I have an HP 200CD ( $15 hamfest special ) that exhibits excessive distortion and bandwidth.
Specifically, about 2.5% harmonic distortion at the frequencies I have checked between 1 and 2 KHz and at any output level. Also, it appears to have an excessively broad signal width - the proverbial bell curve with a flattish top. It appears to make full output voltage. I am using the shorting bar and taking the output unbalanced. I am feeding it 117 VAC off a variac, to eliminate out of spec line voltage as an issue.
Also, the phenolic board in the center bottom chassis shows signs of overheating, although the resistors over it all seem in spec, and not overheated or overheating.
The only mod I am aware of is by me: I robbed the GE 5AR4 ( which is why I bought it in the first place a decade ago ) and subbed in a 5T4. Someone retubed it at some point - it's full of JAN Sylvania's with 1987 date codes. It's possible someone diddled with the alignment - could that explain the broad bandwidth / high distortion ? Should it have been aligned when retubed?
Any common failure points on these instruments, or ideas where to start? Or should I just part it out and look for one that doesn't need troubleshooting?
It's not an urgent matter - I have a Leader generator that is adequate for my present needs, but would like to get this HP in spec at some point - or at least down to my current measurement limits, which is around 0.08% at present. Or reduce it to parts. I imagine that chassis and capacitor would be a good start for a new receiver.
Win W5JAG
Specifically, about 2.5% harmonic distortion at the frequencies I have checked between 1 and 2 KHz and at any output level. Also, it appears to have an excessively broad signal width - the proverbial bell curve with a flattish top. It appears to make full output voltage. I am using the shorting bar and taking the output unbalanced. I am feeding it 117 VAC off a variac, to eliminate out of spec line voltage as an issue.
Also, the phenolic board in the center bottom chassis shows signs of overheating, although the resistors over it all seem in spec, and not overheated or overheating.
The only mod I am aware of is by me: I robbed the GE 5AR4 ( which is why I bought it in the first place a decade ago ) and subbed in a 5T4. Someone retubed it at some point - it's full of JAN Sylvania's with 1987 date codes. It's possible someone diddled with the alignment - could that explain the broad bandwidth / high distortion ? Should it have been aligned when retubed?
Any common failure points on these instruments, or ideas where to start? Or should I just part it out and look for one that doesn't need troubleshooting?
It's not an urgent matter - I have a Leader generator that is adequate for my present needs, but would like to get this HP in spec at some point - or at least down to my current measurement limits, which is around 0.08% at present. Or reduce it to parts. I imagine that chassis and capacitor would be a good start for a new receiver.
Win W5JAG
Does the H-P manual have nominal voltages? I would start by checking those.
EDIT--
http://www.hparchive.com/Manuals/HP-200CD-Manual-SNP_605.PDF
Note the readings are made with a 122Meg(!) meter. Most points won't matter, but power tube grids are near a Meg impedance so a common 10Meg meter will read a bit low. Anyway you are looking for WAY-off voltages, probably not a small shift. (I could accept this thing could be half dead and still sing few-% THD.0
EDIT--
http://www.hparchive.com/Manuals/HP-200CD-Manual-SNP_605.PDF
Note the readings are made with a 122Meg(!) meter. Most points won't matter, but power tube grids are near a Meg impedance so a common 10Meg meter will read a bit low. Anyway you are looking for WAY-off voltages, probably not a small shift. (I could accept this thing could be half dead and still sing few-% THD.0
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200AB is rated 1% but can usually be trimmed better, maybe 0.2% midband.
The 200CD has a better circuit and a lower spec, so sub-0.1% maybe?
Of course these numbers look huge today. An audio DAC should touch 0.01% without doubt, and better numbers exist, so let your computer grind you a Sine.
The 200CD has a better circuit and a lower spec, so sub-0.1% maybe?
Of course these numbers look huge today. An audio DAC should touch 0.01% without doubt, and better numbers exist, so let your computer grind you a Sine.
If you reduce the level of the oscillator loop to 7? volts the distortion can be in the .005% range. Normal setting gets 30V out I think. Its a factory option documented somewhere. You should look at the waveform to determine the cause of the distortion. It may be clipping from being set too high. The light bulb could be open as another simple option. Its a pretty solid robust circuit. HP made those for over 20 years.
I maintained about 100 200AB's and 200CD's at Motorola in the 70's. I don't remember which is which but I think you have the model with 6K6GT output tubes. The other model used 6CW5's and a solid state rectifier.
These things were left turned on 24/7 and about the only thing I ever changed was the 5AR4. OE was a Mullard too! They were used to drive a speaker at 1KHz in the test fixture for the HT220 and MT500 walkie talkies.
There is an incandescent light bulb in the oscillator circuit. It never dies, since it doesn't even light up. It is used as a negative resistance element to stabilize the oscillator level, but things go wrong if the wrong bulb is used.
