You are here

Hum Problem on Ashdown Bass Amp

For all things relating to guitars, basses, amps, pedals & accessories.

Hum Problem on Ashdown Bass Amp

Postby Dan LB » Mon Jul 11, 2011 1:39 am

Evenin' all.

My bass amp (Ashdown ABM EvoII 500) has recently developed a hum problem which it's never had before (I've had it around 5 years), and I wonder if you could help me track it down. Here's what I know so far....

I have tried the amp in several different locations with different power cables so I am presuming the problem lies with the amp.

It hums with nothing plugged into it (only the power cable and a speaker cab)
It seems that the hum is introduced at the start of the signal path as it is affected by all conrols (Pad, Shape, Valve Drive, EQ and Sub Harmonics) and remains there even with all functions bypassed. With the input gain turned all the way down the hum goes away - even with the output wide open.

I have not yet checked whether the DI output is affected but I'd assume so.

Any help would be very much appreciated as it's driving me nuts!

Dan
User avatar
Dan LB
Frequent Poster
Posts: 639
Joined: Sat Jan 28, 2006 12:00 am
Location: Wicklow, Ireland

 


Re: Hum Problem on Ashdown Bass Amp

Postby ef37a » Mon Jul 11, 2011 7:27 am

Hi Dan, The amp has a valve* in it and that is the most likely cause of hum.

If you feel confident to remove it from its sleeve I would bet the valve is easy enough to swop. The snag might be finding a good one!
I suspect it will be a basic ECC83 and I have found the TAD 7025HG a pretty quiet replacement at not too high a cost (~£10).

*On a technical point I find it hard to believe that a company would put a valve in a bass head and feed it with an AC heater supply!If indeed the supply is DC then that blows my diagnosis out of the water but makes it much more an amp tech' job.

Dave.
ef37a
Jedi Poster
Posts: 7200
Joined: Sun May 28, 2006 11:00 pm
Location: northampton uk

#They did not listen, they are not listening still...Perhaps they never will?#


Re: Hum Problem on Ashdown Bass Amp

Postby Dan LB » Mon Jul 11, 2011 7:59 pm

Cheers Dave,
The valve is easily accessible and for the sake of a few quid I'll probably change it anyway. However, I think the valve is only in circuit when the 'valve drive' switch is engaged. Now, I'm not sure if this was a stupid thing to do, but anyway - I took the valve out last night and switched the amp on (it's a solid state amp by the way). Still had hum but when I engaged the valve drive and turned the blend to full valve the hum disappeared. So I presume this means the valve is not in circuit unless I engage the switch. I might not have a clue what I'm talking about though!

Dan
User avatar
Dan LB
Frequent Poster
Posts: 639
Joined: Sat Jan 28, 2006 12:00 am
Location: Wicklow, Ireland

 


Re: Hum Problem on Ashdown Bass Amp

Postby ef37a » Mon Jul 11, 2011 8:56 pm

No, not stupid at all Dan, an excellent bit of diagnosis!

That I am afraid is the only good news. I think you have a filter capacitor or regulator problem and these would both need the services of an amp tech to rectify.

If I was familiar with the amp and had a schematic and you had sufficient soldering skills (or maybe you do?) we might have a crack at it together over email but I don't have such information about the amp.

Sorry, you will have to take it to the amp doc' I am afraid.

Dave.
ef37a
Jedi Poster
Posts: 7200
Joined: Sun May 28, 2006 11:00 pm
Location: northampton uk

#They did not listen, they are not listening still...Perhaps they never will?#


Re: Hum Problem on Ashdown Bass Amp

Postby Folderol » Mon Jul 11, 2011 10:04 pm

As a matter of interest, does the hum still drop as you move to the valve side with the valve present?

Either way, I think you're looking at a preamp cap drying out - Hmmm I would say that was a certainty if the hum has slowly developed over a period of time. If it happened suddenly, it could still be a cap, but I'd be more likely to suspect a bad earth.

As Dave said, without info it's prolly a techie job.

Well, well, well. Look what I just found

Schematics

Look at the first reply.

And having just had a look I can now say I was talking bollox earlier!

