Silvertone 1464 solid state 100 amp

[J M Fahey]

Wow!! So there *is* a "3155" transistor after all!!
Think about a confusing denomination.

That said, I find voltage drop values shown *very* low, in my book they would be called "shorts" .
Yet they behave as diodes. Weird.

I expect some 180/250 mV across a germanium junction, definitely nothing as low as 75 mV or thereabouts.

To be more certain, this is what I do when the amp arrives without the transistors (some earlier "Tech" pulled them, owner recovered amp at gunpoint 6 months to 2 years later (true ) and couldn't find them) or they are a shorted blob of metal no matter what way you measure them:
1) pull both output transistors and measure +v and -V rails
2) *one* collector will go straight to one rail
If it goes to the "-" rail, it's PNP
If it goes to the "+" rail it's NPN
The other collector goes to the speaker output.
3) find the biasing resistors string.
In Enzo's example it's R35/36/37/38 , typically high resistance/low/high/low.
Turn amp on and measure voltage *across* each low value resistor (not relative to ground).
I expect around .2V for germanium and .5V for silicon.
6) if outputs are PNP Germanium and dead, convert it to PNP Silicon, much cheaper and robust.
You'll need to raise bias to about .5V .
As a rough guide you'll need resistors 2.5X the original Germanium value.
In Enzo's example, you'll replace R36 and R38 (2.7 ohms) by 2.5 x 2.7=6.75 ohms, meaning 6.8 ohms.
You can check this bias voltage without the transistors in circuit.

I think you'll need to make this conversion/upgrade because I find your original transistor values too low ... unless there's some measurement error we don't see.

Ok, measure and post before replacing anything .... but measure with both transistors out.
Good luck.

[g1]

My take is both transistors are shorted, and either may have been silicon or germanium.
Don't you normally get .6 or .7V reading for good silicons (not .06 or .07) ?
The readings seem really odd to me, it would be nice to see what kind of readings you get from a good transistor.

Or is it possible you have the pin-out wrong?

[The Dude]

If indeed your readings are out of circuit, those transistors are toast be they ge or si.

[52 Bill]

Odd readings or not, they appear to be Germanium PNPs. You are getting good diode junction readings from B-E and B-C, it's the E-C readings that are odd. I have seen readings from C-E before, but yours don't make much sense to me.

I'm with g-one here, what brand meter do you use for these readings? When you test a known good silicon transistor what value readings do you get?

[DrGonz78]

Well I am positive the pinout is accurate, collector is the case and the emitter & base are clearly marked on the transistor. Perhaps I thought it might be okay as the B to E and B to C readings were okay. Sometimes I forget exactly the numbers I want to see on each junction point.

I am using an Extech true RMS 430 meter. It's not a fluke but it has served me very well.

I am attaching some readings from a TIP142. This is not a brand new part but as far I know it does not measure any shorts. It's all I could find really quick for something to measure, I hope it helps.


I will follow Juan's procedure's in a bit, but first I need to finish attaching the 3-prong power cable. From a previous post of mine I was wondering about removing a cap that went across neutral and hot. Should I remove C47 that was in my example?

[52 Bill]

Those values are what I would expect to see on a good transistor, but a TIP142 is a Darlington. I would expect to see a diode junction reading from C-E. So I don't know where the numbers that you got on the outputs came from. Is it hot or cold where you are testing?

You can remove the cap from the transformer primary of leave it in if it is a properly rated cap. It is meant to remove line noise from the amp.

[g1]

Strange readings for all, including the Tip142. Normally a darlington will show a double diode drop from B to E, 1 to 1.2V.
For germaniums, I agree with JM, expect to see at least .1 or .2 when forward biased. Make sure to not touch any connections with fingers while testing.
However, I'll still agree with 52Bill that the readings do imply PNP germaniums, if those B-E junctions were short they would read short either way, but they forward bias one way and open the other.

[DrGonz78]

Yeah Gone I guess my old TIP142 was shorted out, sorry. I just got home and remembered I had some TIP142T TO-220 types to test that are brand new. So this will better confirm my meter readings, as it is a new part. I see now the short between E to B on my other TIP142 posting. Here is the readings for the new part for testing.



