HP 182T 15V rail - Page 2 - UK Vintage Radio Repair and Restoration Discussion Forum

RogerEvans · 26th Jan 2022, 5:12 pm

Bill,

CR4 is only there to protect Q7 against some types of failure and it doesn't look like it did its job very well! Putting a Schottky diode in that circuit will not do any harm and might help in future. I don't know what part HP specified originally but it probably had a current rating of more than 30mA. I can send you a couple of 1N5189 diodes that I have to hand, they would be overkill at a 3 Amp rating but again they should do no harm. Send me a PM if you would like them.

Regards,

Roger

Bufo Bill · 26th Jan 2022, 5:21 pm

Roger, I have a parts list, and it does give a rating of 30mA. Thanks for the offer of the diodes, I will try one of my Schottsky ones and I'll get back to you if I have problems. Many thanks, it's a really kind offer.
Cheers from Bill.

Bufo Bill · 26th Jan 2022, 8:41 pm

Hi, we are still a bit low at the 15V rail. I am unsure about the variable resistor controlling the voltage. It only seems to adjust by about a Volt. I will check the voltages on the legs of Q7 and Q8, to check all is well there.
Bill.

frsimen · 27th Jan 2022, 5:49 pm

Hello Bill,

The adjustment that will be provided on the 15V rail is only around 1.4V when working normally, so the 500 ohm potentiometer is probably working correctly.

The 10.5V reference on Q7's base is obtained from the 100V supply, divided down by the resistors R14 and R15. If the 10.5V reference is low, that would make the output voltage low too.

If the reference voltage is low, check the 100V line is 100V and that R14 and R15 measure correctly.

If the reference voltage is correct, the problem could be caused by R16 increasing in value, R13 increasing in value or leakage in Q6.

Paula

RogerEvans · 27th Jan 2022, 6:18 pm

Bill,

You haven't said how far off spec the +15V supply is. Just to add a little to the previous posts:

If the base of Q7 and the base of Q8 are at the same voltage (within 100mV or so) then the feedback regulator is doing its job and the problem is that one of the two voltage dividers (R14/R15) or (R19/R21) has drifted off spec.

If the base voltage of Q7 is close to the nominal value of +10.5V then the problem is with R20 and R21, or just possibly that C4 is leaky - I have had problems with ceramic capacitors of that sort of vintage. If you try to check in circuit then measure with the leads both ways round since the collector base junction of Q8 will conduct in one direction.

Regards,

Roger

Bufo Bill · 27th Jan 2022, 9:30 pm

Paula, many thanks for the help. Q6 has been replaced and R13, R14, R16 are fine. R15 is a bit iffy, so will be replaced. Bar the 15V rail, Test Points all measure correctly. We now have a higher voltage than required on Q6 (12v) and Q7 Collector is at 11V! The 15V rail is higher now but still not 15V. Roger mentioned something about high voltages on the transistors so will have a read.
Cheers from Bill.

Bufo Bill · 27th Jan 2022, 9:32 pm

Roger, the rail is currently 11.3V. See above for R15. Will check Q2 Base voltage.
Cheers from Bill .

Bufo Bill · 27th Jan 2022, 9:41 pm

Q2 is very poorly as well. Can replace that tomorrow.
Cheers from Bill.

Bufo Bill · 28th Jan 2022, 4:57 pm

Both R15 and R21 have drifted significantly, with R21 only a few kOhms in resistance. I have been measuring some of the transistors in this area of the circuit. A1A2Q2 which should be a 2N3904 measures 98V on the base! I would like to check A1Q2 also ( one of the ones on the big heatsinks), but cannot seem to get to it in circuit. Any suggestions for rigging it up on the bench? My tester only does NPN & PNP transistors.
Should have the resistor replacements by Monday.
Cheers from Bill.

factory · 28th Jan 2022, 5:46 pm

A1A2Q2 is in the +100V rail and +98V on the base is correct, I thought you said only the +15V rail was faulty.

Drifted voltage divider resistors was a common problem with the older 140/141 series.

I guess they would have used an extender board to get to the big transistors while powered up, the leads might be a bit short with just the heatsinks removed.

David

factory · 28th Jan 2022, 6:01 pm

Here are a few pictures of the 00184-66509 board from my 181T for reference, not sure is yours is the same or not. The components for the 15V regulator seem to be mostly in the lower right section of the board.