You have a twin pot on the valve-clean control, so without the valve in, it would simply act as another volume control

Looking at where that input control is there is just one device before it, TR4.

As this is a source follower, my suspicion then would be either C19 or, more likely, the earth point of the input jacks (look for cracks in the solder).
User avatar
Folderol
Jedi Poster
Posts: 4694
Joined: Sat Nov 15, 2008 12:00 am
Location: Rochester, UK

Save paradise, Pull up a parking lot!


Re: Hum Problem on Ashdown Bass Amp

Postby Dan LB » Mon Jul 11, 2011 11:11 pm

Will, Dave, thank you both very much!

I haven't much experience reading schematics, but I'm fairly handy with a soldering iron. I had to replace the 'Bass' control pot the other day (the hum isn't related as it was there before I did the repair).

Actually, how to read schematics would be something I'd love to learn (it's the signal flow I have problems with) - I presume I'd need a good grasp of basic electronics first though.

Anyway I'll have a look at the joints on the input jack like you suggested Will - I suspect this might be the culprit as I've accidentally yanked the instrument cable a couple of times in the past. I must remember to tie it off to the handle first

Thanks again for your help and I'll let you know how I get on. Thanks Will for the schematic - it'll come in handy if I do need to bring it to a tech.

Dan
User avatar
Dan LB
Frequent Poster
Posts: 639
Joined: Sat Jan 28, 2006 12:00 am
Location: Wicklow, Ireland

 


Re: Hum Problem on Ashdown Bass Amp

Postby Dan LB » Mon Jul 11, 2011 11:22 pm

I just had a look at that drawing - it's actually an earlier version of the amp I have. Mine only has one input jack with a pad switch but I'd imagine it's fairly similar.

The date on the PCB is 2002 and also bears Clive Button's name.

Dan
User avatar
Dan LB
Frequent Poster
Posts: 639
Joined: Sat Jan 28, 2006 12:00 am
Location: Wicklow, Ireland

 


Re: Hum Problem on Ashdown Bass Amp

Postby Dan LB » Tue Jul 12, 2011 12:40 am

Well, I just checked the the solder joints on the input jack and all of them were OK. Just to be sure, I heated each joint to melting point and let them harden again. No joy! I still have the hum.

I have, however, located the transistor 'TR4' and the cap 'C19' like you suggested Will. Do you think I should swap them out??

Dan
User avatar
Dan LB
Frequent Poster
Posts: 639
Joined: Sat Jan 28, 2006 12:00 am
Location: Wicklow, Ireland

 


Re: Hum Problem on Ashdown Bass Amp

Postby ef37a » Tue Jul 12, 2011 4:58 am

Well done that man!

Yes Will, C19, maybe C22 or earthing. Might be one of the zeners gone o/c? Quick check with a voltmeter will prove that.

And my apologies to Ashdown. Not only have they made a bang up job of a regulated heater supply but there is a dedicated 350volt anode supply to boot. Be very careful in there Dan that 350V rail has more than enough juice to kill you before the 100mA fuse pops,even if it did.

Dave.
ef37a
Jedi Poster
Posts: 7200
Joined: Sun May 28, 2006 11:00 pm
Location: northampton uk

#They did not listen, they are not listening still...Perhaps they never will?#


Re: Hum Problem on Ashdown Bass Amp

Postby ef37a » Tue Jul 12, 2011 5:42 am

Dan LB wrote:Well, I just checked the the solder joints on the input jack and all of them were OK. Just to be sure, I heated each joint to melting point and let them harden again. No joy! I still have the hum.

I have, however, located the transistor 'TR4' and the cap 'C19' like you suggested Will. Do you think I should swap them out??

Dan
Don't change the transistor (FET)it will not of itself cause hum. The caps you can check by simply tacking a good one across the old, if that works chop out and replace.
But first put a voltmeter on the 15volt rails, probably easiest to find at PL1 the only DC on that will be either side of the centre 0V pin. If you have more than 15V there one or other of Z1,Z2 has gone o/c.

