Bogen J330 Mic Input Problem

[Sylvaniac]

I just recapped a 1955 Bogen J330. The inputs are XLR low Z. However, my low Z mics give little or no signal through the Bogen inputs, and there's a good deal of buzz too. I hear FM radio coming through too. The Bogen sockets are correctly wired as per schematic. The only mic that works with the Bogen is a low Z XLR with an inline transformer to hi Z phone plug, to which I connect an inline transformer back to low Z XLR into the Bogen. Very strange and frustrating. Why don't known working mics of the same time period work in this?

[g1]

Do you mean sockets for transformers? And are those transformers missing?

[Leo_Gnardo]

Thanks for jogging my memory g-one. Not about this particular amp, but lots of PA amps & preamps had plug-in mic input transformers back then. You could select say 50 or 150 or 600 ohm transformers to match your mics. Altec had ones that plugged into octal sockets, Bogens fit 9-pin sockets. If no transformer, there was an adapter to plug into the octal or 9-pin socket so that it would accept a "bridging input" IOW unbalanced, OR wires soldered across the socket internally if someone wanted to save a buck or 2 on adapters.

[g1]

Most Bogen's had the transformers optional. You needed them to run anything lo-z, or they had jumpers or switches that made those inputs hi-z without the transformers.
The plug-in transformers are usually the first thing that disappears in units that were equipped with them. They show up on ebay a lot.

[Sylvaniac]

Thanks, members, that's helpful info. Fortunately my Bogen came completely stock, equipped with one plug-in transformer in mic 3 socket. According to schematic (attached) that plug is for 200 ohms. Still, even in that socket none of my low Z mics work. I guess I should try cleaning the pins and socket? But are transformers known to go bad?

Also, here's an article that touches on (I think) what you're saying about rewiring the socket. I would need more specific guidance though. Bogen Lo-Z Transformer Options Also, I can tell you there is a wire jumper across pins 2 and 6 of all three of the mic preamp tubes 6SF5.

[g1]

The socket that has the transformer should not have the jumper between pins 8 & 9.

[Sylvaniac]

The socket that has the transformer should not have the jumper between pins 8 & 9.

OK, it does not. None of them have that shorting jumper.

[g1]

So the ones with no transformer need that jumper to act as Hi-Z inputs (pin2 hot).

You can check for any opens in the transformer with an ohm meter. With transformer removed, check the windings as shown in the lower right area of schematic.
pin 2 to 9
pin 5 to 8
pin 6 to 8
pin 6 to 5

[Enzo]

Your schematic shows transformer sockets, but not the transformers. Are there sockets inside the amp that look like tube sockets? And if so are there can shaped transformers plugged into them?

[Sylvaniac]

So the ones with no transformer need that jumper to act as Hi-Z inputs (pin2 hot).

You can check for any opens in the transformer with an ohm meter. With transformer removed, check the windings as shown in the lower right area of schematic.
pin 2 to 9
pin 5 to 8
pin 6 to 8
pin 6 to 5

Never thought of checking that; new to this type of mic transformer. Well, the meter does nothing. However, the meter shows a small impedance between pins 8 to 9. By the way, thanks to your help, I spotted that the other two mic sockets' pins 8 and 9 are jumpered via a plug-in jumper, or, "shorting plug" as illustrated in the schematic (attached photo). Now I'm beginning to visualize how all this works together.



EDIT: I'm wondering if what appears to be the plug in transformer is just another shorting plug, as only pins 8 and 9 have continuity. I guess I can carefully open it and see; and if it's the transformer, maybe I'll get very lucky and see the break.

EDIT: Sure enough, as you can see by the jumper wire and ground to can, this is not the needed transformer after all, but another jumper, I guess.

[Sylvaniac]

Your schematic shows transformer sockets, but not the transformers. Are there sockets inside the amp that look like tube sockets? And if so are there can shaped transformers plugged into them?

Hi, yes, mic socket 3 has the can shaped 200 ohm transformer plugged into it.

[Enzo]

The wire jumper plug makes the channel an unbalanced, Hi-Z input. That explains why your low-z balanced mic doesn't work.

[Sylvaniac]

The wire jumper plug makes the channel an unbalanced, Hi-Z input. That explains why your low-z balanced mic doesn't work.

Thank you. But I'm a little confused. I thought that the socket having the transformer plugged into it makes it 200 ohms, low Z.

EDIT: With the transformer unplugged from mic 3, I tried a low Z mic. No signal at all. The transformer itself is jumpering pins 8 and 9 (because when tested for DC resistance, 8 and 9 were the only pins to show continuity); so with the transformer unplugged, nothing is jumpering any pins. Still no signal. On mic 1 and 2, there are jumper plugs (as I showed in the previous post in reply to g-one). Those are very hard to get of hold of to pull out. But as I say, I unplugged the transformer from mic 3 and nothing was jumpered yet no signal.

[g1]

Ok, so you have no transformer, just another plug-in jumper wire. When that can is plugged into #3 socket, all 3 inputs should work the same, as Hi-Z inputs, XLR pin2 hot.

[Leo_Gnardo]

Thank you. But I'm a little confused. I thought that the socket having the transformer plugged into it makes it 200 ohms, low Z.

EDIT: With the transformer unplugged from mic 3, I tried a low Z mic. No signal at all. The transformer itself is jumpering pins 8 and 9 (because when tested for DC resistance, 8 and 9 were the only pins to show continuity); so with the transformer unplugged, nothing is jumpering any pins. Still no signal. On mic 1 and 2, there are jumper plugs (as I showed in the previous post in reply to g-one). Those are very hard to get of hold of to pull out. But as I say, I unplugged the transformer from mic 3 and nothing was jumpered yet no signal.

So lets make this clear, that was no transformer but a removable jumper. Without a pin 8-9 jumper, whether in an adapter plug or stuck on the socket or wired-on the socket electrodes, signal can't get to the first triode.

With the jumper in place, XLR pin 2 is your input, pin 3 not connected, pin 1 ground.

Input impedance is nominally 2.2 megohm and if you wish, you can load down the inputs with resistors for a lower R in. This should accept an unbalanced hi-impedance mic or any other unbalanced signal of any level up to the point the preamp triode clips.