200 capacitor troubleshooting question

[citysoundman]

Hi all, I've completed the regulation of my Wurly 200 (early model amp), and now looking to fix problems with my amp. I've disconnected the pickup, and wrapped and isolated the transformer leads in copper tape. So what I'm hearing is coming from the amp - a constant ground hum regardless of volume, and a lot of constant noise that increases with volume. The noise kinda fluctuates and sounds a bit crunchy. But the volume pot is not scratchy sounding. This is my first time troubleshooting PCB components. I'd love to get some advice, starting with troubleshooting and replacing  capacitors.

I want to learn to test the capacitors before just replacing them. I started by removing and testing three: the two large ones on the right side of the board (6 & 9), and one small one (11-1007, external amp). I tested them with a DMM set to capacitance, then also charged them with a 6V battery to see if they hold the charge
 
#9, the 500mfd blue one is in good condition (I think it must be newer).

#6, the 1000mfd silver 'Mallory' reads 1440mfd, a much higher capacitance value than expected. But it holds the correct charge.

The small 5mfd one seems bad. (pic #2) It reads 11mfd and only charges to 1.4V. Not sure if it looks leaky.

So...does the high capacitance of the Mallory mean it should be replaced? Or maybe it's OK?

And as for the small 5mfd, I'm looking for a replacement on Mouser's site and there are a few different types of electrolytic capacitors. But are all these considered aluminum?

[sean]

Don't kid yourself that these 50-year-old capacitors are still great. Just because a DMM measured their capacitance does not mean that they are not leaky at higher voltage.  They could have elevated ESR, they could change in value very significantly with temperature.  They are not young anymore, so they may fail or falter soon after you start using the Wurly regularly.  If this were my old Wurly preamp and amplifier, I would replace each and every electrolytic capacitor in one afternoon and never worry about it ever again.

That blue #6 capacitor is directly connected to the speaker output.  I would want a capacitor with the lowest possible ESR, and a good rating for ripple current.  The schematic says 500μF 50V.  I would search Mouser and Digikey for one rated for 100V (because it won't cost much more), and get an axial-lead can that is similar in length to the original for mounting on that old circuit board.  (The mounting clip makes little difference, but it is cute.)

The big silver (1000μF 50V) cap #9 spends all-day every-day at 42V DC (plus ripple).  I would order a 1000uf cap rated at 100V, and get the lowest ESR rating and highest ripple-current rating I could find. 

For less than ten bucks each, you can get a low-ESR (less than 200milliOhms, some below 100mΩ) axial-lead aluminum-can capacitor with good temperature rating (85°C is fine)(longest hours is best).


ALL of the small electrolytic capacitors should be replaced with modern radial-lead caps that can be mounted standing up without their lead wires exposed.  Yes, you may have to bend and spread the lead wires to fit.  Get capacitors that have the same farad-rating, with a voltage-rating that is equal or above the original capacitor's voltage-rating.

Before you remove all the caps from the board, take a photo of the board, then print the photo and mark it for + and - polarity on all of the caps.  Then you can double-check yourself when installing the new caps.

Sean

[citysoundman]

Hi Sean,

Thanks so much for taking the time to give me such a detailed reply! I know that VV sells an amp repair kit that includes all the caps plus some other components. Are you familiar with this? For a newbie like me, would it make sense to go with this, or might it be better to order them individually, seeing that you mention some things like looking for low ESR, high ripple current and V ratings than listed, and radial-lead caps for the small ones?

[sean]

You should probably get the VV kit. 

The only thing to beware with the VV kits is that they supply transistors that have the wrong pinout order, so you have to make sure that you re-orient the transistor to get the leads installed in the right place.  Look in the "instructions" tab to see their only guidance related to this.

Look at the "what's included" tab.  Their standard kit includes all the caps, and also the 5W power resistors.  Very nice.

If you are new to working with a soldering iron, find a friend who has more experience. 

Personally, I would want to clean up that ugly mounting job done on the power transistors, and the VV "complete" kit would take care of that.  You should only do this if you are confident that you can do a much cleaner job than the upside-down mounting shown in your photo.  The schematic indicates that you might have to change some resistor values to get the bias right (R23 and R25).

After you are done soldering, clean off the flux with alcohol and q-tips.  The local drug store should have 91% Isopropyl Alcohol (70% works okay).

Don't forget to inspect your work as you go.  Make sure that every single capacitor is oriented with the right polarity.

Sean

[citysoundman]

Very good, thanks again Sean. I've done lots of cable soldering, just not much PCB stuff. I feel confident that I can get the components replaced properly and cleanly.

But I'm going to give this some thought. I'm weighing the cost of the complete kit + reed bar shields against something like the RetroLinear EP200 board which is $425 but includes the shields. Then, what would the results be? I've read a number of posts here where people have done pretty thorough overhauls of the amp components but still end up with a noisy amp. From what I understand the design of the old 200s is just not good. So I'd hate to put the time and energy into replacing components if I don't know that I'll get noticeably improved sound. thanks

[sean]

I think that getting the newly-manufactured amp is a great choice.  The best choice.

Sean