Help requested with Fender princeton Reverb, odd reverb problem

[Ethan w]

Hi Guys, I have a '66 Fender Princeton Reverb up on my bench. The chassis is totally stock. It has a new reverb tank and good tubes, I double checked this. What's going on is the reverb won't adjust. The reverb works, but it's at a fixed level and the reverb control doesn't do much. It has a lot of reverb when set at 1 or 10. I checked the pot in circuit. I got about 100k with the control on 1, decreasing as I turned the control up. If it was some other amp I would just replace the small amount of components in the circuit and call it a day. I don't want to do this with a valuable vintage amp so I'm hoping to narrow it down a little. I attached a pic, we'll see if it works. Any suggestions are much appreciated.

One funny thing: the can filter cap has been replaced-with a unit dated June, 1959!

Ethan

[Enzo]

Hi Ethan, welcome here.

I am not sure what you measured on the pot. You should have about 100k between the two end legs of the pot. It should vary as you turn it, measuring from the center to either end.

For it to work as a volume control, it needs to be a voltage divider, and to do that, the lower end of the pot needs to be grounded. So set your meter to ohms, ground one probe to chassis, and measure resistance at either end leg of the pot. One end leg should be soldered right to the back cover of the pot. Is it? Does that leg measure zero ohms to ground, or at least a VERY low resistance?

And for that matter, is the nut on the reverb control tight?

[Dave H]

The reverb works, but it's at a fixed level and the reverb control doesn't do much. It has a lot of reverb when set at 1 or 10. I checked the pot in circuit. I got about 100k with the control on 1, decreasing as I turned the control up.

It looks like you are measuring it from the wiper to the cw terminal. What does it measure from wiper to ccw terminal? Is the ccw terminal grounded?

Edit:
Enzo you've done it again!!

[Ethan w]

Hi Guys, thanks very much for the replies. From the front of the chassis, the right hand lug is grounded to the brass strip by a wire. I get about .5 ohm to ground from this connection. The pot is attached tightly via the nut. I'm using a Fluke meter and it's really jumping around a lot. With the pot on "1", I get about 100k from the wiper to left hand lug (not grounded). From wiper to grounded lug it jumps around to all kinds of values, occasionally zero but up to 300k. It seems like the pot is OK. I guess I could solder another one in to be sure.

[Enzo]

Hey Dave H, when there is low hanging fruit, I am right there to pick it off the tree.

[gui_tarzan]

"Jumps around" sounds like the pot is hosed.

[Ethan w]

Looks like a new pot solved the problem! I just halfass tacked one in and it's working correctly. I just didn't think the pot was the problem as the other pots seem OK and I thought the control had worked properly yesterday. Thanks for your help gentlemen, much appreciated!

Ethan

[Chuck H]

Cool! Now maybe address those 50 to 55 year old electrolytic caps

If they're working, they're working. Are they working? Bias caps that old are always a concern. Leaving them in for the sake of authenticity is asking for the sort of trouble that might smoke a much more "vintage value" important transformer. It looks like someone has added a grounded AC cord so bone stock vintage is out the window already. I can't agree with using NOS electrolytic caps as it seems the previous service tech chose to.

Electrolytic caps go bad with age. Even if the amp hasn't been used much. In fact, long periods of no use can age the electrolytic caps faster. Occasionally a typical electrolytic cap will still be doing it's job at this age. Not often though. Most last about ten to twenty years. The symptoms of failing caps include, but aren't limited to, too much hum and odd harmonics that seem out of tune with the fundamental note. If this amp has these symptoms ALL the old electrolytic caps should be replaced. Failing to do so may cause a failure that compromises parts that would ruin any vintage value the amp has forever.

[Ethan w]

Cool! Now maybe address those 50 to 55 year old electrolytic caps

If they're working, they're working. Are they working? Bias caps that old are always a concern. Leaving them in for the sake of authenticity is asking for the sort of trouble that might smoke a much more "vintage value" important transformer. It looks like someone has added a grounded AC cord so bone stock vintage is out the window already. I can't agree with using NOS electrolytic caps as it seems the previous service tech chose to.

