Walter Woods MI-100 swan song

[elgecko]

I retired my beloved MI-100 from main amp duties about 9 years ago and have only used it a handful of times since then. I tried to fire it up about a month ago and it was acting strange, giving me low, distorted output. Also, the speaker would randomly start spasming, moving violently yet silently. About a week ago, I fired it up again to confirm that it had gone bad. This time, there was absolutely no output and it blew a fuse. I cleaned all the jacks and pots, replaced the fuse, and this is what I got:





This leaves me in a weird spot. I bought it after Walter stopped including schematics in the innards so getting it repaired locally is unlikely. I could send it to Walter but I hate to admit that it might not be worth it. I'd probably be better off using that money to buy a more modern, more powerful amp for a backup. I've always loved the amp but I'm not sure it's worth fixing. Do I really need a 100 watt backup amp?

Are there any other Walter Woods aficionados out there? What would you do?

 

[BassmanPaul]

As there are no WW schematics available then Walter is the only one who can supply a remedy for your amp.

 

[agedhorse]

As you are on the fence about how useful the amp is to you, you might get more value for it by putting it up on ebay or such as a non-working amp. It's possible that it might be worth more than you think (but maybe not as well)

 

[beans-on-toast]

A tech doesn't necessarily need a schematic to check the amp out. There are basic tests that can be performed to check out the building blocks and narrow down the problem.

If you can find the right tech, it might be worth an hour of bench time.

 

[elgecko]

As you are on the fence about how useful the amp is to you, you might get more value for it by putting it up on ebay or such as a non-working amp. It's possible that it might be worth more than you think (but maybe not as well)

I did that unremorsefully recently with a Markbass amp that gave up the magic smoke. My issue with doing that to the Woods is 1) it has sentimental value, it was my first "good" amp and 2) I do love the tone I got from the amp unlike the Markbass, which I always found just okay.

 

[elgecko]

According to the word on the street, sending amps to Walter for repair has become pricey. I think the idea of finding a qualified local tech is a good one and a bench test will likely be my first course of action.

 

[elgecko]

This era WW amps has Walter's SMPS but the amp, itself, should be pretty standard stuff for a good tech to diagnose and repair. My guess is the output stage is blown as there must have been DC on the speaker for it to pump like that.

I believe your diagnosis is 100% correct. I was goofing with it today and by running the XLR out into a PA, I've determined the preamp section is working perfectly! Disconcertingly, it was emitting magic smoke but I'm guessing that was the remnants of the output stage or cleaner residue(?)

If anything, it looks like I can pair it with a power amp and still enjoy Walter Woods goodness but I'll probably still look into getting the output section repaired!

 

[Jeff Scott]

I am happy to see I may have figured something out, however, I wouldn't run the amp like that unless all of the power supply to the power amp section is disconnected!

 

[agedhorse]

Cleaner residue? How did that happen? Some cleaners can damage internal parts, not all cleaners (in fact many) are not safe.

 

[elgecko]

The stuff I used to clean the pots last week. I would've guessed it was volatile and all but evaporated by now.

Invalid Link Removed

 

[agedhorse]

I wouldn't recommend that for anything electronic of any value. Notice that it doesn't say "safe for carbon film and conductive plastic controls"? That's because the carriers can damage the parts and migrate bushing lube to places it shouldn't be.

 

[elgecko]

The compartment housing the pots is well isolated from the rest of the innards so hopefully, I didn't muss things up too much worse then they already were.

 

[beans-on-toast]

Cleaner residue? How did that happen? Some cleaners can damage internal parts, not all cleaners (in fact many) are not safe.

I've seen amps destroyed by cleaners that leave either a blue or red residue. One such product was a tuner cleaner sold by Radio Shack. It absorbs into circuit boards, jacks, and resistors and changes their conductivity.

 

[beans-on-toast]

The compartment housing the pots is well isolated from the rest of the innards so hopefully, I didn't muss things up too much worse then they already were.

View attachment 929666

It is good to remove the dirt and dust. It can affect the electrical and cooling performance of the amp, even cause a short. I use a soft shaving brush, it works well in tight areas.

Prior to performing any maintenance, the electrolytic caps -- the ones in the metal cans, should be discharged.

 

[agedhorse]

The reason I mentioned it is that the pots on this amp are pretty much unobtanium in small quantities. There are workarounds, but very labor intensive.

 

[agedhorse]

The 90 degree extended leg PCB mount pots are always hard to find because there were so many different, customized versions. Peavey used to use them a lot and sometimes they can be crossed that way... but the shaft (and thus the knobs) may be different, as well as the bushing itself.

I restored an early MI-100 a few years back, a complete restoration for a very good client. Unit was in good cosmetic shape, and in this case it was worth restoring the original pots one by one, disassembling, cleaning and reassembling because I couldn't source a set that would work and this amp needed to look 100% original and like new. Turned out great, but was not an inexpensive venture at all.

 

[beans-on-toast]

FYI, this is a good place to check out when looking for harder to find amp parts: Guitar Amplifier Repair Parts. They buy inventories of discontinued and no longer supported product parts such as pots, jacks, switches, etc.

Here are their pots:
Guitar Amp and Pro Audio Potentiometers

Invalid Link Removed has a good selection of the Peavey spider pots.

 

[Ric Vice]

All good suggestions, but as most of you know, Walter scraped the serial no's off of many of the key components in his amplifiers. So, repairing one without a schematic, isn't for the faint of heart and will take considerable bench time.

Ric

 

[Hubert]

Hi Elgecko, IMHO, the power amp of this WW is not shorted. If it was the case, the power supply were immediately blown because there is no protection against short circuit in WW amps. On these amps, when the amp burns (shorted), power supply burns. But when power supply has a failure and burns first, power amp is not impacted. The fact you mention preamp is ok means power supply seems to work correctly. My analyze : something is wrong in the power amp at the differential input amp stage. Also the power supply (+/- 45V DC section) must be checked (Bridge rectifier and filtering caps). This failure looks impressive, but the amp is far from dead. In the WW line, I personally like this MI 100 model (analog power amp + preamp with sweeper) which I consider as a must in terms of sound quality.

 

[agedhorse]

IME differential input stages are rarely a problem, there are many more likely causes in the driver and output stages but even that is a guess. The way to know for sure us to have the amp diagnosed and repaired by somebody with the proper tools and knowledge to do this correctly. This is not an easy amp to work on due to the tight conditions within the amp's chassis.