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NPN TRANSISTOR QUERY

Davidr

New Member
Hi. Newbie here. I need to replace a faulty NPN transistor in a guitar pedal. It's a mid 1970s C828 and 1 of 4. I have a modern replacement 2SC828 which appears compatible but on my DMM the hFE for the new one is about 300 and the older transistors around half that. So the question is whether modern equivalents are the same spec as older components? Thanks for any help.
 

Beau Schwabe

Active Member
"I need to replace a faulty NPN ... on my DMM the hFE for the new one is about 300 and the older transistors around half that." .... If the old one is faulty, then how can you be sure you are getting an accurate measurement? The hFE is the amount of gain of the transistor and can vary widely from one transistor to the next even from the same manufacturer, and especially in older transistors. The C828 is the same as the 2SC828. According to the datasheet it has a minimum hFE rating of 130 and a maximum hFE rating of 520 depending on the current gain group your transistor falls in (Q,R,S).

Note: Ambient temperature can also affect the hFE

Second Note: If you are able to measure the hFE on the 'faulty' transistor then is IS working based on how hFE is measured. Something else is wrong in your circuit.



Data Sheet Reference:
 

Diver300

Well-Known Member
That happens a lot. Transistor manufacture gets better over time and the gain increases.

The major problem comes when you have transistors that have to be similar to share the load. That isn't the case in guitar pedals where it is a low power application.

It will most likely be fine, and highly unlikely to cause any damage. If the circuit is well designed, it will allow for any gain from, say, 100 to infinity.

Can you share pictures or a circuit diagram?
 

Ylli

Active Member
Ditto on what is mentioned above. If your 'transistor tester' can give you an hFE, why do you think the transistor is defective? Got a link to the service manual for the device in question, or at least a schematic snip?
 

Davidr

New Member
Thanks for the replies. I replaced all 4 transistors and the hFE measured was typical for the old ones which were OK. However its still not fixed the problem. I have attached the circuit diagram. When R24 sustain is turned fully left, the pedal starts behaving like a tremolo unit. Overall it sounds very little like it used to, even with the 3 controls all up full. There's life Jim, but not as we know it........

I have tested each of the diodes and they seem OK. So I'm stumped now as that's all the most likely components replaced or tested
 

Attachments

Diver300

Well-Known Member
There are many electrolytic capacitors in there. They are low value, and modern designs would probably use ceramic capacitors.

It is highly likely that they will have all dried up, given the age of the device. Change them all.
 

rjenkinsgb

Well-Known Member
Most Helpful Member
What voltage is it running on? A near-dead battery could give that effect.

There are many electrolytic capacitors in there. They are low value, and modern designs would probably use ceramic capacitors.

It is highly likely that they will have all dried up, given the age of the device. Change them all.
I'd agree on changing the caps, but never ceramics for audio coupling - they can introduce distortion.
(Though many oriental makers of ultra-low cost stuff do use them).
 

atferrari

Well-Known Member
Changing components upfront makes no sense and it could create new problems.
 

Ylli

Active Member
My opinion, take it for what it is worth...

1. Assume the unit used to work correctly.
2. Assume the battery is good.
3. Assume the source and load have not changed.

A. All transistors have been replaced with 2SC828, the original parts. Assume they have been installed correctly.
B. Diodes have all been checked. Assume they were properly tested.
C. Resistors, in such low powered applications, seldom fail. Assume they are all good.
D. There are seven 1.0 uF electrolytic caps (and one 10 uF e-cap). I am not usually a fan of wholesale replacement of parts, but in this case replacing these caps may be quicker and simpler than extensive toubleshooting using test equipment the OP may not have.

Note that if the capacitor convention of showing a straight line for the + lead and a curved line for the - lead is followed, C13 is shown backwards in the schematic. C13 + should go to Q3 collector. If you order replacement caps, stay with electrolytic caps and order an extra 10 uF. Install that extra 10 uF between the +9 volt power buss and ground, with the capacitors + terminal on power and the - term on ground.

If you wish us to help troubleshoot more deeply before you replace the caps, then with the unit operating, measure the voltages on each of the 3 pins of the 4 transistors (12 measurements), annotate them on the schematic and repost it.
 

rjenkinsgb

Well-Known Member
Most Helpful Member
Install that extra 10 uF between the +9 volt power buss and ground, with the capacitors + terminal on power and the - term on ground.
Agreed- or bigger (100uF or more) would not hurt.

The "tremolo" effect could be a description of motorboating, possibly from something modulating the battery voltage and upsetting the bias somewhere?

If you have a multimeter I'd re-check all the transistors in circuit (with power disconnected), to ensure the junctions are OK (reading as diodes from the base to the emitter or collector) and there are no emitter-collector shorts.

Is there any possibility the new transistors have a different lead arrangement from the originals?
Or one has ended up the wrong way around?
 

Davidr

New Member
Thanks all. I should add that I have now put the original transistors back in place as the replacements made no difference. The end effect is the same. I've tried various batteries all with the same end result. Its a very good call with the caps. I have ordered direct replacements (electrolytics) and will seek to replace over the next week and get back to you. Thanks again.

I should add that I've had this pedal since new - bought it about 1976 or 77, so it is over 40 years old. It was working OK until a few months ago. Ageing capacitors is a very good call indeed and not one I would have thought of. Now if you've got a fix for the ageing owner of the pedal, I'd love to hear it.......
 
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Davidr

New Member
UPDATE. Just to say thanks very much for the excellent recommendation about the capacitors. Its taken me a while but that's all of them now replaced and the pedal sounds great ! So thanks again. I didn't add the large capacitor across the 9v supply, as there is very limited space inside the pedal. However I did add a 9v adaptor socket to allow connection to an external PSU. There is quite a hum when I connect the external 9v supply but lovely and quiet from battery. So would an extra capacitor help ?
 

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