These things were left turned on 24/7 and about the only thing I ever changed was the 5AR4. OE was a Mullard too! They were used to drive a speaker at 1KHz in the test fixture for the HT220 and MT500 walkie talkies.
There is an incandescent light bulb in the oscillator circuit. It never dies, since it doesn't even light up. It is used as a negative resistance element to stabilize the oscillator level, but things go wrong if the wrong bulb is used.
> I think you have the model with 6K6GT output
No, his CD has two resistor-loaded sweep-like tubes, all one push-pull stage.
And FWIW, the CD has two lamps series. True they never light or burn-up, but I have seen one quit (after being thrown in a dumpster). With two lamps the chance of incidental breakage doubles.
However if the lamps go open, it won't be a stable Sine wave. You can hand-trim it to critical oscillation but it will drift to dead or clipping.
H-P does suggest tarnish in lamp sockets.
Better stock-up on these lamps (230V 7W??)XXX 250V 10W while you can still buy incandescents.
No, his CD has two resistor-loaded sweep-like tubes, all one push-pull stage.
And FWIW, the CD has two lamps series. True they never light or burn-up, but I have seen one quit (after being thrown in a dumpster). With two lamps the chance of incidental breakage doubles.
However if the lamps go open, it won't be a stable Sine wave. You can hand-trim it to critical oscillation but it will drift to dead or clipping.
H-P does suggest tarnish in lamp sockets.
Better stock-up on these lamps (230V 7W??)XXX 250V 10W while you can still buy incandescents.
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I was going from memory, based on the large quantity of HP tube oscillators we had in the plant.
We had both 200AB's which used 6K6GT's and had 5AR4 rectifiers. They were probably early 60's vintage.
We also had 200CD's which had solid state rectifiers, 6CW5's and 6AU6's in the oscillator. The manual you linked shows that early vintage units indeed used 6AU5's which are a sweep tube related to the 6AV5 (same pinout even). The manual shows two different oscillator tubes, but not the 6AU6, which our oscillators were built after that manual was printed.
We had both 200AB's which used 6K6GT's and had 5AR4 rectifiers. They were probably early 60's vintage.
We also had 200CD's which had solid state rectifiers, 6CW5's and 6AU6's in the oscillator. The manual you linked shows that early vintage units indeed used 6AU5's which are a sweep tube related to the 6AV5 (same pinout even). The manual shows two different oscillator tubes, but not the 6AU6, which our oscillators were built after that manual was printed.
Appendix C lists many variations over time.
gain: 6AU6, 6AC7, 6SH7
power: 6CW5/EL86, 6AU5
rectifier: "5AR4 is interchangeable with 5Y3"
So tubes is tubes.
Many minor resistor changes.
They probably had a full-time tweaker rolling tube-types and changing resistors to improve field performance, or just keep the manual editors busy.
gain: 6AU6, 6AC7, 6SH7
power: 6CW5/EL86, 6AU5
rectifier: "5AR4 is interchangeable with 5Y3"
So tubes is tubes.
Many minor resistor changes.
They probably had a full-time tweaker rolling tube-types and changing resistors to improve field performance, or just keep the manual editors busy.
I would bet that when Barney Oliver was bored he would tweak it. And then there is always the question of available parts. Its amazing to think these were built in Palo Alto in the 1970's (as well as the 1960's and 1950's) in the center of transistor and IC development. But a cash cow is a cash cow.
This one uses (1) 5AR4, (2) 6AU6, (2) 6CW5, and (2) lamps in series.
The brutal reality of things is that my test equipment is, on its best day, modest, and I am nowhere close to building anything audio related that gets within orders of magnitude of even those measurement limits. So, I'm not sure where I want to go with this device.
I was looking at it last night, in the aftermath of an experiment that went explosively awry for yet undetermined reasons, and I'm thinking I might build a very low frequency oscillator with the capacitor and dial, maybe use bits of the existing one, and phase lock that to a high frequency VCO that I could use for frequency conversion, and then use this chassis for a hybrid HF transceiver. The power supply might be mostly usable as is, shielding is there, holes for tube sockets exist; it would reduce the build effort by half or more. I have a lot of the circuitry already built up and de bugged, including a PLL, because I have been working on this for awhile.
Win W5JAG
The brutal reality of things is that my test equipment is, on its best day, modest, and I am nowhere close to building anything audio related that gets within orders of magnitude of even those measurement limits. So, I'm not sure where I want to go with this device.
I was looking at it last night, in the aftermath of an experiment that went explosively awry for yet undetermined reasons, and I'm thinking I might build a very low frequency oscillator with the capacitor and dial, maybe use bits of the existing one, and phase lock that to a high frequency VCO that I could use for frequency conversion, and then use this chassis for a hybrid HF transceiver. The power supply might be mostly usable as is, shielding is there, holes for tube sockets exist; it would reduce the build effort by half or more. I have a lot of the circuitry already built up and de bugged, including a PLL, because I have been working on this for awhile.
Win W5JAG
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