Electronics Repair (sci.electronics.repair) Discussion of repairing electronic equipment. Topics include requests for assistance, where to obtain servicing information and parts, techniques for diagnosis and repair, and annecdotes about success, failures and problems.

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Default Mark Bass Combo Head, CMD 121H, of 2005

After half an hour of use a loud consistent pitch , low frequency buzz
Proper solder, from what I've tested .
Mark Bass/ UK have "repaired" this before, changing the tweeter. Even the
owner knows this is ******** as he has a recording of the buzz from the main
speaker, before and after that "repair" .
I've heard it and if it was a conventional power supply I would have said it
was a loose main rail electrolytic or bridge rect and consequental 150 Hz
buzz (uk mains of 50 Hz).
I've heard this recording but have not been able to measure the frequency ,
so may not be 150 Hz.
Of course now its out of the cab and opened out on the bench, no problem
with it, with dummy load, directed hot air over PS/PA and PREA and
"twizzling"/ targetted wrenching. Not brought out the big-gun yet, engraving
tool with nylon active end , and headphones (because of horrendous noise)
monitoring of output.
But as a thought experiment. What sort of fault in a SMPS amp could produce
such a 150 Hz buzz? Anyone ever known a problem with the HV DC to the SMPS
producing interference on the PA rails?
When its occured with the owner , no amount of kicking the cab would make it
stop or even change character. I will have to check with him if the buzz is
total or buzz over the guitar output as recording by owner was obviously not
playing at the time. And try and get a sound file / copy of his recording to
measure the f
ps
the ident of the SMPS driver is ground off , as per usual , it would seem


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Default Mark Bass Combo Head, CMD 121H, of 2005

Not heard back from owner yet, so no sound file yet.
Attacked with engraver and from that , can't imagine the G-forces involved,
passed A1. Whenever I do this with other amps there is always some
microphony showing up with one or more capacitors, but not with this amp.
But...
some things that are not right, if not actually wrong.
The main ps heatsink that carries the switching transistors, has two VR 7812
and 7815 with uninsulated tabs. The second pin of each is taken to ps ground
but also via the tabs and one Allen bolt fixing to the ps ground. Well it
would if not loosened to finger tight , requiring 1.5 turns to tighten. The
other fixing bolt is not bonded. Could some high frequency be induced in the
h/s just by proximity/miniscule leakage of those transistors and affect the
regulator/s ?

The 2 mains caps 200V, 1000uF are 105 deg C and measure excellent ESR and C
cold and hot, but one is slightly domed in comparison to the other. These
caps have ant-vibration mechanical cross bonding to each other with hightemp
hot melt but also fixed and so thermally connected to this heatsink. I can
see that bridge being cut away . Could a problem in one of those caps cause
the owner-observed fault.

The PA is grounded to chassis , but not the PREA, umbilical ribbon
connection only, input is plastic with no grounding ring.
With all the guts of the amp removed from the case then touching the Master
vol pot stem induces a lot of hum. There is no scraping away of paint on the
case for this pot , so could be making and breaking contact .


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Default Mark Bass Combo Head, CMD 121H, of 2005

Hearing it again I would say its more open looped amp motorboating. Cant
analyse for frequency etc until tomorrow. Needless to say have not been able
to induce any noise like this.

Further info from the owner
" It's whenever there isn't a signal that the buzz persists. I first noticed
it when i stopped playing in the middle if a song and i heard this murmur"


140K MP3 , 5 second file, is (theoretically ) now on/off this host site
http://www.diverse.4mg.com/amp buzz.htm

(minimal htm page to get round "forbidden" remote linking stuff )






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Default Mark Bass Combo Head, CMD 121H, of 2005

On Jun 18, 8:03*am, "N_Cook" wrote:
Hearing it again I would say its more open looped amp motorboating. Cant
analyse for frequency etc until tomorrow. Needless to say have not been able
to induce any noise like this.

