Trying to fix loss of volume in a 1978 Randall RG300 Head (SS)

[52 Bill]

I don't think that reading the voltage across a cap will do much good here. You need to trace a signal through the amp and find out where it stops.

Plug a radio or an ipod or any signal source into the input of the amp. Now trace the signal through the amp stage by stage until you find out where the signal dies out. You can either use your ac voltmeter to read the signal level or use a second amplifier to listen to the signal as it passes through the different circuits of the amp.

[Phostenix]

Do you have another amp that you could use an audio probe with? Here's a link with instructions on how to make an audio probe:

http://www.diyguitarist.com/PDF_File...udioTester.pdf

[ParthaD]

Do you have another amp that you could use an audio probe with? Here's a link with instructions on how to make an audio probe:

http://www.diyguitarist.com/PDF_File...udioTester.pdf

Thanks... I do and I will try to follow those instrutions.. will post back!

[ParthaD]

Question on tracing circuit..

As I started doing this a question comes to mind... in numerous previous posts I have seen how when there is no sound output, a user is asked to connect the preamp out to another amp and see if there is output which would mean the problem is in the power amp. Also if the guitar amp is connected to the return and there is sound that would indicate the power amp is fine and there is a problem in the preamp.

In my case the amp has no send and return but there is at least some output at max volume. So in this situation the signal is never going to completely go away as I trace it isn't it otherwise there would be NO output to the speakers. Having said that I am confused as to what I should be looking at as I trace it from the amp input on forward. My guitar pickup output is about a 20-50mv AC signal, so what should I be expecting AC volts to go UP to as it goes thru the preamp and at what point does this happen. The tone stacks are not likely to increase the signal strength, they are just frequency modifiers right?

Thanks.

[Enzo]

The initial preamp stage boosts the signal level up to sometjhing useful, and it mostly just gets massaged the rest of the way. I don;t worry so much about the levels myself, I just look at output stage by stage and look for a substantial reduction after one stage.

Looking at the power amp drawings above, the drive board has the power amp input over at P5, pin 2. Is there reasonable strong signal there? If not, then preamp, if so, then power amp. You cuold also inject a signal there if you wanted.

[ParthaD]

Could that be a wrong schematic??!!!

Mars Amp, as I was messing around with the amp today I noticed that the schemo PDF you provided was marked 99, a full 21 years AFTER the amp date. How can I be so sure of the amp date of 78, it has a label at its side inside the chassis that shows the date it passed final testing!

But what really makes me wonder is that when I look at the voltages inside the circuit in the schemo I see no equivalence in the amp itself. As I indicated earlier the filter caps are providing approx 46 volts. In the 2 plugs that are shown in the pics, the highest voltage is 46 volts. There is no 24 volts and there is no 15 volts anywhere in those plugs. Also the schematic is displaying a line going to a fan, there is no fan in my amp. I think the schematic is for a more recent Randall amp that happens to be called RG300 also.

Perhaps the filter caps while still putting out 46 volts, have actually gone bad and are just not providing enough power. Those caps are marked as 6000 uf and 4-46. Will put up some pics later. They appear to be the original caps and therefore may need to be replaced, something I probably cannot do myself since I don't have a variac to bring the new ones up slowly.

Is there any simple test to determine if filter caps are too weak?

[Enzo]

Weak filter caps do not cause major loss of volume. If your caps are dried out, the amp will start to hum loudly. Or bass notes might get distorted.

Are the power amp and preamp circuits on separate circuit boards? If so, regardless of the schematic, it still gives you a point to break into the signal path and see what kind of signal is passing from preamp to power amp.

[J M Fahey]

IMPORTANT

Sorry for the interruption.
Please stop messing with the amp for now, before a real damage is made.
You have the 1978 RG-300, not the 1998 one.
I have the schematic somewhere, let me find and post it, but *stop* touching it.
Yours is +/-47V powered, 6 power transistors, 300W into 2 ohms, just by heart I seem to remember the output transistors were 2N6254 and , obviously, all NPN.
The one kindly posted by Mars is modern, with complementary transistors, probably +/- 60V to 70V and 300W/4 ohms.
You can't repair an amp just measuring random voltages or looking for discolorations but testing whether it works as intended or not.
To name a pair, Enzo's and Bill52's suggestions pointed in that way.
Will try to get that schematic tonight.
Meanwhile "map" the full chassis taking 4 or 5 overlapping pictures, left-to-right, plus some of the front and back panels, from the inside, showing pots, jacks, etc.
You can also build the "audio signal tracer" suggested, I'd add a 1Mlog/audio taper pot as an input volume control and use a .01x400V cap instead of the .1x50V ceramic.
You can plug it into any amplifier you have around, or build something like a Ruby, with a small speaker and a 9V battery, to make it independent and, most important, "floating"/isolated from mains.
Juan Manuel Fahey

[J M Fahey]

