they have any others. In the meantime, can anyone offer any help? Maybe at least a source for obscure DOD schems? The G7 seems to be The DOD Product that Time Forgot.. It's not mentioned anywhere on the DOD "discontinued products" support page.

The symptom is no output with the noise gate active. With the gate bypassed, if I crank up the level I can hear the input signal very faintly along with a lot of hiss. The odd thing is that the input signal seems to be intact through the first few input stages, past the point where the schem says the gate sense is taken, so I don't know why the gate is shutting it down. I'm wondering if the gate sense is really taken somewhere else in my version.

Thanks for any help.

Reid

[Enzo]

LF353 -- 4560... both are dual op amps and are more or less interchangable. In fact I just installed a 4560 in place of a LF353 last night in a GK amp. Works fine.

Look at your old schematic. What in there uses that -5v? What now does that same job in your newer product? Without looking, I'd be betting the -5 was part of some FX circuit, maybe a chorusing circuit.

Otherwise is there great similarity? Enough to steer you through?

And do give them a chance to correct their oversight. You told them your board number, right?

[Reidk]

Enzo, thanks for replying.

I know the op-amps are interchangeable. I just gave that as an example of the diffs in case it rang a bell with anyone.

The -5 supply in the schem is used all over the place. The opamp chips, for example, are shown with +5/-5 supplies in the schem. On my board, they're at +9/0, with 4.5 V bias on the inputs and outputs.

The circuits are similar enough that I can get by, and I did give DOD my board number and am awaiting a reply.

Thanks again.

Reid

[HipCat5]

I know this thread is quite old ... but I bought a DOD G7 recently with the same issue. I'm in the middle of debugging this problem and so far I've figured out that the LM1973N integrated circuit is basically "clamped down" on all its outputs. The input signals are all ok ... but the outputs have no signal. I'm not an expert in digital electronics, so I'm just going by what I can see with an oscilloscope ... but the LM1973 is running properly (or it appears to be) ... but my guess is that there's an issue with the SAA7360. These two chips apparently control the GATE and COMPRESSOR functions of the DOD G7. The Gate Sense line from the input section appears to be getting signal ... but the SAA7360 doesn't think the gate should be open. I have a hunch that the LM1973N is working properly and the issue is somewhere around the SAA7360.

If anyone else has dealt with this problem of a flaky gate/compressor on a DOD G7, I'd love to hear how you fixed it.
Thanks in advance for any assistance anyone might offer.

[Jazz P Bass]

Are you sure that the clock is present (pin 9) on the LM1973?

[HipCat5]

Are you sure that the clock is present (pin 9) on the LM1973?

Am I sure? not 100% as I don't have anything other than an old tube Tektronix oscilloscope to test with. But I can tell you that there is signal on pin 9 ... its not flatline low (0). If I put the scope on a really low setting, I can "see" what I believe is data on that pin.

I plan on spending a little time checking the SAA7360 tonight to make sure that it is working properly and getting a proper analog signal from the input for the gate sense line.

Thanks!

[HipCat5]

Am I sure? not 100% as I don't have anything other than an old tube Tektronix oscilloscope to test with. But I can tell you that there is signal on pin 9 ... its not flatline low (0). If I put the scope on a really low setting, I can "see" what I believe is data on that pin.

I plan on spending a little time checking the SAA7360 tonight to make sure that it is working properly and getting a proper analog signal from the input for the gate sense line.

Thanks!

Update: The original poster was correct ... there was a revision change somewhere along the line because the SAA7360 layout is quite a bit different than the schematic DOD (Harman) sent me. I do think the issue is a bit deeper in the digital electronics part of the unit, however, as ...if my theory was correct about the gate simply not getting any source signal, setting the gate to **bypass** should have had some measure of success ... and it doesn't.

[HipCat5]

Final update: I wasn't able to figure out the root cause of the issues but I found a relatively simple fix:
1) solder a wire between Pin 2 and Pin 20 on the LM1973
2) solder a 470k resistor between pin 18 and pin 16 on the LM1973
3) solder a 180k resistor (roughly) between pin 4 and pin 6 on the LM1973
These values were found by taking 3 500k pots and using them as attenuators in place of the LM1973. These values may not be 100% perfect, but at least the DOD G7 is useful again.
Cheers!

[Reidk]

This thread refuses to die (but so does my G7).

I eventually fixed it! The problem was indeed the lm1973. Apparently they were known to fail often in this circuit, because later revs changed from a soldered-in chip to a socket.

The lm1973 is no longer available in a DIP package, only SMD, so I bought the SMD chip, an SMD-to-DIP adapter, and (planning for the future) a DIP socket. The G7 has worked like a champ ever since.

I have schems for both versions if anyone's interested.

Reid

[sonicgene]

Hi Reid,

i just won a bid on Ebay for the G7, itīs the second revision, because of the 10v 500ma AC Plug thatīs for sure, while itīs on the way to me, was checking infoīs and being very interested in having a schematik, and or even a Handbook.......

could you send me the schematik pleazzze ;-)

Mat

[Jazz P Bass]

Those SMD to DIP adapters are great.

[Fragger]

Thank you to those who posted in this thread. I recently bought a G7 off Craigslist built in 1995. It worked a few times for me before falling silent. The display and all the controls seem to function properly but all I could hear was groundfloor noise like listening to a cassette tape between songs.

Problem seemed to fit the description of the previous posters, that and a date written in pen, 7-02, next to a socketed LM1973 suggested to me someone had been in there before. I did try reseating the chip first but had no luck.

I know little about SMD to DIP adapters so I found a seller in China on ebay with good reputation sold me a DIP LM1973 for $6. Waited 3 weeks for delivery. Up and working again!

If anyone has a suggestion for different op amp, maybe lower the groundfloor noise a bit? It really isn`t as bad as cassette tape background noise so I shouldn`t complain.

[Juanpivlok]

Hi, is there a way that you can share the DOD G7 schematics, itīs giving me a hard time trying to get it from the company, specialy that I dont live in USA, any help will be greatly aprreciate it. thanks.

[Juanpivlok]

I have the hand book do you still need it?

[Tudy]

HipCat5: Thank You very much! It works! :-)