Vox AC30 Rose Morris ltd advice needed

[JQuick]

I've been dusting off a friend's RM Ltd AC30. It has required many odds and ends. The amp sat for about 15 years unused. I replaced all electrolytics, fresh tubes, etc. It still has a couple gremlins lurking. One of which is noise and ground hum intermittently when a cable is pulled out of the jacks. From research I know RM AC30s in the mid 80s had this issue for which some kind of kill circuit was employed. I even see reference to it on the PCB with no components fitted. Are the jacks on this unit properly grounded? Would I be wise to fashion an alternate grounding scheme for the jacks? Anyone here have experience working with this era? Thanks!

schematic at Prowess Amplifiers- can't seem to paste it here

[The Dude]

Not sure which schematic you are using. If you can't paste the schematic here, a link will suffice. If the hum is present intermittently when a plug is removed from the input, I suspect your input shorting jacks are not shorting when the cable is removed. Give the jacks a good cleaning. Also, you can check continuity with your DVM to make sure the jacks are shorting to ground when there is no input.

[g1]

This is the 'LTD' schematic from the Prowess site.

[JQuick]

I gave the jacks a good scrubbing and it seems to have settled down. I really love the look and sound of these fawn RM Ltds (I've had a JMI '62 for decades), but lord are they a cheap pain to work on!

[Chuck H]

Always good to check/replace electrolytics in a tube amp with 15 years of disuse. So good call there. Tubes don't age with disuse and new tubes can be a game of roulette for quality. So you might want to revisit the old tubes just to see if any are still good (but see below).

Also an issue with disuse is oxides on non soldered contacts and even poor solder joints. As The Dude pointed out above and your experience cleaning the input jack proved. This also applies to tube pins and their receptacles in the sockets. Methods for cleaning tube pins and sockets as well as "retensioning" have been discussed here and can be found by using the search feature (top of page right of center).

Poke components and leads with a wooden chopstick while the amp is in play mode looking for noises and loose connections. Resolder anything that makes a noise or even just looks bad/funny.

Rotate pots and switches to detect any odd noises or behavior. Clean/replace as needed.

I think these amps used metal film resistors? Which tend to age more gracefully than carbon composition. But I'm not familiar with that particular amp so I don't really know. In any case carbon composition resistors can drift in value. Usually up. This seems to be worse for resistors subjected to higher voltages. Check and replace as needed.

Always use the right cleaner for the right thing. There are cleaners for pots and there are cleaners for switches and jacks. They aren't the same thing. Using the wrong product in the wrong place can cause problems.

This is pretty much chapter and verse for what to do with an old amp that hasn't been used for a while. I've seen here on the forum many times when failure to do these things resulted in misdiagnosis and ongoing problems caused by mistakes in unnecessary repairs. Odd combinations of syptoms start to stack up and often these amps get shelved only to pop back up here on the forum every couple of years for repeated repair attempts. So please do the service outlined above and avoid problems like this.

jm2C

[JQuick]

Thank you for all of your suggestions. The amp seems to be ready for service. These RM Ltds have one main PCB populated mainly with carbon film 1/2 watts and rectangular white and blue capacitors which all seem to be okay. The EL84 section breaks away. One socket wasn't conducting so I replaced all. These had a nasty tendency to become loose due to the pressure of an aluminum guard rail over the tube tops plus poor solder flows. I also replaced all resistors on this board with metal oxides.

The amp was oscillating when the Brilliant volume and Treble were set to maximum. Upon replacing both output and preamp tube cages this disappeared. The only hookup wire to play with inside are the twisted wires from both transformers which are rather short and taught. Fingers crossed there...

The original value 47ohm 10w bias resistor was red plating the EL84s a bit so I changed the value to 68ohm. I don't mind running my JMI like this, but I thought it a good idea to change it in an all PCB mount socket array that has already experienced failure.

Way more so than my JMI, the V1 oscillator position is very critical to get the Vib/Trem on these to sound good. The internal pot on the main board is tricky to tweak. On my '62 it's simply an intensity pot that goes from subtle to thumpy. The RM Ltd requires careful adjustment.

These Ltds sound great. They are very quiet apparently due to a separate bridge rectifier and 15k uF smoothing cap sending DC to all heaters. I realize the merits of this get debated all day long on the internet, but it seems to work in this amp. They also have very early Celestion G12M Greenback reissues that sound wonderful. Being someone with intimate knowledge of the original circuit and speakers I don't find this amp lacking at all despite it omitting the GZ34/Alnico speaker combo. I just wish it was of a little higher construction standard!

[Chuck H]

Well... You didn't come in with a lot of info about what you know (or what you don't). My response above was considered for a worst case scenario and I hope you weren't offended if it seemed patronizng. I only wanted to help. But it seems you have some real chops AND knew where to come with questions Welcome to the forum and I would also welcome your future participation.

[Helmholtz]

It seems this amp doesn't use dedicated 1M grid leak resistors at the inputs grids and rather relies on the input jack switches and the guitar for a grid leak path.
I don't consider this reliable and would add those 1M resistors.
This should also avoid popping when pulling a plug.

[g1]

I don't consider this reliable and would add those 1M resistors.
This should also avoid popping when pulling a plug.

Probably reduce a lot of potential noise/crackling issues (due to oxidized jack switch contacts) as well.

[JQuick]

Thanks for the grid leak resistor observations. I may look into adding those. The amp is doing great right now, but armor plating these is effort well spent.

There's not much service info on the web about this era amp. JMIs are well documented for obvious reasons. When I first had the desire to own an AC30 back in the early 90s these were the first ones I tried out (I'm near Chicago USA). They were very hard to find. Then the Korg models came along and eventually became ubiquitous. I bought a '62/'64 from Guitar Oasis in Huntington Beach CA USA in 1995 and never looked back. A close friend tried mine and immediately bought this RM Ltd. a month later at Make 'N Music in Chicago. I've been musically active all this time, keeping up the gear. He plays at home and at some point burnt the tubes out and 'broke' his ages ago. I'm just happy when I fired it up after the initial diagnosis it all still worked- not optimal, but working.

I chose this forum because y'all seem to be pretty sharp! I'm flying by the seat of my pants really. I don't have much gear- only a few pieces. Each one is priceless (aforementioned AC30, '62 blonde Fender Twin, '66 Marshall 200- the Mick Ronson version) and over the years I've had to learn how to keep them going as I use them all to gig with. I'm a sucker for the old stuff.