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 .

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 buzz.htm

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

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Your search - buzz site:

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Your search - buzz site:

- did not match any documents.

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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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noticed

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Your search - buzz site:

- did not match any documents.

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

maybe an oscillogram there later today

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

But my ears are a bit old.

Gareth.

Cant

undescore,

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/

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

I would say its more open looped amp motorboating. Cant

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amp_buzz.htm

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.

I would say its more open looped amp motorboating. Cant

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both

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

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

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

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

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

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