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

[ParthaD]

Hello everybody, I'm very thrilled to have recently found and joined this forum. I have recently started picking up good guitar amps from Craig's List and Ebay (mostly SS) that have some issues (as a hobby) and trying to fix them (have had a pretty good success rate so far with the help of the MEF archives!!!). Please excuse me if I break any forum protocols!

I am a chemical engineer by profession and have researched all this pretty extensively and do take all the precautions when working with high voltages, draining caps etc. I have a DMM but not a scope. I am quite comfortable with a soldering iron but have only just started learning how to read a schematic and do signal tracing etc.

I am a big fan of Enzo's troubleshooting skills , and had sent him an email and am now posting the relevant details here along with his response in a followup post):

On this particular amp (bought on ebay with a known problem) the problem was that even though this is a 300W amp the max volume when connected to a Crate 4x12 (GX412r - 4 ohms mono, 200W cab - spkr cable and cab are confirmed good as is the guitar and guitar cable) is the equivalent of vol 1. No hum or other problems. Cleaned up jacks with Deoxit, checked for lose connections, no visibly damaged resistors. I did see a pretty good discoloration on 2 of the 6 transistors casings (tray type, RCA, 3 each 7750, & 7807) looks like:

Ebay link

I could feel these 2 getting warm but not hot, and these are the ones connected to the speaker jacks. I would like to try to replace them but drew a blank on google. Each of these transistors has 5 terminals in 3 rows, just 1 top left, 3 in the middle in a straight line, and one bottom right (Enzo already explained that it was really just 2 terminals). Looking at the amp from the top with power transformer on left, the order is:
7750 7807 [7807] [7750] 7807 7750
(Again, Enzo has explained that these RCA numbers stamped were just the date codes and not the trans type and that it is a T-03 pkg type) The two in the middle (bracketed) appear to be the bad ones - significant charring on the metal. The amp has 2 spkr jacks which say 150w each into 2 ohms.

Have no schematic and writing to Randall drew a blank since the original Randall's been bought out and they don't have old records. So if anybody can post/email/lead me to the schematic I'd be extremely grateful.

thanks!

[ParthaD]

Enzo's email response...

Electronic components often have date codes on them as to when they were manufactured. All those 77 numbers you listed are date codes, not transistor types. The code is not secret, 7750 and 7807 simply mean 1977 50th week and 1978 7th week. SO if those are the original transistors, your amp was probably built in early 1978. Oh, there I go, you said 1978 in your header.

SO to identify those transistors, we need some other number from the cases. However, I doubt your low output is due to weak transistor outputs.

I think you will find that the six power transistors are in two groups of 3. Their cases will be wired together, their base connections together, and their emitters each having a low value resistor to the output bus. (something like .33 ohms) Three will work off the +V power supply, and three off the -V power rail. Well, come to think of it, I bet two of each group are wired like that, while the third of each will be wired as a driver.

I am not aware of any power transistors of that era with 5 legs. Make sure two of the terminals you see are not socket mounts or some other component lead. Of course I could be wrong. I think what you are seeing is the two screw connections plus maybe a center pin of a socket. All those would be wired together, and I bet they are the upper left and lower right ones and the center pin.. the remaining two are the base and emitter.

The ebay part you mention is an example of what we call in the industry a "TO3" case. The case is the collector and the two screws are usually how the connection is made. Then two legs stick out the bottom. "TO" means transistor outline - the shape. 2N1489 is the type transistor of your example.

SO we need to know what type yours are if we want to change them.

Far more likely some earlier stage is at fault in the amp though.

I am perfectly happy to help you, but let us continue this in the forum. In the forum, a hundred interested people will follow along and maybe participate. If I do it in private then I have 100 email correspondences over the same thing. SO we can let others benefit from the discussion and add their insights.

I do suggest a thread in the schematics request area for your amp model, a quick google trip for me did not pay off, but our members have many files.

[ParthaD]

More info:

Meanwhile, as I was researching this late last night, I stumbled upon a reference to the T-03 pkg size and also saw that this person on ebay that had that RCA transistor for sale, also had a Marantz 2SC1403 (an audio amp power trans as you may already know) on sale for $29. Also came across many references that said that in the absence of detailed specs sometimes a SIMILAR transistor for the same type of application works since power transistor circuits are supposedly very tolerant!

