Kalamazoo Bass 50 3 Band Equalizer Issue

[Jecarroll7]

Amp on the bench doesn't have any volume or bass response unless you turn the treble up at least 1/2 way.

If treble is at 10 I get volume but volume drops around 7 and picks back up around 9, and volume never zeros out.

Bass response (with treble at 10) kicks in about half way with only minor effect.

Last time I ran into this was with a Prosonic that was wired incorrectly.

All the pots seem to be good, and are clean. All voltages are good, no buzz. Connections appear to be tight, grounds look good.

When you find a working mix of equalizer settings the amp actually sounds pretty good.

Below is a link for the schematic. Any thoughts?

http://www.musicandelectronics.com/m...s-Model-50.pdf

[soundguruman]

If the pots are clean then they are worn out.
All the pots must work correctly, for the EQ to work, at all.

[Jecarroll7]

My thought as well but I guess I needed a bit more validation to move in that direction.

Do you have a source for quality 1M and 2M pots with knurled/splined shafts and solder lugs?

Alpha had one (RV16A-10-15K-A1M) but its showing discontinued.

I'm only finding solid shaft and would like to keep the vintage push on knobs.

[olddawg]

Can't you just float the pots and check them with an ohm meter? Barring that, you could pigtail a rotary pot or two in for test. I would not be so quick to condem the sliders. Also... many times you can take them apart, check the element, retention the contacts, re-lube and reassemble. I wouldnt recommend it necessarily for a mixing console, but it's acceptable IMHO for a 3 band eq on an old guitar amp.

[soundguruman]

My thought as well but I guess I needed a bit more validation to move in that direction.

Do you have a source for quality 1M and 2M pots with knurled/splined shafts and solder lugs?

Alpha had one (RV16A-10-15K-A1M) but its showing discontinued.

I'm only finding solid shaft and would like to keep the vintage push on knobs.

You can try antique electronics supply, but I'm not sure of the physical size. You should measure it out...
I've been using Precision Canada pots, which are really nice but the diameter is large.
They is about 1-1/2" dia, but if it fits the Precision is a great choice.
Solid shaft though.

[52 Bill]

A couple of things here. First test the pots with a meter and see if they are in fact working correctly or not.

If they need replacement and you can't find the correct ones and the style is important to you, rebuild the pots like Old Dawg suggested, but with the carbon elements from new pots in the old hardware. Unless the pots are mechanically locked (some are) you can disassemble them and replace just the wafer with a new one from a matching unit.

If the pots aren't bad you could have a parasitic oscillation causing the volume drop. Have you checked anything else in the circuit like coupling and tone caps? Ground connections?

[soundguruman]

A couple of things here. First test the pots with a meter and see if they are in fact working correctly or not.

If they need replacement and you can't find the correct ones and the style is important to you, rebuild the pots like Old Dawg suggested, but with the carbon elements from new pots in the old hardware. Unless the pots are mechanically locked (some are) you can disassemble them and replace just the wafer with a new one from a matching unit.

If the pots aren't bad you could have a parasitic oscillation causing the volume drop. Have you checked anything else in the circuit like coupling and tone caps? Ground connections?

Without over doing it, the pots aren't making contact, they are worn out.
You have to install good pots to draw any other conclusions, FIRST.
AND yes, you can disassemble and clean them, that usually cures the problem.
If you would like to read my blog on cleaning pots and jacks:
CLEANING CONTROLS AND JACKS (INFO) - Audio-Technology - Audio
It has quite a bit of info...

[Jecarroll7]

Kalamazoo Bass 50 3 Band Equalizer Issue - Back on the Bench

I guess it's really a two band equalizer. Haven't changed the pots yet, a bit more checking first.

I think I have a set freq attenuation problem, specifically in the mid bottom bass area that doesn't seem to be pot specific.

I can adjust the volume, treble, and bass such that I get each pot to operate from low to high with relative smoothness depending upon where the other two are set. I can also get each pot to drop the volume when I hit this narrow freq band in the mid bottom bass range, depending upon where the other two pots are set. So again this particular issue doesn't seem to be related to any one pot.

Having said the above, the amp will get loud but volume won't zero out completely at any of the settings, not sure if this is a separate problem or not. Checking the pot with a meter it looks fine. Pots have been cleaned with 100% DeoxIT and lubricated with Gold DeoxIT, grounds checked, molex cleaned and retensioned, tubes tested, sockets retensioned, etc. I haven't checked the 10uf bypass cap on the first stage as the overall volume isn't low. If I've missed something please let me know.

There are four caps in the equalizer circuit, I have lifted a leg of the 500p ceramic cap and a .0047uf poly off the treble pot, as well as the other .0047uf poly off one leg of the bass pot. All three tested fine for value and leakage on my Sencore. The last one is .01uf off the other leg of the bass pot. I haven't tested this yet but clearly someone has been looking at this issue or something very similar before me as this .01uf and the .022uf coupling cap off the plate of the first stage both look like new Sprague orange drops. They are the correct values and the circuit matches the schematic.

Since replacing caps is easier on this amp than replacing the pots I'm going to try that first (cramped space/extra ground lugs/AC switch/etc).

BTW, not sure who assembled the amp but every component is wrapped two-three times around its connection lug, then soldered. This has brought a whole new level of pleasure to working on it for me.

So here are a few questions:

- Any suggestions on which cap(s) to try first and why?
- Suggestions on replacment caps (orange drops to replace the black poly ..47uf, line on one side) keeping in mind that while the signal wires are shielded I do have ac running close to the pots (treble pot is also on/off switch) so noise is a consideration. I plan to use silver mica to replace the 500pf ceramic.

Appreciate the help as always.