Peavey ValveKing 100 Overhaul

[physics]

While I had the amp open last night, I also desoldered C106 and replaced the lead tonestack's "slope" resistor with a 100k unit. Caps should get here Thursday.

Edit: Realized I didn't explain the caps bit...

One of the input filter caps had a mystery liquid leaked and dried on top of it around the top of it where the edge of the plastic wrapping is. That and another cap are beginning to bulge on top. Measuring across the input filters (with the dropping resistor to the screen node lifted to isolate them from the circuit) shows about half the expected capacitance (200uF). Removing the leaky cap and measuring again shows almost the same value (off by about 20uF if I recall). No change if I remove the bleeder resistors either. There is also a persistent hum in the output of the amp, which would seem to correspond to inadequate filtering. So I ordered three new filters to replace them all instead of playing whack-a-mole later. Oddly enough, the leaky cap appeared to measure fine by itself out of the circuit on my DMM, but back on the PCB (with the screen resistor still desoldered) where it's in parallel with the other input filter the readings are off again. Huh.

[K O'Connor]

Hi Guys

Two cap-related points:

First, you cannot make in-circuit capacitance measurements if you expect accuracy.

Second: When it comes to assessing electrolytic health, the first step is visual. ANY sign of other-than-normal appearance tells you to replace the cap. These caps have been stressed and are likely old. Any electrolytic older than 14 years should be replaced for optimal performance.

Have fun

[physics]

Brief update: Installed some new Kemet filter caps. Measured in-circuit capacitance and the (albeit inaccurate) value is much closer to what I'd expect. Mostly, I wanted to make sure I saw a change more than I wanted a specific value. I also removed that capacitor I mentioned going into stage 3, changed stage 1 and 2's plate resistors to 100k 1W each down from 150k 1/4W, and bridged the lead tonestack's mid pot leads so it's wired as a rheostat now. I also lifted a few more pads, yippee. I can hold the board up to the light and look through it at the traces on the opposite side, and I can see spots I've reworked delaminating. In most cases not even from what I'd call abuse either. It seems that dwell times need to be extra short on this board, and even then it's not guaranteed pads won't lift. Every time it happens I think more and more about making my own eyelet card.

[K O'Connor]

Hi Guys

Here's where you have a set of completely opposite expectations.

PV is vertically-oriented company, meaning they make their own PCBs, wind there own transformers, make their own chassis, they do all the stuff from as basic a level as is practical, all the way up to assembly, shipping, an designing everything. They carefully design the production procedures to make money and the balance between that and having things be durable or last a long time are at odds with each other.

So, on the one hand, you would expect them to design their stuff to last as long as is "reasonable" - not necessarily a long time, but long enough that people do not have an impression that "those don't last". On the other hand, they want you to trade up and buy a newer PV product, suggesting that the first one definitely should not last too long. They are not going to do anything that has not proven to be absolutely necessary as far as toughness of the assembly is concerned.

It used to be that 2-ounce copper was the industry standard, but this is now at half the value. Traces are thinner, and of course boards can be many different thicknesses: 1/16th inch used to be standard but again thinner is becoming more usual, especially if the boards are covered in surface-mount components and/or nothing of much weight. Following this, solder connections become smaller and life expectancy deteriorates.

PV cannot blame "cheap Chinese PCBs" that traces lift from since they make them themselves. Besides, as far as Chinese parts go - or sourced from any country other than your own - you can get fantastic quality or poor depending on who you buy from.

How old is this amp? It is probably well past when PV thought it should be scrapped.

When dealing with any PCB, you have to follow the soldering advice given in our Tech Articles. NEVER try to remove components by removing the solder first. ALWAYS add copious amounts of fresh solder and pull the part wet, then add more solder to clear the holes. I've used power desoldering tools and frankly do not like them at all. Nothing works better than a Soldapult.

[physics]

That does seem to be the conundrum with PV products. I'm not sure how old the amp is, I'll have to look into how to date it, but I think it's no later than 2010, probably earlier.

Thanks for the advice on desoldering. For my process so far, I've been using a soldapult III to clear the hole first which usually leaves the lead completely unattached so that the component either falls out or can be freely removed. If it has a small joint holding it onto the pad then I'll either reheat (and maybe add solder) and pull the component out while wet or if small enough break the joint and remove the component. The pads have been lifting during the clearing of the hole before even removing the component, despite working fast and with only one try. I didn't find a dedicated desoldering article, will pulling the part out wet first and then clearing after (maybe from the opposite side so as not to suck the pad off the board?) make any difference over what I'm currently doing since the holes are completely cleared by the soldapult? I'm also concerned about dwell times if I'm keeping the solder melted long enough to pull out the component, given some have bent leads that require some finagling.

I also had one pad lift when stuffing a replacement component it. Tried stuffing it wet since the part fell out a bit the first time around leaving solder in the hole, and pushing the part through pushed the pad up. Doh! Won't be doing that again.

I'm also curious what makes power desoldering tools so bad. I would have thought they work better than a hand tool, and have been eyeing the venturi workstand for my metcal station (which I got cheap).

Thanks!

[K O'Connor]

Hi Guys

Desoldering techniques depend on what you are trying to save: the board? the component? or both?

If all you care about saving is the component, then go ahead and use a power desoldering tool. They extract things dry - too dry - and often damage traces as a result.

If you want to save the board, you MUST EXTRACT THE PART WET.

If you want to save both the board and the part, do it wet.

There are a lot of clever devices and things out there, bu8t some are simply "too clever". Desoldering guns/stations are supposedly faster than hand methods and allow "production" removal of a lot of components. This suggests that the board is sacrificial and in my experience that is the case. For the time it takes to extract a component for the average person working on an average task, hand methods win every time. Don't spend money to be lazy and ruin a lot of boards in the process. It is the proverbial false economy that ends up costing more.

Have fun

[physics]

Spending money to break more things, faster. How efficient.

Noted, and thanks for sharing your experience with them!