Mark 3 Preamp Problems

[dwman3]

Ok so my Mark 3 has been in the local music store shop for over six months now and they just can't seem to fix it and i always get the same BS excuse from them that they always get a defective replacement part from Boogie like pot or something small like that and have to order another one.
Now the problem my amp has is that when you are plugged in the front and turn the channel up past a certain point it starts to attenuate and same thing happens to channel 2 and 3. I want to say its a preamp problem cause I'll plug into the effect loop return and the master volume works just fine from 1 to 10 gobs of clean headroom that we all know and love but plug into the front like normal turn it up and attenuation :x
So examining the amp (way before i took it in and let the tech guy know what was going on) the preamp tubes don't seem to light up like the power amp tubes or any other amp for that matter. Any one ever experience the symptoms that my amp has?

 

[McBarry]

Preamp (12ax7 here) and power amp tubes (6L6 or EL34 here) receive same heater supply but 6L6 is bigger, and heater filaments bigger so look brighter..

U could swap the pre's to another amp and observe the heater glow intensity, or photograph and compare..

Was the PI same also ie: dull glow..??

I'd be a little suprised if the heater supply was failing after the power tubes - MB traditionally used huge amt's of solder for this very reason.
Not saying it ain't so.. just seems unlikely IMHO..

The shop would/should be checking voltages and then signal chasing.. No rocket science here..

What actual parts are they replacing - this'll tell you what they're finding and/or thinking..

Sorry can't be more help...
Regs, Dave

 

[kdorsey]

Ok so my Mark 3 has been in the local music store shop for over six months now and they just can't seem to fix it and i always get the same BS excuse from them that they always get a defective replacement part from Boogie like pot or something small like that and have to order another one.

Where do you live? I want to avoid this shop!

Find someone you can trust or pack it up and ship it to Petaluma.

People don't thank you enough, McBarry, for your in-depth replies. I certainly appreciate the responses.

 

[stokes]

Did you try connecting the loop in and loop out jacks with a cable?Sometimes the shorting jack gets dirty and will not feed the signal thru,very common

 

[dwman3]

Preamp (12ax7 here) and power amp tubes (6L6 or EL34 here) receive same heater supply but 6L6 is bigger, and heater filaments bigger so look brighter..

U could swap the pre's to another amp and observe the heater glow intensity, or photograph and compare..

Was the PI same also ie: dull glow..??

I'd be a little suprised if the heater supply was failing after the power tubes - MB traditionally used huge amt's of solder for this very reason.
Not saying it ain't so.. just seems unlikely IMHO..

The shop would/should be checking voltages and then signal chasing.. No rocket science here..

What actual parts are they replacing - this'll tell you what they're finding and/or thinking..

Sorry can't be more help...
Regs, Dave

To be honest I'm not sure what they replaced, I know they replaced a pot maybe a cap or a resistor who knows. Is it possible to trouble shoot the heater supply on my own? And I had checked the amp with two sets of pre amp tubes the originals and new set all Boogie branded tubes.

 

[dwman3]

Ok so my Mark 3 has been in the local music store shop for over six months now and they just can't seem to fix it and i always get the same BS excuse from them that they always get a defective replacement part from Boogie like pot or something small like that and have to order another one.

Where do you live? I want to avoid this shop!

Find someone you can trust or pack it up and ship it to Petaluma.

People don't thank you enough, McBarry, for your in-depth replies. I certainly appreciate the responses.

Alamo Music Downtown San Antonio Texas

 

[McBarry]

Kdorsey.. U humble me.. just imparting what I can.. and if it helps.. all the better !!

Dwman3 - yes it's def possible to troubleshoot on your own, BOTH with chassis in amp and out on a workbench..
Stokes nailed a terrific trick - def worth a try..

A careful, methodical person can do this and stay safe, BUT clearly, working on a live chassis is inherently dangerous.
If U hear yourself saying "I think......" STOP. You're about to be vague/overly subjective. Try be as precise/accurate as possible.

If U paid money to the shop, they bloody well should tell U exactly wot they've done. Write notes. Don't confront then, but calmly get the info. U may not understand the lingo - we can easily help with that. Insist on the dead parts.
I know some folks are not user-friendly, but none of us set out to be clowns.
Lastly, (longish post,, sorry..), defective parts do happen. MB will likely twig something's amiss and start asking questions/helping out the shop.. Hoefully they're receptive enough to listen and heed any comments/advice..

Let us know how U progress.. Down Under is a little far to drop in.. but if yr Lear jet is idle I'd do it gratis as you've had some **** "help" thus far..
Dave

 

[dwman3]

So I got the amp back today and upon looking at it they put in a new power cord for some reason
and one of the EL34s is a Electro Harmonix not the Boogie tube I had in there and of course they threw it away, maybe it got fried some how when they were trying to fix it who knows. So the explanation I got was basically they couldn't figure out what was wrong with it and the dudes at Boogie couldn't help them and more or less that it was just a old piece of gear or something to that effect, but he did give me his card and told me to bring in anything else that needed to get fixed cause they can fix anything... just not my amp his words exactly. I am beyond pissed!!
So before I box it up and send it off to Boogie, McBarry how would I go about checking the heater and what tools would I need to this?

 

[stokes]

Before you go any further bridge the effects loop with a cable.

 

[Agustín Collia]

one of the EL34s is a Electro Harmonix

If there's only ONE Electro Harmonix EL-34 in there, I'd just ship it to Boogie instead of firing it up hoping to diagnose anything... Those turds at the store don't even seem to know that power tubes come in matched pairs, so only god knows if they modified the bias supply or if they just put that EH tube in there without even checking the bias... My god, and they say they can fix "anything"...

 

[McBarry]

I'm with Stokes- go simple stuff without pulling chassis and get it to a reputable tech or MB if that's plausible..

Def love the FX loop check, and then check basic function to get RELIABLE symptoms and report these to tech.

Heater colour is vastly misleading as diff tubes have diff physical design and so heaters look different.
I/we can walk U through checking heater voltages but truly, I'd suspect it's a red herring, and I'd bet the shop checked anyway. Hopefully.
Even if the voltages are OK, current checking needs desoldering and is a bit involved.. not to mention dangerous..

Regarding diff brand tubes - matching for current draw should be poss, but things like gain, transconductance - I'd seriously doubt they matched these..

Leave 'em in, get it to a hospital.
It needs a laparotomy and then whatever shows up.

A new power cord... FFS.. how magnanimous of them...

Sorry not wot ur wanting to hear, but def the safest course of action.. Dave