Hoffman Amplifiers Tube Amplifier Forum
Other Stuff => Solid State => Topic started by: Lou Popadoo on March 28, 2018, 03:03:15 pm
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So I was given this amp that looked as though it had been in a flood, literally. I didn't hold out much hope, but, the main transformer was good, and after replacing nearly all the components including the Germanium transistors, all of the voltages are spot on, and it works well, sort of. Every thing on the amp works, good volume, good eq, but it sounds crunchy, as if it is being over driven, in either channel. It's not unpleasant and makes for a really neat guitar sound, but it's the only sound it makes and I know it is not right for this amp whose reputation is "clean". One more thing. The preamps, the interstage amp, and vibrato circuit are all in epoxy filled modules, built like Ft Knox. They are not accessible or replacable and it is very possible that one of these has an issue. But before I call it a day I thought I'd reach out and see if someone has a clue they could share with me.
Nick
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"Crunchy" can also be the speaker. You don't say you tried various speakers, so just asking.
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Yes, 3 different cabs.
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Is it crunchy at low levels and less-crunchy when LOUD?
There is (for related models) an idle bias adjustment to reduce crossover distortion.
http://www.standelamps.com/support/schematics/index.html
http://www.standelamps.com/support/schematics/1967.html
http://www.standelamps.com/support/schematics/1968.html
While this is not the Imperial, I would expect the company recycled designs; also the practical/popular topologies were not so varied as later in transistor history. (The transformer-driven plan on the 1968 models apes designs used by VOX and Rhodes in the same era.)
If you have the SIG OUT jack, use it. First: speaker power passes *through*, and disused/muddy contacts corrupt the sound, some exercise may help. That signal can be fed to a power (not instrument) amp and speaker to see if the crunch is before or after that jack.
As I am sure you know, there is very little out there for the SS Standels. Even less than for the custom tube Standels. Apologies if yours is *not* based on the 67-68 designs.
EDIT: an "Imperial" is listed for 1970. The power section is more modern.
http://www.standelamps.com/support/schematics/1970.html
http://www.standelamps.com/support/schematics/documents/1970/power_circuits/ic005.pdf
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PRR,
A guitar playing associate describes the sound as more like "light distortion", and that sound persists at all levels. The speaker jack has been cleaned and exercised and is in good shape. There is not a direct out on this amp, although I read that the speaker out can double as a direct out (?), but that would negate your test. This amp is "The Imperial" Model S110-V, and there is not a schematic on the Standell site that is close. This amp has one driver and two power transistors. But I did find a schematic that I believe is the correct one, or very close, at kevinchant.com. I've attached it and some before and after pics to this post. My hope is that all of the epoxy bricks are good, and that someone with more SS experience than I can point out what might be the problem. This amp's reputation, is a"CLEAN" sound, not what I currently have.
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a scope would be ideal, short of that, I would probably do something like I sketched
you can probably replace everything in the PA section, (except tranny), for ~$25
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We know it is OK up to the Green module because both inputs work.
We can't do anything about a sick module (or hope not to have to do so).
Measure the DC voltages around the power stage. Before you connected a speaker you should have checked for "low" DC voltage across the output. This is not a precision bridge: I would not panic about a few Volts if it came down to <1V with a 10 ohm resistor load.
The driver needs a part adjustment for the specific transistor used. This "should not" go out of whack even in 50 years, but check it. Also there's a lot of new-looking parts. Examine the whole rig for mis-repair.
Measure the DC voltages *across* (not to chassis) the two 0.47r resistors, no signal, no load. We want small but non-zero. There is a general optimum *near* 30mV, however this depends muchly on other circuit details and is not real critical on a stage-amp. 5mV to 100mV may be all fine. It's not adjustable, we just want to know if it is *wrong*.
What's with the attic insulation in the 3rd pic?? This amp should have all the cooling it can get.
> you can probably replace everything in the PA section, (except tranny), for ~$25
No. It is all Power Germanium. Very hard to find authentic replacements today. We "could" redesign for Silicon but that means a major re-design of the biasing, which I gather is not Lou's strong-suit. I thought "LM3886" but it won't fit the existing heatsinking. Anyway any radical change will leave doubts about "authentic sound". If it isn't an as-new Standell, then it is some hack of no special value.
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It is all Power Germanium
I failed due diligence class :think1:
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PRR and Shooter,
So the first thing I did after reading your posts was to check my work, and....I had bought NOS Germanium transistors per the schematic and mistakenly put an ELM 121 (driver) where a DG110 (power) should have been. DUH! I corrected that and lo and behold, the amp sounds beautiful. Very Clean. Also the voltage across the two .47r resistors is 29.9 and 28.2 respectively.