Electrolytic caps go bad with age. Even if the amp hasn't been used much. In fact, long periods of no use can age the electrolytic caps faster. Occasionally a typical electrolytic cap will still be doing it's job at this age. Not often though. Most last about ten to twenty years. The symptoms of failing caps include, but aren't limited to, too much hum and odd harmonics that seem out of tune with the fundamental note. If this amp has these symptoms ALL the old electrolytic caps should be replaced. Failing to do so may cause a failure that compromises parts that would ruin any vintage value the amp has forever.

I'd love to yank out those old electrolytics, but this amp is going on the market and I think it would decrease it's resale value. I usually yank that bias cap and put in a Sprague 100/100. The old caps do seem to be working-no weird noises or humming, surprisingly. I checked the plates for AC and I think I saw .2 VAC, very little. The can cap surprised me when I saw the 1959 date stamped on top, some tech using up his old ass parts I guess, probably years ago.

[Enzo]

Everyone has an opinion on vintage parts versus good parts. My own view is this: Would a 1957 Chevy be less valuable if it did not still have its 1957 tires?

[Chuck H]

Everyone has an opinion on vintage parts versus good parts. My own view is this: Would a 1957 Chevy be less valuable if it did not still have its 1957 tires?

To a driver, and sold as a vehicle to be driven at a corresponding price? No.

To a collector that wanted to put it into a garage and just show it off? Yes.

But this is where amp collecting gets confused IMO. There are very few collectors that don't use their amps, ever. Where museum type car displays might be more common per capita in the genre. So it's not an apples to apples comparison, but many non tech amp nuts seem to treat it the same. With the exception that they know what a crappy old tire looks like better than they do a crappy old filter cap

You don't see it as much now, but once upon a time it wasn't uncommon for guru type, mojo oriented amp "tech's" to get/use NOS filter caps. There were threads and tutorials about how to "rejuvenate" them by baking them or sticking them in a shoe box and heating it with a hair dryer and then applying voltage in stages like you're recharging a battery. Sometimes it worked well enough to produce a "not bad anymore" part. I tried it once with a pair of identical amps that had been new in their boxes for 25 years. I powered one up slow with a current limiter and let it "cook" for about ten hours. The other got nothing. The one I didn't burn in just popped the main filter within five minutes. The one I burned in worked, but sounded bad and exhibited symptoms of failing filter caps. That's my one experience (and will remain so) with trying to make old filters back into working filters.

[guitician]

I'm more apt to trust the caps from 1957 then the ones made after 2000. I've got this radio from 1929 that I checked the old oil caps in it and they're still good. I know, they're sealed and won't dry out, but the rubber/aluminum shouldn't let the electrolytic dry out. New/Old... it's all a crap shoot.

[Chuck H]

I'm more apt to trust the caps from 1957 then the ones made after 2000. I've got this radio from 1929 that I checked the old oil caps in it and they're still good. I know, they're sealed and won't dry out, but the rubber/aluminum shouldn't let the electrolytic dry out. New/Old... it's all a crap shoot.

I've read that the liquid electrolyte "crystalizes". Whether that means it dries out, polymerizes, separates to form curds and way, etc. I don't know. I do know that when they pop there sure seems to be liquid inside

And I've also noticed that there are more old school filter caps that seem to have energizer bunny genes. Where new caps seem to have more predictable life span. Perhaps the old caps were made with some awful carcinogen that worked better, but is now illegal in all the developed nations for health reasons. I never looked into it. But you can bet that if the phenomenon is true it has something to do with materials and not about the quality of the craftsmanship. My perception (= use salt liberally) is that new caps work better than old caps, but old caps last longer than new ones.

[guitician]

Yeah, and who needs lower ESR for a 50 year old circuit... Why doesn't Sprague open a vintage division for all those RI amps being marketed now? There's no EPA in china...

[52 Bill]

While you guys debate the cap question, I'll ask the original poster:

Did you try to clean and or service the original pot? That is one of those parts that everybody seems to like to be original.