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 Post subject: First amp recap
PostPosted: May Fri 10, 2019 2:16 pm 
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Joined: Oct Mon 29, 2018 9:57 pm
Posts: 19
Good morning,

Recapped my first amp. A Westinghouse hf101. Replaced the can with 2, 40uf and 2, 30uf. Triple checked all the connections and checked all the resistor values. Powered it up. ( I do not own a variac yet) stand alone with a dummy load.. tubes warmed up, one of the 40uf capacitors arced followed by the noise that was no longer being filtered. Any ideas

Thanks,

Mike


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 Post subject: Re: First amp recap
PostPosted: May Fri 10, 2019 2:31 pm 
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Joined: Feb Thu 28, 2019 7:48 pm
Posts: 351
Location: Lawrenceville, Illinois 62439
First, double and triple check that you had the polarity correct.
Second, if you have a way to check the new capacitors before installing (especially cheap ones) that can save some grief.
After that, there may very well be a problem in the circuit that led to it, requiring a schematic.
Dan


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 Post subject: Re: First amp recap
PostPosted: May Fri 10, 2019 3:17 pm 
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Joined: Oct Mon 29, 2018 9:57 pm
Posts: 19
Thanks Dan,

Had the schematic prior to, it shows the can caps negative to chassis ground. I did not think to check them first and they are cheapies.


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 Post subject: Re: First amp recap
PostPosted: May Fri 10, 2019 5:52 pm 
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Joined: Oct Mon 29, 2018 9:57 pm
Posts: 19
Here is the schematic for the filter cap.. They are all negative to chassis ground correct? I double checked the capacitors were in their correct positions


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 Post subject: Re: First amp recap
PostPosted: May Fri 10, 2019 6:41 pm 
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Joined: Feb Thu 28, 2019 7:48 pm
Posts: 351
Location: Lawrenceville, Illinois 62439
One thing to verify; that the 'chassis' is indeed the same as the 'ground' (upside down Christmas tree in diagram). Can be quite confusing if you were raised reading modern diagrams, in 3 wire circuits, where the ground symbol indeed is the metal chassis of an appliance, or modern TV radio etc. If the old electrolytic was can mounted on a phenolic wafer which was riveted to the chassis, then the chassis and ground are separated.
Just one more thing to check!
Dan


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 Post subject: Re: First amp recap
PostPosted: May Fri 10, 2019 6:49 pm 
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Joined: Oct Mon 29, 2018 9:57 pm
Posts: 19
Yep no washer. The outer can tabs were locked and soldered to the chassis. I pulled the caps, they all test within range. This was a working amp before I dove in... it was free so no biggie, and Im learning. Maybe i had a bunk cap. Ill try again and get a variac...


Thanks for the help


Mike


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 Post subject: Re: First amp recap
PostPosted: May Fri 10, 2019 7:08 pm 
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Joined: Feb Thu 28, 2019 7:48 pm
Posts: 351
Location: Lawrenceville, Illinois 62439
Learning is half the fun!
Good luck. Might check out making a dim bulb tester - examples of that on this site or youtube - probably have all the items needed laying around.


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 Post subject: Re: First amp recap
PostPosted: May Fri 10, 2019 7:53 pm 
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Joined: Nov Fri 10, 2006 12:24 am
Posts: 2441
Location: Thornhill, Ontario, Canada
These is some merit in replacing one component at a time with a power-up test between replacements. But do at least replace the main B+ caps, e.g. your C1 A and B, before any power-up so as not to find a defective cap takes out the (in your case) EZ80 or power transformer.
Yes, get a Variac, too!
Cheers,
Roger

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Thornhill, Ontario
Ontario Vintage Radio Assoc. http://www.ovra.ca


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 Post subject: Re: First amp recap
PostPosted: May Fri 10, 2019 11:13 pm 
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Location: Vancouver Island, Canada
Lacking a variac, you can use several power transformers to produce lower voltages and put the voltage up in several steps.

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 Post subject: Re: First amp recap
PostPosted: May Sat 11, 2019 2:38 pm 
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Joined: Feb Sat 12, 2011 2:29 pm
Posts: 551
Location: Fayette County, Pa
One more thing to consider. The EZ80 itself uses a cathode / heater combination. A short between heater / cathode would put a short on the B+ to ground.

I always monitor B+ when I first power a set like this. If I have a problem that puts undue load on the B+ the voltage will not reach stated value and I power down immediately to isolate the problem. Usually a supply can tolerate such a condition for a couple seconds or at least long enough for you to pull the plug. The issue with depending on a variac in a case with a tube rectifier is you have to reach at least 50 - 60% of rated voltage before the rectifier will start to conduct. Thus a shorted filter cap or B+ line will not even show up till that happens and by then it may be too late unless you react quickly to pull the plug.