The third pic is a before pic and that is the condition I received the amp in. Once again many thanks to you for pushing me to think a little harder and making me a better tech. I am now going to attempt to recreate the Red Module (vibrato) which is definitely toast. I've attached a drawing that I got off of the Standel site. Can anyone explain the "light bulb" to me?
Nick
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> the voltage across the two .47r resistors is 29.9 and 28.2 respectively.
Volts? Kilovolts? MilliVolts?
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> explain the "light bulb" to me?
How you gonna swell/fade the audio? One way is a controlled resistor. In this period light controlled resistors were new technology. The CL702HL is such a thing in a box with a light-bulb. The light is driven by a low frequency oscillator to tremulate. The twin-T network on the right transistor suggests an oscillator, and uFd caps suggest sub-audio rate. The lamp is a heavy load so the right transistor buffers the heavy load from the weak oscillator.
I guess being Standell you have no choice but to do it EXACTLY. Good luck on a CL702HL. That's a likely fail. But do the electrolytic caps first, cuz they are generic fails.
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Sorry, mV, and thanks for the explanation.
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29mV is 62mA which makes 1.5 Watts/transistor at idle. The full-power dissipation is over 15W so we don't worry about idle heat. The transistor re is very close to the added 0.47 Ohms so the crossover distortion should be fine. I think you are done there.
I have no idea why it was sour with output/driver interchanged. _I_ would have guessed it would work fine until the undersized driver melted.
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Great, because it sounds really nice.
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why it was sour with output/driver interchanged
gain? :dontknow:
If I crossed 'em correctly, the driver has an HFe of 80, and the PA 90.
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Now that I have the Standel up and sounding great, thanks to help from PRR and Shooter, and being a glutton for punishment, I am going to attempt to reconstruct a "Red Module", which is the tremolo module, which does not work. I worked my way through the expoxy and have gotten a decent look at the circuit board and its components. It is "nearly" identical to the handwritten schematic I've attached, which is "nearly" identical to the printed schematic I've attached. To point out a couple of things, the 25K pot is the intensity pot, and the 1k pot is rate. The LDR is not located in this module, it is in the "Black Module". And, on my amp the footswitch was connected to the wiper of the intensity pot. I'm confused about two components, which I've circled in yellow on the hand written schematic. One is a transistor, and the identifier associated with it. Does that number mean anything to anyone out there, because I cannot find a part associated with it. And the other circled component appears to be a ceramic non inductive resistor that reads 256K, question being, can that be correct? Thanks,
Nick
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I've a attached a better view of the second schematic. Also the transistor in question has no markings except a 440 on the top, which only comes up as an NPN in my searches.
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> ... appears to be a ceramic non inductive resistor that reads 256K, question being, can that be correct?
The CMI factory schematic clearly shows a 1K trimmer, with a note /1\ saying how to adjust it.
Perhaps they got a run of optos so consistent that they switched to a fixed value. Clearly 256 Ohms is in range of a 1K pot, 256K is not. However start large, so you don't burn the lamp.
I would use a 1K resistor in place of the lamp until you have stable wobbling at controllable strength.
> One is a transistor
The transistors are PNP jellybeans. Specs not fussy. IF truly bad, I would use 2N3906. However they are unlikely to be bad.
MUCH more likely are the electrolytics. Leaky C16 C17 C19 will stop the action. May as well do C20 too.
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PRR,
I'm doubtful I can reuse any of the components. Getting through the epoxy is nasty and destructive. I'm doing a complete rebuild, so your suggestions are a great help. I'll keep you posted. thanks
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Getting through the epoxy
In the way back days, I was the ships "guy" for burnt, epoxy'd, or other. we had 2 spooks living above our work center, and they came to me with an epoxied board that fried, I worked it to the charcoal, and legs and gave up, I told them I could fix it if I had a schematic, an act of Congress later I got upgraded from Secret to Top Secret, and a schematic snippet, couple days and their Dec Alpha look-a-like was runnin, I know what a :cussing: that is
glad you preserved, there's a perverse happiness when you nail it :laugh:
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Thanks Shooter. It's good to know that I 'm not the only one to have endured that horrible smell.
