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Thread: Lindell No. 603

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    Member patlaw's Avatar
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    Lindell No. 603

    I've never heard of Lindell amps before, but I'll ask anyway. Does anyone have a schematic for Lindell Model No. 603?

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    Lifetime Member Enzo's Avatar
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    Probably nothing to do with present day Lindell Audio.

    Looks like a generic basic amp from the 1970 era. Volume, tone and trem controls.


    It looks like any of several others, maybe someone can suggest a particular model. But the circuit ought to be pretty basic. What is wrong with yours?

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    Member patlaw's Avatar
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    I've been asked to take a look at it. Here's what the owner says it's doing. I haven't seen it yet. "Sometimes it transmits the sound quietly and sometimes not at all. The red light comes on when turned on. It's a cheapo amp, but I played it for many years. The thing I really like about it is how light it is – about half the wt of the little Fender Sidekick."

    I'll probably take a look out of curiosity.

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    Lifetime Member Enzo's Avatar
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    There you go, it's an old Teisco.

    Five transistors.

    My first suspects would be the contacts on that jack just above the speaker on the schematic. Or whatever takes its place in your amp.

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    Member patlaw's Avatar
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    Quote Originally Posted by Enzo View Post
    There you go, it's an old Teisco.

    Five transistors.

    My first suspects would be the contacts on that jack just above the speaker on the schematic. Or whatever takes its place in your amp.
    I finally got the amp. While the schematic above is similar, it's not correct. Plus, I can't read most of the values. The jacks on the speaker are not present in the amp. The speaker is directly wired to the circuit board.

    When I first connected the amp, the volume was low, but the audio was getting through. The problem is in the input stage because the residual hiss is what I would consider about normal for an amp with germanium transistors. One thing is clear. This input circuit design is terrible even for a dirt cheap amp. The volume control is on the input to the first transistor.

    While the amp was out of the cabinet, I poked around a little and tightened the nuts on the input jacks. Now the volume is what I think is correct. I don't think the jacks were the problem, but it sure means there is an intermittent connection somewhere. The tremelo is VERY slow. There's no off switch. Even with the depth at zero, the output of the amp (with a sine wave input) still has a slight tremelo effect. The transistors are 2SB54s.

    If anyone finds a better circuit with readable values, it would be helpful. If not, I'll just have to do it the hard way.

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    Senior Member vintagekiki's Avatar
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    Unfortunately there are no readable schematics.
    Lindell amp is in front of you, and the old Teisco schematics is also in front of you.
    Simply compare the layout amplifier with the schematic, and on (paper) schematic near component written values which you have found.
    And do not forget. Upload your schematics.

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    Member patlaw's Avatar
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    I found what appears to be a closer but not totally accurate schematic. This amp doesn't have a reverb tank. The only challenge I'm facing now is the tremolo. It starts fine, but it is REALLY slow. When the 0.47uf caps dry up, the tremolo should get faster. What causes a tremolo to get slower? A resistor increasing in value? If so, which one? I'm going to check them all, but they're a pain to get to since they're standing on end.

    Click image for larger version. 

Name:	Teisco CM88 Schematic.jpg 
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    "Thermionic Apocalypse" -JT nickb's Avatar
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    Quote Originally Posted by patlaw View Post
    The only challenge I'm facing now is the tremolo. It starts fine, but it is REALLY slow. When the 0.47uf caps dry up, the tremolo should get faster. What causes a tremolo to get slower? A resistor increasing in value? If so, which one? I'm going to check them all, but they're a pain to get to since they're standing on end.

    Click image for larger version. 

Name:	Teisco CM88 Schematic.jpg 
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    Perhaps the section of the tremolo speed pot that is in the oscillator is open or partially so. It's common for old pots to be scratchy.

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    Last edited by nickb; 05-02-2019 at 08:09 AM.
    Experience is something you get, just after you really needed it.

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    Senior Member vintagekiki's Avatar
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    Quote Originally Posted by patlaw View Post
    ... What causes a tremolo to get slower?
    Tremolo work slower if any component in the phase oscillator network due to aging has changed its value.
    Check C18 - C20, R34, R35, VR4 that they have not changed the value.

