Ampex AM-10 Mic Preamp Repair

January 2020

Here is a vintage Ampex AM-10 Mic Preamp of ours we worked on.

Ampex AM-10 Mic Preamp

We inherited this nice old mic preamp a while ago, but were inspired to take a closer look at it after working on the Ampex MX-10 for a customer (here). The MX-10 is tube based, but this AM-10 is transistor based. Otherwise, the configuration was almost identical (4 mic preamps, with gain control, and switch for left, right, or center (both) channels, and a master stereo gain control).

The AM-10 looked to be in good condition, except for a lot of rust on the XLR input jacks.

Rusty XLR Input Jacks

Rusty XLR Input Jack

We were able to get a mic cable plugged into one of them just enough to find that the preamp seemed to be working. However, the rust on the input jacks was bad enough to prevent mic cables from easily being plugged in (the inner part of the XLR jacks were rusty as well, so it wasn't just a mater of cleaning off the surface rust). We decided to take a chance and just replace the 6 XLR input jacks and see what we ended up with. It looked like there would be enough slack in the wires connecting the jacks, so we could solder the wires to the new jacks before mounting them.

Original XLR Jacks - Inside View

Original XLR Jack Removed

We started by removing the old jacks from the chassis, and unsoldering the shielded cables. We used a small vise to hold the jacks while unsoldering, and soldering.

Old Jacks Unmounted from Chassis

Using Small Vise to Support Jacks for Soldering

Here you can see the old jacks removed, and two new ones installed. Installing the new jacks was a bit tricky due to the placement of the nuts. A small hemostat worked well for this.

Old Jacks Unmounted from Chassis - Two New Jacks Installed

Small Hemostat Used to Hold Nuts During Re-Install of Jacks

Below shows the new XLR jacks installed, along with the old jacks. We cleaned the switches with contact cleaner, as well as the preamp module edge contacts, which plug into a motherboard of sorts. The pots are sealed, so can't easily clean them. We just acquired some adapters that screw over the pot threaded sleeves and allow cleaning fluid to be forced down the shaft, so will give them a try later.

New XLR Imput Jacks Installed

After reassembling, we connected this to two line inputs on a small mixer, connected up a mic, and tested. Initial testing revealed that the channel 3 mic input wasn't working. Swapping the mic pre boards didn't make any difference. After removing the top plate again, we discovered that one of the wires on the new XLR on channel 3 had broken off, probably while installing the XLR jacks onto the chassis. After fixing this issue, channel 3 was working properly. We haven't tested the two line level inputs yet, but the 4 mic inputs are all working.

After reinstalling the top plate, we installed the unit back into its road case along side the VU meter unit, and tested it again. The VU meters both seem fine as well. Looking forward to trying this unit out with the vintage RCA ribbon mics we have (which, at a minimum, need new cables).

Back in Travel Case with VU Meters

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