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Shooter,
Regarding Standel "The Imperial" vibrato circuit; I've reconstructed the "red module", which was basically an LFO circuit, per a combination of schematics that were nearly identical. I built exactly like Schematic#1. The LDR and Bulb part of the circuit are in a separate epoxy module, the "black module", which apparently also does not work. This is where I've hit the wall. That part of the circuit on the "red module" drawing is incomplete, and that part of the circuit also lacks detail on the drawing I used to build the LFO, Schematic#1. The drawing of the "black module is equally as confusing, as it shows 5 wire connections, while it in reality only has 4. And in yet another drawing Schematic #2, which could include my amp serial number, is yet another configuration for the LDR circuit. The schematic of my amp is MyStandelAmp. Oh boy, I need advice. I am in possession of a new Clairvex CL705HL LDR, and a bulb that is close to spec. I'm just not sure how to complete.
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Shooter,
Here are the other drawings, sorry.
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You get extra credit for hanging tough! :laugh:
I'm out the door for work, I grabbed your schematics, when I get home tonight I'll try and make sense of them
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Your "black module" drawing shows a buffer transistor which clearly does not fit your "MyStandelAmp" drawing.
I think your Black Module is "just" the optocoupler.
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I LIKE it !
fwiw I started "straighten'n out" the hand-drawn, still a work in progress
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That oscillator is a brain-buster.
Rather than try to understand, I drew it for the idiot assistant. With one sim tweak, it fired right up and a likely frequency and amplitude. Adding the Inten network, another transistor, and resistor (lamp stand-in), I get likely-looking wobbly current. So I believe my re-draw is a correct implementation. Study of the patient is needed to see which parts are "module" and which parts are off-module (clearly the knobs; the lamp may be over in the Black module as previously guessed, its series resistor is somewhere). "My" R36 R37 (lamp and its series resistor) may be in either order; whatever order the factory did it is fine.
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PRR and Shooter,
Thanks for the great response. Yes, this did bust my brain. To address your questions, and ask a few more; The "black module" does appear to have a transistor in it as I can see the outline of it in the epoxy. All the components except the pots ,the LDR/bulb circuit, and the 10K resistor on the low side were in the "red module". Per PRR's changes;
1. you said you added a transistor but i don't see it on your drawing
2. I'm a little confused about how the pots are wired in your drawing. Right now, per the MyStandelAmp drawing, the Speed pot wiper is going to Q3 emitter, and the other leg is going to R30. The intensity pot has the footswitch tied to the wiper which goes to the base of Q4. If that is wrong, could you clarify?
3. If you look at Schematic #1 you'll see that 270ohm resistor is connected to the footswitch. (Where the footswitch connects has also busted my brain)
4. What does the SET=0.5 mean?
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> What does the SET=0.5 mean?
Sim notation for variable resistor setting (halfway).
I'm baffled by the apparent extra transistor in the black module.
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I'm still playing with the puzzle :icon_biggrin:
red module = tremolo osc circuit? = part function board?
Black module = trem input?
I like the "master schematic.pdf", yourstrandel, mostly "lines up"
does any input go to the black module 1st?
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The Black module has 4 wires attached.
Yellow = -4V
Gray = Signal from Volume pot channel 2, the Vibrato Channel
Orange = From Red Module, Q7 Collector
Black = Ground
Also I have attached a photo of the circuit board i removed from the Red module, and a reconstruction of the components I found. They are marked with the component numbers from Schematic#1 which I have attached again to this reply. And of course what i found in the Red module doesn't exactly match that schematic or the red module schematic. I hope this helps.
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I think it's getting closer;
the red and black pdf's each show an ldr, I'm assuming there is only 1 in your amp?
This is a re-draw of the hand sketches. What if the light is in the RED and the R is in the black?
I'm thinking the black is a gain stage that "kicks" in to mix with the dry signal by "shorting the Base R (R inside the ldr) :dontknow:
EDIT: added schematic my block diagram view :dontknow:
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Shooter,
There indeed is only one LDR/Bulb in the amp, and it is in the black module. In your block diagram is the orange wire the -4V connection?
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the -4V connection?
no clue :icon_biggrin:
I'm with PRR, not sure why your black module has a transistor when it can clearly be done like schematic 2.pdf where the R part for the PCA-1 does the wiggling of the dry signal
my guess is the R now wiggles the base of the transistor in the black module which amplifies it then wiggles the dry signal, but what wire goes where....... :dontknow:
have you successfully got the red module to oscillate?
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I have not, which is confusing as I believe my circuit is correct. I'm not sure the circuit is being activated properly as the placement of the foot switch in the circuit is confusing. MYStandelAmp drawing has it connected to the wiper of the intensity pot, which is also where i found it originally, yet in all other drawings it is connected directly to the bulb.