Further info from the owner
" It's whenever there isn't a signal that the buzz persists. I first noticed
it when i stopped playing in the middle if a song and i heard this murmur"

140K MP3 , 5 second file, *is (theoretically ) now on/off this host sitehttp://www.diverse.4mg.com/ampbuzz.htm

(minimal htm page to get round "forbidden" remote linking stuff )


Your search - buzz site:www.diverse.4mg.com/amp - did not match any
documents.
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Default Mark Bass Combo Head, CMD 121H, of 2005

spamtrap1888 wrote in message
...
On Jun 18, 8:03 am, "N_Cook" wrote:
Hearing it again I would say its more open looped amp motorboating. Cant
analyse for frequency etc until tomorrow. Needless to say have not been

able
to induce any noise like this.

Further info from the owner
" It's whenever there isn't a signal that the buzz persists. I first

noticed
it when i stopped playing in the middle if a song and i heard this murmur"

140K MP3 , 5 second file, is (theoretically ) now on/off this host

sitehttp://www.diverse.4mg.com/ampbuzz.htm

(minimal htm page to get round "forbidden" remote linking stuff )


Your search - buzz site:www.diverse.4mg.com/amp - did not match any
documents.

+++++

unfortunately owner's choice of file name, i convert spaces to undescore, I
should have changed it to _.
There is a space between buzz and amp in the file name

I don't know how fidele this recording is, phone - email - MP3
player -DSO
but I make that a principal repeat of 103 Hz and next most observable repeat
of 475 Hz.
So is it mains initiated ?
103100 , I will assume that is twice mains frequency as principal
component




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Default Mark Bass Combo Head, CMD 121H, of 2005

spamtrap1888 wrote in message
...
On Jun 18, 8:03 am, "N_Cook" wrote:
Hearing it again I would say its more open looped amp motorboating. Cant
analyse for frequency etc until tomorrow. Needless to say have not been

able
to induce any noise like this.

Further info from the owner
" It's whenever there isn't a signal that the buzz persists. I first

noticed
it when i stopped playing in the middle if a song and i heard this murmur"

140K MP3 , 5 second file, is (theoretically ) now on/off this host

sitehttp://www.diverse.4mg.com/ampbuzz.htm

(minimal htm page to get round "forbidden" remote linking stuff )


Your search - buzz site:www.diverse.4mg.com/amp - did not match any
documents.





+++++
I've added an underscore version there of both
http://www.diverse.4mg.com/amp_buzz.htm

maybe an oscillogram there later today




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Default Mark Bass Combo Head, CMD 121H, of 2005


"N_Cook" wrote in message
...
spamtrap1888 wrote in message
...
On Jun 18, 8:03 am, "N_Cook" wrote:
Hearing it again I would say its more open looped amp motorboating. Cant
analyse for frequency etc until tomorrow. Needless to say have not been

able
to induce any noise like this.

Further info from the owner
" It's whenever there isn't a signal that the buzz persists. I first

noticed
it when i stopped playing in the middle if a song and i heard this
murmur"

140K MP3 , 5 second file, is (theoretically ) now on/off this host

sitehttp://www.diverse.4mg.com/ampbuzz.htm

(minimal htm page to get round "forbidden" remote linking stuff )


Your search - buzz site:www.diverse.4mg.com/amp - did not match any
documents.

+++++

unfortunately owner's choice of file name, i convert spaces to undescore,
I
should have changed it to _.
There is a space between buzz and amp in the file name

I don't know how fidele this recording is, phone - email - MP3
player -DSO
but I make that a principal repeat of 103 Hz and next most observable
repeat
of 475 Hz.
So is it mains initiated ?
103100 , I will assume that is twice mains frequency as principal
component




By ear, I make that 100Hz and 500Hz.

But my ears are a bit old.



Gareth.


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Default Mark Bass Combo Head, CMD 121H, of 2005

Gareth Magennis wrote in message
...

"N_Cook" wrote in message
...
spamtrap1888 wrote in message
...
On Jun 18, 8:03 am, "N_Cook" wrote:
Hearing it again I would say its more open looped amp motorboating.

Cant
analyse for frequency etc until tomorrow. Needless to say have not been

able
to induce any noise like this.

