Randall RG100 Head ... power loss

[kernalflagg]

Hello, newbie here
I've been playing guit-fiddle for more than 3 decades. Got me a Randall RG100 150w head from the 80's (not sure what year). Been a vintage Marshall guy my whole life - love my Marshalls which I'll never sell! But, a couple years ago I got rid of a guitar at a music store and wound up with this RG100 head as part of a trade.

It worked fine for the first 6 months, but it was quite noisy at idle with a loud "waterfall" with "a little hum to it" sound - even on the clean channel and no pedals or anything even plugged into it. Kinda weird. By contrast, my Marshalls are very quiet at idle going through the same Marshall cabinets. I've tried the Randall into other cabs, like Fender, and Ampeg - same result.

Anyways, the clean channel quit working completely. The green light on the front lights up - but no sound, even with volume turned all the way up. Another peculiar thing is that the distorted channel has lost major tone. It no longer has any power and sounds very "tinnny."  This gets worse after about 15 minutes of constant playing, until there's barely any sound at all coming out. Both of these symptoms occured at the same time. It has been this way for 2 years - but hasn't gotten any worse.

I'm speculating that the idle noise may be some kind of grounding issue. The power loss may be the capacitors. I really don't know.

I've taken it to several amp repair shops, and the techs at two different shops said that they weren't sure what was wrong with it, and didn't want to take it on. They said the amp wasn't worth it. One of the techs said that it had an RG80 board inside of it. But, another tech said it was normal for Randall to put RG80 boards inside RG100 heads in the 1980's. I've seen the RG80 board inside of it.

The techs took it apart and couldn't visually see anything wrong. They told me that this amp is just junk and not worth fixing, and somebody may have done something weird to it before they traded it in at the music store (like swapping the boards of 2 incompatible amps or something along those lines). Now I'm stuck with it. The guitar I traded in was worth about $200. The value of these RG100 heads are about $200-$250 when nothing's wrong. Mine is now worth $0 since it barely works. 

I've considered sending it off to an electronics repair shop rather than going the guitar amp repair shop route. But, that may not work out. I really would hate to just throw it in the garbage or give it to the Goodwill. But, I don't know how to fix it myself either.

I'm very technical minded. I've been an auto mechanic for 15 years, and I have pretty good knowledge of electronics and electrical theory and troubleshooting basic DC circuits. (I know amps are AC) I have a DVOM but no lab scope. But, I'm no amp repair tech - that's for sure!

Does anybody have any ideas? I'd love to fix it for as cheap as possible (or have it fixed as cheaply as possible). I don't have that much money these days, kinda broke with the high gas prices and crap. So, a "quick" and cheap fix is the best for me. I'm not a milliionare!

If I knew what exactly would fix it for sure and the cost is under $100, I'd probably go for it.

Advice and ideas much appreciated.

Thank you in advance,
Kernal Flagg   

[teemuk]

Well, the RG80 and RG100 are technically almost the same circuit. The amps have slight differences - like different mains transformers – but technically the circuit and the boards are just the same for both.

You can find a schematic for a '81 model from schematic heaven but there might have been some revisions since then so I wouldn't trust it blindly. The schematic covers models RG80-122SC, RG80PH, RG80ES, RG100ES and RG100H so you can see that they used the same circuits/boards in a very wide line of products. Most manufacturers do this even today since it is very cost-effective.

These are highly rated amps in circles that appreciate brutal distortion tones (e.g. death metal and such genres). Definitely not junk - far from it, actually.

I really don't have any ideas except the usual things: Measure the rail voltages and see if there's something wrong with them. If not proceed to trace the signal from the input towards the output and see where it breaks/goes bad. Sounds like a component failing – could be a contact problem as well.

[kernalflagg]

Thanks Teemuk.

I have downloaded the book you have on here and I will read through it. I watched one of the techs make some checks on my Randall. He grounded out the caps on the bench (they sparked almost like an arcwelder). He said that was good - shows good capacitance. The voltage on both sides of the two caps was 48V +/- 0.1V. He said that was good - no problem there. I'm skeptical on this though. 

I'm with ya on checking the whole circuit. He must have overlooked something.

I'm not sure of the scematics. Mine has  "RG100 HB" stamped on the back of the amp. Is that the same as "RG 100 H" ??

If I recall, the last time (a couple years ago) I tried to fix this amp, I ran across this problem. I wan't able to get schematics for an RG 100 HB - couldn't find them anywhere. I contacted Randall and they said that there was an "HB" model made that was different than the "H" model. I looked at the schematics of RG80 and RG100 and they are different (quite a bit different if I recall).  But, I may be wrong.   

[J M Fahey]

Dear kernalflagg. I think I can help you with your amp, but to make sure we´re talking the same, post here the schematic *you* have, so I´ll suggest you some things to check, to see how close it is to your actual amp, and then suggest some voltage and resistance measurements. To begin with you´ll need a "series lamp" fixture, and your digital multimeter. Bye.

[kernalflagg]

oK, thanks JM

The schematic is saved in a pdf. I don't know how to post that on here.

[J M Fahey]

No problem, mail it to: juanmanuelfahey@gmail.com   I do have a schematic and a couple of board pictures, but I want to be sure that we are talking about the same, so when I ask you to measure , say, voltage on Q104´s base or something like that, we agree.

[J M Fahey]

I split my PDF into a few functional blocks:

[sixgunner]

I split my PDF into a few functional blocks:

J M Fahey,
What is the trimmer pot for?

[J M Fahey]

Bias.
Don't mess with it needlessly.
If in doubt, set it to its minimum value, it's the failsafe position.

[sixgunner]

Looking at the second pic (red channel), you can see where the clipping diodes are switched with the "pull" treble knob.
When you do this there is a huge volume drop in that channel (by design).
I'm trying to wrap my head around a fix for this issue.......keeping the clipping diodes on but compensating for the huge volume loss.

[J M Fahey]

Are you repairing or building one?

[sixgunner]

Neither.
I own several and they all have this "feature"......and I'd like to figure out a mod that would retain the extra diode clipping in the red channel, but compensate for the extreme volume loss.
That way I could keep the extra gain/compression at a giggable level.

[J M Fahey]

That brutal volume loss comes from the fact that it really does not *increase* sustain, because it does not increase gain when you pull that knob, but it switches those diodes in.
The original Zeners clip at 4.7 V peak; the diodes at 650mV; so you increase clipping but your signal drops to .65/4.7= 0.14 the original level.
You need 7x gain added after that to compensate.
The only way is to pull R28 (10K) and add there a mini-board with something providing that gain.
It might be a FET or bipolar transistor, or a TL071 or similar.
Increasing gain somewhere else would not do, because it would also affect the Green channel.
The amp design is so simple (good) that it does not have many options for tweaking.
Try to get a link to somebody talking about the mod you mentioned earlier, maybe reading it I might have some other idea.

[BMWFREQ]

Did you ever get any resolve to this issue?  If not, let me know.  I just may have the anser for you.

BMW FREQ

[J M Fahey]

Dear BMW.
Randalls are popular here (SSGuitar  ), so even if we never again hear from the original poster , I'm *sure* your posting that solution here will certainly help many, so please do.
Thanks in advance.