Hi again,
I'm fairly new to transistor amps and am looking for some tips of locating a bad hum emitting from the first few gain stages in this old amp (1969). Schematic here. Sorry, thread title mistakenly says 200a, the circuit is indeed the earliest rendition of the 200 series.
- all capacitors have been changed, power supply and coupling.
- all voltages check out
- hum is persistent with no input to the amp (reed input disconnected)
- hum stops if the volume pot is removed from the circuit
- I measure around 2 to 7mVac at the volume pot depending on its position.
Any help is appreciated. thank you!
I'm fairly new to transistor amps and am looking for some tips of locating a bad hum emitting from the first few gain stages in this old amp (1969). Schematic here. Sorry, thread title mistakenly says 200a, the circuit is indeed the earliest rendition of the 200 series.
- all capacitors have been changed, power supply and coupling.
- all voltages check out
- hum is persistent with no input to the amp (reed input disconnected)
- hum stops if the volume pot is removed from the circuit
- I measure around 2 to 7mVac at the volume pot depending on its position.
Any help is appreciated. thank you!
The first 3 transistors are OK with regards to DC voltages - all within 1V of spec.
The 170V supply before R56 shows a strange ripple on the scope of about 20mV p-p. The digital meter alternates between 0 and 20mV. The scope image isn't stable...it seems to dance around a bit, +/- 5mV. The images are taken with the probe at x1, 10mV per division, with delays of 5 and 2ms.
http://farm8.staticflickr.com/7367/9972914663_c39218739b_c.jpg
http://farm6.staticflickr.com/5326/9972914403_1d54c3a45d_c.jpg
The 170V supply before R56 shows a strange ripple on the scope of about 20mV p-p. The digital meter alternates between 0 and 20mV. The scope image isn't stable...it seems to dance around a bit, +/- 5mV. The images are taken with the probe at x1, 10mV per division, with delays of 5 and 2ms.
http://farm8.staticflickr.com/7367/9972914663_c39218739b_c.jpg
http://farm6.staticflickr.com/5326/9972914403_1d54c3a45d_c.jpg
While bad ground connections and disconnected or gappy trace volume pots can cause hum, I would start with replacing 6, the 1000 mf (UF these days) power supply filter cap. If that doesn't help I would replace 17 the 25 mf cap. The calender says they are suspect. Only then would I start removing and cleaning ground connections to the reed and monitor. Then whether the hum was there or not I would replace every other electrolytic cap (ones with plus on one end or a red end) .
I have a Wurlitzer 4500 from 1964 that is totally dead, probably because all the little electrolytic caps are dried up, according to Morelock's organ service. Sixties Wurlitzers did also sometimes have a bad small signal transistor, but the caps were twenty year ****. The e-caps with a red end are especially known for high mortality at this age. You can buy parts from Morelock's Organ Service of Rienzi, MS, who can be contacted through the mitatechs.org website. they bought up all the Wurlitzer spares when they were sold out, and can get things noone else can.
I have a Wurlitzer 4500 from 1964 that is totally dead, probably because all the little electrolytic caps are dried up, according to Morelock's organ service. Sixties Wurlitzers did also sometimes have a bad small signal transistor, but the caps were twenty year ****. The e-caps with a red end are especially known for high mortality at this age. You can buy parts from Morelock's Organ Service of Rienzi, MS, who can be contacted through the mitatechs.org website. they bought up all the Wurlitzer spares when they were sold out, and can get things noone else can.
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Sorry for the delay... I'm getting closer but still no cigar.
So, if I short the +15V supply to ground I can kill the buzz, but I'm not sure where the problem may lie...here are some observations;
- at R51 (10K). The left side reads 15v as it should. That's the side I can short to ground and silence the hum. The right side (coupling TR4, R54, etc) does not read +7V (TR4 collector). The voltage is jumping all over the place, and grounding this spot does not stop the buzz. On the scope it shows a rather straight line in the AC spectrum, but the line is rapidly oscillating up and down over the range of about 5V. Base and Emitter of Tr4 show correct voltages.
So I'm thinking that the problem lays to the left of R51 since that's where I can stop the buzz by grounding it. However, I'm totally confused because TR4 isn't showing the correct voltage at the collector. I've replaced R51. My initial thought is that TR4 is doing exactly what it is supposed to as it is part of the vibrato circuit.
So, if I short the +15V supply to ground I can kill the buzz, but I'm not sure where the problem may lie...here are some observations;
- at R51 (10K). The left side reads 15v as it should. That's the side I can short to ground and silence the hum. The right side (coupling TR4, R54, etc) does not read +7V (TR4 collector). The voltage is jumping all over the place, and grounding this spot does not stop the buzz. On the scope it shows a rather straight line in the AC spectrum, but the line is rapidly oscillating up and down over the range of about 5V. Base and Emitter of Tr4 show correct voltages.
So I'm thinking that the problem lays to the left of R51 since that's where I can stop the buzz by grounding it. However, I'm totally confused because TR4 isn't showing the correct voltage at the collector. I've replaced R51. My initial thought is that TR4 is doing exactly what it is supposed to as it is part of the vibrato circuit.
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It doesn't look like AC, but the whole straight line wobbles up and down.
At collector of TR6 I read about 26vdc. The scope shows a straight DC wave that is wobbling +/- 1v (2v p-p). When I switch to AC coupling I see the same straight line but with a 1v wobble (2v p-p). My meter reads it as a ~250mVac at that spot.
At collector of TR6 I read about 26vdc. The scope shows a straight DC wave that is wobbling +/- 1v (2v p-p). When I switch to AC coupling I see the same straight line but with a 1v wobble (2v p-p). My meter reads it as a ~250mVac at that spot.
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