[pH]

I'm helping a friend restore an old Gates Dualux console full of 5215A preamps, I'm stripping the cards down and replacing all caps and resistors.

The problem I'm having is upon testing with the restored cards there is some serious motorboating happening. I've tested with two different known working power supplies and the problem is still there (and I tested with an un-restored card with no problems, so it would seem that the PSU is not at fault).

When I remove the feedback circuit the problem goes away. I've tried reducing feedback by upping R1 to 100k and lowering C4 as low as .1 but the motorboating is persistent. Since the original caps seem to have a shield I tried mounting C4 under the board, but no change.

I'm wondering if anyone (Doug?) has any experience with this happening with Gates preamp cards, or any other type for that matter.

Thanks,
Paul

[mjrippe]

Been a while since I had my hands on one, but there were a few versions of these preamps.  The 5215 (non-A) used 5879 tubes, for example.  The schematic I have for the A version shows C3 and C4 both as 0.47uf but you said you tried changing that, so not sure where to point you.  Maybe double check all your values against the (two) schematics and make sure that you are not a decade off somewhere (10k vs 100k, 0.1 vs 0.001, etc).

[EmRR]

There's probably a ground or shield that isn't right.  I recall motor boating problems, but they will run properly with the stock values.  It's been 20 years since I messed with one of those, so.....

[rmburrow]

I have some of those preamps and worked on them at various radio stations.  The HT power supply MUST be well filtered.  The original Gates supply was regulated.  If there is an electrolytic "can" type capacitor on the PWB, it must be good or otherwise motor boating will occur.  One overlooked item:  On the original board, the capacitors may be marked "outside foil" or a line indicating  "outside foil"... replacement cap must be installed with the outside foil as the original... the outside foil can be a surface for interstage coupling (especially on a printed board)....

More important, the preamps were installed on a metal rail inside the metal housing of the console.  Don't attempt to install these preamps in an unshielded environment or use long leads from the ground connection on the card to a "real" ground....Long ground return leads are nothing but trouble....

[pH]

Thanks for the responses.

My thinking was regarding the internal shields of the film caps as well, but the replacements I'm using don't have a shield marked. Might try reversing them just to see.

I also have some older 5215 cards using 5879 tubes, re-capped one last night and tested and I got no motor boating. Similar topology but different component values and layout of traces.

[rmburrow]

Ever thought of cloning those preamps by putting the circuit board on a copier and getting the foil pattern?  The original transformers are hard to get now, but that circuit should work with other transformers with modification of the traces and grounding...

[pH]

I'd like to solve this problem before thinking about cloning! Plus a ~40dB preamp like this likes a program amp to boost it up to gain levels most of us are used to.

I'm tearing my hair out, I've tried modifying the feedback resistor and cap, moving the ground traces around, shielding the tubes, and we're still getting the motor boating...

[rmburrow]

pH:  I have some of those Gates preamps.  If you are still having trouble with motor boating,  get two good quality 30 uF (or 40 uf) and connect them across the sections of C5 (which is a metal can capacitor).  Observe polarity and connect the ground end of the substitute capacitors to the same point on a lug of the metal can capacitor.  If you can replace the metal can capacitor, that's also fine but make sure it is good, or  otherwise the preamp will motor boat.  If C5A is bad, the preamp will definitely motor boat.   I would not mess with any other component values in the preamp until you deal with C5.  C4 is a feedback capacitor (value selected).  Set up your audio gen through an attenuator to give mic level at the input transformer, connect a 150 ohm resistor across the output, connect a reliable  decibel meter, and sweep the response before messing with C4.   You might get better low frequency response by increasing C3 (0.47 uF) to 1 uF.  Sweep the response when you make changes to avoid "surprises"... You should easily get flat response 30 Hz to 15 kHz or better through that preamp.

BTW if you have a Gates Dualux board, the program amplifier contains a capacitor that rolls off the high end around 10 kHz.  (This assumed the board was used at AM stations where full response out to 15 kHz wasn't needed.)  Lift one end of this capacitor and the program amp will go full response.  I don't have the program amp schematic in front of me but it uses a 12AU7 working into an output transformer to 600 ohm line.

[dmp]

The problem I'm having is upon testing with the restored cards there is some serious motorboating happening. I've tested with two different known working power supplies and the problem is still there (and I tested with an un-restored card with no problems, so it would seem that the PSU is not at fault).

The first place I would look is coupling through the power rail. It is C5A's job to prevent this. Maybe try making that cap bigger. Does the motorboat freq decrease? Old electrolytics are usually larger value than labelled.

In general, approach it methodically and you can figure out what component started causing the problem. Replace the electrolytic caps first and test.  It it doesn't motorboat, replace the other caps & resistors and test as you go (you may not need to replace everything... especially film caps in old gear are usually good). If you don't have a unmodified board, start swapping parts back on the board until the problem goes away.

My thinking was regarding the internal shields of the film caps as well, but the replacements I'm using don't have a shield marked. Might try reversing them just to see.

I remember reading about a way to test to determine which lead is connected to the shield in film caps. Helps reduce the noise in tube amps to have them connected the correct way. You could probably find it with a search.  That should relate to RF pickup, not motorboating.

[pH]

Update here.
It seems that re-routing the grounding has solved the problem. The two points where grounds were tied to the capacitor can tabs I had tied together with a wire underneath the board since I replaced the can with a pair of 30uF caps on a piece of perfboard, so I moved this ground wire above the board away from crossing B+ and it quieted down. I still got a bit of motor boating in my unshielded environment but my friend got it to run quiet in a shielding sled and he ran sweeps on it and it looked good. We decreased the feedback slightly by upping R1 to 100k to bring the gain closer to 40dB without suffering anything in frequency response and distortion so I think we'll call it good from there.
Now I've handed it all back to him to wire all the routing

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