NEED HELP on a 1978 Randall Commander II Repair



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NEED HELP on a 1978 Randall Commander II Repair

Postby Porkchok » Sat Mar 18, 2017 1:35 pm

I scored this Randall 2x12 combo a couple weeks ago. Thing's been thrashed. Both speaker cones were torn around nearly the entire circumference, cab was wobbly, carry handle was about to fall off, and the reverb/trem channel was not working properly. The clean channel functions perfectly. The reverb/trem channel is super quiet even with the volumes at 10 and 10, no trem and reverb barely audible with it at 10 as well. Minimal hum on either channel. No abnormal distortion or anything like that. It's just really quiet.

I'm alright with a soldering iron. I build dirt pedals to entertain myself but when it comes to troubleshooting those my strategy is usually just to rebuild them haha. I just got this newfangled auto ranging DMM for this job. It reads capacitance and what not, but I'm not all that hip on using it.

This is my first attempt at an amp repair, so I'm going to need some help. Not sure if I've been going about any of this correctly but Here's what I've come up with so far...

So far, I've done all the easy stuff, repaired the speaker paper speaker cones, hosed it out with compressed air, cleaned all moving parts with deoxit and compressed air, and done a few little maintainence/repair things to the cabinet. Now, I'm working on diagnosing the rev/trem channel issues. I'd like to test anything that I can prior to ordering any parts so that I can make one order and have everything I need before I actually start replacing components.

Schematic can be found here : Page 1 ---> http://music-electronics-forum.com/attachments/7507d1262874308-rg120-300a.gif Page 2 ---> http://music-electronics-forum.com/attachments/7508d1262874440-rg120-300b.gif

Upon inspection of the chassis I see a few obvious bulging/leaking electrolytic caps. I've compiled a list of all the electrolytics on the board. Should I go ahead and replace them all or just the obviously bad ones? They're more or less all the same Japanese made "SS" caps so I would assume they're all about at the end of their lives.

Guts.jpg


My research leads me next to attempt to test the output transformers, which by my estimation are those 4 little guys at the bottom of the picture in the TO3 package. My limited experience in diagnosing/trouble shooting has led me to test them this way : I tested them on the diode test function on my DMM with the base desoldered from the circuit but the other 2 leads still in circuit. Don't remember where I read it, but somewhere I read that I could test them this way. I got these values which I believe read out as expected :

Values.jpg


Filter Caps.jpg


These guys are power supply filter capacitors? These are also electrolytic? Upon desoldering the leads and hooking up the DMM set to read capcitance, I receive a reading of OL on both of them after about 10-15 seconds no matter the orientation of my pos. and neg. leads. With minimal apparent PSU noise I will assume these are working properly and that I must just be testing them wrong.

Again, I'm probably going about this all wrong. Any advice or help in any way is appreciated. Will update as I go.

Cheers!
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Re: NEED HELP on a 1978 Randall Commander II Repair

Postby Porkchok » Thu Mar 30, 2017 7:34 pm

Caps should be here tomorrow. Going to do the recap over the weekend. Wish me luck!
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Re: NEED HELP on a 1978 Randall Commander II Repair

Postby fcknoise » Fri Mar 31, 2017 8:01 am

Good luck!!! I have the same issue with channel 2. I've never used it. If it's a relatively easy fix let me know and I'll try it
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Re: NEED HELP on a 1978 Randall Commander II Repair

Postby Porkchok » Sat Apr 01, 2017 11:10 am

Welp replaced all the electrolytics on the board and HUUUUGE improvement. Channel 2 is usable now, however, the HI input is not adding +6db as it should. There is no difference between HI +6db and LO 0db inputs.

Also, the reverb unit will start to feedback if it and the treble are turned up too high. Which isn't much of an issue since I like a pretty dark tone. I also keep my amp reverb down fairly low and use a pedal if I need massive verb. Plus I like having feedback on tap whenever I want it. =D

There is also a tiny bit of power supply hum that I must not have noticed before but that's not much of an issue and easily fixed. The circuit itself is SILENT though. Definitley going to be recapping a couple of the other orange stripe collection as they have quite a bit of noise/hiss/hum etc.

Now, to figure out this input issue...
Last edited by Porkchok on Sat Apr 01, 2017 3:24 pm, edited 1 time in total.
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Re: NEED HELP on a 1978 Randall Commander II Repair

Postby Porkchok » Sat Apr 01, 2017 3:23 pm

After playing on the amp for a couple of hours I've decided there is a noticeable volume boost on the HI input. It's still the entire channel that is not nearly as loud as it should be. It's loud enough play around on but not gut rumbling loud like channel 1.

This will require more testing...
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