HP 200CD
- Thread starter CJ
- Start date
Help Support GroupDIY Audio Forum:
lamp sockets are a problem area, looks like some mods going on there,
second set of metal tubes just went in the other day,
works great but some times takes warm up time to put out 24 volts at 20 Hz,
most labs just left those darn things on all the time, electricity back then was probably a penny per kilo,
second set of metal tubes just went in the other day,
works great but some times takes warm up time to put out 24 volts at 20 Hz,
most labs just left those darn things on all the time, electricity back then was probably a penny per kilo,
Attachments
> dont understand the 40Uf with the 10K ohms in series .
> Is hundreds of Uf ok for second and third smoothing cap ?
They are not "in series". They both go to ground. The only other part of the guts which goes to ground is V1 V3 cathodes. The 10K is the "tail resistor" of a diff-pair. Ground was moved to the top of the tail so the grids V1 V3 would be near-ground and less subject to stray influence.
I see no reason to re-think the filtering. These things did hot have a hum problem. Yours does because the caps have gone bad. At this point you should just replace with similar size values.
Hum WILL be bad if tested with the case open. That big tuning cap absorbs all the hum in the room. Fully assembled and screwed it will be much less.
Slow to stabilize (an old CJ comment) suggests more leaky e-caps.
> Is hundreds of Uf ok for second and third smoothing cap ?
They are not "in series". They both go to ground. The only other part of the guts which goes to ground is V1 V3 cathodes. The 10K is the "tail resistor" of a diff-pair. Ground was moved to the top of the tail so the grids V1 V3 would be near-ground and less subject to stray influence.
I see no reason to re-think the filtering. These things did hot have a hum problem. Yours does because the caps have gone bad. At this point you should just replace with similar size values.
Hum WILL be bad if tested with the case open. That big tuning cap absorbs all the hum in the room. Fully assembled and screwed it will be much less.
Slow to stabilize (an old CJ comment) suggests more leaky e-caps.
Im wondering if some MKP metal foil motor caps would be worth a try in there for psu smoothing , I did a check on the newer motor caps ,sure enough as mentioned by PRR ,the modern ones are back to back Lytics and not paper/foil , as far as I can tell the older types sealed with epoxy are in fact proper foil caps .
I have some 220 or 600 uf electros which should give very low hum compared to the original 10 uf , but I dont want a large first filter cap ,as this would upset my ht volts and send it high , I was thinking a foil motor run of 10uf after the bridge ,followed by a much larger electrolytic after the choke. Is it a potential problem to use a foil directly after a tube rectifier ,Ive heard the very low esr can be a problem . I'll probably keep the silicon rect for now .
Typically the junk box caps Ive found at work are either foil or non polar electrolytic , the true foils dont suffer any of the things electrolytics do , appart from catastrophic failure once in a while .Most of the electrolytic types measure ten times the ESR and are often out of spec in uf also compared to new sample of the exact same type ,but foils remain much more stable and even used ones measure just like new ,if they havent shorted internally . I measured 770mV hum on the output of my gen, thats with the amplitude control fully off . Im very happy its putting out 25volts + , thats hopefully a sign its basically good . Although not having a scope I did'nt look at the waveform yet. I'll probably pop it up on REW later ,once Ive decided on which psu caps to use .I feel like Im desacrating something by having to change things out ,everything is so perfect and so neat from the factory , I want to be sure to do the renovations sympathetically and in keeping with the quality of the instrument itself , no offence for the 'get the job done' style Cj made of his ,luckily mine is close to pristine with all the controls functioning normally ,the frequency is about 10 Hz off at 440 ,as compared to the vernier read out, again thats probably something thats easily fixed with a quick tune of some pots and variable caps .
My box of 6AU6's arrived , sure enough 6136 Jan spec GE made ,with black plate and D getter .
55 cent each is a good deal ,I might try the sony cathode follower found in the C-37 mic, If anyone has other circuits relating to this valve It would be great if you'd point me in that direction .Mic pre's and guitar amp stuff especially.
