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Old 11th April 2012, 06:57 PM   #21
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The voltages suggest Q1 has 8ua flowing into the base ,but does it? 8ua is more than enough to switch it on and so give 0.63 volts .I still believe this is the area where the fault is .
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Old 11th April 2012, 07:37 PM   #22
Simon B is offline Simon B  United Kingdom
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Default Careful with that voltmeter Eugene!



fuzzfacenpnschematic2.gif

Ok, so 8uA base current into Q1 and 0.64v on base...(thanks a bunch Eugene!)

Humble pie for me epicyclic.

About 200uA down the 33k connected to Q1 collector, 0.67v across the b-e junction of Q2 and as with the previous set of err...."readings" (rrrrr!) Q2 b-c junction still forward biased.

Voltage at node between 330 and 8k2 indicates that the gain off the transistor you've used for Q2 is too high for an 8k2 resistor in that location. Either test and select a lower hfe transistor (it is worth the bother sound wise) or just drop the value of the 8k2.

Incidentally, the "C" at the end of BC109C means it's a high gain type, not so good in this application.

If you do want to check the hfe of transistors for this circuit, here's a simple way to do it using just a meter and a couple of resistors,

http://www.geofex.com/Article_Folders/ffselect.htm

it works fine on Si transistors too, leakage shouldn't be an issue with these.


Last edited by Simon B; 11th April 2012 at 08:05 PM.
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Old 11th April 2012, 08:28 PM   #23
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@Simon: Thanks for the floyd reference and the greek lesson, not to mention the nice pic It's not 109Cs though but BC 108 (no suffix that I am aware of).

So, any suggestions as to what values to try based on these numbers?
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Old 11th April 2012, 10:33 PM   #24
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Quote:
Originally Posted by Simon B View Post

If the Fuzz pot is correctly wired and the capacitor on it is good, rotating it will not affect the bias of either transistor.

The voltage at the emitter of Q2 looks about right to me, feeding through the 100k resistor it gives about 8uA bias current into the base of Q1, which should be bumping along the bottom rather. Negative input half cycles get clipped first.

Q2 has reasonable forward bias across the base-emitter junction, 0v68, given the apparent lack of current flowing through it. I'd be looking at the 330R and 8k2 first of all.
Yes, I'm aware of that. With 1.45V across the 1K, Q2 has no choice but to saturate with the 8.53K collector load and 9 volt supply. It would work with a 24V supply, or simply reduce the 8.2K to something like 4.7K to get the collector of Q2 to mid supply. I think the 1K and 8.2K were predicated on using Germanium transistors.

Last edited by Loudthud; 11th April 2012 at 10:42 PM.
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Old 11th April 2012, 11:57 PM   #25
Simon B is offline Simon B  United Kingdom
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Quote:
Originally Posted by Loudthud View Post
Yes, I'm aware of that. With 1.45V across the 1K, Q2 has no choice but to saturate with the 8.53K collector load and 9 volt supply. It would work with a 24V supply, or simply reduce the 8.2K to something like 4.7K to get the collector of Q2 to mid supply. I think the 1K and 8.2K were predicated on using Germanium transistors.

Good point, 1.45mA is too much for the total 8.53k collector load. More humble pie for me Loudthud.
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Old 12th April 2012, 12:25 AM   #26
Simon B is offline Simon B  United Kingdom
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Quote:
Originally Posted by eugene martone View Post
@Simon: Thanks for the floyd reference and the greek lesson, not to mention the nice pic It's not 109Cs though but BC 108 (no suffix that I am aware of).

So, any suggestions as to what values to try based on these numbers?

I'd go with Loudthud's suggestion of 4k7 to start with, but you'll probably need to cut and try for best results.

Any value is ok - even 0R shouldn't blow a BC108, given the 330R upstream. The max current that can flow is about 25mA, and a BC108 is ok up to 100mA & 300mW.

It is worth experimenting. Don't bother getting the old resistor off the veroboard, cut its body out to leave the two leads sticking up.

Then either temporarily solder resistors on or perhaps use a small bit of connector block, until you find what works best for you.
Possibly try a 5k or a 10k pot or trimmer, connecting to the middle and either one of the end terminals to start with.

My mention of epicyclic was referring to a point on which he'd been right and I'd been wrong - though "wheels within wheels" is a bit like this circuit.

Let us know how you get on.

Last edited by Simon B; 12th April 2012 at 12:36 AM.
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Old 12th April 2012, 02:37 AM   #27
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Can somebody help me out here? I have data on just about every Motorola transistor made since the 70s, but nothing on these BC parts. Who makes them and where can I find data? Who made them originally? Any chance of finding a printed data book on ebay?
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Old 12th April 2012, 10:36 AM   #28
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Hi

eugene would you measure the resistance of your 1K0 fuzz pot and repeat your voltage measurements ( with fuzz pot on min fuzz and keep negative lead of meter in same place also include supply rail reading ) as the latest readings dont make any sense ie 1.45V on the emitter of Q2 .

Loudthud ...try www.datasheetarchive.com and search with the trannie number then scroll down for a list of pdfs . I dont know who originally made them .

Last edited by epicyclic; 12th April 2012 at 10:45 AM.
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Old 12th April 2012, 11:03 AM   #29
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Had another thought .....make sure 100k is 100K and not 1M0
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Old 12th April 2012, 01:01 PM   #30
Simon B is offline Simon B  United Kingdom
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BC are Pro Electron type numbers, as opposed to, say, JEDEC or JIS.

Pro Electron

Philips, Motorola and Fairchild all appear to have made them. One thing to watch out for with those in a TO-92 package is that there are umpteen variations of this denoted by a final letter after the TO92, they have different pinouts, of course.

Same goes for BCxxx versus BCxxxL - different pinout again.

Plenty of stuff comes back if you google on eg 'BC108 data', I'll attach a couple that I've got stored.
Attached Files
File Type: pdf BC107_8_9 - 43469001.pdf (100.9 KB, 4 views)
File Type: pdf BC177_Fairchild.pdf (48.4 KB, 0 views)
File Type: pdf BC182_3_4_Motorola.pdf (111.6 KB, 1 views)
File Type: pdf BC184_214_Micro_Electronics.pdf (160.8 KB, 1 views)
File Type: pdf BC237_8_9_OnSemi.pdf (51.7 KB, 0 views)
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