Tube Tech MP-1A - blown fuses

[bram9206]

I have a question concerning the fuses that keep blowing in my new purchased, though second hand Tube Tech MP-1A.

At the last day of an intense two-week studio session (which means the Tube Tech was on 16 hours/day), the main fuse kept blowing, so I checked the other fuses in the power supply, only to discover the 80mA fuse was blown.
I now wonder why the 80mA fuse blew, because fuses don't just blow because they like it... Could it be the rectifier? Both D1 (B380C1000) and D3 (B40C1000) show some heat discoloration on the PCB, not much though.
And then there is R51 (does anyone know the correct value for this resistor?). This one seems to run very hot because the PCB looks really burned here. But R51 looks already replaced by a previous owner, so I'm not sure if it's still the correct value... looks like it was replaced because the original R51 must have been fried. Both traces to and from R51 are 'replaced' by a nice and neat wire, because the traces must have been too fried and therefor probably broken. Is it normal that R51 runs so very hot?

Also, the pilot light that is in the unit is a 36V, 1,8W. Isn't that a strange value? Or am I going too far now...? Just because R51 does have something to do with the pilot light, no?

Any advice is most welcome, but keep in mind that I can't measure any voltages because the fuses blow in 3 seconds after power up.

Thanks a lot!

 

[CJ]

hard to troubleshoot online without a schematic and i can not find one on the web,

can you take a pic of the guts?

could be heater supply diodes, if they use DC on the heaters, what happens is the tubes draw huge current until they warm up, so the rects gradually fry,

if this is the case, replacing the rects with oversize ones will solve the problem,

do not know why the 80 ma fuse blew, unless the diodes feed the B+ and are fried,

 

[bram9206]

I know, seems impossible to find the original schematic on the web...

Here are some pictures. Mind R51, which has been replaced by two resistors in series, and mind the heat marks.
Also, hard to see in the pictures, but all three filter caps don't have a flat top anymore, they bulge a little... Is this normal? Never seen electrolytics with a bulging top like that...

 

[gyraf]

Very good chance that your problem is worn-out electrolytic capacitors. Replace those three.

Jakob E.

 

[bruno2000]

Very good chance that your problem is worn-out electrolytic capacitors. Replace those three.

Jakob E.

+1 Change the caps.
Best,
Bruno2000

 

[bram9206]

Anyone knows a good European supplier for those caps? Can't find that value here in my local store in Belgium... Looking for 100uF radial electrolytics at 450V. Seems to be Panasonics... but now that I'll have to change them, would it be worth replacing them with a more expensive type? Like F&T caps...?

 

[mjrippe]

I know, seems impossible to find the original schematic on the web...

Here are some pictures. Mind R51, which has been replaced by two resistors in series, and mind the heat marks.
Also, hard to see in the pictures, but all three filter caps don't have a flat top anymore, they bulge a little... Is this normal? Never seen electrolytics with a bulging top like that...

Actually, those are plastic covers on the capacitors.  I have seen them in other Tube Tech gear.  Whoever replaced R51 did not clean the carbon off the PCB so that could be causing a leakage path, though it is probably higher resistance than the replacement resistors.

 

[gridcurrent]

I've serviced a few Tube-Tech units including a MP-1a.  The bridge powering the DC heater circuitry gets very hot,  suggesting the possibility of early failure.  Pull the bridge(s) out and power up.  If the defective one is for the heater supply,  heatsink it to the chassis.

 

[mikeyB]

Very good chance that your problem is worn-out electrolytic capacitors. Replace those three.

Jakob E.

+ another 1!
Repaired me mates tubetech summing amp.
Look at the top of the psu electrolytics buldging - need replacing!

 

[sodderboy]

I concur with the cap replacement and bridge suggestions. I give all TT's a recap and an up-rated current rating bridge with an added heat sink. I prefer a sink to chassis mount. Either is good.
The bubble plastic does not necessarily mean a bad cap. They are sealed so gas can get trapped and dome the plastic in a good cap. Besides the value and voltage, is there a 9387 type date code on the caps?

