Fixing a Urei 565T

[HotBluePlates]

Passing along the below to give an example of how to diagnose & fix a (simple) fault in a solid-state circuit.

I picked up a Urei 565T ("Little Dipper") for a reasonable price because it wouldn't pass sound.

Before buying, I checked out the 565T Manual and its schematic.  I noticed that the filtered signal and Bypass path share some components: IC15, IC14, Q1, Q2 and T2.  The previous owner noted it had worked the last time he'd used it, then it suddenly wasn't working the next time he tried it.

Before I even bought the 565T, I'd decided the check-out procedure would be to verify proper voltages at C95 & C97, then check the shared components for the Bypass path.  After the main power supply, there are decoupling resistors and a local filter cap for every 4 IC/transistor stages.  For IC15 (the input gain stage for both the Bypass and filter circuits), these are C5, C6 and R11 for the +17v rail, and C9, C10 and R15 for the -17v rail.

I get the 565T and notice all electrolytic caps appear to be the original units from the 70's.  Nothing looked overly bad on a visual inspection (a good sign since solid state devices tend to fail in dramatic fashion; maybe I wouldn't have to buy any expensive, obscure Hi-Fi IC's).  I considered powering up the 565T on a Variac, but decided if the filter caps were truly failed the fuse probably would have blown.  And if they were gonna fail due to simply having power applied then I'd need to replace them anyway.  So switch flipped on sans Variac.

I proceeded to measure voltage at C95 & C97.  I got ~13v on the +17v rail, and ~-15v on the -17v rail.  Could this unit need voltage-balancing resistors on the bipolar supply, or was there another issue to resolve?  I thought I smelled a faint burning smell, but continued on to check the voltages on IC15.  I actually did this the hard way by looking up the pinout for the LF356H, but quickly got confused when I probed where I thought V+ should be but kept seeing 0v.

Then I did what I should have to start, which was to probe at C5 and C9 after the decoupling resistors.  I found 0v at C5's positive terminal.  After powering off, I checked the resistance from C5 to ground, and got ~0Ω at both ends.  Once I got a bright light on R11, what looked like a shadow cast from a nearby cap was revealed to be a burn mark around the middle of this carbon film resistor.  My guess was C5 failed short, dropping V+ for IC15 and IC1-3 to 0v, and pulling excessive current through R11 to burn it.  I couldn't rule out C6, but it was a small mylar cap and seemed less likely to fail than the electrolytic.

I clipped C5 off the p.c. board.  A resistance check of C5 showed ~0Ω confirming it was bad.  R11 surprisingly measured almost dead-on 47Ω out of circuit, even though it had as much as 3-6w dissipated in this 1/2w part when it was between 13-17v on the + rail and ground (due to the shorted cap).

I soldered in a new 100µF 35v cap for C5 (despite the schematic values, all the factory-original parts were 100µF) and a new 47Ω for R11.  Powered up and first checked voltages at C95 & C97.  They were now a few tenths from +/-17v and essentially balanced.  Looks like the shorted C5 dragged down the positive supply and even threw the -17v rail out of whack.

A quick audio test confirmed proper function, and I buttoned up the 565T.  Now it's ready for sale!

[PRR]

> "Little Dipper"

I'm astonished you even know what that is. I have not seen one in many decades.

"designed with the use of a computer".... oh, the future is here already!! (They probably cribbed a FORTRAN listing out of a paper to figure the filters.)

And a White Cathode Follower output?? Used as a filter as well as the power-out stage? Instead of another opamp?

Dig the crazy fancy(not) power filtering! Someone asked how much filtering opamps need. Regulator? Super-Reg? Cascaded regs? I opined 470u-220r-470u was often fine. This is only marginally fancier (though bigger).

> Could this unit need voltage-balancing resistors on the bipolar supply

Rhetorical? You see the Red/Yel wire. It is hard-centered. Well, as hard as a 10VA PT can be.

> Looks like the shorted C5 dragged down the positive supply and even threw the -17v rail out of whack.

Total drain stock, 95mA. Drain with 47r sucking the rail to ground, near 280mA. Yeah, pull 3X the design load, stuff sags all over.

If it was not drawn so funny, you would see it like a tube amp. You expect say 400V 375V 300V down the power chain. 390V 350V ZERO (and smoke) would draw your attention to that third node, with shorted-cap on your list. Just here there are several "power chains" fragmented all over.

> a few tenths from +/-17v and essentially balanced.

Supplies do not have to be balanced. Each one must be ample for the audio we are trying to throw through it. As we assume audio is symmetric, we nominally use symmetric supplies. But one side more or less is no difference as long as the signal fits.

Rated peak in/out voltage gives 6.5V peak internally. There's a good 14V of available audio swing. It could run on +9V/-27V and meet full specs.

LF356 is what TI targeted with the TL07x types. Very similar performance, but the TL0 is much less expensive. A suitable replacement. Though for what a Dipper should be worth, "authentic" LF356 may be wanted.

[HotBluePlates]

... > Could this unit need voltage-balancing resistors on the bipolar supply

Rhetorical? You see the Red/Yel wire. It is hard-centered. Well, as hard as a 10VA PT can be.

> Looks like the shorted C5 dragged down the positive supply and even threw the -17v rail out of whack.

Total drain stock, 95mA. Drain with 47r sucking the rail to ground, near 280mA. Yeah, pull 3X the design load, stuff sags all over.  ...

Yep, rhetorical statement.  I hoped my walkthrough would help others "see what I saw, think what I thought" on my way to fixing this.

The entire repair was done on Christmas day, though I opened it for a visual once-over more than a week before.  It took more time to take off the cover panels than it did to find/fix the problem.  (And the previous owner assured me, "It's not caps, but I don't know what's wrong with it.")

> "Little Dipper"

I'm astonished you even know what that is. I have not seen one in many decades. ...

I didn't know until I contemplated buying it as a repair/resell job.  But anything "Urei" for cheap seemed promising.

Seems like an excellent filter for video, location recording, and mastering chores.  Plus the odd faux telephone and manual phasing effects.

But now I've got to get get ready to offer it up on eBay.  It might be a good decade before my son gives me enough free time to contemplate any kind of home recording...

[PRR]

> decade before my son gives me enough free time to contemplate any kind of home recording...

Maybe another forum, the 3 year old daughter demanded to be read Merlin's tube-book, and was going around saying "cathodyne!". Your kid ought to be mixing you sooner than you expect.

[sluckey]

the 3 year old daughter demanded to be read Merlin's tube-book, and was going around saying "cathodyne!".

I like that! 

[PRR]

At diystompboxes, though not a public section:

Someday in the future, she may be at a concert, thinking, "That Fuzz Face the lead guitarist is using has a lot of leakage in the first transistor and his amplifier bias is way too cold."

I have a three year-old at home already that will frequently follow me to the basement saying "Bye-bye mommy, I'm gonna help daddy." She will then usually grab my multimeter and start probing things, assuring me "It's okay daddy, I'm gonna fix it."

She also always chooses her own bedtime story from the bookcase, and a couple weeks ago she insisted that we read Merlin's "Designing Valve Preamps." Don't think she got much out of it, but for the next couple days she would point at random objects and say "Look! It's a cathodyne!"

Kids are fun :)

[Willabe]

 

[shooter]

Nice! reminds me of a dilbert cartoon, as the Doc is telling Mom, (as the kid has his head in  a broke X-ray system), don't worry, he's just got the knack