PDA

View Full Version : SB-220 Problems



AH6RR
05-28-2012, 07:12 PM
I am working on a SB-220 that has all of the Harbach Mods HV Cap bank, RM-220 Metering/Rectifier board, Soft Start, Soft Key and fan. I was told it just quit working and while inspecting the amp the first thing I found was one of the 15ohm ceramic wirewound resistors was open and one of the traces on the soft start board was melted and the wire on that trace had gotten so hot that it melted the solder and there was very little connection so I repaired all of that. Then I found that half of the diodes on the HV Cap bank were shorted on the top 4 caps so I replaced them the resistors and caps seem good no shorts or open caps or resistors. Moving on the the Metering/Rectifier board I foumd that the 1ohm 3W resistor was open (waiting on one to get here since I did not have one on hand and Rat Shack does not carry anything like that at all) everything else is good. I know that the 1ohm resistor acts like a fuse but I do not remember what for. Checked the HV Transformer the HV side seems fine with around 13-14 ohms across the windings and nothing to ground I will check the input windings when I get home later today. I checked the tubes for shorts as best I could with a VOM they looked OK but I know that will not show everything. So my Question is could have the 15 ohm resistor opening have caused this or could it have been a tube short that is not detectable with a VOM? Or could it have been something else? The tubes look like the Old Eimac 3-500Z and not the ZG but are made in the PRC.
Roland AH6RR

K1ZJH
05-28-2012, 07:30 PM
Those resistors drop the AC voltage for a short period until the relays pull in. The relay contacts
short the resistors to apply full voltage to the amp. It happens fairly quickly. If a relay fails, it
is likely that the resistor will overheat and fail, and even damage the board. Whether the
Soft Start module creates more problems than it supposedly solves is open for debate :)

R1 is in series with the negative return for the HV power supply. A tube arcing could expain
why R1 opened. I'm surprised the Harbach diodes failed, since they are higher rated than
the diodes used by Heath. A tube arcing over could explain most of the damage. If the
grid circuit at the 3-500Z sockets is original, check the RF chokes and see if they are damaged.

I'd add a properly rated glitch resistor on the B+ feed to limit damage in the future. That is
one modification kit that Harbach should have done.

Pete

AH6RR
05-28-2012, 08:14 PM
The only diodes that failed were the 1n4005 that are on the FB-220. I will check the RF chokes but I was thinking on grounding the grids directly as I have done in my 220 and Yes the Glitch resistor is going in also. Since the tubes are in the same sockets it shoul be easy to tell what tube failed if any. I will check out the relay on the soft start also since I have a few spares I had picked up a year or so ago and if one failed I will replace both of them. Thanks Pete
Roland AH6RR

AG6K
05-28-2012, 10:27 PM
I am working on a SB-220 that has all of the Harbach Mods HV Cap bank, RM-220 Metering/Rectifier board, Soft Start, Soft Key and fan. I was told it just quit working and while inspecting the amp the first thing I found was one of the 15ohm ceramic wirewound resistors was open and one of the traces on the soft start board was melted and the wire on that trace had gotten so hot that it melted the solder and there was very little connection so I repaired all of that. Then I found that half of the diodes on the HV Cap bank were shorted on the top 4 caps so I replaced them the resistors and caps seem good no shorts or open caps or resistors. Moving on the the Metering/Rectifier board I foumd that the 1ohm 3W resistor was open (waiting on one to get here since I did not have one on hand and Rat Shack does not carry anything like that at all) everything else is good. I know that the 1ohm resistor acts like a fuse but I do not remember what for. Checked the HV Transformer the HV side seems fine with around 13-14 ohms across the windings and nothing to ground I will check the input windings when I get home later today. I checked the tubes for shorts as best I could with a VOM they looked OK but I know that will not show everything. So my Question is could have the 15 ohm resistor opening have caused this or could it have been a tube short that is not detectable with a VOM? Or could it have been something else? The tubes look like the Old Eimac 3-500Z and not the ZG but are made in the PRC.
Roland AH6RR

 The 15Ω R was a victim, not the perpetrator.
. . When a hv rectifier diode shorts in a FWD, the soft-start relay opens, the soft-start resistors fry, and half of the polarized electrolytic C-filter bank receives fatal AC current. To guard against potential damage to electrolytics, connect a >600piv diode across each electrolytic filter C. (Jim, VE7RF)
I would replace all of the remaining rectifiers with something better. If 1000piv rectifiers (1N5408) instead of the original 600piv rectifiers are used, only 4 per leg are needed instead of 7.
Free advice: if the Harback mod does not provide them, put 2, back to belly 3A rectifiers across each meter movement and across R2, the grid-I meter shunt R. Add a 15-Ω, 120J surge rated R in series with the HV+ to prevent the hot filaments from bending and possibly causing a filament-to-grid short during the SB-220's intermittent parasitic osc. at c. 110MHz.

