See post #2 for voltage at C17b. Have you replaced the Selenium rectifier (SR5) with a modern silicon rectifier? Could you measure the Vac across the bias coil of the power transformer (WH and WHT/ORN) to make sure your bias supply is powered correctly by the transformer.
I would guess it's somewhere around -75VDC to -100VDC at the diode anode.
If the diode is original, I would replace it regardless, to avoid trouble.
Make sure capacitor polarity is correct.
Check the resistor value.
If the diode is original, I would replace it regardless, to avoid trouble.
Make sure capacitor polarity is correct.
Check the resistor value.
yes, I did replace the original selenium rectifier SR5 with a modern silicon diode.
74.4vac across the bias winding of the power transformer
you can see the diode mounted on a mini turret board on the right of this picture. C17b mounted horizontally to the left of the diode. white wire to the right and behind diode, connected to cathode (top) and white orange to the left of C17B. looks like hell, i know, but this is my first stab at something like this.
C17a is that multi tap capacitor behind C17b (label on top of aluminum mounting bracket C17-15), so tomorrow morning, I will have to disconnect some stuff and check it out.
74.4vac across the bias winding of the power transformer
you can see the diode mounted on a mini turret board on the right of this picture. C17b mounted horizontally to the left of the diode. white wire to the right and behind diode, connected to cathode (top) and white orange to the left of C17B. looks like hell, i know, but this is my first stab at something like this.
C17a is that multi tap capacitor behind C17b (label on top of aluminum mounting bracket C17-15), so tomorrow morning, I will have to disconnect some stuff and check it out.
Looks like C17a is likely to be a goner.
That would lower the bias voltage a lot, which is the other problem that you have.
That would lower the bias voltage a lot, which is the other problem that you have.
According to this description and the picture, you've installed the new electrolytic capacitor C17b wrongly polarized. The positive lead needs to go to ground, the negative one to the diode, see schematics. In reversed polarisation, the cap almost shorts the voltage across it and will pop soon. Now you need to get a new one installed correctly and replace resistor R35 (1.8k) also.
Best regards!
That may be right, please check the polarities carefully.
In any event, replace both caps.
In any event, replace both caps.
This means it's real value is 10 % more than it's nominal one - a clear indication that this 5 % tolerance (if I decipher your pic correctly) carbon comp resistor has been overloaded by the wrongly polarized 'lytic.
Best regards!
I think you are best off building the entire bias circuit on a perf board,
so you can clean up the soldering and be sure to get everything right.
Be absolutely certain about the capacitor polarities.
so you can clean up the soldering and be sure to get everything right.
Be absolutely certain about the capacitor polarities.
if my BM786 multimeter is telling the truth, both C17a and C17b are 23 mf 😳
drawing says 20mf, but I dont know if the difference is enough cause problems.
drawing says 20mf, but I dont know if the difference is enough cause problems.
I'm having trouble trying to figure out how to 'reply' to Kay Pirinha. I dont see a reply button, but I see others have somehow included a quote from me, for reference. I dont know how to do that, either.
I think everyone was right about my capacitor polarities. The picture I posted of C17a does show it installed backwards. Thats the one that initially blew and when I replaced it, I turned it around. However, I did figure out that C17b is connected backwards. C17b is part of a 20-20-20-40 capacitor and they all share the same negative terminal that is grounded. I used one of those 20mf connections and I should not have done that as it is backwards. And the resistor in series with C17a will also be replaced. I just have to rearrange things in there and in my head.
i'm so glad I found this place where everyone is so helpful. you are all truly appreciated. i will have to get to work on this. late start, today!
i'm so glad I found this place where everyone is so helpful. you are all truly appreciated. i will have to get to work on this. late start, today!
I'm down to just 1 spare 20mf capacitor. It will probably take a couple days if I put my order in now. thanks for the help! "I'll be Bach" lol
Walterramjet,
did you install that multiple capacitor named C17-15 in your pic in #23? As usual, the individual units share their negative terminals within this component. Hence, when used in a negative supply bus, the can as the common negative terminal needs to be isolated from the chassis, see schematics. Did you obey this?
Best regards!
did you install that multiple capacitor named C17-15 in your pic in #23? As usual, the individual units share their negative terminals within this component. Hence, when used in a negative supply bus, the can as the common negative terminal needs to be isolated from the chassis, see schematics. Did you obey this?
Best regards!