How to Fix Fender Pro Reverb Crackling and HT Cap Change

[@harryflashman5516]

I really enjoyed watching this repair. It will really help me with fixing my older tube amp. Nice work!

[@stuartukguitarampguy5830]

Thanks Harry that's exactly why I make these vids.

[@monteklein7385]

Thank you Stuart! I enjoyed video. I was changing the electrolytic/ filter caps in a client's 1966 Fender Pro Reverb. Perfect job, with minimal hum. Frustratingly, I came across the similar crackling, popping noises when touching the fiber circuit board. The board looked damp and had a bit of white mold. I wiped the board as clean as possible with rubbing alcohol and Q-tips. I will remove and replace the 100K resistors tomorrow. Hopefully, the crackling demons will be gone.

[@stuartukguitarampguy5830]

I hope that cures it. Sometimes you need to totally decontaminate the board (build up of grease and dirt etc.) Good luck.

[@williswet]

Stuart. I've noticed that at times you use some unorthodox testing methods. They seem to work great and I've actually adopted a few myself. Thanks so much for your time and the great education. Rob.

[@stuartukguitarampguy5830]

Hi Rob. I guess every tech develops their own set of fixes and technieques over the years. Anyway, if they're any good, use them! All the best

[@MichaelSmith-rn1qw]

I always change the dropping and balancing resistors when installing new filter caps. If you're going to disturb the original solder joints to remove the caps, it's just as easy to remove all of the old solder and install new components with fresh solder. On a 1969 Bandmaster Reverb I bought a few months ago, I had a cracked solder joint that I couldn't see with the naked eye. After poking around the eyelet board with a wood pencil I isolated the problem area and found the bad solder joint with a magnifying glass. I did a short YT video entitled "Fender Bandmaster Reverb Troubleshooting." Reading some of the older comments about the wax on the eyelet boards becoming conductive, I assume you've watched Psionic Audio's video on this subject by now. From what I understand, applying the wax to the boards was a practice Fender used for a few years to try and prevent the boards from absorbing moisture, but it created it's own problems as Lyle explains.

[@stuartukguitarampguy5830]

Valuable info thanks Michael.

[@ottomans2876]

I enjoyed this fix. I understand better now where you have said before about a no expenses situation and going down the route where you are changing the whole amp eventually. But as you said, they look bloomin ugly but they work. Keep up the good work Stuart.

[@3MonkeysGarage]

Glad you gave me another video this week. Keep an eye on those waxed boards. sometimes they end up with solder blobs between the boards causing a mess to trace. Kind of a pain to clean out too.

[@stuartukguitarampguy5830]

Ok thanks. I'm after a cure for 'wavy board syndrome'!

[@kengevers8738]

Stewart, I have a SF Champ, and the same condition as this amp. I built a new board from fiberglass epoxy, and that cured the problem. Fender CBS had a habit of spraying bees wax on the boards which in time made the boards conductive. 😳 Nice fix, Cheers

[@stuartukguitarampguy5830]

Hi Ken, nice one, I've never done that. Yes I do know about the cracking etc caused by these old boards but never quite had the nerve to replace one!! Maybe I should.

[@ThomasACarlos]

Thanks for the tip of shorting a preamp valve Anode. It seems like most amps these days come with 1/2 watt resistors - if that !! So you're absolutely right to mention, check the resistor. make sure it is at least 1 watt, and use this trick as a momentary short test.

[@stuartukguitarampguy5830]

Thanks Thomas I'm pleased it was helpful.

[@alexdeleon7135]

Quite a poser of a repair, Stuart. I enjoyed the meticulous deducing of possible causes. That, coupled with lessons regarding voltages, and shorting methodology were top tier. An excellent outcome which gives this amp decades of continued service. This epic episode was very enjoyable. Please remain in good health, and keep safe during these uncertain times. Well done!

[@stuartukguitarampguy5830]

Thanks Alex I'm glad you enjoyed it!

[@dennismasterton3834]

Great video Stuart. Reminds me of the time during the 1970's, I repaired guitar amps for a local shop.

[@stuartukguitarampguy5830]

Happy days I expect - in retrospect of course!

[@johnsalaman503]

Great job Stuart as always,,,these amps will be around forever, nother happy customer...

[@stuartukguitarampguy5830]

Thanks John!

