Sherwood S-1060 Amplifier Restoration

Discussion in 'Tube Audio' started by 365nut, Jun 10, 2018.

  1. 365nut

    365nut Member

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    I’ve had a pair of very rare Sherwood Mono Integrated Amps in storage for many years after purchasing from who knows where. These put out 60 watts of audio from 6 – 7189s. I recently commenced the process of restoring one of these and have some questions but first will describe what I have done.

    Initially powered it up on variac over time to reform caps and got very faint audio output which told me the fundamentals were there. I began by replacing the main C1 Can Electrolytic with one from Antique Electronic Supply which fit perfectly. Bypassed other 2 Electrolytic cans with under chassis replacements. Due to the unusual (“valley”) design of this amp, there was plenty of room for replacement caps on the bottom side of the chassis even though overall the amp has a very low profile. Then, replaced all the caps one would normally replace in an old amp (mostly Orange Drops) and powered it up. Works real well and has a very robust sound. I next proceeded to address power supply issues.

    This amp does not have a rectifier tube (unlike the Sherwood S-1000 which uses a 5Y3). The main rectification in the 1060 is done by 6 diodes located to the left of the power transformer (see pic.). The voltage readings after this are very close to factory specs—so the diodes seem to be good.

    In addition, there is a selenium rectifier (M-7 on pictured schematic) which, according to the schematic receives 25 VAC from the power transformer and is supposed to put out -3h0V DC @150ma. It is getting 25 VAC in but I get a -15 volt reading on the output. Tis tells me the selenium rectifier is in partial failure. I have planned to replace the selenium (for obvious reasons) but am not certain what type diodes to use. I assume the 2 anode ends will be connected to the 2 - 25 VAC blue wires from transformer and the negative ends will be wired together and connected to the C2A terminal of the electrolytic and I may need a dropping resistor. Do I have this right?

    I don’t deal in diodes much and am lacking a bit in knowledge. Any help would be appreciated.
     

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  2. BinaryMike

    BinaryMike Pelagic EE Subscriber

    Not quite right. Rectifier cathodes go to the blue wires, anodes tie together and to added dropping resistor. Resistor value is difficult to predict; it's best determined by experiment. Plain old 1N4007 commodity rectifiers are usually satisfactory, but a lot of folks here have different opinions on that topic. I'll stand aside...
     
  3. gadget73

    gadget73 junk junkie Subscriber

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    I'd get the voltage on the tube heaters proper and maybe sub a trimmer for R73 if it has no means of adjusting bias at all. 5K pot + 2.2K fixed resistor should put the pot pretty much center travel for stock voltages, adjust as needed to get proper current draw on the output tubes.
     
  4. 365nut

    365nut Member

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    Thanks for your comments, guys.
     
  5. 365nut

    365nut Member

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    Follow up Information

    Using 2 1N4007 diodes to replace selenium rectifier, the correct hook up is that the anode (banded) side connects to the transformer AC output (blue) wires. This is the only way they will work. I discovered this by first hooking up the cathode sides to the transformer leads as suggested and had no voltage reading. After reversing and experimenting with various dropping resistors, I got to a -25 volt DC output per the schematic (see in previous post). This little circuit mainly powers the 12AX7s. The voltage readings on the schematic vs. actual are at great odds for the power transformer outputs providing AC power to each of the series of 3 diodes that produce the main rectified voltage for the rest of the amplifier. The factory spec. is 340 VAC. The actual is a whopping 706 VAC @ 122 VAC line in. Reducing the line voltage to 100 brings the power transformer output voltage to 596. The actual DC voltage off the 6 diodes is 430 vs. factory spec. of 465. The 706 transformer VAC is bothersome but guess will have to live with it. It is what it is. The bottom line is that this amplifier sounds great and is very quiet--no hum. I do plan to do some further voltage checks to make sure things are stable as I can get them. I will then proceed to restore my other S-1060--and ultimately 60 WPC stereo.
     
