Serial # 22244

Maybe 22nd week of 72?

I'm working on a Traynor Guitar Mate III that seems to be from 1969-1973 era.

I'd like to pin it down a little closer...

On the pots...

It has Centralab very clearly marked on the front bushing, but the codes on the back confuse me.

Did some googling to no avail...

On back of the vol pot is...    4482  1MEG-A  *then* BD7Q-7  OR   8070-7

The 1MEG-A is easy, the rest...?

The BD7Q or 8070 is hard to read, I'm guessing it's 8070.

The Centralab manufacturer number should be 134 but it's not there...?

Thanks for any help.

Edit; hmmm   8070   8th week of 1970? but why the dash 7?

Quote from: flester on April 03, 2018, 07:24:19 AM
Whats a good way to clean something like this? Its got 69+ years of dust, cobwebs and kitchen grease.

Sent from my SM-G935F using Tapatalk

I use compressed air first to clean old guitar amp chassis.
Just to blow the loose dust, cobwebs, and general grunge out.

Then scrub with Naptha (lighter fluid) and/or alcohol and an old tooth brush,
blowing off the loosened muck with air.

Naptha and alcohol are fairly benign to most things, and certainly ok on metal chassis,
but on something this old,
I'd be cautious about using any solvents near anything that looks like paper or waxed and maybe even plastics.

Test a small area first.

Sure... but a schematic would be a big help.

I realize that may be hard to find.

Can you pull the chassis and take *well lit, in focus, close up pictures* of both sides of the circuit board(s)?

Oh... and what's your experience working with electronics?

teemuk, thanks so much for the detailed and thoughtful reply. 

As you noted there is no thermal compensation so I think I will leave well enough alone.
The slight crossover distortion that I hear at low volume is really very minor and not at all noticeable with a volume increase.
The amp runs "cool"  to "barely warm" and in all other respects, works quite well.

After you posted the pic of the rubber diode I did a search and found this video...

https://www.youtube.com/watch?v=Obh_PIC2qqo

I found it very informative and well presented.
Even I could understand and follow alone!   

I found the video to nicely supplement your reply.

If I feel the need to tweak the bias, at least I know what to do now.
*If* that becomes the case, I may try moving Q7 to the chassis (heatsink) between the 2 output devices.

Thanks again!

PS, when I got this amp it was dead, no sound output at all.
I traced a signal thru the pre amp up to the output stage.

The 2SB507 was fine but the 2SD313 measured open B-E and B-C.
I replaced it with a TIP31C and the amp fired right up.

The original (as far as I can tell) 2SD313 showed no evidence of over heating.
No other components seem to have been damaged.

It's as if the transistor just "went open".
Maybe from a shorted output?

Whatever, I'm not worried about it, just curious about how it failed.
I suppose parts can just "go bad".

Cheers!

Thanks,

So is Q7 in the schematic the rubber diode?

Could bias be  adjusted by using a trim pot in place of  R38, R40?

I have re-drawn schematic output stage for clarity.

C2 definitely looks suspicious.
Should probably be a small tantalum cap like C1 and C6.
Also it looks swollen.
Don't know if this would have anything to do with the distortion you are hearing,
but I'd replace it anyway.

Here's some pics of the chassis.

Added 3 wire power cord and a speaker jack too.

I think this amp may be biased a bit cold.
Even playing clean at low volume there is a decaying "fizz" to the notes.

Also the output transistors never get hot,
barely warm after playing at full volume and full clipping for 1/2 hour.

How would I change the bias on this circuit?

Most of the changes I made are to the tone stack.

With the stock values in the tone stack, there was too much bass at all times.
You just couldn't dial it down far enough. Even at "0" the bass was just over the edge of "enough".
This made the bass control useless. Leave it at 0 and still too much.

So I got out the Duncan Tone Stack Calculator and found that the "James" tone control closely matches the tone stack in this amp.

BTW, if you have never played around with the DTSC it's a fun tool. 

http://www.duncanamps.com/tsc/

Here's some screen shots of the stock and modified tone stack values in the calculator,
showing the sweeps of the controls.

And a schematic with the changes made.

The bass is far more controllable now.
I've found putting the bass at about 2 or 3 and Treble at 6 or 7 works quite well.
Of course there is much more bass on tap if you need it. 

