Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: SleepLess on January 22, 2019, 09:48:00 am
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Hi there guys!A friend of mine dropped his Soldano SP77 at home yesterday with the following symptoms: it works and still produces sound but the sound is muffled and lacks all its dynamics. He told me he tried other tubes to the same results and then being no expert at all, gave it to me for a check-up.
So far I have opened up the SP77 and I see no signs of burnt tracks on the circuit board and no swollen up condensers. I'm totally new to these kinds of amps (preamps actually) since I only build and repair point to point amps, mostly replicas of old Fenders and Marshalls, but I'd really like to help him. Is there any idea that comes to your mind? Do you want some pictures to help?Thanks a lot!
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Hi!Thought I'd send a couple of pictures. As you can see, there is no sign of burnt tracks or components. Don't really know what to start with... Thanks for your help!
https://ibb.co/zZd69j4 (https://ibb.co/zZd69j4)
https://ibb.co/d5DxH5V (https://ibb.co/d5DxH5V)
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Not really helping here but according to schematic the signal goes thru the S2 and S4 vactrols.
Not sure if these goes bad over time but i would probably try to jumper those if that is possible.
Members here who know better can tell you if that is possible to do without braking anything and what pins to jumper.
again not really helping just thinking out loud :smiley:
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That's already helpful. I'll look into the schematics! Thanks!
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Regarding the Soldano SP77 schematics available on internet and in this forum database, the "redrawn" version it's crap.
I currently built a Soldano SP77 clone using that schematic and found a lot of mistakes compared with the original version ( the one from the manual).
For example, the optocouplers that controls the switching channels are not paired properly (according to the original schematic, they must be S2 with S4 and S1 with S3 and also a bias resistor is missing in the switching section of the schematic.
In the tonestack section, the cursor of the Middle potentiometer is connected to GND! I didn't met this kind of tonestack till now :laugh:
So, my advice is to use the original version of the schematic if you want to debug the preamp.
Sorry if you consider this post offtopic.
Goodluck!
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No it's clearly not off topic! Thanks, that will help! Still haven't found the bug...
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In the pictures from your second post, I observed a red wire connected from the R11 to S1.
I'm not familiar with the SP77 PCB, but from what I see, I think that wire bypasses the V2A stage. Am I wrong?
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i was looking at some gut shots of sp77's and to me the red wire looks like a mod where the mute point has been changed X2b grid to X2a grid, i could be wrong too
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I'll look into that... But that wire was already there before the amp started bugging...
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Do you think the issue could be coming from one of the optocouplers?
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@SleepLess
You met this issue at both channels, or only one of them ?
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The clean channel works normally, the high gain channel has the problem...
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OK. If you say that the clean channels works normally, then you have to investigate the problem on the V2a, V2b and V4b stages.
Check the coupling caps, plate resistors, cathode resistors, cathode bypass capacitor.
You must measure the plate and cathode voltages for these stages an see if they're ok.
If you have an oscilloscope and signal generator, you can also track the signal through these stages.
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Before doing all these measurements, be 100% sure that the tubes from the Lead channels are ok. If you have some new tubes, it would be great.
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I actually meet the problem on both channels. I tried it through a poweramp and both channels sound muffled. Don't know where to start, I see no signs of burns or swelled caps...Thanks!
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the fall likely flexed the PCB and cracked one or more traces. the cracks are not always visible. in this case an ohmmeter and a lot of time tracing will help. since you'll be measuring continuity of every trace from end to end, you may as well draft an accurate schematic.
--pete
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It didn't fall. When I said "a friend of mine dropped his SP77" I didn't mean that it fell, I just meant that he left it here for me to check (sorry!). I still have to find what's wrong... Thanks!
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Resolder all solder joints especially tube sockets and big components.
You could have a cold solder even if isn't visible.
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The solder joints are all good. The tubes are all good. Don't know where else to look to fix that muffled sound issue... Is there a voltage sheet available somewhere to check the values that I get?Thanks!
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Could someone tell me how to check the vactrols VTL5C1 optocouplers? I'd like to be sure the issue comes from there...
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I think you can measure the resistance between the leads that are on the side where it reads CELL. You should get two readings depending which channel is on. I suggest you download the datasheet for the VTL5C1.
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Great, thank you!
Update: I just did a diode test on my MM connecting the probes on the emitter and the collector. I get a reading on S2 and S4 but no reading on S1 and S3. Can I conclude that S1 and S3 are dead? Thanks!
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could you post a schematic of the switching circuit you used?
--pete
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Hey Pete. I didn't use any, I tested the optocouplers straight on the board.
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so then you just simply switch both pairs off/on with a SPDT switch? similar to as what is attached?
--Pete
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No Pete, I didn't do any of that, I simply used my multimeter on the diode mode.
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What readings you got measuring resistances at the CELL side?
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oh and Pete, he is trying to fix factory made SP-77. Pictures are on the second post.
in case you are thinking the other thread where vladm made his own.
-with respect
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What readings you got measuring resistances at the CELL side?
Cell sideS1: 0.11MΩ S2: 2.4MΩ S3: 0.56MΩ S4: 0.53MΩ
On the LED side, with the MM on the diode mode I get a reading between the anode and cathode, and no reading the opposite way (or the other way around).Hope that helps, thanks for your help, I'm almost about to give up on that thing and tell my buddy I've got other things to do... :sad2:
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and what are the readings on the CELL side when you switch the channel?
oh and did you have the amp powered when you measured the resistances?
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No the amp wasn't plugged in. Does it need to be?
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Yes, resistance on the cell side should change depending which channel is engaged.
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OK, I'm going back to the amp!
Update: so with the preamp plugged in and turned on, here's what I get at the cells:Green channel (clean): S1: 0.25kΩ S2: 0.26kΩ S3: 0.35kΩ S4: 0.14MΩ
Red channel (OD): S1: 0.11MΩ S2: 0.5kΩ S3: 0.15MΩ S4: 0.44kΩ
Hope that helps, thanks again!
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Thanks, now we just have to hope someone who can interpret those readings reads this :)
link to the schematic is here:
https://el34world.com/charts/Schematics/files/Soldano/Soldano_sp77_preamp.pdf (https://el34world.com/charts/Schematics/files/Soldano/Soldano_sp77_preamp.pdf)
to me they look little odd but then again i have never dealt with optocouplers
:dontknow:
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I haven't been taking notes, so....
just looking at the #'s S2 "stands out" it does double, but the rest are way bigger than 2wice
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I don't know what to make of that...
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try; "S2 stays in one state instead of 2". So "guess" a low R = ON, and since it's still relatively low (compared to the other 3 switch values), it NOT switching fully OFF
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try; "S2 stays in one state instead of 2". So "guess" a low R = ON, and since it's still relatively low (compared to the other 3 switch values), it NOT switching fully OFF
Hi shooter! So if S2 doesn't fully switch OFF, it means it's faulty. Do you really think that could cause the muffled sound issue?
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I finished my Sp-77 preamp clone using relays instead of those VTL5C1. The switching groups are : S1 with S3 and S2 with S4.
There shouldn't be "intermediate states" for them. They must be ON or OFF.
You can also try this:
Remove all 4 optocouplers, and use some jumpers instead of their LDRs. First put jumpers on S1 and S3, check if the channel works ok, then remove the jumpers and put another 2 jumpers to the S2 and S4 and check the other channel.
In this way you can check if the problem is from the optocouplers.
Hope it helps.
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I gave the preamp back to my friend without sorting this issue out...
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Sometimes that's the best thing to do. If there were no charge, you did him a favor and nothing blew up. All good.
silverfox.