VT100 low power / high distortion

[tbonuss]

Hello All --

I posted originally in the schematics section, but since its really about troubleshooting now, I thought I should re-post the issue here. Hopefully, that's reasonable etiquette around here.

So...I have a Valvetronix VT100 which had high DC on speaker wires and ext. spk. out--speakers totally fried.
Preamp section checked out okay all the way through the headphones and effects send/return.
Replaced the power amp IC (STK404-130S).

It is now somewhat alive. Sound coming out is pretty weak and distorted and truncates early--also fades in and out.

I swapped in a known good 12AX7--no effect.

Thanks in advance for any help at all -- and thank you to all who helped with this when it was in the schematics thread.

[The Dude]

It's quite possible that when the IC shorted it took something else with it. If you can measure the voltages on the IC and post them here, we might detect something amiss. Also, I would take the measurements with the speaker disconnected just in case you short something or the amp starts to put out DC again.

[tbonuss]

Thanks The Dude. I went in to check voltages, and when trying to find a convenient circuit ground, I realized that none of the components were labeled like my schematic (I only have one for the VT50)...so I pulled up the board and found that one of the conduction tracks was lifted. I didn't notice that before...weird...it couldn't have been that far off before.

[DrGonz78]

You could possibly trace out the output portion of the amp. It may take a while but you can somewhat use the vt50 as a relative guide. That does not sound like much fun but it might prove to be insightful. I have attached the stk404-130s pin layout with some manufactured suggested circuit design perhaps trace something similar to what this image represents. I am pretty sure the stk404-130s and stk404-130 are interchangeable but the original stk404-130 datasheet is all in Japanese.

Looks like R18 got fried during the failure. What resistor value did you use to replace that component? Also, trace out what R18 connects to in relation the STK404-130 circuit pin paths.

Edit: I had originally posted this to the old thread and did not yet realize that you created this new one. Mostly curious about R18 and some silly thoughts I had... Now I see we had a stressed trace lifted up!! What are your voltages looking like?

[tbonuss]

Thanks Gonz,

I've started tracing with some sketches. I enjoy it -- haven't done serious electronics since the 1980's.

Thank you for the schematic. I think I'll have to compare to the 100S IC (on the VT50) and make some inferences about what should be on the pins.

R18 is okay. That blacking is actually on the shunt next to R18. Its got a little divot but still reads 0 ohms.

Here is another problem.
D3 is open (D14 on the VT50 scat--bottom middle near the fan). I can charge / discharge the parallel caps by measuring then reversing leads (starting in either direction).
C6 has some browning on the glob of adhesive -- my eye is not keen enough to know if thats a bad cap or discolored by D3 burning.
My intent is to replace them both since I believe I am going to have to pull the whole thing off anyhow because of the adhesive mess.
I'll use the same value as the current zener on the VT50 scat, since I doubt I'll be able to read anything from the diode itself.
Is there a better way?

As far as the voltages, I only got as far as checking -/+ VCC between pins 9&10 = 58VDC.
I was getting 0VDC on pin 1, which led me to verify circuit ground, which led me to unraveling all the differences between the schematic I was trying to use (the AD100VTH) and what I am actually working on (the VT100).

Seems like I should fix these before I power on again. This thing has enough faults without generating more...and it will start to get un-fun after a while

[tbonuss]

Okay--I'm able to get back around to this now...

I replaced
C6
D20
D3
And wired across the lifted trace, checking all components along the repaired trace for 0 resistance amongst.

So...it now has more power and somewhat less distortion, BUT, it still has lots of distortion on the clean amps; power seems much better, but I shut it down really quick so I'm not sure if its up to what I would expect from ~100W Vox.

Here are the readings on the stk404-130s

PIN -13---12---11----10---9----8----7----6---5----4----3----2----1----FAN
VDC 2.3 5.1m 61.2 -61.4 6m 2.3 13.6 -60.7 0m 36m 35m 0m 110m 8.5

LATER: Fixed the fan voltage by patching another trace. Fan P/S now putting out 5.2VDC like its supposed to.

