V76 noise problem (pics inside)

[martthie_08]

I have a V76 which has been giving me some troubles, it makes a kind of farting noise every 5-20 seconds. I will get a sound sample up this evening, maybe someone knows what could cause this? I would guess either a tube problem or dried out electrolytics, but have no idea on how to trace this down. I hope it is not a transformer dying out on me, that would be a big problem. Speaking of transformers, one of the trannies (far right on the first pic below) is sitting loose and moves around in the housing, is this a serious problem?

The unit seems to be a very early unit, I got a schematic with it which I can scan tomorrow (haven't seen this one on the net yet), it is close to factory stock condition, all caps (except one) seem to be original. While I'm fixing this problem I'd like to do some "restauration" and replace the four big 32uF HT filtering caps, which you can see in the pics below. One is on the main circuit board and three are upright in a filter daugher board. I guess it is advisable not to change any electrolytics in the signal path - or what do you think?


Here some pics:

The main board:

There is third board which sits in vertically.

This is front, notice no "/80" on the logo, but the 80+300Hz low cuts are there.

any suggestions greatly welcome!! cheers, Marten

 

[chriss]

You should get this one:

LCR-Messgerät


and check the Electrolytics. My experience with the caps in these old VXX-units is that most of them are still closer to their value than all of the new caps you get today. But the described effect sounds a bit like a dead Elyt.
And don't forget: These were bulit for 220V, not 230! The Voltages inside are not very regulated, so you'd have around 7,4V instead of 6,3V on the tubes heating. Way too much. You shouuld place a 5W-Resistor with a value to get the Voltage down to 220. Just use the good old Ohm'sches Gesetz or trial and error :wink: Something like 200 Ohms could be a good starter I think.

regards

Chris

 

[jensenmann]

Conrad sells a 230V to 220V transformer for 50 bucks. Your EF804s will thank you for this investment.

It seems that some caps are dried out. Just recap the thing. No big deal. Any cap you buy today has better specs than these old ones.

 

[guavatone]

It looks like sprague atoms will do nicely for replacement.

 

[Michael Tibes]

I always replace all the Elektrolytics in those units. I am too scared of having the unit braking down during a session. I have never actually tried not to replace them... So I couldn't say whether they 'usually' still are ok or not - if there could be any generalisation after 40 years of usage at all.

I wonder if one can achieve a reliable result from measuring those caps with a usual LCR meter because the caps will neither have the typical operating voltage nor the operating temperature during the measurement. I guess if you want to know whether they are ok, then you'd need to do a long-time recording of the value measured at 300 V in an oven with maybe 50 degrees celsius. Then measure for some weeks...
Ok, maybe I am exagerating, but I hope the point I try to make is clear :wink:

I believe that cleaning does improve reliability as well, especially on old tube equipment. With V76s I have often taken out all the elektrolytic caps and then carefully cleaned the board and tube sockets with isopropyl and lots of cotton sticks. Be careful, some types of resistors are sensitive to the alcohol, others don't mind a bath. Then I have fitted new caps.

Intermittend noise might also be caused by a loose wire or bad contact. I can't clearly see the wires at the tube sockets on your pictures now, many units have a little tube over the soldering point of the connected wires, and the wire can be broken inside that tube. It looks as if everything was ok, but you can just pull out the wire without resistance... They look as if they are corroded, probably the acid from the solder flux has oxidised the copper, just a guess.
And how are the gain selector and filter switches? They have either silver or gold plated contacts, silver ones can build up so much oxide that they become black and don't pass any signal...
And one more, how's the mounting plate of the first 2 tubes? It's usually mounted on 3 (or 4?) rubber standoffs (if this wasn't changed at BR already). This rubber just breaks and the whole plate floats, possibly causing shortcuts somewhere.

Hope this helps a bit, there is a lot to find out about those units. Thinking simple and being prepared for the unexpected can help to fix it, nothing is impossible in there because all parts are around 20 years older than ever expected when they were built.
Anyway, yours looks like it's in a pretty good shape, so congratulations and I hope you get it fixed soon :thumb:

Michael

 

[CJ]

I would leave the lytics in the audio path alone.
The filter caps definately should be changed.
It gets hot inside those boxes, and a lot were on all the time.

Ollie says do not over cap the thing, you will change the sound (impulse) of the unit.

Axial filter caps are not as common as radial, so not as many choices.

 

[martthie_08]

Thanks everybody for the helping words!

@chriss:
Good idea to get something that can measure capacitance, unfortunately your link to conrad doesn't work, but I had a quick look and they sell these units with prices ranging from 37,95€ to 346,84€

about dropping the mains AC with a resistor: do you place this in series with both brown and blue, or just the brown (with german color coding!!!)?

@jensenmann:
This would indeed be a good investment, it is Conrad catalogue number 510188 current price is 74,95€.

@guavatone:
I'd love to use Sprague Atoms, but they are physically too fat. The 30uF / 500V Atom is 26mm, which is 6mm larger than the original. As this might not be a problem for the cap on the top, things will get really tight for the three caps sitting next to each other vertically.
tubetown.de sell JJ electrolytics at a really nice price, but I have to find out the dimensions of these.
edit: they are also too fat, at 25mm. :sad:

@Michael:
good point on the measurement setup, but I don't even have a LCR meter at the moment The unit is actually pretty clean on the inside, the tube sockets don't have anything over the soldering points and the connected wires are all ok, I just checked. The gain and filter switches are silver plated, but not oxidised.
You have a good point with the mounting plate of the first two EF804S and the input transformer and choke, this board is floating loose, it was the loose part I mentioned in my first post. I'll have to take the unit apart some more to see the whole picture...

