Bass Amplifier troubleshooting

[RubiconJoe]

Hey all,

I am pretty new to electronics troubleshooting but I believe I can tackle fixing this one. The issue is with a Bass guitar amp, Fender Bassman 200. When powered on all the amp does is buzz from the speaker whether the input jack is plugged into an instrument or not.

First thing I noticed when removing the chassis is the speaker would sometimes pop when touching the chassis itself or any of the components inside; note that the amplifier was not powered on or even plugged in.

Next I tested the power supply voltages. According to the schematic, there is a +57 and -57 volt rail. The positive rail tested low, about +40V. The negative rail tested at -59V. Maybe an issue with one of the filter caps? It is a typical bridge rectifier with two 3300 uF (63V) filter caps on the output.

There is also a +16 and - 16 volt rail, which looked was very close to expected +-16V. I also tested points throughout the circuit where the rails are connected and looks like power is being supplied correctly.

Where should I go from here? Unfortunately I do not have an oscilloscope or signal generator to further small signal testing. I was hoping it was a simple grounding issue or blown cap. No components are visibly damaged and caps all look good.

Thank you for anyone who can help!

Joe

 

[Martaine2005]

Hi Joe, sounds like an EARTH issue initially.
Is this amp new to you? Or used by you before with no problem?
Reason I say this is because possible interference from flourescent lights etc can cause this kind of issue.
Is it a similar buzz to when you plug a jack in and touch the terminals?

I would first check all EARTH points and the connections to the jack .

Martin

 

[RubiconJoe]

The amp actually belongs to a friend of mine, and I am trying to fix it up for him. I don't believe it to be interference. I do not have any fluorescent lights in my apartment. The buzzing occurs if there is nothing plugged into the jack. Also when a cord is plugged into the jack, sometimes momentarily stops but starts up again. No sound from the instrument is sent to the speaker. Only buzzing.

I will test all the earth points and let you know what i find.

Thanks!

Joe

 

[Martaine2005]

Hey Joe, I shouldn't have said EARTH!! It isn't spiked to EARTH!!
It's kind of frowned upon around here.
I should have said 'Ground' or 'common' or 'neutral'.
Anyway, yes check all the 'common connections' especially the input jack.
There are large capacitors on the board too which can vibrate loose, so might want to give them a wiggle too.

Martin

 

[RubiconJoe]

I went through the PCB's and checked all signal and chassis grounds. All seemed to have continuity.

Shouldn't I be concerned with the low voltage on the positive rail? Something seems to be pulling it down. I tested the rectifier diodes and I believe all four are good. I also tested the output transistors and none seem to have any shorts.

Where should I go from here? Any suggestions would be greatly appreciated!


Thanks,

Joe

 

[Martaine2005]

Hi Joe, have you checked to make sure the mains cable earth is intact?
Does this buzz happen with no volume or can you control it by the volume knob?
Also, you said it sometimes stops. So intermittent buzzing must have something to do with a dry/broken joint.
Can you power it up and poke the caps with something? (not your hands).
Also check the pots properly.

The voltage may be lower on the supply rail for a few reasons. Check the caps for dry/broken joints first. And check the secondary voltages on the transformer.
Intermittent faults are a real pain.
Has this amp got a separate power supply board?

Martin

 

[Martaine2005]

And I forgot to ask,
When the buzz stops, does the amp act normally?

 

[RubiconJoe]

The volume knob does not have any effect on the buzzing. If I unplug/re-plug the input jack the buzzing stops sometimes, but not always. When it does stop, the amp does not produce any sound/act normal.

The power supply board is separate from the input/preamp board. The caps do not show any signs of failure and all solder joint look okay. I will power it up and poke the caps. This is to look for loose joints?

How would I go about checking the pots? Measure the resistance of the sweep? All pot cases seem to be ground.

 

[Martaine2005]

Hi Joe,
So just to clarify, when it buzzes, and the volume is on zero the buzz can still be heard?
Did you check the secondary voltages?
And checking all the pots just to make sure there are no dead/dirty contacts by a squirt of switch cleaner or similar.
Poking around the caps (and the caps) to determine a loose/dry joint.
Also check all cable connectors by disconnecting and reconnecting.
It sure sounds like a loose connection somewhere.

Has the amp got a headphone jack? Do you still hear the buzz through them?

Martin

 

[RubiconJoe]

Hey Martin,

Sorry about the delay. I haven't had much free time lately to play with this amp.

