Mark IVa help needed, Chronic Parts Failures!

[Monsta-Tone]

This was originally a PM to BoogieBabies, but I thought I should post it here, just in case someone else has had this issue....



I recently picked up a Mark IVa combo (serial # in the 700's, can't remember what it actually is though). The guy only wanted $700 because it was not working right.

It wasn't working, but I thought I would have a pretty decent amp for a decent price if I got it up and running......
Originally, it had leaky PI coupling capacitors. Both of the .047 caps were leaking DC, which was causing the power tubes to red plate.
I replaced the caps and the tubes and the amp sounded great. For a week......

The gain pot stopped working on the lead channel. Then, the tone pots stopped working. I figured it was leaky tone stack caps, and sure enough, it was. I ended up replacing 3 of the 4 orange drop caps in the tone stacks and the amp worked fine again.....for a week. (I'm starting to see a pattern here.....)

It started making a horrendous crackling sound and had a lot of white noise. I ended up chasing my tail because I had already cleaned and re-tensioned the power tube sockets.
When I finally got around to that being the only thing left to check, I re-cleaned them and re-tensioned them and the sound went away and the amp sounded great again......for a week.

A buddy of mine called me up and said that he had my old PRS McCarty guitar and asked if I wanted it back. I told him that I was broke, and he asked about the Mark IV.
We set up a trade and he drove over to my house to check the amp out.
One of the inner power tubes shorted out and made a beautiful hum sound, just when he turned it on.....
I replaced the power tubes again and the amp sounded phenomenal for about 10 minutes. Then, it just kept sounding worse and worse and there was no volume.
This time, it was one of the plate resistors (the one related to the tube that died), no surprise there....
And the 82k resistor in the PI was open, that was a surprise.
The amp sounded phenomenal! Much better than it ever had since I owned it. I'm guessing these 2 resistors were slowly going and the power tube shorting out just finished them off.

So......I call the guy up and meet him in town (it's an hour drive from his place to mine). We make the trade, without him playing the amp.
He did not contact me that night. I figured he was up late playing his new baby.
The next morning, he calls me up and plays the amp over the phone.
Now.....
There is a horrible hiss in the preamp. It is noticeable on R1 & R2, but is unbearable in the Lead channel.
This noise is somewhere in the preamp section (not a tube) as the tone stack will barely affect it, but the gain pots will lower the volume of the noise.
And....now there is a huge popping sound when you use the footswitch to change channels. That was never there before!

I was planning on sending the amp back to Boogie because it has chronic failure issues, but with shipping and repairs, it would run me well over $600! It would be cheaper for me to just re-populate the board!
Has anyone ever seen a Mark IV (or any other Boogie) that has chronic parts failure?
I'm wondering if the guy I bought it from did something stupid to the amp or??????

Sorry for the novel, but any help would be incredibly appreciated.
Thanks,
Andy

 

[Monsta-Tone]

I'm now wondering if maybe the original fan (I changed the original one out because it was running a little slow, but it still worked) was not moving enough air and the amp simply got too hot over long periods of time?

I'm guessing that I will end up changing out lots of parts in anticipation of them failing.............

Anybody had this kind of issue before????

 

[mark2boogie]

Well, you may have tried this before, but if not, then :

1 - with the schematic on hand, measure all the voltages on different test condition,
2 - try some signal-tracing and/or signal-grounding to localize the faulty section,
3 - of course test the tubes (with a correct tube tester, not a compass) or at least try a swap with known-as-good tubes,
4 - have a look to the wiring and inspect the solder joints (cut, bad or even unsoldered are common).

You should be able to draw some conclusions...

I only had 3 MKIV for service these past years : they had some faultly preamp tubes (hiss, noise, microphonics, ring effect) and power tubes (dying by loss of their vacuum, which increases the plate current and so causes thermal runaway, and potential damage to the circuit like screen resistors), they also had faulty channel and functions switching (notably dying Vactrols, detected by measurements and signal-tracing), some non-soldered joints (discovered by a magnifier, by pulling on components...) , and some other idiosyncrasies tied to the complexity of these amps (notably many dirty switches and/or pots affected by cigarettes smoke residues).

MKIV are "houses of cards", and this kind of amp will probably not age pacefully...

I let BoogieBabies give you other ideas, if he has...

A+!

 

[Monsta-Tone]

Thanks!