I have some 220 or 600 uf electros which should give very low hum compared to the original 10 uf , but I dont want a large first filter cap ,as this would upset my ht volts and send it high , I was thinking a foil motor run of 10uf after the bridge ,followed by a much larger electrolytic after the choke. Is it a potential problem to use a foil directly after a tube rectifier ,Ive heard the very low esr can be a problem . I'll probably keep the silicon rect for now .
Typically the junk box caps Ive found at work are either foil or non polar electrolytic , the true foils dont suffer any of the things electrolytics do , appart from catastrophic failure once in a while .Most of the electrolytic types measure ten times the ESR and are often out of spec in uf also compared to new sample of the exact same type ,but foils remain much more stable and even used ones measure just like new ,if they havent shorted internally . I measured 770mV hum on the output of my gen, thats with the amplitude control fully off . Im very happy its putting out 25volts + , thats hopefully a sign its basically good . Although not having a scope I did'nt look at the waveform yet. I'll probably pop it up on REW later ,once Ive decided on which psu caps to use .I feel like Im desacrating something by having to change things out ,everything is so perfect and so neat from the factory , I want to be sure to do the renovations sympathetically and in keeping with the quality of the instrument itself , no offence for the 'get the job done' style Cj made of his ,luckily mine is close to pristine with all the controls functioning normally ,the frequency is about 10 Hz off at 440 ,as compared to the vernier read out, again thats probably something thats easily fixed with a quick tune of some pots and variable caps .
My box of 6AU6's arrived , sure enough 6136 Jan spec GE made ,with black plate and D getter .
55 cent each is a good deal ,I might try the sony cathode follower found in the C-37 mic, If anyone has other circuits relating to this valve It would be great if you'd point me in that direction .Mic pre's and guitar amp stuff especially.
I don't understand why you think the stock values will hum.
Fix what's broke. It will be FINE. This (and the 200AB) was THE standard of excellence in general-use oscillators for a decade. If yo do not need point-oh-oh THD, or square waves, or sub-20Hz, I dunno what more you could want.
Fix what's broke. It will be FINE. This (and the 200AB) was THE standard of excellence in general-use oscillators for a decade. If yo do not need point-oh-oh THD, or square waves, or sub-20Hz, I dunno what more you could want.
Yeah I could just replace like for like , but modern 10uf /400v electrolytic caps are tiny , maybe I'll try a 10 uf foil after the rectifier then a 20 uf after the choke to replace the 3 section cap ,sizes will fit in place of whats there already .
There seems to be a mod for it to get thd down , I saw a reference to it somewhere .
And in fact it does give useable output down to 5 hz ,the wide range model at least , thus explaining the need for the 50 watt size output transformer
Well , in any case I do trust the values engineerded in by the makers ,but of course ESR with electrolytics is generally much higher than foils , especially old electrolytics , comparing used motor foil caps vs used electrolytic ,the Esr is ten times higher in the electro's .
Can we get 'too much of a good thing ' in relation to recapping old units and using vastly bigger/lower esr caps to clean up the power supply , is using a bigger cap going to give better stabillity in the ultra low frequency range and possibly even reduce distortion ?
This morning I'm verging towards the 10uF foil after the rectifier ,then a fat electrolytic bypassed by a few uf of foil after the choke, I'll do it step by step though ,I'll add the 10 uf first ,keeping two of the three terminals of the original electro in place after the choke , then see how were looking before considering the next move .
I have a 5Y3 and a GZ32 to try in the rectifier slot also .maybe with the addition of a two pole relay the tube rectifier could be used only on the power up cycle to nice and gently ramp up the ht ,and then cut out after a predetermined time or voltage is reached . If you cut in silicon diodes when the tube is cut out you get the benefit of slow ramping the ht ,but you dont have the long term consumption of the rectifier tube burning away . Thats all a bit fiddly for now ,and would involve fitting a relay board , the full consumption of the unit with tube rectification is 75 watts , ;D ,
And in fact it does give useable output down to 5 hz ,the wide range model at least , thus explaining the need for the 50 watt size output transformer
Well , in any case I do trust the values engineerded in by the makers ,but of course ESR with electrolytics is generally much higher than foils , especially old electrolytics , comparing used motor foil caps vs used electrolytic ,the Esr is ten times higher in the electro's .
