Bogner Uberschall channel switching problem

[six el six]

I have an Uberschall I'm trying to repair. It does not switch properly between the two channels (dirty/clean). The dirty channel is fine but the clean channel when engaged has very low volume, approximately 10% what it should.

It uses the Vactec 5c1 optocoupler (6 of them) and Omron relays. It uses three of the  Omron G6A-234P-ST and one Omron G5V-1.

Voltages on the LED side of the optocouplers when switching between channels are for the most part 1.6vdc except for one that reads 12.5 vdc when the clean channel is engaged. Some of them have voltage on both the anode and cathode of the LED and some of them don't.

I have no schematic for this amp and I can't locate one in spite of trying.

I am attaching a Bogner Triple Giant preamp schemo for reference. I would like to get a very thorough understanding of how this very common switching system works so that I can get these switching amps like this repaired quicker. How do the relays fit into this system exactly?

Am I possibly right in guessing this is a relay or optocoupler failure?

Does anyone have any suggestions on fixing this thing?

Where should I be looking?

Thanks,
six

[birt]

maybe if you want to understand it better you should go step by step in checking what is causing the low volume. trace the signal with an audio probe for example. see if the voltages in the audio circuit are what you expect them to be. if you find something wrong, follow that lead and you might get to an optocoupler or a relay. this way there is a bigger chance of solving the problem than immediately focusing on these specific components.

[six el six]

maybe if you want to understand it better you should go step by step in checking what is causing the low volume. trace the signal with an audio probe for example. see if the voltages in the audio circuit are what you expect them to be. if you find something wrong, follow that lead and you might get to an optocoupler or a relay. this way there is a bigger chance of solving the problem than immediately focusing on these specific components.

I've traced the signal with my audio probe and I certainly understand how that troubleshooting technique leads to success on simpler amps.

In this case however I have NO SCHEMATIC and as I understand it powering on the LED in the optocouplers causes the LDR to go up to a very high resistance and this resistance is in turn used to do several things like shunting the audio signal to/away from ground AND altering the cathode circuit in the preamp tubes amongst other things.

[six el six]

There are 6 -12ax7 tubes you can see in this shot. All the voltages on these tubes seem fine EXCEPT the first half of V2. Pins 1,2, and 3 on this tube have no voltage at all.

Signal makes its way throught the whole amp. It's just the clean channel that has a problem.

[sluckey]

You really need a schematic and layout with parts locator to properly service this amp, especially with the problem you have. You may have to buy a service manual from the company.

[John]

I'm sure you already found this one, but figured I'd post it up anyways.

http://diyfactory.ru/forum/uploads/img-1774-85104c65f0.GIF

[six el six]

You really need a schematic and layout with parts locator to properly service this amp, especially with the problem you have. You may have to buy a service manual from the company.

Thanks Sluckey. I'm following your advice right now. I just emailed Bogner and hopefully I'll get some assistance from them. A statement on their website alludes to the possibility that they might assist.

I'm sure you already found this one, but figured I'd post it up anyways.

http://diyfactory.ru/forum/uploads/img-1774-85104c65f0.GIF

That does not appear to be the beast I have before me!

[John]

Yeah, I thought it was much too simple compared to the gutshot you posted! 

[HotBluePlates]

I agree that you want the actual Uberschall schematic, as our guesses from a different amp's schematic may not be right.

But...

The Triple Giant schematic shows series and shunt optoisolators for every channel except the clean channel; that one uses only a series opto. The scheme seems to be when you want "not clean" the LED is turned off and the opto becomes a big resistance. At the same time, one of the distortion channels has its pair of optos switched to the opposite state. I mean, the shunt opto is turned off (no longer shunting signal to ground) and the series opto is turned on (becomes a low resistance and lets the signal through).

My guess is that the opto you measured the strange voltage on is a bum opto. Or, the relay driving that opto isn't giving a low resistance path for current. But I strongly suspect the opto, because generally LEDs clamp an applied voltage to a specific level, and you merely adjust the current through them through an external resistor. That resistor could be made bigger to lower the LED current and brightness. I'm guessing the LED in the suspect opto is bad, and isn't turning on bright enough to reduce that opto's resistance very low. That might account for the weak, distorted sound of the clean channel.

But you'll want to review the schematic, and cross-check the on-voltage of the opto in question. Of course, there's a chance the driving relay is faulty, etc.

[Jack1962]

Good luck getting the schematic from Bogner , I repaired a uberschall that had been in a fire , I had to reverse engineer the entire amp to do so.

[six el six]

Good luck getting the schematic from Bogner , I repaired a uberschall that had been in a fire , I had to reverse engineer the entire amp to do so.

Yes. I've found high gain amp culture to be rather "special".

On a positive note they are providing some troubleshooting assistance via email.

[The_Gaz]

If you haven't yet, trying asking over here:

http://www.slocloneforums.com/

[six el six]

Good luck getting the schematic from Bogner , I repaired a uberschall that had been in a fire , I had to reverse engineer the entire amp to do so.

I don't feel good about charging someone to reverse engineer their amp. I've talked to someone on the phone at Bogner and tried to get some assistance that way. This person admitted to me on the phone that there were some production problems with this amp and that these problem amps made it to the public. They don't seem to be interested in really helping with my problem though. No schematic, no technician help, nothing of any real substance at all sadly.

Two thumbs down on Bogner.

If you haven't yet, trying asking over here:

http://www.slocloneforums.com/

Thanks Gaz. I've looked over there.

[mot]

http://tinyurl.com/bqvsv6u