Randall RG140 - need information about a burnt resistor - or schematics ..

[labosecondsouffle]

Hi Everybody,

I need information about Randall RG140 guitar amplifier.
Can you tell me the value, the color code, or take a pic of this resistor?
It's impossible to find any schematics about this amp.

Thank you so much
Scuse my english, i'm french.

[teemuk]

...And if any of that fails, then you can find out what the resistor does in the circuit and we can approximate its value or figure it out from other similar circuits.

Yes, you may not perhaps find a RG140 schematic* but Randall amplifiers have, for example, employed very similar power amp circuits from the late 1970's to about late 1990's. They are not identical designs, but similar enough to usually figure out what value a certain resistor, capacitor or such component should have. Or what semiconductors are used. Same goes with preamps, the classic "FET Preamp" introduced in circa 1981 changed very little throughout its history up to late 1990's. Again, we can probably estimate a ballpark value based on similar circuits if you find out what the resistor is doing in the circuit.

Is this a 1980's / 1990's Randall amp with simple FET common source gain stages, or those earlier designs that employed direct coupled cascade of FET common source and BJT emitter follower gain stages?

*By the way, even if you happen to find a schematic for RG140 do note that Randall, like practically all manufacturers, always revised their amp designs now and then so one schematic may not neccessarily cover all revisions. For instance, RG100ES was introduced in circa 1981 and throughout the history of that model up to late 1990's Randall made changes to power amp circuitry, preamp circuitry and channel switcing citrcuitry. The circuit diagram of the 1981 revision, for example, is not really compatible with late 1980's or late 1990's revisions eventhough the original circuit architecture hasn't significantly changed. You can still use about any revision schematic you can find to estimate component values for about 80% of the circuit. Little less luck with features such as channel switching where they gradually upgraded from pure mechanic switching (within the footswitch) to remote controlling with relays, discrete switching FETs and then TL604 chips.

[labosecondsouffle]

Hi,

This is a 1W resistor in serial wiring with a .1uF/100V film cap, twice  beetween and the 4Ohms Tip speaker output and 3300uF/25V caps...

my DCM tells me 668K ohms for this burnt resistor.. no color code visible, and randall answers me there is no schematic for this model (Randall RG140 - year:1991)

thanks for your help

[Rob Strand]

This is a 1W resistor in serial wiring with a .1uF/100V film cap, twice  beetween and the 4Ohms Tip speaker output and 3300uF/25V caps...

I'm fairly certain it's part of the Zobel network.  A network used to stabilize the amplifier (in an electronic feedback sense).   Typically the resistor is in the 4.7ohm to 22ohm region.   It's a good idea to make some effort getting the right value.   I can't see any Zobels on my Randall schematics.

One thing worth thinking about is why that part failed.  Sometimes it's because someone connected the input jack of the amp to the speaker jack.  Other times it's because someone was touching the speaker jack and input jacks with their fingers at the same time - a good way to fry speakers.    However, the other case is the amplifier has gone unstable.  That could mean the output devices were replaced with incompatible ones during a repair or there is another fault somewhere causing the problem.

[Rob Strand]

Actually when I look at your pic I can see a brown band on the resistor.   You could be fairly confident it's 10 ohms.  10 ohms is a *very* common value in that position.

[teemuk]

Some Randall power amps have a R-C Zobel, some don't. For example, there was no R-C Zobel in the early 1980's RG100ES / RG80ES amps (the revision you commonly find a circuit diagram to) but Randall actually began to employ one in  those since the mid 1980's. Also their late 1970's amps did feature such R-C Zobel too. Actually, by quick glance of my schematic collection the '81 RG100ES series amps seem to be only Randall amps without one.

When they do have it it's more or less consistently 100nF + 10 ohms, 1 watt. In higher power models (e.g. RG300) they seem to employ 100 nF + 4.7 ohms, 2 W, or 100nf + 10 ohms, 5W (e.g. T2). Proper power dissipation rating is probably more of a concern than exact ohmic value, assuming you're in the 4 - 10 ohm ballpark. It's just a series resistor limiting current at high frequencies at which the capacitor characteristics otherwise approach "short circuit". A crude rule of thumb for dimensioning is to pick an ohmic value about equal to rated nominal load impedance of the amp. e.g. 10 ohms for 8 ohm rated load impedance.

BTW, a greater concern - IMO - is that a burnt Zobel resistor may be a sign of high frequency oscillation in the power amp stage.

[labosecondsouffle]

Thank you for your information!
I replaced by 1W 10ohms resistor.
Amp works great.