Mark III rebuild troubleshooting...no output!!!

Are there any specs that are important for ordering the 1n4007? Or is a 1n4007 a 1n4007? Im looking at a fairchild model on mouser but cant post the link because I'm a new member.

1n4005/6/7 at Radio Shack (or whatever they are called today) for about $1 will work just fine.

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Wow. This is crack work Pete. Checks out. Thanks a million for the help so far. I'm very surprised that something once sold as a kit would have such a counter-intuitive marking in it.

I'm dying now to reverse it and try it but your warning is pretty grim. Guess I'll wait until the diode comes in next week.

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This series of posts is a perfect example of why I love this forum some very talented people helping others of varying skill levels from one amateur who normally lurks this site every day THANK YOU!
PS Love my Latino ST-120 and SP-14 Pre amp

Well the saga continues.

I ordered the 1n4007 and replaced the bias caps with two 50uf's which had been a 68uf and a 50. Put all that back in order. Switched it on and......STILL NO OUTPUT.

So everything has been replaced except the power transformer and the choke.
Time to test the transformer.

Across the 2 yellows: 5.24vac
Each red wire to ground: 423vac
across the greens: 6.6vac
Green Yellow to ground - .19vac!!!???
Red/black which has faded to blue/white: 56.4Vac
Red/yellow to ground: 0VAC

Everything there looks right EXCEPT the green/yellow to ground. it should be 3.15VAC.
Could this be a wiring issue? Green yellow goes to the .02 cap and then to the positive of the 50uf cap and to ground. All of which follows the wiring diagram. I'm not clear on how or why the schematic doesnt seem to match that wiring scheme...

Is 30v a high enough rating for the .02 cap?
Do I need a new power transformer?
How do I check the Choke?

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1) Measure GY to G.
2) If you can cobble together a series-parallel combination of resistors to make a 10W, 50 Ohm resistor you can susbstitute that conglomeration for the choke as a test. If the choke is open circuit you should be able to see missing high voltage at the red wire in output transformer. Confirm by lifting one of the choke leads and measure dc resistance.
3) Yes, a 20V cap is fine for the .02µF from GY to ground

What dc voltage do you measure on the wiper of the bias pot?

Petercapo: The banded side of the diode is certainly connected to the faded black/red wire.
I wired the green/yellow to the cap as the wiring diagram shows. I just don't really get the difference with that and the schem.

J Beede: The GY to ground is .19 while hooked to the .02 cap. Should I unhook it and test it?

to both, the cap is not shorted. I'll double check.

Back to J: The reds are checking out as tested earlier anyway in regard to the choke. 423VAC on both sides.
2 - Measure DC voltage from what side of the bias pot? to ground? What measurement do mean? You'll have to be more specific... I might be being a dummy here...

On the choke are there any more measurements that are more straightforward?

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Please be careful and keep one hand in a pocket when working with live tube gear. Advice you acquire here often assumes a basic understanding of simple components, basic measurements, and safe high voltage practices. REMINDER: There is high voltage present on both sides of the input board. There is no way that Dynaco gear could pass industry standards for safety in the 21st century!

Measure GY to G means measure from green-yellow to green. This is an AC voltage. It doesn't matter if the 0.02µF is connected or not. At 60Hz that capacitor acts like an open circuit.

The wiper of most pots will be the middle terminal, it's the one that shows up in the schematic with as an arrow pointing at the resistance.

I am concerned that you had to order a 1n4007 diode. Along with a 2n2222 these are the most common semiconductor devices on the planet. Is there no Radio Shack near you?

Some alternatives to replacing the choke with a 50 Ohm (10W) resistor:
1) Order a replacement choke from www.dynakitparts.com
2) Lift one side of the choke and check for continuity
3) Check for 350-450 VDC at eyelet #5

Do you have spare ac coupling caps? Maybe 0.1 or 0.2µF non-polarized? Do you have a signal generator? If you have these items there are simple/crude ways to bypass the input stage to determine whether your amp has basic functionality.

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My error... I had not seen the photos before I made my comments above.

I am with PeterCapo, spend a day with diagonal cutters, soldering iron, and de-soldering tool to strip that amp down, sand, paint or powder coat the chassis, and rewire it.

You might consider a new chassis (a bargain), choke (cheap), quad cap (annoyingly expensive), and correct output transformer from www.dynakitparts.com. I've not seen many MkIII as "crusty" as yours especially on the underside--this makes me suspect the tube sockets should be replaced as well. Overall this is not going to be a cheap endeavor. Keep in mind that "farm fresh", complete (and sometimes working) MkIII can be had for around $300-400 with tubes and cage.

Unless this particular amp has sentimental or historical value I think I would look for a MkIII with a decent chassis and healthy stock transformers as a faster, better, cheaper path to your desired end point.