94041
QST Magazine photograph.

W1QJ
05-28-2012, 11:53 PM
The step start resistors will act faster than the circuit breakers in a "start up fault". You have to ask yourself if those resistors opened up they had to be in the circuit at the time of the fault. Normally under normal operating they are shorted out by the relays. If the amp "in fact" was in the normal operating mode the resistors would be out of the circuit so you may want to see if the diode or cap in the 120vdc low voltage supply went south, that would cause the step start relays to disengage and the line current will be fed through the resistors, the current through them with the normal standby current draw is enough to heat them up and blow them open after they got real hot. The solder would never melt in a short quick fault so it tells me that the reistors were passing current for some time. That is why I would check the low voltage supply. Also, if that was the case the tubes would loose cut off bias and the tubes would get real hot. Although that does not explain the shorted diodes in the power supply. The Harbach board has the meter protection diodes on the board, better lift one leg and check them all. The 1 ohm resistor is there for metering of plate current. The plate meter reads the Vdrop across that 1 ohm resistor and it is calibrated in plate current. 280mv drop across the 1 ohm resistor equals 1 amp of plate current. If there is a major fault that resistor will open and so will the .82 ohm grid shunt as well. Check the .82 ohm resistor for open as well. Lift one leg to check. it seems you had a chain reaction going on. Take a good look at the pate choke too. The meter protection diodes should have protected the meter(s) they must be checked.

AH6RR
05-29-2012, 12:18 AM
Lou I checked the .82 ohm resistor and it is good still as is the Meter diodes I did as you said because the meter diodes are back to back and show a short unless one leg is lifted. As for the soft start 15 ohm resistors only one blew the other was good and the trace to the amp side was burned up and that is the side with the blown resistor I will check the relays, the 33 ohm resistor on the board is fine. I am starting to think that the owner has a 240V line problem as this is the 3rd amp in 3 months time I have repaired for him. One was a AL-82 with the step start relays fried the other is a LK-550 with the High Voltage board having blown diodes in the full wave bridge and the step start resistor open he also had a Expert Solid State amp die too he sent that to the dealer for repair.

W1QJ
05-29-2012, 12:23 AM
Yikes, Did you check the low voltage PS?

AH6RR
05-29-2012, 12:52 AM
I just got home from work and I am going to dig into it asap.

AG6K
05-29-2012, 12:58 AM
Those resistors drop the AC voltage for a short period until the relays pull in. The relay contacts
short the resistors to apply full voltage to the amp. It happens fairly quickly. If a relay fails, it
is likely that the resistor will overheat and fail, and even damage the board. Whether the
Soft Start module creates more problems than it supposedly solves is open for debate :)

R1 is in series with the negative return for the HV power supply. A tube arcing could expain
why R1 opened. I'm surprised the Harbach diodes failed, since they are higher rated than
the diodes used by Heath. A tube arcing over could explain most of the damage. If the
grid circuit at the 3-500Z sockets is original, check the RF chokes and see if they are damaged.

I'd add a properly rated glitch resistor on the B+ feed to limit damage in the future. That is
one modification kit that Harbach should have done.

Pete

 IF the tube arced, a high-pot tester will detect gas. My guess is that the tube is not gassy.

I agree on the importance of a suitable gliitch-R.

AH6RR
05-29-2012, 02:36 AM
Lou the low voltage supply looks good. One of the relays on the soft start is bad it has a short across the 2 contacts I dug my relays out to replace it with but mine are 24V not 48V So I just ordered some from mouser. Also the RF chokes on the grids are good too so I am leaning in 2 directions 1. The soft start relay failed 2. Flaky 240V line voltages. And Rich if you want to send me a Hi-Pot tester I would be most Happy.

W1QJ
05-29-2012, 10:38 AM
Roland. a gas tube arc will not affect the low voltage PS so one of the relays must have failed but since they are in series both would fsil if one went open coil. I suppose the failure was acutasl contacts allowing the resistor to pass current (not shorted). Your customer seems to have some problem with something causing all these different amps to fail. If it is his 240 line voltage he needs to monitor it or have the POCO check it out.

KM1H
05-29-2012, 03:36 PM
The Harbach and Ameritron step starts have a long history of failure that has nothing to do with the AC line or tube arcs. The Ameritron is a poor design and the Harbach has underated parts that eventually fail.

While the Harbach uses 1N5408's for the B+ they use undersized 1N4005's as the operating bias. I always replace with just enough 1N5408's to get sufficient idle current. That board also likes to blow HV metering resistors and I replace those with 3 real HV 5M Ohmites.

Having a line recorder on your customers circuit would be a good idea before plugging anything in again.

Carl

AG6K
05-30-2012, 10:30 AM
Lou the low voltage supply looks good. One of the relays on the soft start is bad it has a short across the 2 contacts I dug my relays out to replace it with but mine are 24V not 48V So I just ordered some from mouser. Also the RF chokes on the grids are good too so I am leaning in 2 directions 1. The soft start relay failed 2. Flaky 240V line voltages. And Rich if you want to send me a Hi-Pot tester I would be most Happy.

 I will high-pot test your tubes gratis but I will not donate a high-pot. . . I have been testing/autopsying 3-500s and 3-400s for >30-years and I have yet to find a gassy one with an arc mark on rhe grid. . . . If a 110vdc coil dpdt relay is not available for step-start duty in a SB-220, a 12vdc coil relay will do the job if a FWD rectifier-filter is powered by the 5v-rms winding on T1 to produce 12vdc. - note - A series R of roughly 1/3 of the coil Ωs needs to be put in series with the relay's coil to control pull-in.