Further info from the owner
" It's whenever there isn't a signal that the buzz persists. I first

noticed
it when i stopped playing in the middle if a song and i heard this
murmur"

140K MP3 , 5 second file, is (theoretically ) now on/off this host

sitehttp://www.diverse.4mg.com/ampbuzz.htm

(minimal htm page to get round "forbidden" remote linking stuff )


Your search - buzz site:www.diverse.4mg.com/amp - did not match any
documents.

+++++

unfortunately owner's choice of file name, i convert spaces to

undescore,
I
should have changed it to _.
There is a space between buzz and amp in the file name

I don't know how fidele this recording is, phone - email - MP3
player -DSO
but I make that a principal repeat of 103 Hz and next most observable
repeat
of 475 Hz.
So is it mains initiated ?
103100 , I will assume that is twice mains frequency as principal
component




By ear, I make that 100Hz and 500Hz.

But my ears are a bit old.



Gareth.



With this amp and the Return Effects bypass switch to ground. If it breaks
it does not kill the amp but is highly sussceptible to injecting hum into
the amp. There are 6 chances of a break in that line.
That switch
a simple .1 in space 2 of 3 pin jumper in series with it
4x IDC ribbon headers, on two ribbons, links to the PA
Although , as received , the headers and ribbons were made, there was very
little force required to separate them (as well as no glue across them). I
will go with opening of this line somewhere as the fault and something to do
with none of the PREA is directly grounded to chassis, with or withoud
Ground-Lift

Perhaps just replacing input and EFF-Ret socket and a set to these header
pins plus glue spots.
Will try a heavy throughput with amp in the chassis this afternoon , in case
a more electronic type fault.
I don't like the way its consistenetly half-hour in and no amount of kicking
or tugging will change the buxx once it starts/


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Default Mark Bass Combo Head, CMD 121H, of 2005

It seems the ps h/s is heated by the internal air as it is near the fan,
rather than cooled, but presumably within allowable parameters, cannot see
any retrofit way around that.

So with the top removed and replaced with a large book, a thermometer over
the PA hs and another over the ps hs and one in the fan exhaust, ambient 20
deg C

All controls at mid and 4R dummy load
Initially 12.04V ac out for 6 minutes and then reduced to .1V input/ 400 Hz,
and 7.47V ac out,
reducing input to 1/10 while removing top and reading the thermometers for
that 30 seconds or so and loss of internal heat
6 minutes , 11.93 Vac, pa 72 deg C, ps 33 , exhaust not yet monitored
input reduced
15 m (6+9), 7.37V, 72, 45, 33 deg C
23m, 7.34, 72, 47, 36
30m , 7.33, 69,50,36
40m,7.32,77,52,37
50m,7.32,76,53,37
55m , 7.32,75,53,37

I reckon if the ps hs was not being heated by the internal air it would have
been only 35 to 40 deg C

No sound distortion emerging . So will just make those socket changes and
few paliatives like removing the paint on the chassis around the pot bushes
and resolder the usual suspects


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Default Mark Bass Combo Head, CMD 121H, of 2005

On Jun 19, 1:14*am, "N_Cook" wrote:
spamtrap1888 wrote in message

...
On Jun 18, 8:03 am, "N_Cook" wrote: Hearing it again I would say its more open looped amp motorboating. Cant
analyse for frequency etc until tomorrow. Needless to say have not been

able
to induce any noise like this.


Further info from the owner
" It's whenever there isn't a signal that the buzz persists. I first

noticed
it when i stopped playing in the middle if a song and i heard this murmur"


140K MP3 , 5 second file, is (theoretically ) now on/off this host


sitehttp://www.diverse.4mg.com/ampbuzz.htm



(minimal htm page to get round "forbidden" remote linking stuff )


Your search - buzz site:www.diverse.4mg.com/amp- did not match any
documents.

+++++
I've added an underscore version there of bothhttp://www.diverse.4mg.com/amp_buzz.htm

maybe an oscillogram there later today


Just sounds like something's ungrounded, like when you plug a 1/4 in.
phone plug into a jack, before the ground connection is made.