@CJ:
yes, I'll first change everything electrolytic that is not in the audio path, maybe I'll need some help identifying all the caps to change, but I'll post the schematic later this afternoon, that'll help.

Thanks again everybody, I'll report back! cheers, Marten

 

[chriss]

unfortunately your link to conrad doesn't work, but I had a quick look and they sell these units with prices ranging from 37,95€ to 346,84€

yes, sorry. it was meant to be a link to that 37€-thing. i have it and it's exact enough for my purposes.

about dropping the mains AC with a resistor: do you place this in series with both brown and blue, or just the brown (with german color coding!!!)?

you can do both, i think. just for the brown AND blue-version the resistors need to be half the value. i think.

This would indeed be a good investment, it is Conrad catalogue number 510188 current price is 74,95€

i heard they shall not be very reliable , since the output-voltage varies a lot with the load connected. haven't checked it though.

chriss

 

[jensenmann]

Axial caps are cheap at Reichelt. I used them for recapping my V72 PSU. They fitted and it´s tight in there.

Yes, that´s the stepdown tranny I was talking about. Huh, it became expensive in the last year.

 

[jensenmann]

btw, there´s a LCR meter at Reichelt which is around the same price like the one chriss mentioned but it has a wider measurement range

 

[sodderboy]

And I always pitch the hand-sandwiched selenium rectifier. I replace it with a bridge mounted upside-down on a perfboard, sharing space with the original fuseholder, or for mobile units I use an in-line fuse holder.
Be careful with the selenium, and dispose at a battery return center.

Which reminds me. I have started a second trash bag that gets all blown components and sucked solder from the Hakko and Palladins, and I take the bags of crap to either Rat Shack or a supermarket that has a battery return center. I hope they do not in turn toss the $hite in THEIR trash. . . .
Mike

 

[EZ81]

You could hook up a cheap 15V/1A or similar transformer as a Spartrafo to drop the mains voltage by 15V. You need to watch the polarity of the 15V winding, or you get 245V...

      3         +------------------+
      4         |                  |
      5 L-------+-----|| I         |
      6               || I ||------+
      7 230~          || I ||  15~
      8               || I ||------------
      9 N-------+-----|| I
     10         |                    215~
     11         |
     12         +------------------------
     13

Excuse the line numbers.

 

[Michael Tibes]

[quote author="sodderboy"]And I always pitch the hand-sandwiched selenium rectifier. I replace it with a bridge[/quote]

This will result in a significantly higher operating voltage, doesn't it? I did the same on the first units I did, but then didn't continue that road. I never had any of the original rectifiers failing among the at least 40 or 50 Vxx units I handled. Might just have been lucky though.

Michael

 

[Boswell]

[quote author="chriss"]And don't forget: These were bulit for 220V, not 230! The Voltages inside are not very regulated, so you'd have around 7,4V instead of 6,3V on the tubes heating. Way too much.[/quote]
My calculator says 6.59V not 7.4V from the 6.3V winding when using 230V on a nominal 220V primary. Or are the heaters not transformer-fed?

 

[Salossi]

@EZ81: cool idea, the transformer wired in this way.

But in my opinion it´s not necessary to change the voltages.
The V7xx´s are made for a voltage of 220V +5% / -15%. This means, that you are in tolerance up to 231 VAC. On the other hand I would check the actual voltage at the place you´re using your preamp.
e.g. here in Cologne I have usually only about 220 VAC

Greets, Sascha

 

[jensenmann]

tubes aren´t very tolerant in respect to their heatervoltage. 10% more heatervoltage equals 50% less lifetime.
:sam:

 

[Michael Tibes]

I got scared and measured the voltage in our studio - which is 225V right now. I wonder if a little underheating would be better for the tubes instead of the slight overheating we have here? If I used a 230 -> 220 Trafo I'd probably be around 215V...

Or is it irrelevant in the end and I am just thinking too much??

Michael

 

[sodderboy]

[quote author="Michael Tibes"]This will result in a significantly higher operating voltage, doesn't it? I did the same on the first units I did, but then didn't continue that road. I never had any of the original rectifiers failing among the at least 40 or 50 Vxx units I handled. Might just have been lucky though.

Michael[/quote]

I guess so. Today I put a EMT V 54S plate amp unit on the bench that had a"noisy right side". Turns out that both are noisy, and voltage out of the rectifier is bouncing around by about 5 volts. That rectifier is DONE. Perhaps they do not like leaving Germany?

I remember a thread in The Lab discussing selenium replacement. I searched but it must be gone since the crash. IIRC the change is not significant, like less than 5%. I will be able to measure it in this instance. And I have never had any tube or unit failures after rectifier replacements.
Mike

 

[Michael Tibes]

[quote author="sodderboy"]IIRC the change is not significant, like less than 5%. I will be able to measure it in this instance. And I have never had any tube or unit failures after rectifier replacements.
Mike[/quote]

I remember that it was something like maybe 50 V in the case of the first 2 or 3 V76 on which I replaced it with silicon diodes. I ended up using a series resistor to get the voltage down again, because it was higher or close to the max voltoage of the caps... I don't remember how much it was exactly, but I remember that it was so much that I was worried about it.
It does also sound different then, definitely when the unit is overdriven the character is very different.

It would be very intersting to work out how to replace the selenium ones and maintain the character.

Michael

 

[mikka]

10% more heatervoltage equals 50% less lifetime.

Huh? The specs in the tube books usually give the heater voltages as +-10%.