The amp buzzes no matter what position any of the volume/tone controls are in. Just a constant loud buzzing.
I tested the two secondary feeds from the transformer. Both looked good, 88 Vac and 41 Vac feeding the two rectifiers.

I also checked/ disconnected and reconnected all cables. Poked around the caps looking for loose joints, but nothing seemed to effect the buzzing.

The amp does have a signal line out, which I tested and actually get instrument signal there! So that should eliminate the pre-amp board and any faulty grounds there!

I guess the problem is with the power amp/power supply board. Any other ideas?


Thank you again for your input!

Joe

 

[Martaine2005]

Hi Joe, I was kind of hoping you had sorted it.
Thanks for the update.
I had to read the whole post to see what was already said and done.
Ok, I think I am with it..
Where exactly are you measuring the secondary outputs? You should check them directly on the transformer.
Try lifting a leg of the first component to test the transformer secondary output directly. (just a check)
Just out of curiosity, if you can't check the ESR of the caps, can you replace them or swap them round?
If the service manual says +-57volts on the rails, we still don't know if this is with load or no load.
Having said all that, it would either work or not..Not just buzz.

When you used the line out, it worked fine. So, the psu seems fine. But worth checking anyway.
I still think it's a GND issue, and we may have to check the output stage for shorts or open circuits..
But as you say, wiggling the input guitar lead sometimes stops the buzzing. Can you feel anything with your fingers getting really hot? Have you got a can of freezer? to spray the individual transistors?

I will D/L the schematic tonight and have a look. But I am probably getting out of my depth here now.
I would check everything because it was mine. But trying to explain that over text is something else.

Let's hope that somebody else pitches in with some other ideas or checks.
And as happens on all forums, let's hope they read it from the beginning!!!!!

Martin

 

[Martaine2005]

Ha' yes I did D/L it..It is all sideways.. Let me have a play..

Martin

 

[RubiconJoe]

Yeah, I had to print it out the layout!

The secondary outputs of the transformer were connected to the power supply board via snap connectors. I removed the wires from the board and tested the transformer outputs directly.

I could order new caps and swap out the old ones, I wanted to try and figure out the issue before ordering parts and throwing them at it. Maybe its time to start doing that! haha

 

[Martaine2005]

SO, did you get the different voltages directly? or from the rails?

 

[RubiconJoe]

I don't understand the question. I measured 88 Vac from the secondary of the transformer disconnected from the PS circuit. This feeds the +-57 Vdc rails. Connected back up I measur +40V and -59 V on these rails. So far cannot find where the extra 17-19 V are being dropped.

I measured 41 Vac off the other tap of the transformer feeding the +-16 Vdc rail which measured +- 16 Vdc connected back up.

The only way to measure ESR of the caps is with an ESR meter?

 

[duke37]

It looks as if the transformer is 44-0-44 V. 44 * sqrt(2) = 62 according to my slip stick. This is the peak value which should be obtained with no load and no diode drop. The centre tap must be connected to earth/ground/common.

A faulty reservoir capacitor will not hold the peak so the voltage will be lower and hum would come from the amp. I would tack another capacitor acros the suspected faulty cap and see if the voltage rises.
You could also do what Martin suggests and swap over the reservoir capacitors.

 

[RubiconJoe]

The transformer has 6 wires on the secondary side. Two measuring around 44 from the ground wire in between and two measuring about 20 from center wire. There's about 88 Vac feeding the 57Vdc rectifier, and 40Vac feeding the 16Vac rectifier. This was measured without any load

 

[duke37]

The amp will not work properly until you get the DC conditions right.
It looks as if the reservoir capacitor is faulty or excess current is being drawn, if the latter, something will be getting hot.

If you have a meter which will measure AC voltage and ignore DC, then you can measure the ripple on the capacitor.

 

[dorke]

Try measuring the DC voltage on the speaker,you should get 0v.

The +rail is too low if you are getting +40v(should be +57).
Measure the AC volt on the +rail and compare it to the -rail.

 

[Martaine2005]

Hi Joe,
Some more good suggestions above..
I had a sideways peak at the schematic.
Something else to try.....Because I still think it is loose /faulty caps or GND issue.
Just after the bridge rectifier there are two caps in series. C3 and C4 Both 3300μf. There are also two test points there.
You have the + Tp at C3 + and the - Tp at C4 -. According to the diagram you should get + - 50Vdc.
If not, your problem may well be there somewhere.

Also, on a side note. This amplifier has an FX input. Have you tried the guitar in that and get signal?

Martin