[@edwardhannigan6324]

Fab job Stuart..love all the tech info and running commentary..Great learning curve..Thanks for sharing..Ed..UK..😁

[@stuartukguitarampguy5830]

@@edwardhannigan6324 Thanks Ed.

[@michaelevans3852]

Another excellent video! Nice to see a cap job. I particularly enjoyed seeing you preform the process of changing and soldering the caps. The testing of the caps was interesting as well. What type of meter is that you use to do that? Thanks for the lesson.

[@sparkipeat2255]

Hi Stuart. Very informative, thanks. I'm an Electrician with a bit of electronics knowledge but I've learned loads watching this video. The plan is to buy one of these vintage silverface amps not working and fix it. Would you mind saying what you were doing with the freezing spray and heat gun 👍

[@stuartukguitarampguy5830]

Hi. Yes, suppoose you have a noisy resistor which is producing an annoying crackle on the signal. It can be hard to track down WHICH resistor (or capacitor) is giving the problem. You don;t want to start sequentially swapping out components. So you can use a freezer spray to narrow down the area. Cooling the offending component will lower the noise. Sometimes it occurs with heat, so you can use the heat gun again to try and narrow it down to an exact component. It's pretty tricky though!

[@sparkipeat2255]

@ Thanks 👍

[@stewarttyler6445]

For what it costs i always change all the HT caps. Pot scratching can also be dc voltage on them. Usually faulty caps. Misleading as everybody thinks it just a clean job.

[@stuartukguitarampguy5830]

Hi Stewart. I'm always mindful of the final bill for the customer so tend not to do more work than required. Yes, it can be confusing about DC on pots but as you know there's an obvious difference in sound compared with dirt. Dirt tends to give a more lumpy noise, happens at certain positions on the dial. DC gives a fairly smooth continuous noise as you trun the pot. All the best.

[@stevehead365]

I'd also go with the conductive board camp, moisture and general crap. I also just watched Lyle at Psionic's video on this subject. One thing he did not mention is that alcohol is flammable, so be careful with the hairdryer. The amp sounded like it was going up in flames but literally would be bad. Cheers Steve.

[@stuartukguitarampguy5830]

Hi Steve. YEs this is certainly possible but flexing the board slightly caused the problem and that feels more klike dry joints. Also, after doing the joints it was 100% solid. But I agree it could come back.

[@cliverkay]

I had a similar problem a few years ago and it turned out that the boards had absorbed water and partly became conductive and made a hell of a noise.. Had to replace the whole board .. Used a pre made turret board to save time !! Keep well !!

[@stuartukguitarampguy5830]

Hi that's VERY interesting thanks. Do you have any cures for those wavy boards like in the one I showed?

[@gringopig]

I had time travel audio in Edinburgh sort my fender champ out. They had to stick the board in an oven to dry it out and reform it straight. Had to take all the components off though.

[@weschilton]

@@stuartukguitarampguy5830 isopropyl alcohol to dissolve the contamination and force the moisture out. Sometimes it even happens with the 60s cloth wiring as well.

[@stuartukguitarampguy5830]

@@gringopig Hi. Wow that must have cost a fortune! Just the strip down and rebuild would have cost a fair bit.

[@stuartukguitarampguy5830]

@@weschilton Okay thanks Wes. Just watched a v good vid on this from Psionic.

[@johnc.4625]

Only my reverb hums loudly . Have some leaky brown drops any suggestions besides changing those. Also some blue sprague filter caps at 10 to 20% over. Thank you.

[@bobc.5698]

The test strum sounds like 'The Hard Days Night' intro......in a few of your vids...lol.

[@stuartukguitarampguy5830]

Ah yes it's just an open string strum of course and they did use that as the opening chord in A Hard Day;s Night. God that seems so long ago now!

[@axemonkey]

Sounds like dc in the signal path. Leaking capacitors most likely.

[@suso_alonso]

Hi Stuart? Do you think that the layer of resin spread throughout the chassis can create some conductivity when dirt adheres and can generate certain noises? It was another great video. I learn a lot with you. Cheers

[@stuartukguitarampguy5830]

Yes it defintely seems so. Have a look at some of the other commenst about this video. Thanks.