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So I hooked up my scope to the collector of Q7 and got this picture of a jittery sine wave. The amp was not even plugged in. Am I oscillating?
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got this picture of a jittery sine wave
:dontknow:
I would NOT expect a transistor circuit to work without power, BUT maybe you cracked the perpetual motion nut :icon_biggrin:
take a look a the image from PRR's sim, I would expect something close with power applied
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So after solving the mystery of Perpetual Motion, I plugged the amp in and turned it on and got this image from Q7's collector.
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I'd say PRR nailed it, nice modeling
sooo, as long as you're bread boarding,
wire up like schematic_2.pdf, ala, no black module.
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So I did the following with PRR's drawing as a guide; Using alligator clips I connected the collector of Q7 to one leg of my bulb. The other leg of the bulb i connected to a 270ohm R and then to the -32V power rail. My bulb did not light, but the bulb in the black module did, and when i plugged in a speaker and a guitar I had VIBRATO! So I guess my black module works. But, I had no control, that is, the footswitch, the intensity pot, the speed pot and my 1K VR, had no effect on the vibrato.
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The other leg of the bulb i connected to a 270ohm R and then to the -32V power rail
If you look at schematic2, your handdrawn red, they show "Q7" with +32 at the emitter, and ground at the collector via bulb and FS. if you OPEN the FS it should turn off Vib, shorted turns on.
still looking at the pots..................
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So as usual PRR and Shooter made me think a little harder and triple check my work, which led me to discover that I had made a couple of errors in the LFO circuit, so I rebuilt it exactly to PRR's drawing. See attached. Now let's pretend that the Black Module does not work. If I attach one leg of a bulb to the Collector of Q4 on PRR's drawing, and the other leg to -32v via a 270ohm R, and step on my footswitch (which is connected to the wiper of my intensity pot and the base of Q4), the lightbulb lights up, but does not pulse. If I put a meter on the Collector of Q4 and engage and disengage the footswitch, it reads from 0V to a fluctuating voltage of between 4.6V and 3.1V and everything in between. And if I put a scope on the Collector the LFO is clearly being activated by the footswitch. So why aren't I getting the bulb to pulse? I wish I knew.
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If I put a meter
with FS engaged
what do you get with a scope there, Q4 collector
how about Q3 collector
in PRR's schematic 10k int is drawn as a fixed R, did you use a pot
if not, short out the 10k (gator jump across the 10k) and recheck Q4's collector
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Shooter,
On my amp, PRR's R33, is the 25K Intensity pot. Attached drawings show how it is connected on my amp and recreated Red Module. I'll be back with scope answers.
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I've been using the scope on the two Collectors, and I believe the Vibrato circuit may be working in the opposite manner than it should. Again bypassing the black module and using an external bulb as i described in yesterday's email; When the bulb is on the Speed and Intensity knobs seem to have no effect on the scope. When the bulb is off, the Speed and Intensity knobs affect the scope's waveform in the way you would expect. Only in the opposite direction. That is Speed and Intensity in the waveform slow down as I turn the up, and seed up as i turn them down. Also, the waveform on Q3 is a rolling wave, on Q4 it is not.
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my footswitch (which is connected to the wiper of my intensity pot and the base of Q4)
you have the FS wired with ring to ground, or one of the rail volts?
Q4 is doing what I expect, looks to be ~~8vac if you have a 1x probe
q3 might be "influenced" by the scope
what is your scopes time base set at, wanna make sure we're not seeing some external influences
Freq = 1/period time, so if you are 1ms/div, 1/.001 = 1000hz
using an external bulb
is this your lrd, or just a bulb you have laying around
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I'm using the footswitch jack on the back of the amp, so the ring/sleeve is going to ground
Scope is at 2ms/div.
I'm just using a bulb
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2ms/div.
I might be rusty, my scope just displays the data, so I got lazy, but I come up with ~~~166.7hz, (1/6mSec)
soo, switch your scope to "AC coupled", probe PS points B C D,
I would expect a SS trem to be in the same range as tube, 3 - 10 hz. If I slipped a bit, we could be at 16.7
this is a rabbit hole, but ohm out the resistance of your bulb and compare it with your ldr "bulb"
EDIT: just went back to PRR's plot, get about ~~150mS 1/.15 which get about 6.7hz, so I think we already jumped into a hole
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The resistance of my bulb is 48.4ohms. There is no way I can measure the resistance of the bulb in the black module.