IMPORTANT

Sorry to interrupt.
You have the 1978 one, not the 1998 one which is a very different beast.
Yours is +/- 47V, 300W/2 ohms, 6 output transistors, all NPN's, if I remember correctly all 2N6254 or equivalents.
The "modern" one is 300W/4 ohms, NPN/PNP complementary output, probably between +/- 60V to 70V supplies.
Please stop touching it for now, before real, expensive damage is made.
I'll search my overflowing and very disorganized library and try to post tonight or tomorrow.
Meanwhile you can "map" the chassis (everything, not just the boards) with 5 or 6 consecutive overlapping pictures, plus a couple of the inside part of front and back panels, showing pots, jacks and switches.
Repairing by taking random voltage measurements and looking for discolorations and such or changing parts at random is incredibly more tedious and difficult than studying the schematic, following the signal path and checking measured voltages against what they "should" be.
Your power amp seems to be working, but not receiving signal.
Good luck, will try to post soon.
Juan Manuel Fahey

[Gtr_tech]

Have we verified the front end is getting proper supply? I bet its zener regulated off the output v rails. Those zeners tend to be the cause of problems quite often. Is there op amps in there or is it all discrete? Op amps like 741,1458 make nice test points to check preamp voltages.

[Jag]

I don't have a variac to bring the new ones up slowly.

Just one quick addition, you don't need to bring new caps up slowly.

[ParthaD]

Sorry for the interruption.
Please stop messing with the amp for now, before a real damage is made.
You have the 1978 RG-300, not the 1998 one.
I have the schematic somewhere, let me find and post it, but *stop* touching it.
Yours is +/-47V powered, 6 power transistors, 300W into 2 ohms, just by heart I seem to remember the output transistors were 2N6254 and , obviously, all NPN.
The one kindly posted by Mars is modern, with complementary transistors, probably +/- 60V to 70V and 300W/4 ohms.
You can't repair an amp just measuring random voltages or looking for discolorations but testing whether it works as intended or not.
To name a pair, Enzo's and Bill52's suggestions pointed in that way.
Will try to get that schematic tonight.
Meanwhile "map" the full chassis taking 4 or 5 overlapping pictures, left-to-right, plus some of the front and back panels, from the inside, showing pots, jacks, etc.
You can also build the "audio signal tracer" suggested, I'd add a 1Mlog/audio taper pot as an input volume control and use a .01x400V cap instead of the .1x50V ceramic.
You can plug it into any amplifier you have around, or build something like a Ruby, with a small speaker and a 9V battery, to make it independent and, most important, "floating"/isolated from mains.
Juan Manuel Fahey

Thanks Juan, will do... by the way this amp from the looks of it was sitting in someone's basement for a couple of decades or more.. I got it for next to nothing off ebay because of the harmony central favorable reviews. I don't think I have done anything to damage it (yet). I have already posted overlapping pics of the circuit board (there is only 1 board btw, Enzo had asked about separate boards). I will take some more pics of the pots from the inside and the front panel and post soon.

This may be ridiculously obvious to most of you but when one is tracing a signal with a meter, one would place the black probe at the chassis ground and the other at the point where one is trying to measure the signal, and I'm measuring AC volts right?

Also, in an amp, I know that the original guitar amp signal is AC and the final output to the speakers is AC, but am confused by all the DC in the circuit both in an SS and tubes although in tubes I know that the high DC volts are needed to drive the tube itself. Would appreciate some pointers on that.

[ParthaD]

Have we verified the front end is getting proper supply? I bet its zener regulated off the output v rails. Those zeners tend to be the cause of problems quite often. Is there op amps in there or is it all discrete? Op amps like 741,1458 make nice test points to check preamp voltages.

There are 5 wires coming from the xfrmr, 2 green, 2 dark green and 1 yellow. The 2 green are going to what appears to be a rectifier (square box in first pic 92) have about 67 VAC, the 2 dark green going to the switch is standard 123 VAC, the yellow is ground. The red terminals on the rectifier have 91.7 V DC coming out to the filter cap on each side adding up nicely to 46 coming out from each of the filter caps.

You mentioned op-amps. I looked that up (didn't know what an op amp was ) and the only ones that appear to be op amps are in the last 2 pics 95 and 96, they seem to have some kind of date code but you can probably recognise them. They each have 2 legs with one seeming to have been broken off during original assembly. There never was a hole for it to go into the board itself.

[Gtr_tech]

Don't worry about the transformer....the voltage(s) for teh front end are almost always taken from the output section rails. Say your output supply rails are +/- 40v, there will be Zener regulators that knock that down to +/-15v to pwr op amps or other signal level circuitry. Some amps may have a seperate winding for the low current supplies. In that case they'd use 3 terminal regulators (7815, 7915) for the 15v rails.

As far as those transistors on the heatsinks, the leg that's broken off is that way intentionally. They don't have an insulator between the tab and the heatsink, and the tab is internally connected to the collector (the leg that was cut off was the collector leg). So they'll get that connection to the pcb via the bolt that holds it down. Those may be regulator circuits for the low voltage, they may be part of the output section. Without a schematic I dunno.

[ParthaD]

Meanwhile you can "map" the chassis (everything, not just the boards) with 5 or 6 consecutive overlapping pictures, plus a couple of the inside part of front and back panels, showing pots, jacks and switches....

Here are some of them...