    If the C18 - C20 has been dried, oscillator can not oscillate.
    If the VR4 intermittently interrupts or has increased the value, the oscillator works slowly. The same goes for R34, R35.

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    Member patlaw's Avatar
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    Unfortunately, the tremolo portion of the schematic is not accurate either. I removed the three 0.47 uFd electrolytic caps in the tremolo section. One measures 1.8 ufd, one measures 3.2 uFd, and the the last one measures 5 uFd. They're all three are marked and are supposed to be 0.47 uFd. Thinking my meter was acting up, I tested the values of three precision caps, and they were right on the money (1 uFd). I don't think I've ever seen an electrolytic increase in value. Anyhow, is there any reason to replace these with electrolytics? I don't think the circuit cares so long as the value is correct.

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    "Thermionic Apocalypse" -JT nickb's Avatar
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    Leaky caps will give incorrect high readings. It's best to avoid electrolytics. Film would be a better choice for stability.

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    Old Timer J M Fahey's Avatar
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    Agree.
    Cheap built-into-multimeter capacitance meters donīt really measure capacitance but inject some signal through them, usually a cheap squarewave, and measure how much voltage reaches a load resistor, a really crude system.

    In principle: "higher voltage means a larger capacitor" ... sort of.

    Problem is that a very leaky one will also let more signal through, and be incorrectly assumed to be higher capacitance

    Agree that film capacitors are best, no doubt about that, but fear they will be too large to fit and given the cheapish type of design, I would still try a couple electrolytics there.

    Anything modern will be 1000 times better than the original ones, just improve them to bipolars.

    Absolute worst case, 2 of them, those connected to collector and base (C18/19) must be properly polarized; the intermediate "floating" one (C20) *should* be bipolar because it has no DC reference and yet signal through it is relatively large.

    In fact, seeing all others have + signs by them and these do not confirms these *are* bipolars.
    Modern ones should work properly at least quite a few years.

    Not NASA quality parts but then this is not a NASA design anyway

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    Member patlaw's Avatar
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    The meter is a dedicated capacitance meter, but it's certainly not laboratory grade. As you suggest, I can't find a film to fit. The PCB has 4mm hole spacing and no room for a capacitor larger than about 5mm in diameter. So it looks like I have to go with an electrolytic.

    You suggest upgrading the caps to bipolar, but I don't know where to find 0.47 ufd bipolars that will fit. Since the amp has polarized caps (confirmed on removal), and since I marked the orientation before I took them out, it seems to make sense to go back with the same part. Feel free to suggest otherwise, though.

    The schematic is not the right one for the amp, but the tremolo section appears to be close. The caps are definitely 0.47 ufd, but the resistor values are different.

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    "Thermionic Apocalypse" -JT nickb's Avatar
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    Quote Originally Posted by patlaw View Post
    The meter is a dedicated capacitance meter, but it's certainly not laboratory grade. As you suggest, I can't find a film to fit. The PCB has 4mm hole spacing and no room for a capacitor larger than about 5mm in diameter. So it looks like I have to go with an electrolytic.

    You suggest upgrading the caps to bipolar, but I don't know where to find 0.47 ufd bipolars that will fit. Since the amp has polarized caps (confirmed on removal), and since I marked the orientation before I took them out, it seems to make sense to go back with the same part. Feel free to suggest otherwise, though.

    The schematic is not the right one for the amp, but the tremolo section appears to be close. The caps are definitely 0.47 ufd, but the resistor values are different.
    4mm pitch is tough in anything. 5mm is easy enough

    https://www2.mouser.com/ProductDetai...ExePEl6XCE8%3D

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    Member patlaw's Avatar
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    Quote Originally Posted by nickb View Post
    4mm pitch is tough in anything. 5mm is easy enough

    https://www2.mouser.com/ProductDetai...ExePEl6XCE8%3D
    That's a fact. Since my previous post, a buddy found some .47uF caps with 0.2" lead spacing. Those are easy enough to bend to fit into 4mm spaced holes.

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