Can we get 'too much of a good thing ' in relation to recapping old units and using vastly bigger/lower esr caps to clean up the power supply , is using a bigger cap going to give better stabillity in the ultra low frequency range and possibly even reduce distortion ?
This morning I'm verging towards the 10uF foil after the rectifier ,then a fat electrolytic bypassed by a few uf of foil after the choke, I'll do it step by step though ,I'll add the 10 uf first ,keeping two of the three terminals of the original electro in place after the choke , then see how were looking before considering the next move .
I have a 5Y3 and a GZ32 to try in the rectifier slot also .maybe with the addition of a two pole relay the tube rectifier could be used only on the power up cycle to nice and gently ramp up the ht ,and then cut out after a predetermined time or voltage is reached . If you cut in silicon diodes when the tube is cut out you get the benefit of slow ramping the ht ,but you dont have the long term consumption of the rectifier tube burning away . Thats all a bit fiddly for now ,and would involve fitting a relay board , the full consumption of the unit with tube rectification is 75 watts , ;D ,
Be careful tweaking oscillators for lower distortion. One typical approach involves slowing the agc time constant that can increase the settling time and upset amplitude stability. That model was good old soldier for general bench work.
JR
JR
Thanks John,
Just finalising the fitting of the new caps now , the line up will be ,10uf foil at the rectifier , second cap after the choke 330uf electrolytic , and an 82 uf in place of the 40 uf , the original caps date from 1961 , but they were wired differently than the schematic shows , one of the 10uf sections was used in series with the 10k to ground , the 40 uf was used in parralel with two of the 10 uf sections making a value of about 60uf after the choke. If the caps were replaced it was a very neat job , but I dont get why the swapped things around .
I can hide the new electrolytic caps inside the paper tube that contained the old three section capacitor , just two extra holes were needed in the chassis to make it all happen ,
I did'nt touch any of the adjustments on the machine yet , it will be interesting to see how the old calibration stands up with new psu caps , it looked good before the old caps smoked anyway .
The other good news is the seller finally contacted me back , turns out he was under the weather for a few days , were gonna come to a settlement over the other unit ,which hopefully he will be sending to me also .
Thats interesting about the settling time of the machine vs stabillity vs thd , the sound after you change the range switch ,the tone glides for a bit ,is really interesting sonically, with more than one oscillator gliding ,it should sound very 50's sci-fi ,BBC radiophonic workshop etc.
Look they even allowed hippies at the controls ,
details of the low distortion and other noise reducing mods for earlier units to be found here ,
http://www.fulltrackproductions.com/HP%20200CD/mods.html
Hopefully the bigger psu cap will provide more stability and less noise in the low end ranges of the unit , and generally a bit less of the modulation at 60/120 that users report .
Just finalising the fitting of the new caps now , the line up will be ,10uf foil at the rectifier , second cap after the choke 330uf electrolytic , and an 82 uf in place of the 40 uf , the original caps date from 1961 , but they were wired differently than the schematic shows , one of the 10uf sections was used in series with the 10k to ground , the 40 uf was used in parralel with two of the 10 uf sections making a value of about 60uf after the choke. If the caps were replaced it was a very neat job , but I dont get why the swapped things around .
I can hide the new electrolytic caps inside the paper tube that contained the old three section capacitor , just two extra holes were needed in the chassis to make it all happen ,
I did'nt touch any of the adjustments on the machine yet , it will be interesting to see how the old calibration stands up with new psu caps , it looked good before the old caps smoked anyway .
The other good news is the seller finally contacted me back , turns out he was under the weather for a few days , were gonna come to a settlement over the other unit ,which hopefully he will be sending to me also .
Thats interesting about the settling time of the machine vs stabillity vs thd , the sound after you change the range switch ,the tone glides for a bit ,is really interesting sonically, with more than one oscillator gliding ,it should sound very 50's sci-fi ,BBC radiophonic workshop etc.
Look they even allowed hippies at the controls ,
details of the low distortion and other noise reducing mods for earlier units to be found here ,
http://www.fulltrackproductions.com/HP%20200CD/mods.html
Hopefully the bigger psu cap will provide more stability and less noise in the low end ranges of the unit , and generally a bit less of the modulation at 60/120 that users report .