Dave.
ef37a
Jedi Poster
Posts: 7200
Joined: Sun May 28, 2006 11:00 pm
Location: northampton uk

#They did not listen, they are not listening still...Perhaps they never will?#


Re: Hum Problem on Ashdown Bass Amp

Postby Agamemnon » Tue Jul 12, 2011 6:09 am

ef37a wrote:Well done that man!

Be very careful in there Dan that 350V rail has more than enough juice to kill you before the 100mA fuse pops,even if it did.

Dave.

So is it advisable that he drains the caps before going in?
Agamemnon
Regular
Posts: 54
Joined: Wed Mar 24, 2004 12:00 am

Re: Hum Problem on Ashdown Bass Amp

Postby ef37a » Tue Jul 12, 2011 6:28 am

Agamemnon wrote:
ef37a wrote:Well done that man!

Be very careful in there Dan that 350V rail has more than enough juice to kill you before the 100mA fuse pops,even if it did.

Dave.

So is it advisable that he drains the caps before going in?
Yes if the amp is off and the easiest way is a croc' lead from pin 1 or 6 of the triode to chassis.
In fact the best way to work would be to remove Fs3 and leave the 12AX7 in place.
I do find it exasperating that in an otherwise very well designed piece of kit they have not fitted a 0.1p HT drain resistor!

All that said the "LT" supply to the power amp stage must be pushing 100volts and the current capabilty is such that even tho' lethal shock is unlikely a slipped meter probe could result in some expensive fireworks and nasty burns.

There are no old, bold amp techs'!

Dave.
ef37a
Jedi Poster
Posts: 7200
Joined: Sun May 28, 2006 11:00 pm
Location: northampton uk

#They did not listen, they are not listening still...Perhaps they never will?#


Re: Hum Problem on Ashdown Bass Amp

Postby Dan LB » Wed Jul 13, 2011 12:10 pm

Thanks for the advice Dave, and thanks Agamemnon for raising a very good point. I have been working on the amp so far with it unplugged from the mains, but I am well aware of the dangers of stored current in capacitors.

ef37a wrote:

But first put a voltmeter on the 15volt rails, probably easiest to find at PL1 the only DC on that will be either side of the centre 0V pin. If you have more than 15V there one or other of Z1,Z2 has gone o/c.

Dave.

Thanks for this Dave - I presume I would have to do this now with the amp plugged in and switched on.
I'd like to be completely clear about what I'm looking for and where to place the probes before going in.

Tell me if I've got this wrong - I measure across the +15V and 0V, and then across the -15V and 0V? Also in the pic below, have I labelled the rails correctly?

Image

Also, what does o/c mean in relation to a Zener diode?

Thanks,
Dan
User avatar
Dan LB
Frequent Poster
Posts: 639
Joined: Sat Jan 28, 2006 12:00 am
Location: Wicklow, Ireland

 


Re: Hum Problem on Ashdown Bass Amp

Postby . . . Delete This User . . . » Wed Jul 13, 2011 12:20 pm

o/c = open circuit s/c = short circuit std shorthand for electronics geeks
. . . Delete This User . . .
Frequent Poster
Posts: 2368
Joined: Sun Jun 22, 2008 11:00 pm

Re: Hum Problem on Ashdown Bass Amp

Postby . . . Delete This User . . . » Wed Jul 13, 2011 12:26 pm

yes plugged in and powered up....


across those 3 terminals, with your negative (black) probe on the 0V pin, you should measure + 15V between the +ve rail and 0V rail, and -15V between the -ve rail and the 0V rail.

or, if you place the red +ve probe on 0V, then the opposite should be true...

essentially the polarity of the supply across those is opposite... giving a 30V differential between the 2 15V pins.

any other result means a fault in the power rail supplying the pre-amp circuitry somewhere.

possible, as Dave says, a failed Zenner Diode...
. . . Delete This User . . .
Frequent Poster
Posts: 2368
Joined: Sun Jun 22, 2008 11:00 pm

Re: Hum Problem on Ashdown Bass Amp

Postby Dan LB » Wed Jul 13, 2011 12:42 pm

Cheers ODBBQL,

Just checked the 15V rails and got readings of
+14.78 and -14.93 so I presume that's close enough to rule out a problem on the power rail.

