Hi Folks; Minor panic here; A VOX Escort Bass 50 landed in my lap and is quit sick.
It is a capacitor coupled amp so the voltage between the output transistors should be half of B+. (80 V) It was around 19 V until some probing and now it is closer to only 1 volt.
All the transistors seem to test OK in circuit. It is a very simple amp but it has me stumped at the moment. Here are my voltage measurements;
Tr 1 E 2.1
B 9.4
C 0.0
Tr 2 E 0.7
B 0.0
C 0.9
Tr 3 E 1.5
B 0.0
C 0.9
Tr 4 E 2.3
B 0.9
C 72.0
Tr 6 E 0.01
B 0.58
C 1.4
Tr 7 E 1.8
B 2.3
C 72.5
It is a capacitor coupled amp so the voltage between the output transistors should be half of B+. (80 V) It was around 19 V until some probing and now it is closer to only 1 volt.
All the transistors seem to test OK in circuit. It is a very simple amp but it has me stumped at the moment. Here are my voltage measurements;
Tr 1 E 2.1
B 9.4
C 0.0
Tr 2 E 0.7
B 0.0
C 0.9
Tr 3 E 1.5
B 0.0
C 0.9
Tr 4 E 2.3
B 0.9
C 72.0
Tr 6 E 0.01
B 0.58
C 1.4
Tr 7 E 1.8
B 2.3
C 72.5
Thanks Bone;
Removed Tr 1 and Tr 2, Tr 1 did not conduct enough in the forward direction to make my diode checker beep. Tr 2 was shorted E to C .
I may have damaged Tr 2 in my previous testing....(confession). I have replaced Tr 1 and Tr 2 and now I am back to how the amp was when I received it; Tremendous turn on thump and hum in output, but amp passes signal. Voltage readings to follow
Removed Tr 1 and Tr 2, Tr 1 did not conduct enough in the forward direction to make my diode checker beep. Tr 2 was shorted E to C .
I may have damaged Tr 2 in my previous testing....(confession). I have replaced Tr 1 and Tr 2 and now I am back to how the amp was when I received it; Tremendous turn on thump and hum in output, but amp passes signal. Voltage readings to follow
tr 1 E 13.8 B 13.2 C 1.34
tr2 E 0.71 B 1.3 C 19.0
tr3 E 20.0 B 19.4 C 73.0
tr4 E 20.7 B 21.2 C 73.0
tr6 E 0.0 B 0.5 C 19.8
tr7 E 20.0 B 20.6 C 72.4
tr2 E 0.71 B 1.3 C 19.0
tr3 E 20.0 B 19.4 C 73.0
tr4 E 20.7 B 21.2 C 73.0
tr6 E 0.0 B 0.5 C 19.8
tr7 E 20.0 B 20.6 C 72.4
"PRESET" under TR1 is supposed to pull more current in R18 to raise the output from 13V to 41V. Math says PRESET is 119 ohms but they did not slap a 120r resistor so the circuit is less predictable than it looks. However a "center" voltage of 20V suggests PRESET may be far too high value (Cracked? Roasted? Broken? Mis-set?).
Me, I would use a FIXED resistor here. Try 120r and see how it sits.
Me, I would use a FIXED resistor here. Try 120r and see how it sits.
Hi PRR;
After replacing Tr 1 and Tr 2, I guess I neglected to retry the 'preset'. I tried the pot and now it seems to be in range! I set the pot for 39 V at the reference. (My thinking was; half of the B+ of 78 V) I see now that the voltage is 42, somewhat higher than half of 80 V on the schematic.
The answer may be beyond me, but could you explain the math that determines the value of the preset to be 119 ohms?
Thanks, Peter
After replacing Tr 1 and Tr 2, I guess I neglected to retry the 'preset'. I tried the pot and now it seems to be in range! I set the pot for 39 V at the reference. (My thinking was; half of the B+ of 78 V) I see now that the voltage is 42, somewhat higher than half of 80 V on the schematic.
The answer may be beyond me, but could you explain the math that determines the value of the preset to be 119 ohms?
