| Replacement diodes [message #23130] |
Thu, 18 June 2015 00:22  |
bluezebra
Messages: 89
Registered: February 2014
Location: Lafayette, LA
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I don't see a suggested replacement for this large board mounted diode.
Labeled 366-D AR diode.
I see where Westinghouse replacements are no longer around. What is an
equivalent replacement?
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| Re: Replacement diodes [message #23133 is a reply to message #23130] |
Thu, 18 June 2015 06:09  |
stevem
Messages: 4773
Registered: June 2004
Location: NY
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Senior Member |
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No need to replace them unless any of yours have shorted out and blown the fuse
If you are looking to up grade you can use modern fast switching types but you will have to mount them on a terminal strip or two.
[Updated on: Thu, 18 June 2015 06:12] Report message to a moderator |
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| Re: Replacement diodes [message #23136 is a reply to message #23130] |
Thu, 18 June 2015 11:28 |
stevem
Messages: 4773
Registered: June 2004
Location: NY
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Senior Member |
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You could install the most massive filter caps in the world and still those diodes would only get smaked with the same amount of current that the power transformer can pump out like when the amp is up full tilt!
That 1.5 volts across the speaker output should be making for a big hum out of the speaker no?
In regards to this problem here are some things to check.
1) how well balanced is the + and - voltage across each main filter?
Are all the 4 output transistors still the original matched set of RCA brand?
The resistors on the output transistors need to be checked to see how far they have drifted, in fact by the time you have unsoldered one end of each to check them, you should just replace them with 1% tolerance ones.
The big 5 watt ones can be 10 % but buy 5 or 6 of them so you can use 2 of the closest matched ones and up the wattage of the replacements from 5 watt to 7 or even 10 watt.
I have found that small differences from the schematic called for values of these resistors will knock 35 watts off of the amps output and the imbalance will make the amp run hot!
In regards to the diodes once again you can help them and take some switching noise hash out of the amp by wiring a .01uf 400 volt ciraimic cap across each diode if you care to.
[Updated on: Thu, 18 June 2015 11:31] Report message to a moderator |
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| Re: Replacement diodes [message #23137 is a reply to message #23130] |
Thu, 18 June 2015 11:42 |
bluezebra
Messages: 89
Registered: February 2014
Location: Lafayette, LA
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Thanks Stevem...interesting notes.
I decided to check the output since one of the 15's was found DOA in the second cabinet.
While playing it after 20 minutes or so I find some heat coming off the back which prompted
my investigation.
All the transistors were original RCA. I checked all the ceramic resistors connected
to the transistors and all were pretty much spot on. There are two 8.2 ohm resistors
on some terminal strips that have drifted to 10 and 12 ohm and need to be replaced as soon as I get to
the store today. I also found two 3 watt carbon comp 820 ohm had drifted to 948 ohm and those were
replaced.
While I had everything unsoldered I changed all six transistors on the chassis. I was
getting 2.2 vdc on the output. The new transistors (and possibly the resistor changes)
brought it down to 1.5 vdc. All the electrolytics
were changed out. Strangely enough there is no hum on the amp. Just a little bit of buzz when the
treble is turned up. Both sides of the main caps were at -39 and +39.
[Updated on: Thu, 18 June 2015 11:50] Report message to a moderator |
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| Re: Replacement diodes [message #23141 is a reply to message #23130] |
Thu, 18 June 2015 17:55 |
chicagobill
Messages: 2006
Registered: April 2003
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Senior Member |
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The Frankie power amps are completely different from the later series amps. They are very similar to the power amps in the Vox Beatle amps.
The two 8R2 ohm resistors set the bias of the output transistors. One is 25% high and the other is 50% high, replace them and I will guess that the overheating will stop.
Steve is right, the output offset voltage is controlled by the matching of the output transistors. If you have the time to swap them in and out, you might be able to get it down to less than 1/2 volt dc. If your meter has a gain test setting you might be able to try and match 4 outputs to make it easier.
Of the six transistors one is the 24 volt regulator another is the driver transistor and there are 4 in the output section, two for each half of the signal. You only need to match the 4 outputs.
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| Re: Replacement diodes [message #23142 is a reply to message #23130] |
Thu, 18 June 2015 19:37 |
stevem
Messages: 4773
Registered: June 2004
Location: NY
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Senior Member |
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And all the other Thomas Vox amps that have a duel rail power supply.
Last winter on a boring cold day I go the itch to change my Vox reverb twin over to silicon type outputs from Germinum type for better life, which ment having to rebias them for more idle current to turn them on.
After jerking around with resistor values for one hour I got it set only find that the better specs of the new transistors made for 2 more watts of peak power, but less clean RMS power due to the greater idle current draw!