I talked with client today and asked him about the amps behaviour. He said it was passing signal but the sound was horribly distorted/farty. Time to get the grounded plug on there and start testing voltages.

TYou can remove the cap from the transformer primary of leave it in if it is a properly rated cap. It is meant to remove line noise from the amp.

Cool So the cap is for filtering noise and it is functioning so I will just leave it be, thanks Bill.

Make sure to not touch any connections with fingers while testing.

Yeah I will be using clip on probes to measure safely, thanks for the tip too.

[DrGonz78]

Wow!! So there *is* a "3155" transistor after all!!
Think about a confusing denomination.

That said, I find voltage drop values shown *very* low, in my book they would be called "shorts" .
Yet they behave as diodes. Weird.

I expect some 180/250 mV across a germanium junction, definitely nothing as low as 75 mV or thereabouts.

To be more certain, this is what I do when the amp arrives without the transistors (some earlier "Tech" pulled them, owner recovered amp at gunpoint 6 months to 2 years later (true ) and couldn't find them) or they are a shorted blob of metal no matter what way you measure them:
1) pull both output transistors and measure +v and -V rails
2) *one* collector will go straight to one rail
If it goes to the "-" rail, it's PNP
If it goes to the "+" rail it's NPN
The other collector goes to the speaker output.
3) find the biasing resistors string.
In Enzo's example it's R35/36/37/38 , typically high resistance/low/high/low.
Turn amp on and measure voltage *across* each low value resistor (not relative to ground).
I expect around .2V for germanium and .5V for silicon.
6) if outputs are PNP Germanium and dead, convert it to PNP Silicon, much cheaper and robust.
You'll need to raise bias to about .5V .
As a rough guide you'll need resistors 2.5X the original Germanium value.
In Enzo's example, you'll replace R36 and R38 (2.7 ohms) by 2.5 x 2.7=6.75 ohms, meaning 6.8 ohms.
You can check this bias voltage without the transistors in circuit.

I think you'll need to make this conversion/upgrade because I find your original transistor values too low ... unless there's some measurement error we don't see.

Ok, measure and post before replacing anything .... but measure with both transistors out.
Good luck.

Okay finally got the AC power cord installed. Had some fussing around getting the gromet to fit into the chassis and had to enlarge the hole a bit. So, now it is safety grounded I got to testing voltages.

We can positively state that they are PNP type transistors as I measure -25vDC on one of the collectors pin socket(transistors are removed for testing). We can also use the 1423 schematic as a reference to this output section as tracing it out it appears to be nearly identical. So I will reference R36 and R38 as 2.7ohm resistors as they are color coded Silver/Gold/Violet/Red. However measuring them in circuit they are 3.5-3.6 drifting a bit or I need to pull a leg to really see what they read. Anyhow I measured the vDC across each one. One is reading 141.5mvDC and the other is reading 145.2mvDC at 120vAC, no light bulb limiter. So pretty close and not quite close to the 200mv mark but definitely confirms that it is Germanium. So I will pull a leg on each of the resistors to check them next, figuring we need to know exactly what they are measuring to accurate adjust the bias voltage. Will post that soon...

Update: The resistors are measuring in at 3.5 ohms so they have definitely drifted just a bit. Also here are my voltage readings for each transistor just in case it comes in handy. This is with the transistors removed from circuit. EDIT: I forgot to note where there is negative voltages on my original post. I can't remember which ones might be negative but definitely the collector at -26vDC.
Transistor 1
E = 546mvDC
B = 431mvDC
C = -26vDC ***Edit*** This is negative voltage here...

Transistor 2
E = 26vDC
B = 26v
C = 566mvDC

[Jazz P Bass]

My take on this is, the 0.2 volt bias on the one output transistor (per the schematic) is screaming 'Germanium'.

I would simply replace then with silicon transistors, rebias the output section & be done with it.

The germanium amps that I have worked on usually have a gritty fuzz on note decay.