Did some more digging and found a vendor who had the same item on sale for $4 at RC Electronics! That's where I found some rating info also so I ordered it based on the fact that this appears to be an audio amp application transistor, and the power ratings also seem to match my application since this part is rated for 180 V 16A 150W while I measured my (giant) filter caps putting out +/-47 volts and Power=V^2/R and V=IR calcs using my rated output of 150 W into 2 ohms leads to an I = sqr root(150*2) = 17.3 amps and the corresponding V = 34 volts at peak power at 2 ohms. Of course this may not be as loud if it works since the amp is actually rated for that much through EACH channel so if I connect both channels I will only get 75W/channel but since I am not going to be playing anywwhere near those power levels, I thought it worth a shot, even at half power. If it doesn't do anything then at least we'll know its not the transistor.

Some pics to follow soon...

[Gtr_tech]

Of course this may not be as loud if it works since the amp is actually rated for that much through EACH channel so if I connect both channels I will only get 75W/channel but since I am not going to be playing anywwhere near those power levels, I thought it worth a shot, even at half power. If it doesn't do anything then at least we'll know its not the transistor.

Some pics to follow soon...

Its a *mono* amp...there's no 2 channels, just one single output. The 2 speaker jacks are parallel connected to the same output.

[Enzo]

What he said.

There are two jacks wired together for your convenience. The ratings are for the total connected to the two jacks, not per each jack.

[Mars Amp Repair]

I'm Randall authorized & have access to the schemos. they are handled now by US Music corp.
Here's an RG-300. BTW, I'm with enzo, I'd suspect bad bypass or coupling capacitors in the preamp stages. ...g

[Phostenix]

Partha, you now have part numbers thanks to Mars, but just for future reference... Those ratings on output transistors mean little in the real world. What you care about is the SOA - safe operating area. Those ratings like 16A & 180V are only if you keep the silicon die at 25 degrees C (about 75 degrees F). The also are mutually exclusive - you can put 16A through the xstr only if the voltage across it is something like 10V. If you put 180V across it, the current can only be some small mA number. Again, all of that is if you never let the temp rise on the xstr - at all. Never gonna happen in the real world. That's why there are 6 xstrs to produce the power (160W) that it sppears 1 could provide. Google SOA on transistors & look at some datasheets. It's good info to know,

You say that the power rails are +- 47VDC, right?

Check for DC voltage between the speaker leads with no audio input.

I had a similar problen with a Crate SS amp & it turned out to be the muting FET. Q10 on your amp appears to be a muting FET to me. A quick check would be to just remove that FET & see if the output comes back up.

There are quite a few xstrs in what looks to me like a protection circuit, too. (Q11-Q14).

[Steve Conner]

I've never in my life seen a TO3 transistor that "discolored". They usually short out internally and blow fuses. If you're having a really bad day, they can explode and blow their tops off.

By all means pull them and check them, but that can be a hassle.

For some reason, when a tube amp goes down, Joe Public always blames the output transformer. And when a SS amp goes down, he blames the output transistors. Maybe because they have the word "output" in their name.

But there are dozens of other parts you should check first before replacing these particular ones, they happen to be expensive. If the amp has no blown fuses, no DC on the speaker, and produces some sound, the output transistors are highly unlikely to be blown, and you should trace a signal through the amp to check out all of the stages.

The previous poster was right, it's not crucial to get the exact replacement part, as long as it's the right polarity (NPN/PNP) and the ratings are the same or better. The MJ15024 and MJ15025 from OnSemi (was Motorola) are a popular pair of 16A, 250V TO3 transistors that will work in just about any amp up to 600 watts.

[ParthaD]

Thanks Mars Amp... will start tracing the circuit... having never done something like this before, may come back with some questions!

[ParthaD]

Check for DC voltage between the speaker leads with no audio input.

I had a similar problen with a Crate SS amp & it turned out to be the muting FET. Q10 on your amp appears to be a muting FET to me. A quick check would be to just remove that FET & see if the output comes back up.

There are quite a few xstrs in what looks to me like a protection circuit, too. (Q11-Q14).

Thanks for that info Phostenix.... checked the DC voltage.. was 0.04V, that is within the spec hopefully... have to first locate Q10, parts are not marked on the board like newer ones.

[ParthaD]

I've never in my life seen a TO3 transistor that "discolored".

The MJ15024 and MJ15025 from OnSemi (was Motorola) are a popular pair of 16A, 250V TO3 transistors that will work in just about any amp up to 600 watts.

