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Post subject: Dual Showman SR (red knob) Power Transformer Question
Posted: Mon Nov 02, 2009 6:57 am
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I picked up a Dual Showman SR (red knob) cheap to repair. The power transformer has an open primary and must be replaced. The original part is F026477. This has a 120V primary with 6.85v filament tap, 56v bias tap, and 175/0/175v taps. A new one will run about $160. The F026554 transformer has exactly the same output taps, but can be wired for 100/120/240 voilt input. A new one of these would cost $99. The dimensions and weight of the two transformeers are virtually identical.

I can't find any more detailed specs on the F026477 versus the F026554. Can I substitute the less expensive 554 for this amp???


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Posted: Wed Nov 04, 2009 1:09 pm
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Yeah I see what you mean about the specs(no amp figures). The catalog I checked out shows the 026477 as the one used in the Tone Master and the 026554 for the Vibro King. I would do a little research before going for one of those trannies. Do you think that the 026477 is actually the original or possibly a replacement? ART

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Post subject: Some New INformation
Posted: Wed Nov 04, 2009 1:35 pm
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I have been continuing to work on this and have some new information. I believe that the F026477 is the original transformer. My understanding is that the Dual Showman SR is basically the same amp as the combo amp "The Twin" of the same vintage (both in the red knob versions). I have the schematic for the latter and it indeed shows the F026477 power transformer.

The tubesandmore website lists the F026477 and the F026554. They show violtage sfor the outputs of the 477. The angela.com website lists the F026554 transformer and they give voltages that agree with the voltages fgiven by tubesandmore for the 477. So far so good. However, NEITHER of these transformers can be the correct one for the Dual Or the Twin. My guess is that the voltages provided for the F026477 are wrong. The first DC voltage indicated on the schematic for the Twin (at the first filter cap) is +397 volts! No way you can get that our of a 175/0/175 AC output. Perhaps that explains why the F026477 costs $60 more than the F026554. It is a beefier transformer. BTW, The F026477 is on backorder with no estimated delivery date.

I contacted a local Fender repair shop and they contacted Fender for me. Fender was not helpful in providing the specs for the F026477 and they also said it was not in stock. I know that Fender would rather sell me a PT , but it is unfortunate that they will not provide the specs. SO..... I have contacted Heyboer Transformers here in Michigan. I will be sending the bad PT to them along with the information I have to date. They will analyze the old transformer and determine if they can make me a new one. WHen they get inside it they may actually be able to repair it for cheap! BTW, their seat of the pants cost estimate for custom making the PT is less that either tubesandmore or Fender are charging. Heyboer makes replacement transformers that are sold by many companies including Mojotone.


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Posted: Wed Nov 04, 2009 6:32 pm
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Yep Yep, I was going to suggest contacting Heyboer, Mercury Mag, or Weber for more info, but wanted to leave that up to you. I think Heyboer is the right choice. Yeah, Antique Electronics is a great vendor, been doing business with them for more than 20yrs. Good luck with that amp! Almost time to put the rake away and dust off the snow shovel. ART

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Post subject: Hey Dick Thomas Kalamazoo - Your old open pri transformer...
Posted: Thu Jul 08, 2010 9:07 am
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Hello Dick Thomas

Do you still have your original 026477 power transformer from your Dual Showman SR? I have a fix for it if the rest of the amp electronics are in good condition.

I too just bought a "The Twin" Red-Knobber cheap to fix up and after troubleshooting a bit, I too found the primary open on the 026477 power tranny. I work as an electronic tech and have seen lots of stuff over the years...including this problem on a piece of gear where there is no apparent reason for the power tranny to be dead.

Anyway, it occurred to me later that night after t-shooting that the primary may have an internal thermal one-shot fuse. First I completely removed the tranny from the amp chassis. Them, I took off the tranny end bell (the side with the wires coming out, and carefully peeled back the insulation to reveal where the external wires are attached to the internal winding wires. And there it is....a small square black U23 130-degree-Celsius one-shot thermal fuse. After researching the part and the application notes, I find, as I have in the past that , these fuses "age" over time and can fail at random even on an otherwise fine amp. This is what happened to my amp....the guy I got it from was jammin along and the amp suddenly died completely.

I bench tested the raw tranny last night. I temporarily bridged the legs of the thermal fuse and brought up the primary AC voltage slowly. Well the tranny works!!! I now have some of the original thermal fuses on their way to me to reinstall for proper thermal protection (don't wanna burn my own house down).

So, if you still have your original, don't toss it as it's probably repairable and just a victim of an aged part..... just like worn out capacitors can kill your amp too. Email me at cavemaaan@comcast.net if you want to chat some more. I have pictures of the process, the $2 part, dc-resistance measurements, and working (unloaded) AC voltage measurements.

Dan


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Posted: Thu Jul 08, 2010 4:32 pm
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I'll confirm what Dan has indicated. And in your case, the symptoms seem to mimick his tranny with the open thermo-couple. Should be an easy fix (and a helluva lot cheaper than replacing or rewinding a tranny) too.

Best of luck, HTH

Arjay

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Post subject: Replacement of Thermal One-Shot Fuse in 026477 Transformer
Posted: Fri Jul 09, 2010 6:54 am
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I installed the new U2X (RoHS replacement for U23) thermal one-shot fuse into the 026477 transformer last night. I then re-installed the tranny into the chassis and flipped the switch....VOILA!!! The Twin lives again!!! I ran it for about 20 minutes jammin a bit through it at low volume with the two-tube option at 1/4 power...enough for me in my lil living room

Anyway, the fix worked as I thought it should since there were no burnt smells in neither the tranny or the chassis.

I hope this helps some more people as the replacement tranny is now around $180.

Dan


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Posted: Fri Jul 09, 2010 7:47 am
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And I can think of a few shops who'd charge some poor shmuck that $180 bucks after swapping out a two-dollar T/C too.

Ya done good, Dan!

Arjay

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