I'll go out and see if I can get the correct cap and replace C19

Any other suggestions?

Dan
User avatar
Dan LB
Frequent Poster
Posts: 639
Joined: Sat Jan 28, 2006 12:00 am
Location: Wicklow, Ireland

 


Re: Hum Problem on Ashdown Bass Amp

Postby Folderol » Wed Jul 13, 2011 9:09 pm

Voltages look fine so far.
Before actually pulling out C19, do as Dave suggested and try 'tacking' a new one underneath across the terminals of the original. Be careful to get the polarity the same!

If that doesn't make a significant difference, then the cap is not to blame and you can try the same trick with C22, although I think this much less likely to be the problem as the configuration of TR4 would tend to reject hum from the positive rail quite well.

If there is still no difference, then it looks like an earthing problem. Have a very close look at the trackwork around all those input earthing points (the big fat downward arrows). Preferably use a strong light and a magnifying glass. Look for poor soldering and hairline cracks. Be especially suspicious of joints that have a round ball of solder instead of the usual tapered shape.

As a long shot, check the voltages across C47 and C46, you should get a little over 20V across each of these, and the voltage across C47 will probably be slightly lower as this line also has to feed the valve heater. If the voltages are significantly lower then it's possible the caps are drying out.
User avatar
Folderol
Jedi Poster
Posts: 4694
Joined: Sat Nov 15, 2008 12:00 am
Location: Rochester, UK

Save paradise, Pull up a parking lot!


Re: Hum Problem on Ashdown Bass Amp

Postby Dan LB » Wed Jul 13, 2011 9:46 pm

OK I got a replacement cap of the correct value but I don't know what way round to install it. The existing cap has had its leads cut to the same length so I can't tell from that. Can anyone tell from the schematic linked above?

Dan
User avatar
Dan LB
Frequent Poster
Posts: 639
Joined: Sat Jan 28, 2006 12:00 am
Location: Wicklow, Ireland

 


Re: Hum Problem on Ashdown Bass Amp

Postby Dan LB » Wed Jul 13, 2011 9:49 pm

Sorry Will, I wrote that last post while you posted yours.
I'll tack it initially. Do you know which way round from the schematic? I can take a pic of the PBC too if that's any help

Cheers,
Dan
User avatar
Dan LB
Frequent Poster
Posts: 639
Joined: Sat Jan 28, 2006 12:00 am
Location: Wicklow, Ireland

 


Re: Hum Problem on Ashdown Bass Amp

Postby Folderol » Thu Jul 14, 2011 7:07 pm

The caps have a stripe down one side, usually with a row of '-' signs. These are nearest the negative pin (in fact you can actually see these on the caps in the picture you put up).

If you can't work out where the original C19 cap is when looking from underneath, shine a powerful lamp through the board and everything will be silhouetted

If you're still in doubt, the positive of C19 goes to the negative of C22 and also to ground (earth), so a continuity check should quickly reveal which is which.
User avatar
Folderol
Jedi Poster
Posts: 4694
Joined: Sat Nov 15, 2008 12:00 am
Location: Rochester, UK

Save paradise, Pull up a parking lot!


Re: Hum Problem on Ashdown Bass Amp

Postby Dan LB » Fri Jul 15, 2011 12:41 am

Thanks for the tips Will.

OK I've replaced cap C19.

No difference. I have a new development now though.
The hum pretty much disappears when I plug in a bass.
This presumably means it's an earthing problem.
With the bass unplugged from the amp, the 'Pre Shape' button is making loud switching noise across the output when operated but with the guitar plugged in it's pretty much silent

The amp is usable but I'd love to get it sorted! I've checked continuity between the chassis and the earth of the mains plug so I am assuming the amp is safe to use - please correct me if I'm wrong!!!!

Anyway, I'm not sure what to do now - maybe it's just beyond my capabilities and I'll have to bring it to a tech. I've learned a lot though so thank you all!

If you have any suggestions as to what to try next I'll give it a go!

Have a very close look at the trackwork around all those input earthing points (the big fat downward arrows).