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Man you guys move fast. There's little chance I'm going to replace the chassis or rip down everything now. The entire amp has been rebuilt according to the diagram and schematic except the choke and the power transformer. I might have screwed up with the diode but I have checked and rechecked everything and it all looks right to me.

Using an audio probe I know that good sound is coming through the input stage into the 6550's. It stops at the green and blue and green/white blue white wires where the audio probe only picks up hum/buzz. I'm not sure what that indicates, if anything at all, but I understand the signal should be flowing from there into the output transformer which is a brand new like for like replacement and no good signal is coming into the speakers.

As far as I know the jacks are wired correctly and the 680 resistor is in fact there and tests good. Also the green yellow has a good connection to the cap.

I will check the bias pot wiper, the mains, and pins 5,6 of each socket and recheck the green-yellow to green within a few hours. I will also post more pictures of everything including the power supply caps which I have checked and checked, and the output jack wiring too.

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Yep. I just finished debugging a pair of MkIII since my last posting. One amp had two bad segments in the quad cap. The other had been "tampered with" to add ac balance and bias circuit to work with EL-34. New caps, yank the selenium stacks, "yellow sheet" pre-rectification, and sweet music as I type.

A couple things I learned today:

1)A 450V cap really won't work in place of a MkIII quad cap segment very long. About an hour or so.
2) Svetlana EL-34 in MkIII does not make the MkIII sound like a ST-70 (that's a good thing). In spite of my years of running MkIII I have never tried them with EL-34 before. I like the sound enough to take the Svetlana's inside and try them with my primary MkIIIs.

FYI: I am no fan of quad caps. I usually build my own cap boards using series/balanced 450V electrolytics. Today I bypassed the bad segments with fresh caps and left the working quad cap segments in place. Adequate for now.

By the way... the failed 22uF cap still measures 25uF on my (LV) cap meter.

Steve, A couple things:

1) I don't think you are going to make much progress unless you can post the make and model of the output transformer that you are using.
2) Hum, bad output, low output, distorted output... these are not the same thing as "no output".
3) If someone mentions a voltage at a single pin you can assume a ground reference
4) If you see 5-6VAC between pins #2 and #8 on your rectifier you can probably assume that the GY-G voltage is okay and so is the 0.02uF on the center tap.
5) I hate quad caps. All my MkIII have one--but they are (usually) disconnected. I keep them for cosmetic reasons.
6) Even the "wrong" output transformer can produce sound. Multiple requests have been made by the patient folks here for details on the transformer that you are using. Maybe I missed those details.

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Ok, first some pics:

https://imgur.com/a/bAG4m

J Beede:

1. The transformers are both the correct make and model. Posted earlier in the thread. I know there's a lot of posts.
2. There is NO OUTPUT at the jacks. No hum, buzz, hiss etc. There is a buzz on pins 3,4 of the 6550's on my audio probe though.
5. The only changes to any values in the amp are the power caps. I replaced them all with 2 68uf 400v's. Negative to positive style.

Pete: The Leads on the output look fine. I had a real close look and there was a shaved off bit of insulation on the orange one which made the yellow beside it look frayed. All are intact and no bare wire exposed. I don't think the choke is wired any differently. One side to a power cap set and one to pin 8 on the rectifier.

Promised readings coming next.

Mains voltage checks out at 119vac.

Eyelet 2 only has one wire connected to it but bias pot wiper(middle lug is connected to the 50uf bias cap.
It reads -59vdc 0vac

Pins 5,6 of the 6550's:
1(5)- -58.5vdc
1(6)- ""

2(5)- -58.5vdc
2(6)- -58.5vdc

I'm now getting 10.7vdc at the g/y wire! Low low output(with a guitar in) at max volume and a low hum!!! Maybe I was measuring the green/yellow a little high close to the insulation before. Maybe that connection is loose. I'll check. Also fiddling with the output tranformer wire might have done it.

Please take a good look at how my jacks are wired up. I've had a gut feeling about that since most voltages check out, good audio comes through the input section and nothing for output happened. I have continuity from ground to all the grounds on the jacks and continuity from all the hot leads to the tips. Is it possible that the 16ohm jack is wired wrong because its got three lugs!?!?! I smell blood.

Sounds like you're using the guitar as a preamp. Many guitar pickups have a very low voltage output.

Wait a goddamn minute. Just hooked up my cellphone and its giving out clean audio and sounding great. Low signal but maybe this thing just needs a real stereo preamp to push the input??? I'm pumped. Seems like in in business?!!?!

I'm going to do the Limbo rock after giving a huge thanks to you fine wizards out there in internet land. This thing sounds as good as I feel right now. In spite of all the little hangups I wasn't putting the right kind of source in! I'm a dumbass. Much love gentlemen!