Feeling pretty discouraged with this--maybe I'm in over my head without an accurate schematic??? Haven't done this kind of thing in 30 years, maybe this isn't a good "review" project

Thanks in advance for any help at all!!!

[DrGonz78]

Is there a relationship between certain frequencies and the distorted sound? Using a signal generator, try to find a certain frequency that might make the clean channel demonstrate the distorted output. At that point signal trace from before and after the tube section to hear if there is a difference in the signal. For example, I have had amps where the coupling caps are bad film caps that distorted at certain frequencies but that's just a thought. All the voltages on the STK404-130s look pretty good to me. Is pin 13 >>> 2.3v or 2.3mv?

[tbonuss]

1. I'll need to get an o-scope. Seems like there are some cheap ones, but I'll have to do some research.

2. Its 2.3V, so I pulled the output resistors (R17 and 18 on scat; R24 and R28 on the board). One was open, one was at .5--they should be .22

Now I'm trying to figure out where to get the right resistors, R22, +-5%, 3W. Where have all the electronic stores gone?


Thank you!

[DrGonz78]

2. Its 2.3V, so I pulled the output resistors (R17 and 18 on scat; R24 and R28 on the board). One was open, one was at .5--they should be .22

So when you measure each of those resistors we have one that is basically OL (or infinite resistance) and one that says 0.5 ohms? Nobody here will typically recommend Ebay, but for resistors it might an okay source. I sometimes hit up Fry's Electronics for items such as resistors, but they might not even supply these 3 watt types. Often stores will only have 1, 2, 5 and 10 watt resistors. What I am seeing on Ebay are all from China and will take weeks to arrive. Mouser and Digikey are good sources but you have to pay $5 for the shipping, so it is wise to order extra supplies at the same time.

[oc disorder]

Where have all the electronic stores gone?
Two places..In your phone and China!

Well me thinks You need 2 x Metal Oxide Resistors.

Hopefully you don't need a roll of 1000!

Well here's a start for your quest!

Note: Australian version of Mouser It asked me if I was in Australia or Denmark.
I'm surprised it didn't know!

Mouser Part #:
660-MOSX3CT631RR22J

Metal Oxide Resistors RSS3 .22 5%TR

Available in MultiSIM BLUE or Melting Pot Green

MOSX3CT631RR22J KOA Speer | Mouser






Footnote: I made that bit up melting pot green.
The way things are going wouldn't surprise me if they came in designer colors !
Actually Multisim Blue appears to be a free cad type package.
Maybe worth checking out so I can waste even more time on this computer!

[tbonuss]

Just got off the phone with Mouser. They were great. The markings are a bit weird, but we finally verified that they are .22 ohm / 3 watt. Need two. Bought ten.

They said that I should be looking for the Korg (vice Vox) VT100 schematic, which makes some sense.

[tbonuss]

Here's what they are sending me:

ERX-3SJR22 Panasonic | Mouser

Says its a metal film resistor, guess that's okay?

BTW: I couldn't work on this before because I was in Canberra.

[DrGonz78]

Here's what they are sending me:

ERX-3SJR22 Panasonic | Mouser

Funny that when you look at the link of that part it says 2 watt in the description but then in specifications it says 3 watt. I wouldn't worry about it but just noticed the typo. Also, when you measure the 0.22 ohm resistors on your meter the leads typically add in about 0.3 ohms of resistance, so hence the 0.5 ohm reading.

[tbonuss]

Yes-I noticed that too. Checked and they said its just a typo on the top section. I read the Explanation of Part Numbers, and the 3 means 3 watts. So it'll prolly be 2 watts

Thanks for the reminder about the leads. Makes perfect sense.

Thanks for all the help!

[J M Fahey]

Where's R18?
Where's the schematic(s)?