Kendrick 1000 Reverb Tank - Dwell Knob

[HowardMusic]

Hi All,

I seem to be having trouble with the Dwell knob on this Kendrick reverb tank - it seems that the level / length of the reverb effect is unchanged from zero to 12. Regardless of what setting the dwell knob is at, the amount of the reverb decay is the same. FYI, I had not used the reverb tank in around 10 years, and just pulled it last week use it. This one is from 1992, and I last had it serviced around 2007/8-ish. Here is a link to the Fender 6G15 schematic, which I believe the Kendrick 1000 is based.

https://www.thetubestore.com/lib/thetubestore/schematics/Fender/Fender-Reverb-6G15-Schematic.pdf

Any suggestions are very much appreciated! FYI, I am not so much a DIY'er so some suggestions may be difficult for me to implement, but I am willing to give it a try. Thanks!

 

[pdf64]

With a 6G15, the only way I can see that behaviour, ie no issues other than an inoperative dwell control, being caused is if the dwell pot, or a connection to it, was faulty.

 

[Jeff Gehring]

With a 6G15 you can easily pull the back panel off and get eyes on the dwell pot. The dwell pot is DC isolated, so with the unit powered off and unplugged, you can put an ohmmeter between the dwell wiper and ground and check that the resistance goes from about zero to about 250k. You can also verify continuity from the top lug of the dwell pot back to the 0.010uF cap on the board. Assuming you are getting reverb, you might try another 12AT7 there.

 

[HowardMusic]

Guys, thanks very much for your comments and suggestions, I really appreciate the help! Should have noted this in the original post, but I am still getting reverb, but just can't seem to control the dwell. Sorry about that.

Let me check these things out, and hopefully I will be able to get a better understanding of what the problem is. The connection looks good, and I will see about how to check the voltages on the dwell wiper and the 0.010uF cap, although I'm afraid that I am not entirely sure how to do this, although I do I have voltmeter. I've only got an ECC81 at the moment, no 12AT7, so will also give that a shot instead.

Thanks again!

 

[HowardMusic]

Just a quick update, Changing to the ECC81 did not have any effect - still no control of the dwell setting. I also checked the sweep of the dwell knob and it goes from 0 to about 230k. Will check the cap now, but will need to figure out how to do this. Thanks again all!

 

[pdf64]

It would be helpful if you could provide some detailed photos of the chassis internals. Upload them to a hosting site, eg imgur, and post links.
12AT7 is just a different name for an ECC81, it’s a Notth American - European naming scheme thing.

 

[HowardMusic]

Here is a link to a picture - not sure if this will work:


Please let me know if you can see this or not. Thanks for your help.

 

[pdf64]

I can see it, but a downside is that I can’t zoom in, when viewing via safari on an iPhone.
It may be a flickr thing?
EDIT - The 'zoom in' is fine using Chrome on a Win10 pc.

 

[Jeff Gehring]

The one thing I can think of that would make the setting of the dwell control irrelevant, is loss of the ground connection to the 'low side' lug of the pot, or an internal problem with the pot itself that duplicates that condition.

 

[HowardMusic]

Thanks again everybody. I took a close-up picture of the connections on the dwell knob - not sure if you can tell if the ground connection is ok or not, but perhaps you can see from this?
Thanks again for your help!

 

[HowardMusic]

Strangest thing - understanding that the ground on the dwell knob may be the problem, I did the most scientific and technical thing - I tapped on the dwell knob several times from below to possibly re-engage any bad connections, and after tapping the knob several times, the knob is working as expected. Maybe being idle for so long had caused the problem, but it is now working fine. Thanks everybody for your help and suggestions to troubleshoot this problem. Really awesome guys, thanks so much!

 

[Steppin' Wolfe]

Good find. Sometimes cleaning/lubing the pot will cure such a problem...or simply working the pot back and forth across its sweep will clean the contact surfaces. And....the case of a pot can be loose and cause problems. If the pot sounded ‘solid’ when you tapped it, you probably just had some dirt or oxidation on a contact surface.

 

[Jeff Gehring]

If you've never seen a pot taken apart before, it is basically a housing containing a central shaft (with the knob attached) that has a radial arm that has one piece of metal on it fashioned into two sets of contacts, and a horseshoe-shaped resistive element whose ends are crimped to the outer two lugs on the pot. I made a pic that 'splains it better/more visually:

So with your problem, the issue was that the ground connection which was made at the pot lug (by soldering it to the pot case, which is mashed up against the chassis metal) was NOT making it to the resistive element. There are no moving contacts involved with the outer two (end) lugs on the resistive element, just that crimp connection between the lug and the element end. Possibly a loosening crimp, or some contaminant in there was causing your problem. If the problem ever comes back, don't bother busting a think on it, just replace that pot.

 

[Steppin' Wolfe]

Jeff, thanks for the break down display. It appears to me that there are ‘wiper contacts’ that ride on that resistive track and provide the connection between the resistive track and the slip ring contacts that ride on the slip ring.

 

[Jeff Gehring]

Yep, sure are. But he's not losing the wiper connection -- if he did the 'verb would be dropping out on him, which is not happening. Instead, the pot was losing the ground connection (at the end crimp on the horseshoe), causing the pot to stop being a voltage divider, and instead become a variable resistor in series with the grid, (which has scant effect reducing the dwell signal, owing to the large impedance presented by the grid dropping virtually all of the dwell signal).

 

[ac427v]

The ground terminal on the pot appears to be grounded to the chassis through the pot body and the star washer-pot nut. Corrosion between those parts could make for a poor connection. Easy to visually check by removing the knob and pot nut. Or measuring resistance between the pot body and the chassis.

 

[HowardMusic]

Jeff / All, thanks very much for taking the time to help, and to explain in detail the technical aspects of the seemingly simple pot. For me, as a non-technical person, I really appreciate the detailed explanations you have provided, and this will really help me (and hopefully others) to troubleshoot in the future if a similar issue arises again. The generosity of the folks here is one of the reasons why I joined TGP so many years ago in the first place, and I’m happy to see that spirit continues here today. Hoping that I can contribute something useful in the future as well. Thanks again. Howard