I wasn't entirely sure where to be looking here.
I can see no big fat downward arrows on the PCB


Dan
User avatar
Dan LB
Frequent Poster
Posts: 639
Joined: Sat Jan 28, 2006 12:00 am
Location: Wicklow, Ireland

 


Re: Hum Problem on Ashdown Bass Amp

Postby Dan LB » Fri Jul 15, 2011 1:17 am

Ah! I see them on the schematic!
There's f#%*+^g loads of them!!!!!!

Dan
User avatar
Dan LB
Frequent Poster
Posts: 639
Joined: Sat Jan 28, 2006 12:00 am
Location: Wicklow, Ireland

 


Re: Hum Problem on Ashdown Bass Amp

Postby ef37a » Fri Jul 15, 2011 6:26 am

Aha!
One of the input jacks is not shorting when there is no plug inserted.

Quick fix: Amp off, plug in a jack, push the top contact leafs down a tad with a small screwdriver tip then give the contacts a spray of De-oxit, Maplin switch cleaner or WD-40 (and before you all pile in, WD has worked fine for me as a GP switch and pot cleaner for 40+yrs and for another amp tech of my (forum) aguaintance!)

Better fix of course, replace sockets, but be advised, make carefull note of the resistor positions and DON'T try to use the originals, go nuts and buy new!

Dave.
ef37a
Jedi Poster
Posts: 7200
Joined: Sun May 28, 2006 11:00 pm
Location: northampton uk

#They did not listen, they are not listening still...Perhaps they never will?#


Re: Hum Problem on Ashdown Bass Amp

Postby Dan LB » Fri Jul 15, 2011 7:15 pm

ef37a wrote:Aha!
One of the input jacks is not shorting when there is no plug inserted.

Quick fix: Amp off, plug in a jack, push the top contact leafs down a tad with a small screwdriver tip then give the contacts a spray of De-oxit, Maplin switch cleaner or WD-40

Dave.

That's fixed it!!!!!!!!!

Wow! So it was such a simple thing in the end! After all that! Sorry about the wild goose chase folks but I have to say I had fun trying to solve the problem. Thank you all for your help - namely Dave, Will and OdBBQl. It really is much appreciated!! I have learned a lot this week.

Dan
User avatar
Dan LB
Frequent Poster
Posts: 639
Joined: Sat Jan 28, 2006 12:00 am
Location: Wicklow, Ireland

 


Re: Hum Problem on Ashdown Bass Amp

Postby Folderol » Fri Jul 15, 2011 7:51 pm

If I had £1 for every 'simple' fault that ran me round in circles I'd be a rich man indeed!

Well spotted Dave. I tell you, we'd be masters at a pheasant shoot - one of us starts the birds, the other shoots them down
User avatar
Folderol
Jedi Poster
Posts: 4694
Joined: Sat Nov 15, 2008 12:00 am
Location: Rochester, UK

Save paradise, Pull up a parking lot!


Re: Hum Problem on Ashdown Bass Amp

Postby ef37a » Fri Jul 15, 2011 8:10 pm

Folderol wrote:If I had £1 for every 'simple' fault that ran me round in circles I'd be a rich man indeed!

Well spotted Dave. I tell you, we'd be masters at a pheasant shoot - one of us starts the birds, the other shoots them down
Quite a common problem Will but one the owner rarely spots because they tend not to crank amps with no input* whereas it is standard noise test for B's. NOT that the jacks were crap! No, I suspect that they got a pretty hard life, not all plugs are bang on 0.25inches and I dare say the leads got wrenched about a bit.

Pheasant shoot W? Do you know I have shot all the common pistol calibres from 0.22 to 44mag' but never shouldered a shotgun!Never had the urge TBH.

*Shameless plug: The Series one and HT range of amps bias off the op valves unless a plug is present at speaker and input jacks so no hum would be heard anyway.

Dave.
ef37a
Jedi Poster
Posts: 7200
Joined: Sun May 28, 2006 11:00 pm
Location: northampton uk

#They did not listen, they are not listening still...Perhaps they never will?#



Who is online

Users browsing this forum: No registered users and 1 guest