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 Post subject: Re: First amp recap
PostPosted: May Sun 12, 2019 5:13 pm 
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Location: Thornhill, Ontario, Canada
CaveRat wrote:
One more thing to consider. The EZ80 itself uses a cathode / heater combination. A short between heater / cathode would put a short on the B+ to ground.
I always monitor B+ when I first power a set like this. If I have a problem that puts undue load on the B+ the voltage will not reach stated value and I power down immediately to isolate the problem. Usually a supply can tolerate such a condition for a couple seconds or at least long enough for you to pull the plug. The issue with depending on a variac in a case with a tube rectifier is you have to reach at least 50 - 60% of rated voltage before the rectifier will start to conduct. Thus a shorted filter cap or B+ line will not even show up till that happens and by then it may be too late unless you react quickly to pull the plug.
A couple of points...
The EZ80 (and EZ81) seem to be quite robust regarding heater cathode shorts... better, I think, than the 6X5. I have two audio amplifiers using the EZ81. That said, I don't like any of these 6 volt heater rectifiers.
Re. using a variac: get one with an ammeter. When you get used to it you can recognize the "proper warm up" pattern of various radios and amplifiers as you advance the AC voltage. In my experience (few occasions only), there is plenty of time to switch-off before any damage is done.
I am a big fan of the variac, far better than the "dim buib" approach. A variac can be used on several bench activities... you never know when you need a variable AC source.
Cheers,
Roger

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Thornhill, Ontario
Ontario Vintage Radio Assoc. http://www.ovra.ca


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 Post subject: Re: First amp recap
PostPosted: May Sun 12, 2019 9:13 pm 
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Joined: Oct Mon 29, 2018 9:57 pm
Posts: 19
Appreciate all the input from you guys. Waiting on the variac to come in the mail. The good news is I tested the power Transformer as well as the output Transformers and they are fine. Double checked all the wiring. The R-25 12 K Ohm resistor that was tied into the filter cap is spent. I'm not sure about the tubes. But again they were working before I touched it :D


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 Post subject: Re: First amp recap
PostPosted: May Mon 13, 2019 5:02 pm 
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Location: Thornhill, Ontario, Canada
Mikemackd wrote:
... Waiting on the variac to come in the mail...
Congratulations! Don't forget to add an AC ammeter to aid a general variac diagnosis, a 0-3 amp meter works well for radios and audio amplifiers.
Cheers,
Roger

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Thornhill, Ontario
Ontario Vintage Radio Assoc. http://www.ovra.ca


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 Post subject: Re: First amp recap
PostPosted: May Tue 14, 2019 1:52 am 
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Joined: Nov Wed 30, 2016 7:35 pm
Posts: 5012
Location: Sunbury, Ohio 43074
Do you know which of the caps blew up? I'm guessing that it may have been installed backwards, or perhaps shorted inside the can during your re-stuffing operation, which does happen occasionally to all of us.

I base this on the statement that the radio was working before the recap. That probably indicates an issue with something that went wrong during your repairs.

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 Post subject: Re: First amp recap
PostPosted: May Tue 14, 2019 8:11 pm 
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Joined: Oct Mon 29, 2018 9:57 pm
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It didnt actually pop or swell I saw an arc followed by a hum. I think it was one of the 40uf. After I pulled the caps I tested them and they are all in spec but who knows under a load. I have 4 new ones, and yes I definitely caused this issue. :D


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 Post subject: Re: First amp recap
PostPosted: May Tue 14, 2019 8:17 pm 
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Joined: Nov Wed 30, 2016 7:35 pm
Posts: 5012
Location: Sunbury, Ohio 43074
An arc, especially if you saw it, probably means one lead of one of the caps shorted out to something. You most likely would not have seen a cap arc like this. Perhaps it was a bad solder connection, or maybe even a solder bridge.

I don't restuff cans ... I just cut them loose and put the new caps under the chassis. You can still use the old can for ground connections. Might have to add a terminal strip, depending on your physical layout

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 Post subject: Re: First amp recap
PostPosted: May Tue 14, 2019 8:23 pm 
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Joined: Oct Mon 29, 2018 9:57 pm
Posts: 19
I cut the can off, installed axial cap from the top and soldered to the existing tabs with the negatives tied together and grounded to the chassis. Here is a question. I didnt clean off the flux can that bridge and cause a short?


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 Post subject: Re: First amp recap
PostPosted: May Tue 14, 2019 8:37 pm 
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Joined: Nov Wed 30, 2016 7:35 pm
Posts: 5012
Location: Sunbury, Ohio 43074
Most flux is non conductive. It wouldn't hurt to scrap or spray it off though. Flux Solvent is available in spray cans, or you can use a Q tip and some alcohol with some rubbing after you scrape away most of it with a sharpened wood stick or even a small screwdriver.

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 Post subject: Re: First amp recap
PostPosted: May Thu 23, 2019 8:33 pm 
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What is the voltage rating of the capacitor that failed?

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 Post subject: Re: First amp recap
PostPosted: May Fri 24, 2019 2:06 pm 
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Joined: Feb Sat 12, 2011 2:29 pm
Posts: 551
Location: Fayette County, Pa
engineer wrote:
A couple of points...
The EZ80 (and EZ81) seem to be quite robust regarding heater cathode shorts... better, I think, than the 6X5. I have two audio amplifiers using the EZ81. That said, I don't like any of these 6 volt heater rectifiers.
Re. using a variac: get one with an ammeter. When you get used to it you can recognize the "proper warm up" pattern of various radios and amplifiers as you advance the AC voltage. In my experience (few occasions only), there is plenty of time to switch-off before any damage is done.
I am a big fan of the variac, far better than the "dim buib" approach. A variac can be used on several bench activities... you never know when you need a variable AC source.
Cheers,
Roger


Agree regarding the variac. I never use the "dim bulb" either. My variac has a current limiter on it as well. It consists of a sense coil on the load that goes to a comparator circuit. This feeds a trip relay so it acts like a variable current circuit breaker. that allows me to set the "trip point" anywhere between about 100 ma. up to 7 amps. (The rating of the variac.) It also has a delay adjustment so I can allow for over current spikes from about .1 seconds up to 5 seconds. to delay the trip out on initial start up. Usually I set that for about .5 seconds. I set the current limit for whatever the load is expected to draw plus about 10%. A problem radio can be expected to endure a 10% overload long enough for the current limit to trip if a short is present.


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