Click image for larger version

Name: DSC_7002a.jpg
Views: 47
Size: 154.7 KB
ID: 250641

Click image for larger version

Name: DSC_6999a.jpg
Views: 41
Size: 88.1 KB
ID: 250645

Click image for larger version

Name: DSC_6998a.jpg
Views: 37
Size: 166.2 KB
ID: 250642

Click image for larger version

Name: DSC_6997a.jpg
Views: 34
Size: 131.5 KB
ID: 250643

Click image for larger version

Name: DSC_7005a.jpg
Views: 37
Size: 125.9 KB
ID: 250644

David

Bufo Bill · 28th Jan 2022, 6:21 pm

Hi David thanks for posting. It should hopefully be a matter of R15 and R21 then. Thanks for the help.
Bill.

Bufo Bill · 28th Jan 2022, 9:05 pm

A1Q2 seems fine, so hopefully A1A2Q5 and R15 and R21 should sort things out. Won't have the parts until Monday, so you can all have a whole weekend without me pestering you!
Cheers from Bill.

Bufo Bill · 30th Jan 2022, 3:40 pm

Hi guys, got it sorted! I found some 5k1R resistors that I'd hoarded, and stole a Q5 from a parts machine. Will probably put a new Transistor in when they arrive.
Thanks for all the help everyone. Roger and Paula: your contributions were especially important in understanding the cause, so special thanks. And thanks for the schematic Bill.
Cheers from Bill.

factory · 30th Jan 2022, 4:20 pm

That's what I would have done too, but would have maybe checked the common E24 5k1 resistor measured between 5059 & 5161 ohms.
Makes you wonder if the circuit could still be adjusted to +15V with standard E24 (5%) resistors, instead of the E96 (1%) series resistors they used for a lot of them.

David

Bufo Bill · 30th Jan 2022, 6:32 pm

Thanks David, that's very reassuring to know.
Bill.

26th Jan 2022, 5:12 pm	#21
RogerEvans Hexode Join Date: Oct 2014 Location: Wiltshire, UK. Posts: 381	Re: HP 182T 15V rail Bill, CR4 is only there to protect Q7 against some types of failure and it doesn't look like it did its job very well! Putting a Schottky diode in that circuit will not do any harm and might help in future. I don't know what part HP specified originally but it probably had a current rating of more than 30mA. I can send you a couple of 1N5189 diodes that I have to hand, they would be overkill at a 3 Amp rating but again they should do no harm. Send me a PM if you would like them. Regards, Roger

26th Jan 2022, 5:21 pm	#22
Bufo Bill Heptode Join Date: Jan 2016 Location: Wolverhampton, West Midlands, UK. Posts: 777	Re: HP 182T 15V rail Roger, I have a parts list, and it does give a rating of 30mA. Thanks for the offer of the diodes, I will try one of my Schottsky ones and I'll get back to you if I have problems. Many thanks, it's a really kind offer. Cheers from Bill.

26th Jan 2022, 8:41 pm	#23
Bufo Bill Heptode Join Date: Jan 2016 Location: Wolverhampton, West Midlands, UK. Posts: 777	Re: HP 182T 15V rail Hi, we are still a bit low at the 15V rail. I am unsure about the variable resistor controlling the voltage. It only seems to adjust by about a Volt. I will check the voltages on the legs of Q7 and Q8, to check all is well there. Bill.

27th Jan 2022, 5:49 pm	#24
frsimen Heptode Join Date: Jan 2011 Location: Croydon, London, UK. Posts: 773	Re: HP 182T 15V rail Hello Bill, The adjustment that will be provided on the 15V rail is only around 1.4V when working normally, so the 500 ohm potentiometer is probably working correctly. The 10.5V reference on Q7's base is obtained from the 100V supply, divided down by the resistors R14 and R15. If the 10.5V reference is low, that would make the output voltage low too. If the reference voltage is low, check the 100V line is 100V and that R14 and R15 measure correctly. If the reference voltage is correct, the problem could be caused by R16 increasing in value, R13 increasing in value or leakage in Q6. Paula

27th Jan 2022, 6:18 pm	#25
RogerEvans Hexode Join Date: Oct 2014 Location: Wiltshire, UK. Posts: 381	Re: HP 182T 15V rail Bill, You haven't said how far off spec the +15V supply is. Just to add a little to the previous posts: If the base of Q7 and the base of Q8 are at the same voltage (within 100mV or so) then the feedback regulator is doing its job and the problem is that one of the two voltage dividers (R14/R15) or (R19/R21) has drifted off spec. If the base voltage of Q7 is close to the nominal value of +10.5V then the problem is with R20 and R21, or just possibly that C4 is leaky - I have had problems with ceramic capacitors of that sort of vintage. If you try to check in circuit then measure with the leads both ways round since the collector base junction of Q8 will conduct in one direction. Regards, Roger

27th Jan 2022, 9:30 pm	#26
Bufo Bill Heptode Join Date: Jan 2016 Location: Wolverhampton, West Midlands, UK. Posts: 777	Re: HP 182T 15V rail Paula, many thanks for the help. Q6 has been replaced and R13, R14, R16 are fine. R15 is a bit iffy, so will be replaced. Bar the 15V rail, Test Points all measure correctly. We now have a higher voltage than required on Q6 (12v) and Q7 Collector is at 11V! The 15V rail is higher now but still not 15V. Roger mentioned something about high voltages on the transistors so will have a read. Cheers from Bill.

