Hello all. Been reading these posts for years but I never posted before. I was wondering if anyone could help with an issue I'm having with an old tub amp. The amp will work and put out good power if I power the relays with an external 12vdc source. I checked the line that is supposed to supply the voltage but it is dead. There is a red wire that comes off one of the tube sockets that goes to what looks like a series of rectifier diodes and from there runs through the standby switch and on to the relays. This line shows 0 volts right off the tube socket. I tried rotating the tubes around with the same results. Anyone know how I can fix this or do I need to run an external 12v source? Any ideas would be greatly appreciated. Thank you.
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D&A maverick 250 No relay voltage
- Thread starter ssb4all
- Start date
Can't remember seeing a diagram for the Maverick with a 12-Volt relay and transistor keying circuit. All of them I have are for older ones with a tube to key a 110-Volt relay.
The relay in your Maverick is powered from NEGATIVE 12 or 15 Volts DC. This comes from rectifiers fed by the 12.6-Volt AC heater supply.
Here's a late-version Phantom diagram. The keying circuit in this one should be close to what's in your Maverick.
http://robco.nitwitz.org/cb_tricks/...ge/graphics/phanton_triple_stage_sch_8-11.pdf
If the filter capacitor for this circuit has failed, it may keep the relay from keying. A meter probe to the standby switch won't show zero Volts DC, but it won't be near 12 Volts, either.
73
The relay in your Maverick is powered from NEGATIVE 12 or 15 Volts DC. This comes from rectifiers fed by the 12.6-Volt AC heater supply.
Here's a late-version Phantom diagram. The keying circuit in this one should be close to what's in your Maverick.
http://robco.nitwitz.org/cb_tricks/...ge/graphics/phanton_triple_stage_sch_8-11.pdf
If the filter capacitor for this circuit has failed, it may keep the relay from keying. A meter probe to the standby switch won't show zero Volts DC, but it won't be near 12 Volts, either.
73
d&a maverick 250
i have a d&a maverick 250 can anybody tell how many watt it do and how many watt it take to drive it
I'm sure the link is broken , but...
d&a maverick 250
i have a d&a maverick 250 can anybody tell how many watt it do and how many watt it take to drive itwww.worldwidedx.com
I'm sure the link is broken , but...
Okay, so I missed a couple of subtle hints here. You mentioned a measurement of zero Volts at either pin 4 or 5 of a tube socket.
If the heater in that tube is lit up, you must have had your meter set for DC Volts. The power at the heater pins of a tube socket is AC. What comes out of the rectifier diodes and into a filter capacitor is DC. That's what feeds to the standby switch, and from there to the relay coil.
And if the heater in that tube is NOT lighted, you have a different problem besides a relay that won't respond.
73
If the heater in that tube is lit up, you must have had your meter set for DC Volts. The power at the heater pins of a tube socket is AC. What comes out of the rectifier diodes and into a filter capacitor is DC. That's what feeds to the standby switch, and from there to the relay coil.
And if the heater in that tube is NOT lighted, you have a different problem besides a relay that won't respond.
73
Thank you for the reply and great info. Yes, this amp does in fact have a transistor keying circuit.
Thank you for the reply and great info. Yes,this amp does in fact have the transistor keying circuit. I did find a couple of the rectifier diodes in that circuit to be shorted and replaced them but still no DC voltage output and no ac input. All the tubes light and put out normal power if I activate the relays externally. Could a tube still function without the presence of the 12vac heater voltage? When I test for ac voltage I put the positive lead on the pin socket and the negative lead to chassis ground. Is this correct?
Oddly enough, D&A did not wire the tubes' heaters the same way every year. To check AC voltage on one tube socket, the meter leads must go one each to pin 4 and pin 5 of the tube socket. Good chance that neither of those pins is grounded. And if one of them is grounded, your amplifier is one of those, so to speak. The tubes' heaters use 6.3 Volts. The transformer winding is 12.6 Volts. Pairs of tube heaters in series split this voltage 50-50.
The Phantom schematic I linked above has an error on the power-supply half. The polarity of the four diodes in the bridge rectifier that powers the relay are all backwards, if you believe the "-16V", or minus 16 Volts marking. And that's how the keying transistor is wired, to match a negative DC supply. That voltage would be measured with one meter lead grounded.
Did you replace the 250 uf filter capacitor along with the diodes?
73
The Phantom schematic I linked above has an error on the power-supply half. The polarity of the four diodes in the bridge rectifier that powers the relay are all backwards, if you believe the "-16V", or minus 16 Volts marking. And that's how the keying transistor is wired, to match a negative DC supply. That voltage would be measured with one meter lead grounded.
Did you replace the 250 uf filter capacitor along with the diodes?
73
I checked the rectifier diodes again and found another bad one. I replaced it and now I have output but it's showing 15.8 volts DC on my meter. Is that too much for the relays?
That's about what you should see. The bridge rectifier will put out 1.414 times the 12.6 Volt RMS AC feeding into it. Subract from that the approx 2/3 of a Volt each you lose in two rectifier diodes at a time, and your 15.8 reading is spot on. This is just a bit hot for a 12-Volt relay coil, but you'll lose another Volt in the keying transistor. The relay will be running from just under 15 Volts DC. Not enough to overheat it quickly. And if it stayed keyed up long enough to overheat the relay, the rest of the amplifier would have melted down long before.
73
Ok. Well I'm going to leave it as is then. Seems to be working fine now. Thank you for all your input and valuable information. 73's and happy DXing!!
Okay, so the "If it ain't broke" rule might seem to apply here. And if old electrolytic caps have ALL been updated in the last 30 years, this should be a safe assumption.
It would be wise to obtain a replacement for the filter capacitor that supplies the relay if it looks factory-original. Eventually it will fail. Could be that it shorted, blew out a diode, and then "healed" internally. This is a common failure pattern for very-old electrolytic caps. They'll short, and then the electrolyte chemistry inside puts a layer of oxide across the short circuit. This will temporarily appear to "heal" the shorted part. But it's only temporary.
Maybe what went on here, maybe not. But I'm just pessimistic about parts that always cause trouble eventually.
73
It would be wise to obtain a replacement for the filter capacitor that supplies the relay if it looks factory-original. Eventually it will fail. Could be that it shorted, blew out a diode, and then "healed" internally. This is a common failure pattern for very-old electrolytic caps. They'll short, and then the electrolyte chemistry inside puts a layer of oxide across the short circuit. This will temporarily appear to "heal" the shorted part. But it's only temporary.
Maybe what went on here, maybe not. But I'm just pessimistic about parts that always cause trouble eventually.
73