Attachments
Got the job done ,
Final line up was 12uf Mkp foil after rectifier , then 330 uf electrolytic ,with a 82 uf epoxied to the top of it ,clamp mounted and hidden under the original cardboard tube .
Voltages were spot on , 250 volts at the rectifier output ,-160 volts on the negative rail , I dropped the silicon diodes for a Gz32 instead.
Last night after a prolonged warm up I set the controls to 250 hz at 9 volts , this morning I switched it on, its slowly getting up to temp and the frequency raises as it goes , after about 2 hours its approaching the setting I left it at , Its certainly within spec , but the warm up drift is a bit longer than spec'd 30 minutes .
There is also some charging and reforming action going on in the caps and I expect it will take some hours use before this settles down.
Just as you said Prr and Cj ,
The unit with the lid off drifted at 50 Hz , it slowly perambulated up and down from the middle frequency and amplitude modulates to a degree as well , once the lid went on ,much more solid performance .
The tube rectifier raises the mains transformer temperature a good bit over the first few hours after switch on ,maybe this translates into an increase of temperature in the chassis over time and thus the longer time for it to settle .My multimeter indicates a non 50:50 duty cycle ,so I can only assume some imbalance in the sine output . I think they made a mod for dynamic balance in the later units ,the hum-dinger pot is deleted in the same update .
The heat output from the unit is enough to raise the temp in one of the colder rooms in the house very noticeably . Space heater that doubles as tone generator, its performs well at both tasks ;D.
I hear the alarmists going off about energy effeciency , but isnt the reality that bringing this unit back to life pays for all the extra energy it uses in its second lifetime. Wasnt that big a job really , spent more time considering options ,than cutting or soldering . Between the cleaning and time to fit the new parts was only a few hours work , of course the bench testing and calibration takes time , but its all been very worthwhile for me , I do wish I had one of these long before now .
I might sub in a couple of random specimens of the 6AU6wa/6136's later and see if it improves stability .
Thanks all for switching me onto to this great bit of kit .
Final line up was 12uf Mkp foil after rectifier , then 330 uf electrolytic ,with a 82 uf epoxied to the top of it ,clamp mounted and hidden under the original cardboard tube .
Voltages were spot on , 250 volts at the rectifier output ,-160 volts on the negative rail , I dropped the silicon diodes for a Gz32 instead.
Last night after a prolonged warm up I set the controls to 250 hz at 9 volts , this morning I switched it on, its slowly getting up to temp and the frequency raises as it goes , after about 2 hours its approaching the setting I left it at , Its certainly within spec , but the warm up drift is a bit longer than spec'd 30 minutes .
There is also some charging and reforming action going on in the caps and I expect it will take some hours use before this settles down.
Just as you said Prr and Cj ,
The unit with the lid off drifted at 50 Hz , it slowly perambulated up and down from the middle frequency and amplitude modulates to a degree as well , once the lid went on ,much more solid performance .
The tube rectifier raises the mains transformer temperature a good bit over the first few hours after switch on ,maybe this translates into an increase of temperature in the chassis over time and thus the longer time for it to settle .My multimeter indicates a non 50:50 duty cycle ,so I can only assume some imbalance in the sine output . I think they made a mod for dynamic balance in the later units ,the hum-dinger pot is deleted in the same update .
The heat output from the unit is enough to raise the temp in one of the colder rooms in the house very noticeably . Space heater that doubles as tone generator, its performs well at both tasks ;D.
I hear the alarmists going off about energy effeciency , but isnt the reality that bringing this unit back to life pays for all the extra energy it uses in its second lifetime. Wasnt that big a job really , spent more time considering options ,than cutting or soldering . Between the cleaning and time to fit the new parts was only a few hours work , of course the bench testing and calibration takes time , but its all been very worthwhile for me , I do wish I had one of these long before now .
I might sub in a couple of random specimens of the 6AU6wa/6136's later and see if it improves stability .
Thanks all for switching me onto to this great bit of kit .
hitchhiker
Well-known member
Nice work tubetec!
Great thread. I bought one of these lovely beasts locally last year. It is the version with the 6AU6 and EL86 tubes. I tested it when I got it and it works but it does have the hum problem so I will be doing some cap replacement too.