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Default Mark Bass Combo Head, CMD 121H, of 2005

spamtrap1888 wrote in message
...
On Jun 19, 1:14 am, "N_Cook" wrote:
spamtrap1888 wrote in message

...
On Jun 18, 8:03 am, "N_Cook" wrote: Hearing it again

I would say its more open looped amp motorboating. Cant
analyse for frequency etc until tomorrow. Needless to say have not been

able
to induce any noise like this.


Further info from the owner
" It's whenever there isn't a signal that the buzz persists. I first

noticed
it when i stopped playing in the middle if a song and i heard this

murmur"

140K MP3 , 5 second file, is (theoretically ) now on/off this host


sitehttp://www.diverse.4mg.com/ampbuzz.htm



(minimal htm page to get round "forbidden" remote linking stuff )


Your search - buzz site:www.diverse.4mg.com/amp- did not match any
documents.

+++++
I've added an underscore version there of

bothhttp://www.diverse.4mg.com/amp_buzz.htm

maybe an oscillogram there later today


Just sounds like something's ungrounded, like when you plug a 1/4 in.
phone plug into a jack, before the ground connection is made.

+++

When I've a bit of time , will add a few more sound snippets to be able to
direct owners to
Any more to add to list of , simulated or actual , faults ?
"static" excessive amp hiss noise
howl-round / squeel
simple mains hum
mains buzz from failure of bridge rectifer
clipped signal
one polarity only of push-pull output
"motorboating" of open-looped amplifier
compression/ lack of compression



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Default Mark Bass Combo Head, CMD 121H, of 2005

I think I will try a baffle in this amp to keep the PA heat away from the
PREA and interboard headers etc . Those ribbons were surprisingly hot from
that high power test. Obviously if on repeating that high power test , the
PS h/s climbs excessively then my thinking is suspect and can easily revert
to as "designed". Incidently this amp , no load, SMPS runs at 380KHz , much
like the other Mark Bass that required serious work to a month or 2 back


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Default Mark Bass Combo Head, CMD 121H, of 2005

I don't know what the extraction force should be , whether this is good or
bad.
Decided to pull out the PREA-PA 10 way ribbon IDC connector, when cold, one
took 550 grams , the other at least twice that. Going by how hot these
connectors got I would not be at all surprised one of the spring metal
contacts relaxed , either in this ribbon link with loss of a DC rail or the
front to back ribbon board carrying the EFF-RET


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Default Mark Bass Combo Head, CMD 121H, of 2005

the weaker header conn was the one further into the heatflow from the PA
heatsinks


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Default Mark Bass Combo Head, CMD 121H, of 2005

another one is the ting-ting noise introduced by an audio valve/tube with
C/H leakage and then highly susceptible to vibration, can be self-induced
via combo speaker vibration, giving those tings




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Default Mark Bass Combo Head, CMD 121H, of 2005

I think that fireproof card will stay in there. A long half-swastika one
along the front edge of the front PA H/S across the amp and to rear of the
ps H/S and a smaller L-shaped one along the rear of the rear PA H/S. That
one removing the free path run straight to the fan, ie forcing air through
the h/s vanes.
The other one stopping short of the inlet grill to allow some unobstructed
cool air along the long run over the PREA and the ps to the fan

Todays results, adjusted down for an ambient today of 22 deg C, and starting
with similar 7.5 V ac over 4 ohm
5min , PA temp 64, PS temp 26 , exhaust 25 deg C
13 min, 74, 37, 35
fan running noticeably faster by now
20m, 74, 40, 37
30m , 74, 42,37
40m , 74, 44, 38
50m , 74, 45, 38
considering the stem of the ps hs thermometer I had to make a hole in the
card and the end of the stem was then in hotter air than the bulb end temp
maybe. The interboard connectors etc were noticeably not so hot. Makes
thermal runaway to destruction of the SMPS less likely if the amp is driven
excessively hard, hopefully. As far as I can tell with these amps , thermal
monitoring is only on the PA heatsink , not the ps one

yesterdays figures
15 m , 72, 45, 33 deg C
23m, 72, 47, 36
30m , 69,50,36
40m,77,52,37
50m,76,53,37
55m , 75,53,37


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