Thanks, Peter
So I'm still sort of back to square 1. There is a sharp, (kind of a double) turn-on thump, I guess that has to be tolerated with a capacitor coupled output....but this one is nasty!
Also, and more objectionable is a hum. This was the owner's biggest complaint. Most of the electrolytics were bulged, so I have replaced them all. The hum persists. As an experiment, I connected a 100 ufd cap from the Base of Tr 1 to ground---All silent, so can I assume the hum originates in the IC section.
I assume the IC wants a + and - 15 V supply. It looks like +30 V is applied to pin 8 and grounnd to pin 7. The centre of the 30 V voltage divider is applied to all the "+" inputs. Is this a 'legal' way to provide a + and - supply to the IC?
Also, and more objectionable is a hum. This was the owner's biggest complaint. Most of the electrolytics were bulged, so I have replaced them all. The hum persists. As an experiment, I connected a 100 ufd cap from the Base of Tr 1 to ground---All silent, so can I assume the hum originates in the IC section.
I assume the IC wants a + and - 15 V supply. It looks like +30 V is applied to pin 8 and grounnd to pin 7. The centre of the 30 V voltage divider is applied to all the "+" inputs. Is this a 'legal' way to provide a + and - supply to the IC?
Yes.
Op Amp by itself needs no voltgw reference,it is floating.
So all it wants is up to 30V between its positive and negative power legs.
+30V and ground or +/-15V are the same to it.
Of course, it´s doing some job for others, who will want some signal reference, but as far as the op amp is concwerned, its own reference must be halfways between those power pins, which will be either +15V (what we have here) or 0V, what we are used to see.
Just one question, I wonder how that chassis is mounted in the cabinet, it being a Bass amp, so presumably sealed.
Can you please post 2 pictures, one showing full chassis and its mounting screw holes?
And another showing cabinet from the back, showing the area where chassis mounts?
I wonder about chassis vibration and rattling.
As a sidenote, I was intrigued by the PCB including an *old* late 70s early 80s) brazilian made Siemens 1000x63 electrolytic (you are in Canada) and a couple very Oriental blue radials with legs extended to fit in an axialfootprint.
Also the (barely visible) speaker *looks* like a Peavey Scorpion, any labels on it?
Too shallow a cabinet for proper Bass duty, *looks* like they created a Bass amp to fill a catalog hole by repurposing existing Guitar cabinets.
Thanks for your help! The turn-on thump is almost a non-issue now. There is still a minor hum, reduced by adding another 1000 ufd across C 24 and C 26. It seems to be a design issue(?) that the Volume is on the very first stage and the whole rest of the chain is running wide open?
That was a Peavey Spider driver. The owner wanted the original driver put back.
That was a Peavey Spider driver. The owner wanted the original driver put back.
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Hi, in the hope that the original posters (or others also more knowledgable than I) might pick this up, I am repairing a Vox Escort Bass 50 and have fixed everything (it was very poorly) except I am having the same hum problem that Peter S had previously in this thread. I note from the thread that you eventually said Peter that the hum was now a non-issue but I couldn’t see in your posts how you got from your “back to square 1” post to your next post where you said you had resolved it. Was it as simple as increasing the capacitance of C24 and C26? Also one other thing has me stumped. The C26 1000uF 63V electrolytic cap that is supposed to be across the Zener diode just doesn’t seem to actually be there on my PCB, unless I’m missing something obvious. There is a single 1000uF 63V cap on the PCB and that’s C22, which is in series with the speaker. I also can’t see it in the photos you posted Peter. Of course this thread is 4 years old so I’m kind of clutching at straws, but in the hope someone has some advice I’m posting this. Many thanks in advance!
Hi Marshall, I am the "OP" but certainly a Vox expert. Regarding the cap (C26), seems to me that it would be vital to hum free operation. How about trying a temporary shunt across the zener with a cap?. I don't recall the gruesome details of that amp but I have another Escot Bass 50 here a month ago. Hum was reduced on this unit by place a sheild on the inside of the rear wooden cover and connecting it by wire to the central ground terminal on the chassis. This design seems to be vulnerable to hum since the power transformer was located in the bottom of the cabinet. I used an steel screen as a shield, thinking it might be a better shield than something non-ferrous.