Sadly, since then while sitting on shelf it was attacked by something and it now has a broken binding post.
Does anybody by chance have a parts unit that could give up a red knob binding post?
Great thread. I bought one of these lovely beasts locally last year. It is the version with the 6AU6 and EL86 tubes. I tested it when I got it and it works but it does have the hum problem so I will be doing some cap replacement too.
Sadly, since then while sitting on shelf it was attacked by something and it now has a broken binding post.
Does anybody by chance have a parts unit that could give up a red knob binding post?
scott2000
Well-known member
hitchhiker said:
Here's some on Ebay
https://www.ebay.com/itm/5060-0633-HP-BINDING-POSTS-5-PCS-NEW-OLD-STOCK-/251215442584
If you want room heaters, buy some old school Tektronix scopes... on cold days we would put one scope cart on either side of our chair to keep us warm. :
JR
JR
> non 50:50 duty cycle
Do 'scope. Do THD meter.
However, first attack should be to note the plate/cathode voltages, all around. Unbalance will make 2nd harmonic (non-50:50 duty). Maybe tubes, maybe drifted resistors, and always leaking caps.
Do 'scope. Do THD meter.
However, first attack should be to note the plate/cathode voltages, all around. Unbalance will make 2nd harmonic (non-50:50 duty). Maybe tubes, maybe drifted resistors, and always leaking caps.
not a scientific instrument, but reliable and will drive a tank, nice sine wave, nice output voltage for testing transformers, 50/60 Hz hetrodyne not a big issue , not much audio testing at 60 Hz,
you might notice that the range switch does not track in a precise manner, 500 Hz on x 10 will not = 500 Hz on x 100, so a freq meter sitting on top is a welcome addition,
also, voltage is not stable as you go up and down with the freq adj knob, which can make transformer sweeps tedious,
maybe "hit and git" a soldered lead to the tip of the light bulbs to take resistance of the lamp sockets out of the equation,
you might notice that the range switch does not track in a precise manner, 500 Hz on x 10 will not = 500 Hz on x 100, so a freq meter sitting on top is a welcome addition,
also, voltage is not stable as you go up and down with the freq adj knob, which can make transformer sweeps tedious,
maybe "hit and git" a soldered lead to the tip of the light bulbs to take resistance of the lamp sockets out of the equation,
I started checking the composition resistors , +10-+20% off , maybe even more with higher values .
There are a few matched pair resistors in there too, so I suppose at very least these should be put right in order for correct operation . Many of the larger end capped resistors are spot on also ,but their counter parts on the frequency range switch ,the smaller composition resistors are marginal at best ,seeing all these resistors are +/- 10% anyway it probably doesnt matter so much on the greater scheme of things .
A funny thing happened , I reached into my box of 100 6AU6's ,plucked out two at random , popped them in and poooof ,the fuse blew instantly , when I examined one of the tubes it had a tiny red paint mark between the pins , the next two I tried worked but didnt make much difference from the original tube in it . Still drifts slowly up up up for hours ,mind you it could still be within the 2 % spec , just 5 digit multimeter makes it look bad , of course with a standard meter of the time ,moving coil ,the eye wouldnt notice the movement .
Last night after hours and hours it finally seemed to stabilise it went upto about 611hz while set at 600 on the dial , and tonight after I turned it on for 10 minutes it measured 597.8 hz , so maybe just over 2 % drift from just after switch on too when it stabilises .
I understand as you switch through the ranges theres going to be some descrepency in the dial read out , So yeah I have been using it with my multimeter counter to verify .Looks to me like the dial is calibrated to the output after 30 minutes warm up time ,not when the unit actually settles ,which in my case takes hours . The other thing is this unit was most likely not used a very long time , so I expect a certain amount of settling in time at least initially ,fingers crossed with some usage ,it will level out quicker , we'll see how tonights power up goes in terms of stabillity , if the time to stabilise starts to diminish I just might let it on longer term power up ,then once its stabilised slip the disk to the correct value and see how we look across the ranges . If I can get a good quality sine out of it I'll be happy with relying on the frequency counter to set the unit , but I rather not start switching out every resistor thats out of tollerance just for the sake of it . I believe the second 200cd I'm due is a lot more modern ,with cal stickers into the 90's so its most likely in better shape tollerance wise .
