messa mkiv and capacitor problems

[mellowamoya]

hi all,
i'm having a bad time with a messa boogie mk iv repair, but the main topic of this post is to see if my theory with regard to capacitors is correct. i replaced a number of components in the amplifier with new but when i started testing the amp it would red plate one pair of tube on checking 115ma grid current and grid voltage down to 14 volts, the other two output tubes were 42ma and 46 volts on the grids i removed and checked all components in the grid bias circuit but all checked out to be in tolerance. after much messing around i changed C243,C244 now my bias was the same on both pairs, on checking the capacitors they both checked out fine on my capacitance checker 98nf and 95nf, so i tried them one at a time back in the circuit and one would cause the low bias voltage and high current.
so why did they test fine on the tester? i then thought about testing at higher voltage than what a capacitance tester can test at the only thing i could think of was my balco insulation tester this goes up to 1000v when i tested the good one it showed infinite resistance up to 500v but the one that was causing the problem was fine at 50v and 100v but as soon as i tested at 250v it showed 50k and at 500v nearly a short these are .1uf 600v orange drops.
i had purchased 10 of these so i decided to test them all by the same method all showed correct on the capacitance meter but 3 more failed the insulation (mega) test.
any comment would be much appreciated.
thanks
dennis

[g1]

Sounds like you have found a good method for checking caps, by doing both tests.
Regular capacitance meters will not detect high voltage faults as they only test at lower voltages.
Capacitors that measure ok on a cap meter but fail when higher voltages are applied are not as common as leaky caps, but you still come across them occasionally.
If you had 4 out of 10 new caps fail, I would find another supplier. But make sure the high voltage tester is not putting out more voltage than it says it is.

[mellowamoya]

caps

Sounds like you have found a good method for checking caps, by doing both tests.
Regular capacitance meters will not detect high voltage faults as they only test at lower voltages.
Capacitors that measure ok on a cap meter but fail when higher voltages are applied are not as common as leaky caps, but you still come across them occasionally.
If you had 4 out of 10 new caps fail, I would find another supplier. But make sure the high voltage tester is not putting out more voltage than it says it is.

hi g1.
thanks for the reply yes the balco(mega) does put out that voltage and i believe that 600v caps should be able to withstand 1 1/2 time its rated voltage i will do some destructive testing on the remaining caps as i don't trust any of them now, and yes i will stick to a recognized supplier and not buy cheep.
thanks
Dennis

[cyclone]

You should buy an old Sprague TO5or heath or Eico bench capacitance meter that can test capacitors at rated voltage. Fix it up, its fun and a challenge and the only way you can get what you truly need. It only takes a few seconds to test a cap. I test EVERY SINGLE one at rated voltage. You really need old heavy duty test equipment to build and test vacuum tube circuits properly. A 9v battery obviously can't supply enough potential to test the insulation resistance of a 630 volt cap.

I know a noted amp builder that has rave reviews in Premier guitar, accolades etc. He is clueless when it comes to electronics theory, he cuts and pastes together circuits and sticks them together with solder. Mojo makes him pretty boxes to put them in. So, its not necessary to test ANY component to be successful (i.e. giving away amps for parts cost) but it is if you want to be consistent and if you want to truly understand what you are accomplishing. I showed him a pic of my test equipment, he didn't know what it was, any of it. No scope, all he has is a multimeter. So, you can DO it, you just can't do it well or safely or consistently without the right equipment. If you are selling stuff, you dang sure should check caps, transformers for leakage and manufacturing defects before you burn someones house down My lawyer said an LLC will not protect you from liability for stuff like that.

[patlaw]

My lawyer said an LLC will not protect you from liability for stuff like that.

Your lawyer is incorrect. Product liability is one key reason for forming an LLC to insulate the members from personal liability. However, the right thing to do is to buy adequate insurance that allows you to compensate a customer if you burn down his house.

[Leo_Gnardo]

Your lawyer is incorrect. Product liability is one key reason for forming an LLC to insulate the members from personal liability. However, the right thing to do is to buy adequate insurance that allows you to compensate a customer if you burn down his house.

