Randall bass amp

[Valvehead]

I was brought a Randall RBA 500 bass head . The amp powers on, but wont pass signal. It does make a loud low freq hum out of the speaker cab.

The 6 output transistors are new . Possibly the 6000uf caps in the power supply ? Expensive lil buggers

One thing i noticed is when i remove the 1 of the 2 connectors from the "preamp" board to the output board , the hum goes away . Looks like 1 con goes to 3 of the output transistors, and the other to the other trio.

[52 Bill]

Disconnect the speaker for now and measure across the output jack for dc voltage.

[Valvehead]

yes sir , it has -48.2 vdc at the jacks

[lowell]

DC on the output = hum. Is this with the cable connected or disconnected? If connected, then the positive portion of the PA isn't functioning...or the + DCV rail is non existent. I'd check the positive rail voltage first.

Then if there is +45 there, check for voltage drops across the following components.

Both 1n4007 diodes across the upper and lower PA.
Check Vbe of all 3 upper MJ15003
Confirm +1.1v and +39v test points to ground

That should give us some clues

[Jazz P Bass]

'Quote: The output transistors are new.'
Meaning, you installed them?

If they where all bad then I would also replace the driver transistors.
It sounds like (-48.2Vdc on the output) theTIP32C is shorted.

[lowell]

Good call JPB,

Absolutely that could be shorted. I posted late and was tired. Any component in the negative portion could be shorted as well!

[Valvehead]

thanks guys , i will test more tonight

there is 52v at the 6000uf caps and 52v at the output transistors

[Valvehead]

took some quick measurements , i hope this helps

TIP31c has -48v / -48v

TIP32c has -32v / -32v

TIP32c has -49v / -48v

volts at the 1N4007's -33v / -48v / -48v / -52v

there is -49v at all for points of the 1N914 's

[lowell]

took some quick measurements , i hope this helps

TIP31c has -48v / -48v

TIP32c has -32v / -32v

TIP32c has -49v / -48v

volts at the 1N4007's -33v / -48v / -48v / -52v

there is -49v at all for points of the 1N914 's

Little confused by these. Can you differentiate between the 2 TIP32Cs?

Please also specify base emitter or collector and for diodes, specify probe orientation.

How about those test points around the first (leftmost) 32C? +39 and +1.1v?

If I interpeted your readings right then JPB is probably right!

[Valvehead]

ok sorry about that,

the top/left 32c on the scem , im getting the following --looking at the 3 legs left to right in the amp, = all 3 have -48vdc

the bottom 32c on the scem im get , -29/-33/-49vdc at the legs , left to right , measuring to ground

so no , the test points arent jiving at that top 32c !

at the 1N914 , looking at the scem , the top one has -49 cathode / -49 anode

the bottom one has -49 cathode / -49 anode

[lowell]

Yah pull that first 32c and check for shorts.

"Left to right" still doesnt differentiate the pins. The emitter is on top, base on the left, and collector on bottom. This is a PNP Bipolar Junction Transistor. It is drawn upside down as usually the emitter in a transistor diagram is at the bottom. The 31C as well as the MJ15s are drawn right side up and are NPN transistors.

Best to be super clear on this when working on troubleshooting as even the slightest mistake can result in false hypotheses.

the top/left 32c on the scem , im getting the following --looking at the 3 legs left to right in the amp, = all 3 have -48vdc

Not sure if youre saying the emitter (top leg) has -48vdc? Pretty sure you said in your 2nd or 3rd post that the +rail was good. ? Which conflicts with this.

[g1]

If that 22 ohm connected to the 32C is open, then you might see -48V at the emitter.

[Valvehead]

the 22 ohm is ok.

and you have me confused Lowell !

the tip32 / 31 are 60 cents so im going to order them tonight just to get em on the way--- not sure when i can get back to the amp

[lowell]

My apologies. What can I clear up?

[Enzo]

If you have -48 on the emitter of the upper left 32, do you not also have +48 on the top end of the 22 ohm resistor? That is the V+ rail. And if that is true, you have about 96v dropped across 22 ohms, and that is over 4 amps. And that is about 400 watts dissipating. So either the 22 ohm is open, or there is no +48 in that part of the circuit.

You have all V- at the TIP31, again suggesting the V+ is missing. The 1N4007s you report at V- all across, again telling me there is no V+. You said there is 52v at the 6000uf caps. Well I HOPE that means +52 on one and -52 on the other. If +52 is really there, find out why it is not reaching the power amp circuits.


I don't know what you mean by left to right on the readings. Left to right on the drawing? On the part itself? Left to right as it sits in the chassis or left to right looking at the front of the part?

If you look at a TIP31, there is a tab at the top, the tab has a mounting screw hole in it. Then the flat surface of the body has the print on it that says TIP31 etc. Finally a row of three legs. The legs are identified from looking at the printed surface. Left to right they are Base, Collector, Emitter. Look at the picture in this data sheet:
http://www.mouser.com/ds/2/149/fairc...31a-549394.pdf

So please get into the habit of identifying readings at the emitter or collector, rather than left or right.