Mike

 

[bram9206]

Just to make this thread worth reading, here's the end of the story, the answere to my problem:

I did'n have the time to work on the TubeTech myself, so I sent it to an official dealer (with official service tech) and after some measurements he found out the mains transformer was dead after all... He replaced it with a brand new one and the unit is now back in working order. So that was it, smoked mains...
Anyone knows if this is a common problem with TubeTech gear? The unit was only about 10 years or 15 years old I think...

 

[outoftune]

i have serviced many tube tech units over the years (i work at a dealer)

i have only seen the PSU transformer need to be replaced in one case...all other problems were either tube related or PSU related (caps / bridge replacement solves most)

 

[mjrippe]

I had one on the bench that failed - after it was filled with water during Hurricane Sandy!  But a thorough cleaning and new power transformer got it up and running again.

 

[Nanitlig]

Anyone know the correct value of R51.  I'm working on MP-1A with the same problem.  R51 is toast and bridge got really hot.  All the cathode caps were over 40 ohms ESR, so I replaced those!  The plastic toppers on the 100uF 450V caps bubbled up, but cap measurement and ESR are within spec..

 

[gyraf]

R51 is 215 Ohms - dropping voltage for the front-panel pilot light. Replace with 220 to 330 Ohms /2w.

Ignore the marking in red in the schematic, it's just a correction for the original schematic.

Jakob E.

 

[mjrippe]

Anyone know the correct value of R51.  I'm working on MP-1A with the same problem.  R51 is toast and bridge got really hot.  All the cathode caps were over 40 ohms ESR, so I replaced those!  The plastic toppers on the 100uF 450V caps bubbled up, but cap measurement and ESR are within spec..

If you mean the black plastic discs (which can actually be removed), they often are bowed upwards while the aluminum top of the cap is still flat.  A lot of folks take this to mean that the cap is ruptured but this is not the case.

 

[Nanitlig]

Thanks for the info!  Also, Courtesy of Tony at Transaudio Group, early units SN 08065 and lower have an update to increase heater voltage.  D9 in PSU is replaced with a BZX79C22, and on each channel R52 (30R1) is removed and replaced with a wire jumper.  This increases the voltage for the heater supply to 37V.  Some newer tubes won't emit if the heater voltage drops below 11.5V.

This is the only Tube Tech unit that uses a 36V .05A lamp in the pilot, so don't do what my customer did and throw a 12V .1A bulb in.  It will try to catch itself on fire!  ;D

 

[Nanitlig]

I found some 24V E10 LED bulbs that are 420mW.  The secondary there is about 39VAC with the bulb unloaded.  The LED bulb only draws 9mA when used with AC supply.  To drop 15V, I come up with a drop resistor of ~168R.  1.34W + .42W(bulb) for 1.76W, so a 2W 168R for R51, does that seem legit?  Not 100% sure about my math on that.  Might be best to just stick with the original 36V 2W bulb.  ???

 

[Gene Pink]

I found some 24V E10 LED bulbs that are 420mW.  The secondary there is about 39VAC with the bulb unloaded.  The LED bulb only draws 9mA when used with AC supply.  To drop 15V, I come up with a drop resistor of ~168R.  1.34W + .42W(bulb) for 1.76W, so a 2W 168R for R51, does that seem legit?  Not 100% sure about my math on that.  Might be best to just stick with the original 36V 2W bulb.  ???

STOP! you might burn out the LED.

15 volt drop at 9ma

15 / 0.009 = 1.66 k

Gene

 

[Nanitlig]

Thanks Gene!  Decimal places matter!?  ???  Luckily I haven't done anything to the unit yet.  Is it correct to use the current of the bulb then in determining the wattage? 9mA on 1660R with a 15V drop across the resistor would be about .14W and adding the bulb at .42W would be .56W?  So a 1W 1.6K resistor should be ok?  Or, should I be using the full 39V supply voltage value to figure the wattage of the drop resistor?  Then it would be more like a 3W resistor.