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 on: Today at 11:20:58 AM 
Started by gearhead63 - Last post by Jazz P Bass;topic=3501.0;attach=4607

All the information that you need is on the schematic.

 on: Today at 08:21:18 AM 
Started by gearhead63 - Last post by mrounsavall

 on: October 20, 2018, 08:55:00 PM 
Started by bobster - Last post by bobster
hi G1 ,
i took these voltage readings a few days ago and have copied from another post earlier in thread...

TIP29c [ BD711 on my amp ]    C = 46.5v  B = 1v      E = 0.5v
TIP30c [ BD712 on my amp ]    C =    - 46.1v  B = - 0.6v   E = 0v

DC mv across  R28    = +0.6mv     [ on 200mv scale ]
DC mv across  R34    =  +0.6mv

Power Q's
Q1    2N3773      C = 46.5v    B = 0.5v     E = 0v
Q2    2N3773      C = 0v         B =  - 46.8v   E =   - 46.8v

2N3440     C = 46.9v   B = 0.6v   E = 0.1v            [ all positive voltage readings ]
2N5415     C = 1.2v     B = 46.8v     E = 47.4v      [ all pos voltage readings ]

*2N5145 is now heating up fast and the Tip29c is no longer heating up since i replaced C5 of 150pf as suggested in case of high freq osc.... [ it's possible my replacement C5 is faulty so will will try another one there soon ]

R10 of 100 ohms has +47 volts at either end of it
C5  of 150pf [ replaced ]  has +47 at one end and + 1.2v at the other end which is collector of 2N5415 i think

the junction between R35 of 100 ohms and D3   has + 109mv which then after a short time drifted down to around +100mv

According to the power amp sim Phil put up this reading between R35 and D3 should be negative and around -155mv?

hope all this gives a clue as to what parts may need replaced or are causing issues...

thanks rob

 on: October 20, 2018, 08:29:19 PM 
Started by blackcorvo - Last post by blackcorvo
You all might be familiar with the Marshall Lead 12 preamp. I've heard it and I've seen it recommended, but I'd like something I could use with either guitar or bass. This idea came about after I saw some people using it's little bass counterpart with guitar, and it sounded very interesting.
I wanted to try and make a more readable version of the preamp schematics available online, as this is the only version that seems to be available online:

So, to the best of my abilities, I tried to read the values in it, and I ended up with the following:

I was not sure if it used +/-15v or +/-18v for the rails, but since the datasheets for the 1458 ICs these amps usually had in them calls for +/-18v Max, I went for the more conservative value of 15v.

My original objective was to simply clone this preamp and use it as a preamp for a Class D Power Amp, but I believe it would be helpful to make the schematics available online for repair techs.

I'd like to ask, if any of you owns this amp or knows someone who owns one, who would be willing to have it open, to please take some detailed photos of the board, so I could do a better job of re-drawing the schematic.
That is, unless someone has a more readable version of that schematic they would be willing to share.

 on: October 20, 2018, 01:50:45 PM 
Started by bobster - Last post by g1
The VS musician can not work if it has 0V at the lower 3773 emitter.
So you may be mixing up the lower and upper 3773's, or mixing up emitter and collector.
The 'lower' will have -48V on one of it's terminals.

How about posting your E,B, & C idle DC voltages for the 2N5415, Tip29, Tip30, upper 3773, lower 3773.

 on: October 20, 2018, 11:48:50 AM 
Started by bobster - Last post by bobster
Hi Phil , thanks again for your help.
I never got an alert for a PM but will check the messages here...
i have also ordered two new 2N3773's and a 2N5415.....

the other amp i made the comparisons with is from the same stable and maker and it's power board is very similar in design and interchangeable as far as i am aware and also has the same two big electro caps and general topology   .
i did suspect that little 150p cap [ threw the old one out unfortunately ] and will have a look in my stash for a non ceramic replacement and try it there to see if anything gives..
i only tried the scrap amps 2N3773's as an experiment as i have to wait for parts to arrive but did think they would be likely to be dead ..
i am reading very low mv on speaker outs well within limits according to Kiki...

happy to admit i'm floundering here but am determined to get this amp working again and am not in a major hurry plus i'm learning [a little! ] along the way ;)

 on: October 20, 2018, 09:34:47 AM 
Started by bobster - Last post by phatt
Although both may use the same transistors they could be completely different designs.

Q/ Do both amps use split supply rails?
If the VS head reads 0Volts on the emitter it maybe single supply in which case it would read 0Volts.
Just look for your main filter caps,, does the other VS head only have one big electro cap?
 if so it's a single supply circuit.
You are chasing your tail using second hand parts,, Just go buy some NEW parts,,, build a light bulb limiter  :trouble
(so you don't burn out new parts before you have a chance to check for faults),,
with no speaker,,fire up the amp and check voltages.
If the output is close to zero VDC then read the voltage drop across R28 & R34 do some maths and if all is well then  connect the speaker.
*ALL that is done with limiter in circuit*
When you are confident that the circuit is stable then you remove the limiter and recheck voltages.

Of course with limiter in place voltages will be lower so you have to recheck the voltages and the bias.

I recall you replaced C5 and it distorted badly,, I hope you removed it and reinserted the old one?
The original looks like it could be a mica cap ,, I'm not sure but maybe ceramic won't work there.
Also the ceramic may be a low voltage type and hence it has failed.
Others here may know a more professional way to fix this but that is how I do it.
I run power amp repairs full bore into a load with lamp limiter on and read the voltages while it's working to weed out any lurking problems. ONLY then do I remove the limiter.

As I'm not a teck by trade I don't try to fix exotic circuits that are way beyond my ability to fix.
As I tend to get a fair amount of 60's 80's Amps they are often small fairly simple circuits and hence easy to fix.
I did pm you about this a week back but maybe you have not checked your messages.

 on: October 19, 2018, 01:18:53 PM 
Started by bobster - Last post by bobster
hi guys
just on to ask for some more advice before i attempt any more of this repair with the IC100.....

i have taken readings off the driver and power transistors in circuit in the IC100 and my working VS-Musician head with power on ..

all transistors read similar except for the lower of the 2N3773's which reads - 47.2v on its emitter while the 2N3773 in the working VS Musician amp reads 0v...

I tried another two 2N3773's from a scrap donor PA amp but got the same results with distorted output so they were both likely to be dead....

At this stage the 2N5415 is still overheating but the previously overheating tip29c is now not getting overly warm but they give very similar DCV results to the working VS amp so might still be serviceable

in your experience , does this look like i just need a new 2N3773 [ or pair of ] ?
thanks rob

 on: October 17, 2018, 02:49:35 PM 
Started by blackcorvo - Last post by blackcorvo
Just a small update to this project.

To deal with the noise issue I mentioned on my last post, I tried many things but none of them worked. I ended up using a different DC-DC converter board, which is smaller, and that allowed me to place it further from the preamp tube and, thus, avoid inducing noise onto it.

The board in question is this, from eBay seller VFDClock:
Still has plenty of power to feed this amp, no problems. I just needed to adjust the bias on the 5902, by increasing the cathode resistor from 330R to 1k. Works great!

 on: October 17, 2018, 11:38:16 AM 
Started by Gustaf - Last post by g1
Once I worked on a tube mic that had a higher noise floor because the heater voltage was too high.  Bringing the heater back down to 12.6V had a noticeable impact.
Try it and find out, it's probably something that varies from case to case as far as how much difference it makes.

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