This time, it was.......
Faulty power tube - sounded like the Columbia River coming out of the speaker

Noisy V3 preamp tube - slight feedback and crackling noise

Faulty Tone Stack capacitors - again - I ended up replacing all of the Tone Stack capacitors this time, not just the faulty ones.
- 2 of the brand new ones were leaking DC!
- I also replaced the 150k plate resistor for V1a - I am thinking that this resistor was getting hot and then shorting out. When I put my meter on the output side of the tone stack caps, there was 278vdc! The drawings show that there should only be about 210 on the input side of these caps!
- I also replaced the 1k 2 watt resistor in the power supply for point D because there was 452vdc instead of 378.

I've always wondered what would happen to amps like this when they got old......Makes me love my DC-5 more and more every day!

I played it for a few hours yesterday. The 1st hour, there were absolutely no problems. Then, slowly, a crackling sound started creeping in. It sounds like I have a bad solder connection on a preamp tube socket. Once it heats up, it starts to introduce a tiny bit of noise.
This noise is very bad with an open E string, but is barely noticeable with other notes.
The amp will also make the noise if I stomp on the floor next to the amp.
This noise is on all of the channels, so I'm guessing it's not V3 again......

I would like to get it in top shape, where it won't break every few days, then sell it or trade it off. I'm pretty sure that I can work all of the bugs out of it, but it's just so odd that there would be multiple parts failures (resistors and orange drops) in completely different parts of the amp.

 

[McBarry]

Firstly, never worked on a Mk IV, so I profess zero first-hand knowledge..
With such a wide range of faults, I'd suspect something affecting the whole chassis externally - like heat and the fan Monsta mentioned.

But another thing common to the whole amp is the power supply.

I'm going right outside the square here, but I wonder if there's an intermittent failure causing bizzare spike voltages, or parasitic frequencies on the supply or something else that is not gonna show with a touch of a DMM, but maybe a data logger would record it.

Another, less desireable scenario could be a PAST PSU fault, slowly failing components as they age.

I wonder Monsta, have any replaced components failed again?? - could point to a current cause rather than a past cause..

I know these ideas are a bit "out there", but the symptoms are equally bizzare if indeed they have a common root cause, OR if they are merely coincidental..

Also be very interesed in Ed's ideas..
Dave

 

[Monsta-Tone]

But another thing common to the whole amp is the power supply.

I'm going right outside the square here, but I wonder if there's an intermittent failure causing bizzare spike voltages, or parasitic frequencies on the supply or something else that is not gonna show with a touch of a DMM, but maybe a data logger would record it.

I have replaced the 1st 2 filter caps, and as many resistors in the PS as I have in stock. Have to order the rest, but was broke for the last 2 weeks.

Another, less desireable scenario could be a PAST PSU fault, slowly failing components as they age.

I wonder Monsta, have any replaced components failed again?? - could point to a current cause rather than a past cause..

Dave

This and current intermittent failure of the PS is what I've been thinking.....
The amp looked fairly virginal when I 1st got it. There were only slight signs of repairs in the past, and the solder joints on the PI caps (I had repaired this before the guy sold it to me).

As for replaced components failing....
I had replaced 3 of the orange drops in the tone stack with the last batch of repairs. 2 of them were leaking (or passing) DC this time. I replaced most of the parts around them this time, thinking that maybe the resistor values go way down when it gets warm.

The new crackling noise sounds like dirty preamp tube socket or failing preamp tube. I tried several known good preamp tubes in the amp on Saturday morning before playing it for a few hours. A few of them made this exact same sound. I will re-clean and re-tension the tube sockets sometime this week.
If that doesn't work, I'll take a look at plate resistors, etc.

Keep the suggestions coming! I really wanna like the amp, but so far it's just been a pain in the ***!

 

[mark2boogie]

Monsta-Tone,

The suggestions of McBarry are valid, particularly a parasitic noise coming from the mains : My local music shop had problems with that, sending me amps that I found working perfectly at home. The mains line of the shop proved faulty and polluted by neons or so...

Moreover, reading the description of the last issue you mention, I am akeen to think that there is a vibration sensitive problem in this amp, a common plague in hi-gain tube amps : maybe another faulty tube, but more certainly a bad contact somewhere, for example a power tube in his socket, which would need to be re-tigntened (it's a common thing), a bad solder joint, maybe tied to dilatation when the amps reaches it's nominal temperature... I can't list them all, as they are sometimes so odd and subject to intermittency, making them pure pest to find !

A tip that I frequently use - you may know it : I call it the "playing xylophone"... I take a drumstick and hit everywhere in the amp (sometimes quite hardly), while listening to the speaker, at different volume settings. Doing that, you often localize the source, or at least have an indication about it. During the test, of course turn knobs, switch channels and functions, etc... To help localization : the goal is to make the amp told you where is its issue, it's a kind of "mechanical signal-tracing".

A+!