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Hi,
today a friend came to my home while I was taking some voltage measures in my Sherwood S-5000. We were talking about the hot bias it had (-13 instead of -20) and about a resistor I had to replace maybe together with the selenium rectifier. When I was in another room, he took my digital meter, turned it into the ampere position and tried to "measure" the Amperes that were coming out from the selenium rectifier, but he did this in the wrong way, by using the meter as it was a voltmeter, with a plug into chassis (ground) and the other on the + of the rectifier. Some little sparkles came from the point where the plug touched the rectifier, with some pops. In the meantime I returned and immediately stopped him, but the damage was done. Now the amp has a big hum, and the music from a CD player is low. I tried to measure some voltage but now everything is messed up. The output tubes have 240V plate instead of the 380 that they had yesterday, the bias is non-existing (+3V istead of -13), and I turned all off to avoid other damages. I checked the 12ax7 tubes (that are feeded by the rectifier current) with my tube tester and they seem still all fine.
Now I have to find a technician here that can take a look at it (but in my area there are no good techs that I am aware of).
I think that at least one big capacitor (C3 that was connected to the selenium rectifier) is surely gone to create such a hum, but I don't know if there is some more possible damage (other filter caps?the secondary of the power transformer?). Can a failing filter capacitor explain all that mess?
Any advice? I can email a rather good schematics including the power supply stage if someone is interested. That C3 capacitor is: 150 - 250 - 50uf at 50V
Thanks
Raul
Follow Ups:
Today I replaced the two 1N4007 diodes (in the correct way this time) and all seems to work well now, voltages are in the ok range, the anode of 6BQ5's at 405V, bias at -18 and -20 (not all 4 tubes have the same bias, but it's better than the -13 I had before the "accident").
So it seems that only the selenium rectifier was damaged by my friend (and it had to be replaced as well, for the low bias). No resistor was needed to balance voltage with the diodes.
All can caps seem to be still fine without (Jimmy you were right, no damage by a so little current capacity), so I didn't replace any of them.
Now the amp sounds fine and it is very quiet also with the ear very close to the speakers (only a very little hiss).
So this bad story has its nice end I want to say this is really a great forum, with a lot of nice guys really wanting to help.
I want to thank in particular Steven Oda for the continuous and patient assistance during my repair work, Jim McShane and Jimmy for their kind offerings to repair it for me, merccougar89 for offering to call me from USA to direct my repairing steps, Koo for pointing me to the wrong diodes connection, Russ57 for the many good suggestions, and everyone that answered to my post.
My biggest wishes to you all for an happy and prosperous 2004!
Raul
basically its probably the sellenium rectifier and the filter cap should definitely be changed.I think its an isolated ground on that filter but i would just change the filter and the rect.Dont blame your buddy too much because being the fact your neg voltage was low these parts were do to be changed regardless.Any questions email me at amiejocpa@hotmail.com
I also would offer to repair it for parts cost only, but also live in the USA. It appears the rectifier was damaged. The capacitors should not be damaged. If so, it would be the first time in 40 years I experienced an arc (transient) puncturing the filter capacitor(s) insulation from shorting a power supply of such little current capacity.With equipment as old as it is, unless previously replaced, the power supply filter caps should be replaced anyways.
Many experts on this site should be able to guide you through the repairs.
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.
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.
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.
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.
sure sounds like the rectifier shorted out. If you replace it with silicon rectifier diode, the B+ will be higher becuase of less voltage drop across the silicon diode, you may need to add a dropping resistor.
Raul,Your friend probably blew up the selenium rectifier. DO NOT OPERATE THE AMP RIGHT NOW OR YOU'LL SMOKE THE OUTPUT TUBES OR WORSE. NOT EVEN JUST FOR A MINUTE TO "CHECK" SOMETHING!! The capacitors are probably fine, but there may be other damage. The output tubes may well be toast.
The low plate voltages are probably caused by the severe load on the power supply. With no negative bias voltage the output tubes are running wide open passing as much current as they can, and are loading the power supply way beyond normal; those tubes will (if they haven't already) destroy themselves if this continues.
Where are you located, maybe I can repair it for you. I am really busy but I hate to see more damage done to the amp.
thank you for your offer Jim,
but unfortunately I live in Italy (and I'm a newbie in this field). I don't think that the problem is only the selenium rectifier. I hoped it was so, and I already removed it and replaced it with two 1N4007 diodes with the two positives (band) tied together going to the to the C3 capacitor and the two wires from the power transformer to going the two remaining sides of the Y (it as a three wires rectifier and I read on the Asylum that it can be replaced this way). The hum was still here. No more tests made. Just tested all the tubes with my emission tube tster. They seem still good, no shorts, no leakage and they are still in the "good" range.
nt
REVERSED CONNECTION. THE POSITIVE SIDES SHOULD GO TO TRANSFORMER LEGS SINCE YOU'RE HAVING NEGATIVE BIAS VOLTAGE.
Basically, similar approach as other inmates except starting up with variac after repair.Depending on your transformer configuation, you're right for B+ only with centre tap, hence two diodes will be OK, one at each arm of the transformer output.
For bias voltage without centre tap,(check your original rectifier circuitry) a full wave bridge rectifier is the way to go(four diodes configuation) with standard product from RS or similar mfy. A rectagular or bead like black subject with four legs.(two for ac input, two for dc output)
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