    Last edited: Jun 12, 2018
  6. gadget73

    gadget73 junk junkie Subscriber

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    HV voltage is actually right. Its 340 from ground to either leg, so double that across the whole thing. Add in some for higher line voltage and tolerance and 706 vac is reasonable.
     

     

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  7. 365nut

    365nut Member

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    "Gadget" Thanks. That's comforting to know. The transformer does run very hot. There is a lot of melted wax on the output wires as proof. A beefier transformer would have been better but because of the space compromise with the low profile design, this is what you get. The 7189s are installed at an angle at the rear and have a steel cover over them but do have a lot of ventilation space. The operating instructions specify placing the amplifier at least 2 inches from the wall (I guess to keep from burning your house down). The outside cover (which holds the front plate) adds to the heat encasement but pretty difficult to operate this amp without installing the cover because you do need the front plate. I'm going to figure out a way to run a cooling fan to try to keep the temp down.
     
  8. BinaryMike

    BinaryMike Pelagic EE Subscriber

    The banded end is the cathode connection on all modern diodes.
     
  9. 365nut

    365nut Member

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    Good to know. Thanks.
     
  10. gadget73

    gadget73 junk junkie Subscriber

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    Have you measured the voltage drop across the 33 ohm resistor in the power supply to confirm if the voltage drop jives with what the schematic says? I can't see all of it, but it shows 150ma, so by that logic I'd expect to see very near to 5v across it. If you have significantly more you may need to adjust the bias supply voltage to get it back to stock current demands.
     
  11. 365nut

    365nut Member

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    Gadget, Thanks, I'll check that out.
     

     

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  12. 365nut

    365nut Member

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    UPDATE & QUESTIONS

    Shown in pics are main power supply schematics for 2 Sherwood S-1060 integrated amps I’ve just restored. These consist of diodes numbered M1 through M6. It is curious that the final voltage measurements from both is 447 & 449—virtually identical (see arrows). Yet the intermediate voltage measurements (shown in blue) are substantially different on each of the amps. There must be an explanation. Anybody have one? Also, because house current here is 122 VAC the measured VAC coming from the transformer is around 360 for both amps vs. factory spec. of 340. One would expect the final (after diode) voltage to be greater than the factory spec of 465. Yet the actual measurements are 449/447. Would be interested in an expert’s opinion on this. Because of the 447/449 “final” voltages, the other source voltages are all less than factory spec but both sets sound fine (very quiet no hum) I do not worry a lot. The main worry is the temperature of the power transformers which run hot after a short time of operation. Haven’t gone into the output tube biasing yet—so await results of that.
     

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  13. BinaryMike

    BinaryMike Pelagic EE Subscriber

    DC voltage measurements within the diode strings aren't meaningful.

    The low main B+ voltage despite excessive line voltage is an indication of power supply overload. Don't wait before checking bias current in the finals.
     
  14. gadget73

    gadget73 junk junkie Subscriber

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    Confirm the diodes aren't bad, basically not open and not shorted. Beyond that, excess load and/or low value first cap is the likely reason for low voltage. Check the voltage drop across that first resistor and see what that says. If its more than spec, you have something drawing too much current, and its probably the output tubes. Adjust the bias as needed to get the proper voltage drop.


    If you can control your input voltage, adjust it until the heater supply is at 6.3 vac. From there the rest of the voltages should fall pretty close to schematic values if things are right.
     
  15. 365nut

    365nut Member

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    Appreciate your responses. Have some interesting additional information. Dug up an operating manual I forgot I had--original Sherwood issue with an interesting schematic. It shows the voltage at the end of the 6 diode rectifier string as 440 not the 465 volts indicated on the Sams schematic. Thus my actual readings at this point are much closer to the Sherwood spec. So, that changes things a bit. One wonders how there could be such a large deviation in so-called "factory" specs. All part of the intrigue in figuring these things out. Thanks again.
     
  16. gadget73

    gadget73 junk junkie Subscriber

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    Sams schematics are known to not always be gospel. If you have a factory one, I'd go with those voltages.
     

     

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