Thanks!

I'll post soon some pics and a schematic with the parts value changes I made.

Here's a schematic and layout for the Unicord Stage 65 amp. 
Took me some time to draw this...

I found this on ebay as just the chassis, no cabinet.
Will post more later about this amp and mods I did to it.

Yes, electros for the power supply caps.

For the rest, ceramic or plastic film types are fine.

Don't bother with tantalum. They are polarized and often used only in fairly specific applications.

A word of warning...
There are "tone snobs" all over the internet that claim to be able to "hear" the differences in capacitor types,
and that it makes a big difference in the "tone" of your amp, pedal or whatever....

I say BS.

Maybe, really big maybe,
that you could hear the difference in caps in a high end full frequency music reproduction system... ie, a home stereo.

This is for guitar, it won't matter a hoot.

Also, I don't know how 1W resistors could give "better results".
Better how? Please define "better".

Millions of pedals, amps, etc have been built with 1/8, 1/4, and 1/2 watt resistors in the audio path.

You only need higher watt resistors (1 watt and above) if the circuit demands require it.

Brush up on ohms law and calculate the current flowing in the circuit to determine if the higher wattage resistor is needed.

Now, resistor types are a different story.
Carbon, carbon film, metal film etc.
The differences?
Noise.
Or I should say, potential noise.

Carbon composition resistors (pretty much obsolete) can potentially cause noise in circuits.
The newer film types are quieter.

But again, it's for guitar, I'm betting it won't make a whit of difference even if you get a hold of some carbon types.

Read here for more on this...

http://www.aikenamps.com/index.php/resistor-types-does-it-matter

Edit; please note that in the above link he is talking about resistors for TUBE amplifiers with high voltages in the range of 250V to 450V or higher.
So some of what he says doesn't really apply to low voltage designs in SS amps.

I'm sure 12V would be ok. Probably not sound much different from 9V.
Maybe a little more headroom?

Just make sure the caps are rated for the higher voltage.

QuoteOn Brian Wampler's modified schematic, off pin 3 is reference to 4.5 something. I can't work out if this should read 4.5v or 4.5uF.  On the Effects Layouts version it is shown as a cap rated at 1uF.

Reference to 4.5V. This is the bias voltage needed by the op-amp on pin 3.
The cap is to filter that 4.5 volts and 1uf electro should be fine.
Could go higher to 47uf or even 100uf.

QuoteOff pin 7 on the Effects Layouts version is a diode marked D9. Any suggestion what one I should use?

Could leave that diode out.
It is simply for reverse polarity protection in a pedal, in case some doofus uses the wrong polarity supply. (wall wart)
If you want to add it, it won't hurt anything. Could use almost anything silicon. 1N4001 is common.

QuoteAlso off pin 7 on the Effects Layouts version is a cap the value of which looks to be 47uF.  That seems very high.  I've proposed 0.47uF in my drawing but is 47uF reasonable?

That cap is for power supply filtering. 47uf electro is fine.
Could use anything from 47uf to 100uf.

None of these things will affect the "sound" of the circuit which is why their values are not critical.

HTH

Quote from: SurreyNick on March 11, 2018, 09:40:57 AM
OK I have had a go at drawing the schematic for the Effects Layouts version and this is what I think it looks like.  I have highlighted in pink the additions of the Effects Layouts version.  Is this what Brian Wampler intended?  It looks quite different.

A couple of the component values weren't clear so I have put a ? next to those too.

The 1M at the input jack is the only difference I see as far as the audio path is concerned.

The other two 1M resistors form a voltage divider to provide the bias voltage for the op-amp.
The two caps are for power supply filtering and the diode is for reverse polarity protection.

Since this is a mod for an existing pedal,
it's possible Brian didn't show those components as part of the mod, because they are already there in the pedal.

The storyboardist Effects Layouts blog seems to assume that you will build the circuit from scratch,
rather than modify an existing pedal.

If that is the case then you definitely need those extra components, with the exception of the 1M at the input.
Further suspect that the 1M at the input was added to increase input impedance. (that's a good thing)
Try it both ways, with and without the input 1M.

Are you building from scratch or modding an existing pedal?