Hi Peter
Many thanks, I will try a cap across the zener and a grounded steel shield per your suggestions, is your shield a plate over the whole back of the cab, or just essentially a box around the transformer?
Many thanks
Paul
Many thanks, I will try a cap across the zener and a grounded steel shield per your suggestions, is your shield a plate over the whole back of the cab, or just essentially a box around the transformer?
Many thanks
Paul
Hi Paul, the shield is not around the transformer. The power transformer on my 2 units was at the bottom of the cabinet. The shield was about the same size as the chassis...across the top of the back cover. I used screen so as not to block 2 round holes IIRC. The input side of the pcb was very sensitive.
PS; are you absolutely sure there is no C26?
PS; are you absolutely sure there is no C26?
Thanks Peter, so if I understand correctly (forgive me if not) your shield was screen (steel mesh with holes? Like a Faraday cage?) the length of the back panel? Was it just the back panel covered or was it an L shape and screened the back panel plus horizontally between the PCB and speaker? Sorry for all the questions- just want to be clear, is all. Many thanks for your guidance, Paul.
Yes. Ours was steel mesh, actually some expanded metal from the cover of a scrapped stereo receiver. There was a dramatic improvement with just a flat sheet the same size as the chassis, with a ground wire connection. We did not experiment to see if closing in the bottom would have helped further.
Hi Peter, that’s extremely helpful- will try it and update once done. May be a while as I need to source some cheap mesh, but will definitely try it. Many thanks for all of the good advice! Paul.
PS - just saw your question about being sure there is no C26. The circuit diagram shows there should be 2 x 1000uF 63V electrolytic caps in the circuit, C22 and C26. They are quite large and there is only one such cap on the PCB I have (and also only one I can see on the photo you posted 2020-11-17 ( big orange one). I think that’s C22 as it’s in series with the speaker. I checked my PCB traces and couldn’t find any links to a 63V 1000uF capacitor across the zener. No problem though, I will simply solder one directly across the zener. Just seemed odd to me that I couldn’t find C26. Maybe I was just looking too hard! Many thanks again
Hello all, I found this old thread dedicated to this amp which I have trouble to get running. Which is frustrating because it's pretty simple amp. Any help will be much appreciated, because I have limited time to fix it.
It came to me witch strange "noises" coming from the unit, so I cleaned it, recapped it and changed output transistors to new, as there was one original and one different type that measured way off. I put CDIL there because of cost and overall "level" of the amp so I could buy 10 of them and pick a pair (I measured hfe at different currents up to 3A). Then I put the thing together and since then it gives me pain.
The push-pull stage seems to be failing to work because I got sharp, clipping-like on positive output waveform at half the input frequency. Also I'm not getting signal to the base of the TR6, only to the base of TR7. This problem increases with output load. Both output pairs are new, VAS seems to work, preamp too. I tried different loading of the VAS, different biasing, different NFB factor and nothing works.
I checked all connections around the amp section and didn't find any fault. But something is wrong. The actual amp somehow differs from the only schematic available on the web (the one above) in some details and the PCB is mess, so maybe I overlooked something.
Any ideas what to check? Any help will be much appreciated, thanks in advance.
It came to me witch strange "noises" coming from the unit, so I cleaned it, recapped it and changed output transistors to new, as there was one original and one different type that measured way off. I put CDIL there because of cost and overall "level" of the amp so I could buy 10 of them and pick a pair (I measured hfe at different currents up to 3A). Then I put the thing together and since then it gives me pain.
The push-pull stage seems to be failing to work because I got sharp, clipping-like on positive output waveform at half the input frequency. Also I'm not getting signal to the base of the TR6, only to the base of TR7. This problem increases with output load. Both output pairs are new, VAS seems to work, preamp too. I tried different loading of the VAS, different biasing, different NFB factor and nothing works.
I checked all connections around the amp section and didn't find any fault. But something is wrong. The actual amp somehow differs from the only schematic available on the web (the one above) in some details and the PCB is mess, so maybe I overlooked something.
Any ideas what to check? Any help will be much appreciated, thanks in advance.
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