The attached picture is what PS points B,C, D, and the collector of Q4 all look like, with the scope set at 1V/div. and 1ms/div. The only point that looks and acts like Vibrato is circled in the second attachment. At this point I'm seeing an 8V, 6ms wave modulate and actually react to the Intensity and Speed knobs.
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picture is what PS points B,C, D, and the collector of Q4 all look like
is this a 2 pronger amp
I'm not sure if we have a real problem, or your amp is floating and your scope is grounded, IF, you got a 2 prong.
8vacpp seem way high to me for SS, 1/2 wave rec, will probably let that much through. On the Pwr side you get cancelation, on the pre/trem side :dontknow: I' have a SS audio amp on the bench and it has mVAC on all the rail volts, this is a guitar amp, maybe they designed in hum :icon_biggrin:
having the "same" signal "everywhere" is a good indication there is a missing ground somewhere
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This is a 2 pronger. Also it helps if i set my scope up as you asked. When I set it to AC coupled I'm getting 265mVAC at point "C", no AC at point "D", and 333mVAC at point "B". It definitely looks like I'm getting oscillation at the point I marked on the last attachment, as well as at the Collector of Q4. Also the footswitch and the Intensity and Speed knobs appear to be effecting the signal at Collector of Q4 properly. All voltages are nearly spot on.
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looks like I'm getting oscillation at the point I marked
the problem is what freq it osc. looks to be at 60hz instead of 6hz. So either you are just seeing ripple from the mains power, or you have built a a 60hz osc instead of a 6hz one.
spend some time verifying the AC at point C, is or is not the same as Q4's collector.
PRR shows a period time for 1 sine wave ~~~150ms, that is the ballpark you want with enough amplitude - guessing 3-8vac p-p is close-ish
All voltages are nearly spot on.
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The proper ground point for AC helps...The ACV at point "C" and the Collector of Q4 are the same, 1.7VAC. When I said the voltages are nearly spot on, that meant the DC voltages.
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The ACV at point "C" and the Collector of Q4 are the same
voltage is a secondary issue, frequency is the primary issue, if your oscillator is "syncing" to 60hz, that's a problem, if it's producing 60hz, that's a problem. You are correct, you need a good, solid DC, for proper AC operation, so we've got that far :laugh:
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An easy way to tell if that signal is derived from the power line rather than the oscillator is to set the scope trigger source to Line. If you can then lock the signal display then that signal comes from the power line. If the signal display is unstable or simply drifts slowly rather than being rock solid, it is not synchronous to the line and is probably being generated by the oscillator.
BTW, that's the ugliest trem signal I've ever seen. I would expect to see a very clean sine wave. To produce a useful trem effect, the frequency should be about 3Hz to 10Hz (period .3sec to .1sec). Your 6.2mS (161 Hz) signal is useless for a tremolo effect.
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With the scope on Line, the signal is a "little" jittery, mostly solid. My work is per the drawings I have, with the help from this blog. I will review all again. Any more insight as to why I have an unusable trem signal, or how I could be getting a signal from my power supply is much appreciated .
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did you build your osc on a perf board, or breadboard, If so, try and make it work from an independent power supply. you can "cheat" things like pots by using a R value ~ 1/2 the pot value.
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Are you focused on making that trem circuit work?
Or are you open to using a different trem circuit that will tie into the signal path like the Standel circuit? If so, look at this simple phase shift oscillator that uses common easy to find parts. I put this into a Fender amp...
http://sluckeyamps.com/misc/76_Amp_Project.pdf
This circuit uses a home made opto-coupler with a Radio Shack photo cell and tiny incandescent bulb. But, the opto-coupler can be replaced with a Vactrol VTL5C1 simply by changing the series limiting resistor. You could use the VTL5C1 in your circuit too.
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I just picked up A very nice Imperial (S110-VA) which appears to be working very well - needs a new head cabinet - but is complete.
The odd thing is there is no "Black" module in the amp.
No of the various schematics - The Custom/Imperial one or the "S110" one is quite correct
I don't have the -4 VDC circuit either,
The one Orange lead - apparently from the "top" of the LDR goes to the output of the "Blue" module at the volume control
The other Orange lead - right next to the first - goes to the Ground side of the Volume control.
Not sure of the date of this Standel but this looks like it has a "M6" Red module - found on the Custom/Imperial schematics - that doesn't have a "Black: module in the circuit. At least the wire color codes and hook-up match my Imperial.
I wonder when the "Black" module was added to the Tremolo circuit - maybe it was to make it easier to change a bad Light/LDR when needed - probably a cheaper module.
Mike