Moscode 600 hum/turn off thump

[MattV]

Hello all,

This is my first post/question. I am choosing the tube forum for this question even though the amp is a hybrid because I know you are all highly intelligent and tasteful people.javascript:smilie('')

A (good!) friend of mine recently gave me a Moscode 600 amplifier which I have listened to on a
couple of occasions but have experienced a fairly loud 60 cycle hum and a loud turn off thump. I know my friend retubed the amp not long before he gave it to me so I don't think that is the problem. Do any of you own this amp and have experienced this problem? If so, is it fairly easy to fix or should I seek professional help(no jokes)?

If I do have to have it repaired I would like to do so in the Phoenix, AZ area as it is extremely heavy. Does anyone know of a reputable repair shop which could handle this beast?

Any help you give is greatly appreciated. It seems a shame to leave this amp languishing in the closet.

[dentaudio]

It appears the power supply electrolytic caps need replacement. I experienced the same problem on a friends Moscode 300 amp. Same values, same or higher voltage rating.

For the popping on turn off check if the rocker switch has a ceramic AC rated cap attached.

[MattV]

DentAudio,


Thanks for the info. Any idea where I can find those power supply caps? I looked on the Moscode website's parts section and they only have the caps for the high voltage supply(or are those the caps you are referring to?).

There is a ceramic cap strapped across the power switch. The switch was arcing when I first turned it on after a long period of not being used so I replaced the switch. Perhaps the cap was damaged when this happened causing the turn off thump.

I'll keep surfing around and see if I can find these gigantic caps.

[quinnling]

MattV
The thump is normal for a Moscode 600, but not the buzz.

This guy mods and fixes Moscode in AZ.
http://www.thuntek.net/~bk11/home.htm

Quinnling

[MattV]

Quinnling,

Thanks for the input on the amp. I do remember my friend using this amp (circa 1991) on a pair of Mirage speakers and there was no thump at turn off. I did follow your link to the repair shop in Tucson but all of the links to the various portions of his website seem to be dead. Maybe I'll try giving him a call later. If all else fails I'll follow the "parts replacer" credo of electronics repair and put in new power supply caps and hope that solves the problem. I think I found some reasonable replacements on the Newark website for about $30 each.

[MattV]

If anyone is still looking at this thread that might know;

Can I replace the stock 25,000 mfd 100 vdc caps in the power supply with 39,000 mfd versions or will this cause problems?

[quinnling]

It should not be a problem as long as the voltage is the same or Higher.

BTW, My Moscode 600 and 300 always thumped when the amp turned off. I notice the amount that it thumped depends on the speaker. George Kayes has a modification on the protective relays that caused, I'm not sure if mod removes the thump of not.

[KBK]

I put in a set of 47kuf CDE caps,whcih are a perfect fit. These are fine. This was in the last version of the 600 that came out, the one with the rifa(?) film caps beside the big cans. I moved him up to a 50A 600V rectifier at the same time, the original being 25A or 35A rated

The thump may be the relay circuit not functioning properly, as the relay kills the signal ridiculously fast in that amp, when shutting it down. I don't like the relay being there, but I've seen it save many a speaker at my friend's place where cab drivers sit outside and key their mike on their radio..and he has a full tube system, except fo the moscode. The +400W rf-AM-af blast through the system is not pretty. The Moscode kills it via the relay before it gets through. It may be that the voltage sensing system for theis protection circuit is failing in some manner, the relay might be sticking, etc. If the relay does not fire immediately at the moment when you hit the power switch to the off position....then there may be your issue. (I'm talking about somewhere in the area of a 1/4 second or so after turn off..that relay should slam shut) Dirty contacts on the relay can also allow turn on thumps to come through, as it's job is to cut the tube circuit out of the hitachi fet's input until the tube circuit has stabilized..and to shunt the Hitachi MOSFET power circuit to ground...to keep it quiet and stable..this..until the tube front end is ready and stable. This takes about minute, via a timer circuit. If that relay or it's socket is dirty, some signal may be getting through to the MOSFETs at turn on. This is likely your 'thump' culprit.

Clean and tighten the plugs for the circuitry as well..and consider getting rid of the plugs altogether, via new wiring and solid-soldered connections. It makes a big difference.

The amp also likely needs a bit of DC and offset checking/resetting, as the amp has tendacy to drift over the long term. It likely needs new tubes as well. There are two regulator tubes in there, that should be sourced as soon as possible, they are becoming fairly rare. Use matched sets if you can, of course. Grab 4 or so of those regulators, if you can. That particular tube will never be manufactured again.

IIRC, the entire board is 'floated' via connectors, so no stable low-impedance ground is actually available..thus the potential rises for thumps at turn on. Just a guess.

[Mr. Moscode]

As the Moscode designer and inventor I think I can shine some light on this subject.

First, The turn off thump. On a stock 300 or 600 there is a time delay relay in series with the speaker. In both amplifiers it is controlled by a LM555 timer. It remains open for 45 seconds, disconnecting the speaker in order to allow the tubes to settle to their operating voltages. Without the relay you would get a rather large woofer excursion which could damage the woofer.

In the 600 there is also a DC offset detector that will disconnect the speaker for about 3 seconds if there is more than a 4 volt offset for more that about a tenth of a second. I see in one of the posts a 600 is picking up some mobile radio signals. This may be causing the circuit to trigger as the RF signal may be being fed into the amp through the speaker leads. Remember the negative feedback circuit is also an input. Or the RF signal may be directly triggering the safety circuit.

In any case if there is a small amount of DC offset in the output circuit you will hear a thump when the amp is turned off. There is a small amount of time delay before the relay disconnects when the amp is turned off to keep the speaker connected during a brownout so that the timer circuit doesn't go through its 45 second warm up mute. During this time the High Voltage power supply is discharging and that may cause a transient that gets through to the speaker before the relay drops out.

My upgrade to the amp eliminates the relay in series with the speaker and replaces it with one that shorts the drive from the tube circuitry to ground during warm up. This mod takes the relay out of the circuit while the amp is playing and eliminates the signal degradation caused by the relay contacts and relay socket. However because the relay shorts the gates of the Mosfets to ground during warm up and turn off the mosfets are not fully biased on until a few seconds after the relay opens (disengages) and the bias shifts which causes a pop on turn off.
In all cases the turn off thump is benign and will not harm the speaker unless there is a fault in the output stage-blown mosfet for instance.
I was able to design this transient out of the Moscode 401HR and the soon to be released 402Au.

The hum you are experiencing is most likely due to a faulty high voltage regulator circuit or defective filter caps for the hi voltage regulator. There is a high voltage test point on both sides of the board and it should measure around +320 volts. If it is high, like around 450 volts the regulator is not working. If it is low then probably the filter caps are defective. If you are technically adept you can replace the 2 800 mfd filter caps in the middle of the board.

There is also a possibility there is a bad input ground in your wiring or a heater cathode short in one of the tube, most likely the ones on the outside edges of the 600 or the 2 in the middle of 300.

I hope this helps. If you need more help contact me through my websites.
I service all the classic Moscode products and you can find my service site at http://moscode.home.att.net/ or www.Moscode.com which has a link to the service site.

[Rob11966]

As the Moscode designer and inventor I think I can shine some light on this subject.

Classic opening line Mr Moscode