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In Reply to: A friend just shorted my Sherwood S-5000! posted by Raver on December 29, 2003 at 11:48:39:
FIRST, DON'T turn on the amp! Actually you could if you removed all the output tubes but basically don't until you make repairs. With that out of the way. RELAX, it's very likely that only your selenium rectifier (or solid state diodes) for the bias supply are bad. Sounded like they needed changing anyhow from your voltage readings. The bias filter caps should also be replaced for safe operation. So really the parts that are likely to be damaged should be replaced in any case even if they worked perfectly. Ten bucks in parts oughta do it.This is work that you can do yourself. Or to put it another way, if you are going to get into tubes you need to understand what bias is about and how to solder! Otherwise buy a solid state amp and suffer:) But really, rebuilding a fixed bias supply is pretty much the number one first thing to do if the main filter caps and transformers are okay so it's no big loss.
Now as to your friend....well let's just say he should pay for the parts....maybe even a new meter. And he should be made to understand that he could of very easily killed himself. Regardless of what he thinks he knows about electrical/electronics work anyone knows that they would have to break the circuit to measure current with a regular meter. Besides, there should be very little current draw on the bias circuit by the output tubes.
Your asylum friends can talk you through this repair and even suggest some things to do to prevent damage if anything like this should occur in the future.
Now the bad news....it is possible that your amp is badly damaged. But I truly doubt it. If it was turned off quickly....didn't blow any fuses....didn't smoke and stink like burnt tar...tubes didn't glow cheery red....etc. Then you are probably fine. But do put the fear of God into your friend and make him take some financial burden.
Russ
Follow Ups:
There are three can caps in my Sherwood S-5000:C1: 20uf/500V + 40uf/500V + 40uf/500V
C2: 30uf/400V + 20uf/400V + 5uf/250V
C3: 150uf/50V + 250uf/50V + 50uf /50V
Then there is a small electrolytic (C4) (under the chassis): 50uf/6VShoold I replace all of these?
Which value of separate caps I have to choose to match these?
Thanks
No smoke, no bad smell, no fuse blown, tubes still test good.
My friend offered to pay for repair of course, and even to buy me another S-5000 if the repair cost should be too high to justify it.
I am able to do some soldering (I succesfully replaced small electrolitics in my other amps) but not much more.
Two diodes, say 100 volt and 5 amp or better, and three caps, a 150uF, a 250uF, and a 50uF with a voltage rating of 50 VDC or better oughta do it. Just remember that the caps have the positive lead connected to ground! Remember to pull your tubes when you fire it up and check for proper bias voltage. Once you feel the voltage is okay then put in the 12AX7's and recheck before putting in the output tubes. One pair of 12AX7's gets their heater voltage from this bias supply so they will give you a visual indication of bias voltage. Quite likely that you will have to make some slight resistor value change to get proper heater voltage. It looks like that resistor is R68, a 47 ohm 5 watt resistor. Measuring voltage drop across it will give you current and with that then you can calculate the new resistor value via ohms law (voltage drop across resistor equals amps times ohms).The main can cap is a different job. You appear to have two multi section caps, C53 and C24. The first cap next to the rectifier tube, a 20uF should be no larger than a 40uF and should have a voltage rating around 500VDC. The rest of the caps can pretty much be whatever size you like and 450VDC rating should suffice. You can buy new can caps or you can wire in a suitable radial or axial replacement cap under the chassis. Something like a Solen would be nice for the first cap and Panasonic electrolytics would be fine for the reast...say around 50 to 100uF each at 450VDC. New power supply caps such as I mentioned above will make a big difference in the units performance.
Russ
Hi Russ,
thank you for the infos.
What about C2 (C24 in your schematic)? It's the one with 30uf/400V + 20uf/400V + 5uf2/50V. Should be replaced as well? So all three the multisection caps should be replaced?
Please explain me again the "positive ground" thing: if I use some separate electrolytic caps to replace the multi-can, I have to wire together all the positives to make a single ground with them and use the negatives to connect the various wires now connected to the "figures" (square, triangle, etc.)?
Thanks
ONLY the bias caps have the positive leads connected to ground.....the main power supply caps have the negative leads going to ground. And yes, replace C24 as well. Again you can use whatever values you want just keep the voltage the same or more. I would go bigger on the uF....like 50 to 100 uF on each section....it will make the amp perform better and cost is about the same. Good idea to replace the coupling caps too....the two 0.06uf and the two 0.04uF going to the output tubes. A few other things to do later on but lets get the bias and main filter caps right first.