[@goodun2974]

I hate to be the bearer of bad news, but.....that crackling and rumbling noise will eventually be back, probably sooner rather than later (blame your damp UK climate). The wax-impregnated eyelet board (note how shiny and gunky it is) absorbs and holds moisture, causing leakage current from the high-voltage nodes on the board (primarily the plate resistor feeds) to reach the preamp tube grids. You can actually use your multimeter to measure voltage flowing through the board itself; referencing to ground, you might find several volts on the areas of the board surrounding the high-voltage components, and it only takes 20 to 50 millivolts surrounding the grid resistor connections to generate noise in the tube. The heat from your soldering iron and heat gun temporarily drove out the moisture, but eventually the board will reach equilibrium with the surrounding air and the noise will return. The main reason the controls were so noisy is because the leaky, conductive board put DC on the controls. The only cure is to saturate the board with pure alcohol and wipe / scrub off as much of the wax and old flux and greasy residue as possible using paper towels, Q-Tips and a toothbrush. You might have to clean the underside of the eyelet board as well, and perhaps the insulating sheet underneath. Like I said, go watch some Psionic Audio videos on the subject.

[@stuartukguitarampguy5830]

Yes I have heard about this and it may come back but it was a pretty solid fix after I went over the dry joints, and remember, that gentle flexing of the board caused exactly the same noise. Any thiughts on straightening those wavy boards? I guess not possible?

[@xonxoffxonoff]

@@stuartukguitarampguy5830 Hi Stuart, as a Brit living in New Zealand I really enjoy your videos. There's a video by Lyle at Psionic audio where he goes into HT leakage across the board. Very similar problem to this. kzbin.info/www/bejne/a4C6hIiPgL-ml8k As for flattening the board, I think it's a case of removing all the components and using heat. But once you've done that you might as well put a new board in!

[@kengevers8738]

I had the same problem with a 70's Champ. The board was so conductive after washing the fibre board with pure alcohol. The only solution was to change the board out with a glass epoxy board. Problem solved. I gave the customer the original board back in case he wanted to the amp original.

[@stuartukguitarampguy5830]

@@kengevers8738 Hi Ken..Do you mean waqshing the board with alcohol CAUSED the board to go conductive?

[@stuartukguitarampguy5830]

@@xonxoffxonoff Ok thanks Mark I'll take a look.

[@jjsant3250]

That is likely a bias balance and not a bias level control. Turning it brings one side down and the other up. You placed 100u of filtering on the tube rectifier. OK on SS diodes but not ok on a tube. Schematic is 35u net. 50u net was in there. 100u will shorten the rectifier life and could blow the power tranny (again.)

[@blakesimpson9974]

This is incorrect. He installed two 100uf caps in SERIES which equals 50uf. Also, you can clearly see him dropping the current down on the power tubes and showing them being closely matched. You clearly did not pay attention to the video and or don't understand what you are saying, rather just regurgitating what you read on a forum.

[@gpwx]

Brilliant deductions …

[@stuartukguitarampguy5830]

Elementary my dear gpwx....

[@bobc.5698]

Is this amp the 40 watt with the tube rectifier or the 70 watt with the solid state rectifier?

[@stuartukguitarampguy5830]

Hi Bob.. I can;t remember unfortunately. Stuart

[@johnc.4625]

Normal channel works but but vibrato ch very low out put and sound bad. Where should I look first. Thank you

[@stuartukguitarampguy5830]

Hi John It's so hard to say. Try changing the preamp valves first. Other than that, you'll need a tech!

[@johnc.4625]

Really enjoy your Channel !@@stuartukguitarampguy5830

[@ernestschultz5065]

Those brown caps look a little baked

[@stuartukguitarampguy5830]

I likes 'em nice and crackly...

[@bobc.5698]

I just bought a used 1980 pro reverb. I better buy 2 sets of tubes. Like $400.

[@stuartukguitarampguy5830]

Hi Bob. You're a wealthy guy if you can afford 2 sets of tubes these days!

[@shane011471]

It's leakage on the board because it has become conductive and it will be back sooner than later! You need to clean the entire board with Alcohol and a Toothbrush or whatever you prefer .....

[@stuartukguitarampguy5830]

Thanks Shane good info. My vuew is that on THIS particular amp it wasn't board conduction - but I'll wait and see!

[@shane011471]

@@stuartukguitarampguy5830 You would know more about it than me as you had the Amp in front of you and I did not! Do I still see the EAR PSU in the background? Are you still going to give it a go?

[@stuartukguitarampguy5830]

@@shane011471 Yes still at it! I'll release part 3 soon!!