No such thing as a free lunch yet!
Anyway, I reinstalled the original transistors and let it be, they lived since 1966 in the amp with its 18 watts of clean power and will likely last after my Soul has left this rock!
[Updated on: Thu, 18 June 2015 19:38] Report message to a moderator |
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| Re: Replacement diodes [message #23143 is a reply to message #23130] |
Thu, 18 June 2015 22:21 |
bluezebra
Messages: 89
Registered: February 2014
Location: Lafayette, LA
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Member |
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Stevem,
you had me wondering at the reading since I have an auto-ranging Fluke and
sometimes don't pay attention to what it's reading, however, I just remeasured
and the output is definitely 1.59 vdc...
Couldn't find the exact resistors I needed locally.
Thanks Bill for your input. I have two transistors that are incorporated into
a power board on one side. I'm assuming that the other four are for the output.
[Updated on: Thu, 18 June 2015 22:24] Report message to a moderator |
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| Re: Replacement diodes [message #23144 is a reply to message #23130] |
Fri, 19 June 2015 01:22 |
chicagobill
Messages: 2006
Registered: April 2003
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Senior Member |
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Yes, the two transistors that connect to the sockets on the pc board are the driver and the voltage regulator. The four that are wired directly to the transistor legs are the outputs.
That's part of the problem with swapping outputs, as they are all soldered in.
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| Re: Replacement diodes [message #23149 is a reply to message #23130] |
Fri, 19 June 2015 12:54 |
chicagobill
Messages: 2006
Registered: April 2003
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Senior Member |
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1 1/2 volts dc isn't that much, and it is dc so I'm not surprised that it isn't humming. Remember that this is a pure dc voltage caused by the transistor biasing, not from a circuit problem.
I would expect the speakers to thump when the amp is turned on. Once the two 8R2 ohm resistors are replaced, some of that voltage will probably go away.
The original schematic for this amp shows a -1 volt offset at the speaker jack.
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| Re: Replacement diodes [message #23157 is a reply to message #23130] |
Fri, 19 June 2015 21:11 |
bluezebra
Messages: 89
Registered: February 2014
Location: Lafayette, LA
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Member |
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I stand corrected there is only one 8.2 ohm resistor mounted between two
wires near the small transformer that looks like a choke. Two of the power
transistors have 820 ohm resistors and they are connnected on to the other
two power transistors by a wire. These I also replaced with matched 5 watt cement
resistors. Ooops....I just found the second 8.2 ohm... changing that also.
So now one can assume that I should shoot for more matched power transistors.
NOTE: After changing the second 8.2 ohm (serves me right for not wearing my
magnifiers)...the output voltage is now -.023 vdc
[Updated on: Fri, 19 June 2015 21:54] Report message to a moderator |
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| Re: Replacement diodes [message #23160 is a reply to message #23130] |
Sat, 20 June 2015 02:29 |
chicagobill
Messages: 2006
Registered: April 2003
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Senior Member |
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I think I posted earlier that your two 8R2 ohm resistors were 50% and 25% off value. They were probably 10% tolerance parts to begin with, which would be fine. Even a tube amp with resistors 50% off value would have problems.
In answer to your original question, I pulled one of my Frankie heads and checked the rectifier diodes. This one has Westinghouse 368D diodes. According to my NTE book, they can be replaced with an NTE5896 diode, which is rated 200 volts at 16 amps.
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| Re: Replacement diodes [message #23163 is a reply to message #23130] |
Sat, 20 June 2015 15:32 |
stevem
Messages: 4773
Registered: June 2004
Location: NY
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Senior Member |
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Yup, me bad Bill!
I just did a little test since I had my DC power supply fired up to work on a amp.
I applyed a positive 1.5 volts to a CTS 15" Bass driver I have and it kicked the cone out over 1/32 of a inch!
This offset / new zero position for the cone probibly would have started into cutting into your Amps Bass output once , or if you where to play it at about 50 % of its output, so it's very good you got that voltage down to near zip!
[Updated on: Sat, 20 June 2015 15:33] Report message to a moderator |
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 Re: Replacement diodes [message #23164 is a reply to message #23130] |
Sat, 20 June 2015 21:03 |
bluezebra
Messages: 89
Registered: February 2014
Location: Lafayette, LA
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Member |
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Yep Stevem,
Glad I noticed the heat and checked. It's running a bunch cooler now and
I don't have to worry about reconing more speakers. Well at least not from
dc voltage. 1 was enough.
Now if I could only find a Charcoal bottom for my K200 head....!!! Oopps,
wrong forum for that...!
[Updated on: Sat, 20 June 2015 21:05] Report message to a moderator |
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