In USA I'm sure you're correct. Our OP is in Trinidad WI, not Wisconsin but West Indies. Still a good idea to have insurance plus whatever legal/corporate protection you can get. And parts that don't fail.

[mellowamoya]

thanks cyclone im looking for a tester now it will be very useful still like to know if my method is ok.
leo yes we do not have the sort of problems that they have in the usa. people may say its lax but its just relaxed! so don't need a lawyer sat in my back pocket.
we don't need things like licensed techs to gas up your refrigerator as an example
thanks
dennis

[cyclone]

Your lawyer is incorrect. Product liability is one key reason for forming an LLC to insulate the members from personal liability. However, the right thing to do is to buy adequate insurance that allows you to compensate a customer if you burn down his house.

Well to be honest his speciality is not business law I don't think he really understands what I do. Very good point about the insurance, I wonder what that cost would add to the price of an amp? Its already a money losing venture given the cost of parts and amount of labor involved. Hijack off.

OP: I guess it might be tough to source one of those meters down there. I'd look for an Eico or Heathkit because of the excellent instructions and schematics, layout drawings and troubleshooting steps. If there's a Ham radio fest down there, good place to look.

I know I've found HV cap test equipment DIY project stuff in some old electronics books. I'll see if I can find them.

[mellowamoya]

Which to choose

Well to be honest his speciality is not business law I don't think he really understands what I do. Very good point about the insurance, I wonder what that cost would add to the price of an amp? Its already a money losing venture given the cost of parts and amount of labor involved. Hijack off.

OP: I guess it might be tough to source one of those meters down there. I'd look for an Eico or Heathkit because of the excellent instructions and schematics, layout drawings and troubleshooting steps. If there's a Ham radio fest down there, good place to look.

I know I've found HV cap test equipment DIY project stuff in some old electronics books. I'll see if I can find them.

I have the choice of to5 or HealthKit it-28 which is the better
Thanks

[g1]

The sprague TO-5 has a meter as well as the magic eye tube, the heathkit IT-28 does not have a meter.
The sprague can also tell you transformer ratios.
I don't know which is better, my preference would be the sprague.
Tube-era capacitor analyzers - Sprague Tel-Ohmike
Tube-era capacitor analyzers - Part Two; Knight, Eico, Lafayette, Heathkit

[Tom Phillips]

Of the two choices you have, I vote for the TO-5 too.

[trem]

Is there a modern day piece of equipment that does what the...
Sprague TO-5...does, but does not cost a Whole Bunch (less than 500 bux) of money.?
Thank You

[cyclone]

I have a TO5, replaced the power cord, couple of electrolytics and a couple of others including the calibration caps which are odd values, easiest to make up a couple of "gimmick" caps to calibrate it. Turns ratio measurement works and is handy but I don't usually use it because I usually want to make other measurements on an xformer that necessitate rigging up an ac source and measuring the step up or down, etc. for impedance calculation, identifying taps etc.

Shotgun replacement is the sensible route but expensive and sometimes invasive, you can reform and test old electrolytics, a surprising number of them are good to go. If they are bulging and leaking they are usually bad and will load the meter down before you get close to working voltage.

After a while, you will learn which brands hold up and which ones don't. There are several types of coupling caps I will immediately snip out and replace without testing, others I will not test in initial troubleshooting because they are usually good.



Its a really nice tool to have and the insulation resistance test is cool, you will find WAAAY more leaky caps than you ever thought you would in old but "working" amps. It is also helpful in not "over-repairing" an amp for a player.

[mellowamoya]

Thanks for all the advice i bought the TO5 should be here the end of the week, Then i can get to the messa amp and find out what is wrong I'm sure it more trouble with capacitors never had so much problem with any other amp. I'm hoping the JJ tubes i purchased are not part of the problem I'll have to get another set of tubes for testing.

[cyclone]

Any luck with your TO5? let me know if you need voltage or resistance tests of a working one to get it going.