dbx 386 issues?

ranjam

New member
I just brought home a used dbx 386 from my local music store. I'm thinking clean the pots and tube sockets, tighten up any hardware, and it'll be good to go. Then I get a brilliant idea to put a signal generator through it, and scope it out, checking the waveform and frequency response. Well, channel 1 won't go into the red as easily as channel two :confused:. So, I swap tubes between channel 1 and channel 2. Same difference. I don't see any trim pots around the input section. The waveforms looks great, but just quite a bit lower on channel 1 (for exactly the same settings as on channel 2). With a real microphone it appears to be a bit lower as well, but that may be the power of suggestion :o. Is this tolerance or difference in gain between two channels in the same preamplifier acceptable? I can always return the preamp, but I don't want to look like a worry wart. I can't find any schematic online, so I have no idea what to check other than tubes. Whaddaya think?
 

8bus

New member
I have a 386, but I don't have the problems that you are having. I would say take it back or if you can repair it yourself. It's not a bad pre, but not great either.
 

dodgeaspen

New member
If you do take it back make sure you don't tell them you took it apart. (I assume you already know this) If they find out you did they could say you damaged the unit and give you lots of grief. Try using it and see how it works. You might be pleased with the results.
 

ranjam

New member
If you do take it back make sure you don't tell them you took it apart.........

They know me better than that ;)! I'm known for not having any stock gear. But I had another idea, and I may take it apart again (two screws on each of the sides and one from the front, so it ain't time consuming) and compare tube voltages. If they turn out to be identical, I think it's going back. Beyond that, I'd need a schematic. I have enough test equipment (signal generators, oscilloscopes, dummy loads, distortion analyzers, etc.) that I can test anything, but if I have no idea of the circuit..... :confused:.
 

dementedchord

Psychotic State alumni ch
just how much diffrerence is there??? are you playing with both the inputs and outputs controlls??? also the output meter can be switched between the analog and digital output... i'ld keep it... i'm happy with mine... i even use it in my live keyboard rig...
 

ranjam

New member
just how much diffrerence is there??? are you playing with both the inputs and outputs controlls??? also the output meter can be switched between the analog and digital output... i'ld keep it... i'm happy with mine... i even use it in my live keyboard rig...

Well, with various frequencies, such as 1KHz, channel 1 is 8db lower. I sprayed the heck out the analog/digital switch as well as the 'Line' switch. Using the 'Line In' input or the balanced XLR, and switching from channel 1 to channel 2, I left the signal generator and the oscilloscope alone. Putting the Drive and Output levels all the way to +15, channel two goes into the red at +20db, while channel 1 only gets to +12dB. Scoping the TRS output or the XLR output shows that channel 1 is indeed lower. It looks fine, and with a real mic it sounds like it should, but you know in the back of your mind you have to crank channel 1 and it still never goes all the way to +20dB. I've only tried dynamic mics, but tonight I plan to test tube voltages and try a condenser. I'll know tomorrow whether the old girl stays or goes. I really wanted to like this, I had hopes for it :(.
 

ranjam

New member
As The Good Book tells us... 'Seek and ye shall find!'. Measuring the voltages, I had to move some ribbon cable out of the way, and there underneath the LED panel is a trim potentiometer :)! I honestly did not see it before now. My problems are not solved though :(. The tube voltages are identical (or within a volt or two), so there is a problem somewhere. Turning the Channel 2 trim pot all the way CCW and the channel 1 pot completely CW now gives me +12dB and +16dB. Both pots were set similarly, and almost completely CW. That's why channel 1 didn't really change, but I brought channel 2 down. I know this isn't the way to do it, but it is a step in the right direction. I am not sure it matters, but I am using a 250mVAC signal.
 
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