27th Jan 2022, 9:32 pm	#27
Bufo Bill Heptode Join Date: Jan 2016 Location: Wolverhampton, West Midlands, UK. Posts: 777	Re: HP 182T 15V rail Roger, the rail is currently 11.3V. See above for R15. Will check Q2 Base voltage. Cheers from Bill .

27th Jan 2022, 9:41 pm	#28
Bufo Bill Heptode Join Date: Jan 2016 Location: Wolverhampton, West Midlands, UK. Posts: 777	Re: HP 182T 15V rail Q2 is very poorly as well. Can replace that tomorrow. Cheers from Bill.

28th Jan 2022, 4:57 pm	#29
Bufo Bill Heptode Join Date: Jan 2016 Location: Wolverhampton, West Midlands, UK. Posts: 777	Re: HP 182T 15V rail Both R15 and R21 have drifted significantly, with R21 only a few kOhms in resistance. I have been measuring some of the transistors in this area of the circuit. A1A2Q2 which should be a 2N3904 measures 98V on the base! I would like to check A1Q2 also ( one of the ones on the big heatsinks), but cannot seem to get to it in circuit. Any suggestions for rigging it up on the bench? My tester only does NPN & PNP transistors. Should have the resistor replacements by Monday. Cheers from Bill.

28th Jan 2022, 5:46 pm	#30
factory Octode Join Date: Jul 2009 Location: Cheshire, UK. Posts: 1,737	Re: HP 182T 15V rail A1A2Q2 is in the +100V rail and +98V on the base is correct, I thought you said only the +15V rail was faulty. Drifted voltage divider resistors was a common problem with the older 140/141 series. I guess they would have used an extender board to get to the big transistors while powered up, the leads might be a bit short with just the heatsinks removed. David

28th Jan 2022, 6:01 pm	#31
factory Octode Join Date: Jul 2009 Location: Cheshire, UK. Posts: 1,737	Re: HP 182T 15V rail Here are a few pictures of the 00184-66509 board from my 181T for reference, not sure is yours is the same or not. The components for the 15V regulator seem to be mostly in the lower right section of the board. David

28th Jan 2022, 6:21 pm	#32
Bufo Bill Heptode Join Date: Jan 2016 Location: Wolverhampton, West Midlands, UK. Posts: 777	Re: HP 182T 15V rail Hi David thanks for posting. It should hopefully be a matter of R15 and R21 then. Thanks for the help. Bill.

28th Jan 2022, 9:05 pm	#33
Bufo Bill Heptode Join Date: Jan 2016 Location: Wolverhampton, West Midlands, UK. Posts: 777	Re: HP 182T 15V rail A1Q2 seems fine, so hopefully A1A2Q5 and R15 and R21 should sort things out. Won't have the parts until Monday, so you can all have a whole weekend without me pestering you! Cheers from Bill.

30th Jan 2022, 3:40 pm	#34
Bufo Bill Heptode Join Date: Jan 2016 Location: Wolverhampton, West Midlands, UK. Posts: 777	Re: HP 182T 15V rail Hi guys, got it sorted! I found some 5k1R resistors that I'd hoarded, and stole a Q5 from a parts machine. Will probably put a new Transistor in when they arrive. Thanks for all the help everyone. Roger and Paula: your contributions were especially important in understanding the cause, so special thanks. And thanks for the schematic Bill. Cheers from Bill.

30th Jan 2022, 4:20 pm	#35
factory Octode Join Date: Jul 2009 Location: Cheshire, UK. Posts: 1,737	Re: HP 182T 15V rail That's what I would have done too, but would have maybe checked the common E24 5k1 resistor measured between 5059 & 5161 ohms. Makes you wonder if the circuit could still be adjusted to +15V with standard E24 (5%) resistors, instead of the E96 (1%) series resistors they used for a lot of them. David

30th Jan 2022, 6:32 pm	#36
Bufo Bill Heptode Join Date: Jan 2016 Location: Wolverhampton, West Midlands, UK. Posts: 777	Re: HP 182T 15V rail Thanks David, that's very reassuring to know. Bill.