I thought I would post here, before diving in and stripping down my HP8903B which has been a faithful companion for many years.
As of today, it started blowing the main fuse on the back, properly vapourising it the moment the front power switch is enabled.
Is there are particular cap or filter that fails on these? I hoping its a simple fix once I get it apart, but I also don't have a service manual for it.
If anyone can offer some advice I would be very grateful!
Many Thanks
Pete
Vacuum Tube Audio - Home | Facebook
As of today, it started blowing the main fuse on the back, properly vapourising it the moment the front power switch is enabled.
Is there are particular cap or filter that fails on these? I hoping its a simple fix once I get it apart, but I also don't have a service manual for it.
If anyone can offer some advice I would be very grateful!
Many Thanks
Pete
Vacuum Tube Audio - Home | Facebook
> don't have a service manual for it.
http://www.petergottlieb.com/docs/hp/8903/HP 8903B Service Vol.1.pdf
http://www.petergottlieb.com/docs/hp/8903/HP 8903B Service Vol. 2.pdf
http://www.petergottlieb.com/docs/hp/8903/HP 8903B Service Vol.1.pdf
http://www.petergottlieb.com/docs/hp/8903/HP 8903B Service Vol. 2.pdf
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Volume 2, page 164 of that Vol.2 PDF--- line plug, voltage switching, power switch, main power transformer, then on to J2 on main power board.
Are the voltage switches set for your local voltage? Are you using the indicated fuse ratings? Any sign of distress or burning on the power transformer (PT)? Wire-clippings or pennies fallen through the case onto power traces or connectors?
PT connects to main power board at connector J2 hidden amongst large caps C1 C3. Disconnect here.
Really should be using a Lamp Limiter- if problem not found quick you will use-up all your fuses before you figure it out.
Will the fuse hold (lamp stay dim) with PT disconnected at J2? If not, you have a problem around line switching or inside PT (boohoo).
Still J2 disconnected, probe AC Volts from PT. If 6-4 or 6-5 reads over 45VAC, thyristor Q3 will go short to protect from overvoltage. (I would expect 20VAC here, so an overvoltage would be a Problem.)
Most likely cause of gross overvoltage is PT set for 120V and plugged to 240V. Considering age, a less-likely problem is a shorted turn inside the PT, directly causing high line-fuse current or indirectly inducing overvoltage out of PT exciting Q3 to shut-down.
Are the voltage switches set for your local voltage? Are you using the indicated fuse ratings? Any sign of distress or burning on the power transformer (PT)? Wire-clippings or pennies fallen through the case onto power traces or connectors?
PT connects to main power board at connector J2 hidden amongst large caps C1 C3. Disconnect here.
Really should be using a Lamp Limiter- if problem not found quick you will use-up all your fuses before you figure it out.
Will the fuse hold (lamp stay dim) with PT disconnected at J2? If not, you have a problem around line switching or inside PT (boohoo).
Still J2 disconnected, probe AC Volts from PT. If 6-4 or 6-5 reads over 45VAC, thyristor Q3 will go short to protect from overvoltage. (I would expect 20VAC here, so an overvoltage would be a Problem.)
Most likely cause of gross overvoltage is PT set for 120V and plugged to 240V. Considering age, a less-likely problem is a shorted turn inside the PT, directly causing high line-fuse current or indirectly inducing overvoltage out of PT exciting Q3 to shut-down.
I have had this unit for many years, and its been working without fault until this morning.
So yes the voltages are all set for my local voltage.
We have had voltage fluctuation problems, going as high as 254v recently and a transformer blew up in the substation next door as well (doh!).
This unit wasn't even powered when we had those issues, it just popped today, line voltage (which we have been monitoring) was 235v.
Thankyou "PRR" for the info on the input boards, I'll take a look tomorrow!
P.
So yes the voltages are all set for my local voltage.
We have had voltage fluctuation problems, going as high as 254v recently and a transformer blew up in the substation next door as well (doh!).
This unit wasn't even powered when we had those issues, it just popped today, line voltage (which we have been monitoring) was 235v.
Thankyou "PRR" for the info on the input boards, I'll take a look tomorrow!
P.
A14 F1 is the line fuse, I assume this is the one blowing.
The low-voltage supplies have individual fuses on A11, F1 F2 F3. I assume if these are holding there is no gross fault in or past the regulators.
If when PT is disconnected at J2, A14 F1 holds, and PT LV AC is safe (probably under 20VAC), but A14 F1 blows when J2 is connected, this leads to the rectifier diodes and main filter caps. Any short here would kill the main fuse.
You could try reading DC voltage on C1 C2 C3. If power has not held for a while, they will be very-very small, of course. But dead-zero is suspicious of a shorted cap. Also you could try customary diode-checks on the rectifiers, though with all the diodes and caps connected they may not give a clear answer.
Considering this thing is old enough to vote (older than H-P expected its prime customers to keep using it), AND you have had power troubles, it may be time to just replace all these bits.
A13CR1 DIODE-PWR RECT 100V 1.5A
A13CR2 DIODE-PWR RECT 100V 1.5A
A13CR3 DIODE-PWR RECT 100V 1.5A
A13CR4 DIODE PWR RECT 100V 1.5A
A13CR5 DIODECT-RECT 200V 15A (two rectifier diodes)
The low-voltage supplies have individual fuses on A11, F1 F2 F3. I assume if these are holding there is no gross fault in or past the regulators.
If when PT is disconnected at J2, A14 F1 holds, and PT LV AC is safe (probably under 20VAC), but A14 F1 blows when J2 is connected, this leads to the rectifier diodes and main filter caps. Any short here would kill the main fuse.
You could try reading DC voltage on C1 C2 C3. If power has not held for a while, they will be very-very small, of course. But dead-zero is suspicious of a shorted cap. Also you could try customary diode-checks on the rectifiers, though with all the diodes and caps connected they may not give a clear answer.
Considering this thing is old enough to vote (older than H-P expected its prime customers to keep using it), AND you have had power troubles, it may be time to just replace all these bits.
A13CR1 DIODE-PWR RECT 100V 1.5A
A13CR2 DIODE-PWR RECT 100V 1.5A
A13CR3 DIODE-PWR RECT 100V 1.5A
A13CR4 DIODE PWR RECT 100V 1.5A
A13CR5 DIODECT-RECT 200V 15A (two rectifier diodes)
The first thing to check is the line filter in the voltage selector. Some are notorious for this type of failure. You would start by checking DCR across the line to neutral on the power cable. Then disconnect the power transformer (one lead is enough because the primaries are in series on your line voltage) and check DCR. It should be at least 100K (the bleeder resistor in the filter) with the transformer disconnected.
Good luck!
Good luck!
OK,
the main fuse doesn't blow with J2 out, and the transformer reads 20v AC across the taps. Removed all three big Caps and test them and charged them - no shorts. All the diodes test ok, and all regulator downstream fuses are intact. The triac has a suspciously low impedance of 50ohms across the two main terminals, (not the tag) which are across the PSU. I am wondering if the triac is dead (Q3).
I'll take a look again later...
P.
the main fuse doesn't blow with J2 out, and the transformer reads 20v AC across the taps. Removed all three big Caps and test them and charged them - no shorts. All the diodes test ok, and all regulator downstream fuses are intact. The triac has a suspciously low impedance of 50ohms across the two main terminals, (not the tag) which are across the PSU. I am wondering if the triac is dead (Q3).
I'll take a look again later...
P.
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