When I looked at the transistors at a certain angle from the back those 2 ones I had listed just jumped out at me, they were not charred but significantly blackened and even rough to touch. The others looked shiny and almost new. It is actually the easiest to remove and replace, just removed the two screws and pulled out to check. Pulled out one of the shiny ones also just to see the difference between the resistance (which may not mean anything for xstrs) across the two terminals and it was different. On the "good" one the resistance across them read around 2000+, and if you switched the meter leads around, it read about 700. When I tried that on the suspected bad xstr it read 1400 one way and 700 around the other. Those readings never completely stabilize (I guess because of the way a transistor interacts with the meter voltage) but were fairly stable in the first 5-6 secs when I took those readings. These readings were taken after pulling them both OUT of the circuit.

I even tried switching the "good" and the "bad" one around (remember I am a tinkerer not a professional amp guru like many of you ) and the sound got perceptibly louder equivalent to about a vol of 1.5 instead of 1 at maxed out master and chan volume!

Since I am new to signal tracing, would appreciate some pointers on specific points in the circuit I should look at and what should I be measuring, i.e. V or ohms. There are no bulging caps at all, and I've measured the two large white 0.27 ohm cement Rockwood resistors as well as the two similar 0.6 ohm resistors and they read the right values and also quite a few of the resistors on the board and they all read close to the proper values (had read on some other thread that those types of resistors are prone to failure).

[ParthaD]

About ready to throw in the towel and get professional help...

Here is an update...

The new transistors arrived and (drumroll please).... it did NOTHING! Just as some of you expected! Sound was just like before. From the sound of the guitar it seemed almost as though the PREAMP was dead (which it probably is!). Anyway since the board is not marked with part numbers it will take more than my expertise to go thru it and trace the circuit.

Since caps are usually the culrpits in a 31 yr old amp (although none are bulging or leaking from what I can tell) what I did do was to check every cap that I could recognise as one and measure the DC voltage drop across it with the amp ON. The power caps are steady at 47V. The 3 largest ones (470uf) had a drop of 40, 28, and 24 V across them. There was one small one that was oscillating rapidly between +-3, one between 4-21, one between 6-11, and some had 0 V across it. I took some pics and marked the voltage drops directly on the pics which you can see if you use windows photo viewer to blow them up. Based on this, my gut feel is that probably a large number of the caps may need replacing.

Some of you experts may find in interesting to look at the pics. There are 3 large and 15 small caps that I've seen. At least 5 of the small ones are reading 0 volts across it. At this point should I try replacing those 3 large caps with the highest voltages across them, or those other smaller ones reading 0. The parts themselves will probably not cost much and I can solder them myself but I could use some tips on which ones I should try to replace first rather than use a sledgehammer approach and replace all the caps. I haven't looked at resistors the same way but initial measurements (again with the amp on showed that there was a voltage drop across each of the ones I checked.

Waiting to hear from you...

[ParthaD]

Oscillations and Photomod?!!!

Would somebody enlighten me on why some of these voltages measured across caps appear to be oscillating. (picture 92) and also what is that black device (in pic 92 right above the 2 470 uf caps) that looks like a black cap but has 2 leads on each side with the word "Photomod" etched on it. There is some voltage across the upper left and right leads but nothing on the lower left and right. One of the reasons some of these caps are reading zero volts across them is because nothing is getting to it. Also, wouldn't you be expecting those 2 large caps in pic 92 to have around the same voltage drop across them, they are both rated for 50v but one has 40 and the other 28. Also the same spec/size cap in pic 95 is at 24.

thanks again!

[Mars Amp Repair]

Hello,
The part called 'photomot' appears to be the LED/Photo assy which runs the tremolo. One side (the led drive side) would be oscillating. Its freq depending on speed setting on the front panel.
The other side would be the photo sensor...you won't see any change here as the only change that is taking place is a resistive one to change the volume.

It is a little unclear as to what other voltages you're trying to read. glen

[ParthaD]

Hello,
The part called 'photomot' appears to be the LED/Photo assy which runs the tremolo. One side (the led drive side) would be oscillating. Its freq depending on speed setting on the front panel.
The other side would be the photo sensor...you won't see any change here as the only change that is taking place is a resistive one to change the volume.

It is a little unclear as to what other voltages you're trying to read. glen

Ok that explains the oscillations... the other voltages that I have listed are basically those measured with the amp on and touching the meter leads to the cap leads in each case. I am getting my hands on one of those in-circuit cap testers (an 88A) to see if the ESR measurements identifies some bad caps. Does anyone use those in your shops, are they reliable?