Germanium may be cool for pedals & such, but with there built in self biasing leakage, in an output section they are not up to todays 'ears'.

[drewl]

I have a gutted one around here somewhere so if you need some parts I may have them like the driver trans, faceplate, clean head box.

[DrGonz78]

I have a gutted one around here somewhere so if you need some parts I may have them like the driver trans, faceplate, clean head box.

I appreciate the heads up Drewl. The only part I am looking for is the lamp bulb for the pilot light as mine is dead. As I continue to work on the amp I may hit you up to buy some parts if I need to, thanks.

[J M Fahey]

We can positively state that they are PNP type transistors as I measure -25vDC on one of the collectors pin socket(transistors are removed for testing).

Good.

So I will reference R36 and R38 as 2.7ohm resistors as they are color coded Silver/Gold/Violet/Red.

Please name the colours in due order, which in this case would be:
Red/Violet/Gold/Silver
so as to minimize possible confusions.

However measuring them in circuit they are 3.5-3.6 drifting a bit or I need to pull a leg to really see what they read.

99% of meters have difficulty reading low ohms because they add their own contact resistance which to boot depends on pressure, so ....
Short the meter probes, they will NOT show 0 but some low value between 0.4 to 1.5 or thereaboits.
Substract that number from displayed value.

Anyhow I measured the vDC across each one. One is reading 141.5mvDC and the other is reading 145.2mvDC at 120vAC, no light bulb limiter.

Good. Now this measurement can be trusted.
Thatīs why I asked for it

So pretty close and not quite close to the 200mv mark but definitely confirms that it is Germanium.

Good.

So I will pull a leg on each of the resistors to check them next, figuring we need to know exactly what they are measuring to accurate adjust the bias voltage. Will post that soon...
Update: The resistors are measuring in at 3.5 ohms so they have definitely drifted just a bit.

No drifting but low ohms measurement error, see above.
Voltage measurement is good.
If you will use Silicon transistors (MJ2955 or similar), multiply bias resistor value by 2,5X and choose nearest standard value.

Amp *will* be slightly underbiased bit will be safe.
Annoying in the bedroom but fine at rehearsal or live.

What drewl said:

The germanium amps that I have worked on usually have a gritty fuzz on note decay.

Germanium may be cool for pedals & such, but with there built in self biasing leakage, in an output section they are not up to todays 'ears'.

Really means crossover distortion.
FWIW it might be even harsher on silicon
Later we may try to minimize it, but for now letīs try to make it work first.

[DrGonz78]

Okay I did some more work on this amp tonight and this is where I am at now. I measured those 2.7ohm resistors R36 & R38 to be 3.1ohms currently. So 3.1 x 2.5=7.75 or 8 ohms. I could not find an 8 ohm resistor and have some ordered. I did have a 10 ohm resistor to try out after using ohms law to approximate the rise in voltage.

I first solved for current (I = V/R)
I=0.145/3.1 (which is voltage across the resistor and resistor value)
which would be I = 0.047 (approximate current)
Then I solved for voltage again V=IR
V = 0.047 * 10 (As I am putting a 10 ohm resistor in now)
V= 0.468v to be exact (or 470mv approximate voltage reading across r36 or r38)

So I did that just to help understand exactly what we are doing and how it relates to ohms law. After installing the 10 ohm resistors I measure 449mv across one and 446mv across the other. So, my math was pretty close and I believe we are trying to get that bias voltage to be around 500mv. Is that correct?

Measured the connection points that connect to the transistor again. There was a change in the base voltage on the 1st transistor, but perhaps I need to remeasure to confirm.
1st transistor
E = 570mv B = 115mv C = -26v

2nd transistor
E = 26v B = 25.6v C = 558mv

So I have ordered the MJ2955 PNP output transistors and they are on their way. I also ordered some resistors ranging between 6.8-9.2 ohms for fine tuning if I need to do so.

Does this look good with the 10ohm resistor in there? Thanks again.

[J M Fahey]

Bias with 10 ohms looks good.
If the rest of the amp is healthy, puting those MJ2955 there and biasing as measured should make it work.
You can later tweak, but thatīs minor matter.