Russ
Thank you Russ for this very important clarification about the bias caps as I was very confused about the positive/negative thing in all these can caps.About the electrolytics to replace the can caps, I didn't find anything rated at 500V or more, so I didn't buy the caps for C1 (C53). Any hint is welcome here, maybe to connect some in series?
For the other two cans I found the following, let me know if my choices are ok. They are not special brands, but mainly Elna plus Jamicon, Pec and Samwha.
C3:
100uf/50V + 47uf/50V to replace 150uf/50V
220uf/50V + 33uf/63V to replace 250uf/50V
47uf/63V to replace 50uf/50VC2(C24):
47uf/450V to replace 30uf/400V
22uf/450V to replace 20uf/400V
4.7uf/250V to replace 5uf/250VThanks
Raul
P.S. A thing about the coupling caps to replace: in my amp, the four caps coupling the 7199s to the 6BQ5s are four .068, not two .06 and two .04.
I have two Sherwood S-5000 schematics, the one of rather bad quality that's available on the net (http://www.triodeel.com/s5000.gif ) and another from a 1960's copy of a Sam's photofact. I was using this last one because it's a better quality copy and because the other says "Serial No. 959000 and up" while mine is a 5121XX.
But after all these helpful messages that I got from many of you after my request for help, I found that something was wrong in the schematics I was using. So I carefully compared the two and I found that I was following a wrong schematics.
The two schematics, for example, show the the bias supply rectifier with the two "diodes arrows" pointing into opposite directions: the Sam's (that I followed) has them pointing to the filter cap (as I did yesterday), while the "net's schematics" has them pointing toward the power transformer (as some of you told me to do).
Again: the 47 ohm 5W resistor (that is on my amp) becomes a 33 ohm 7W in my schematics.
Another: in my amp the can capacitor C2 (C24 on the "net" schematics) is a two sections (30/20/400V) can, and not a three sections (30/20/400-5/250) as the Sam's parts list states (and as I reported in my post above).
So, if the correct schematics is the one from the net, yesterday I installed the two 1N4007 diodes in the wrong way, and tomorrow morning I'll install them again as per the "net schematics" direction (arrows pointing to the transformer). AS HERE WE NEED NEGATIVE VOLTAGE, IS THIS THE CORRECT WAY TO INSTALL THEM, RIGHT?
Then I'll should be able to see if only the selenium rectifier was damaged from the short or if also the filter cap(s) suffered. However, as there was a low bias also before the accident, I think that I will replace at least also that bias filter can cap.
Of course, following your advices, I will pull the tubes and I'll check the voltages before putting them on again.
Raul
Or having a response from you, as if I were speaking to the dark blue sea. Maybe, getting over-zealous, my fault again??This phenomenon is common to this asylum. I don't need people to thank for me but at least, I want to know the inmates have read my listing and got better ideas.
The selenium rectifier due to age, would cause a higher voltage drop across itself and hence lower bias. a leaky cap will do the same.
Sorry Koo,
Of course I read carefully your posts and were these that put me in the doubt that something was wrong in my schematics. I have two schematics with opposite directions for diodes and by being a newbie I was very confused by this. I would have written to you to ask some more detail but I saw that you are not accepting emails. So after reading all the other emails I began to understand how all these things of positive/negative, bias and not bias caps work. Then I posted my conclusive opinion that my schematics was wrong.
Today I will replace the wrong diodes and I'll see what happen, then I would have posted the results and of course I'm very grateful to everyone that answered to help me. Sorry if I seemed ungrateful to you.
Raul
Sorry Koo,
Of course I read carefully your posts and were these that put me in the doubt that something was wrong in my schematics. I have two schematics with opposite directions for diodes and by being a newbie I was very confused by this. I would have written to you to ask some more detail but I saw that you are not accepting emails. So after reading all the other emails I began to understand how all these things of positive/negative, bias and not bias caps work. Then I posted my conclusive opinion that my schematics was wrong.
Today I will replace the wrong diodes and I'll see what happen, then I would have posted the results and of course I'm very grateful to everyone that answered to help me. Sorry if I seemed ungrateful to you.
Raul
I am a professional engineer and I am willing to give my advice (though the risk is on your side) and share my experience. Nowadays, this asylum is overwhelmed with advertisments(or better say, pre-advertisements) and Ebay grieves and it's driving me mad, too.When you post some topics really pertinent to the Asylum, very few people would respond.
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