I'll most likely use these as tone generators more than test instruments in any case ,
I'll get a waveform up on screen and possibly through phones later also ,if tiredness doesnt catch up with me first
There are a few matched pair resistors in there too, so I suppose at very least these should be put right in order for correct operation . Many of the larger end capped resistors are spot on also ,but their counter parts on the frequency range switch ,the smaller composition resistors are marginal at best ,seeing all these resistors are +/- 10% anyway it probably doesnt matter so much on the greater scheme of things .
A funny thing happened , I reached into my box of 100 6AU6's ,plucked out two at random , popped them in and poooof ,the fuse blew instantly , when I examined one of the tubes it had a tiny red paint mark between the pins , the next two I tried worked but didnt make much difference from the original tube in it . Still drifts slowly up up up for hours ,mind you it could still be within the 2 % spec , just 5 digit multimeter makes it look bad , of course with a standard meter of the time ,moving coil ,the eye wouldnt notice the movement .
Last night after hours and hours it finally seemed to stabilise it went upto about 611hz while set at 600 on the dial , and tonight after I turned it on for 10 minutes it measured 597.8 hz , so maybe just over 2 % drift from just after switch on too when it stabilises .
I understand as you switch through the ranges theres going to be some descrepency in the dial read out , So yeah I have been using it with my multimeter counter to verify .Looks to me like the dial is calibrated to the output after 30 minutes warm up time ,not when the unit actually settles ,which in my case takes hours . The other thing is this unit was most likely not used a very long time , so I expect a certain amount of settling in time at least initially ,fingers crossed with some usage ,it will level out quicker , we'll see how tonights power up goes in terms of stabillity , if the time to stabilise starts to diminish I just might let it on longer term power up ,then once its stabilised slip the disk to the correct value and see how we look across the ranges . If I can get a good quality sine out of it I'll be happy with relying on the frequency counter to set the unit , but I rather not start switching out every resistor thats out of tollerance just for the sake of it . I believe the second 200cd I'm due is a lot more modern ,with cal stickers into the 90's so its most likely in better shape tollerance wise .
I'll most likely use these as tone generators more than test instruments in any case ,
I'll get a waveform up on screen and possibly through phones later also ,if tiredness doesnt catch up with me first
Hmmmmm,
curious circuit ,the dc cross coupling network is a masterpiece ,but an awkward one to troubleshoot,any cap leakage will have a compound effect on the volts you see , I suspect both main 0.5uf caps to be leaky to some degree , one I have removed ,and replaced , Everything bar the cathode volts on the El86's is bang on , the 1 meg (formerly 10 meg ) strapped across the coupling cap I suspect to be badly out of wack ,Ive also weeded out three duds from my 6au6 Pile ,two tubes out of about 7 pairs ruptured the fuse ,its not looking good ,and I need not be trying out any more dead short, dud tubes on a dc coupled instrument with transformers ,mind you the designers fuse choice is very good ,if theres more that a certain % unbalance ,bzzfffct, the 600ma fizzles and the transformers are saved .
I might try to use the logging function on the multimeter to capture the graph of the drift , its looking good now, all voltages are within spec , and equal on both sides of the circuit , Im getting -24.5V on the cathodes of the EL86's ,but that roughly correlates to the Ht volts Im using which is 250 v instead of 215 v the later version which specifies -19.5 volts .
I also note that in the manual ,if symetry is required an external balanced attenuator is recomended , I am able to get a 50:50 duty sine ,but its not at full output ,between about - 20 and -30 db , at full attenuation or max out more like 51.3% out of balance, I havent eyeballed the waveform yet :
New 1meg cap bypass resistors as well as 820K grid leak and .5uf cap all need to be replaced for proper sine wave opperation , possibly 22k and 68k in the oscillator too ,a little slip on the dial is acepatable if your using a freq counter to measure ,but you still want a proper sine ,with proper stabillity .
curious circuit ,the dc cross coupling network is a masterpiece ,but an awkward one to troubleshoot,any cap leakage will have a compound effect on the volts you see , I suspect both main 0.5uf caps to be leaky to some degree , one I have removed ,and replaced , Everything bar the cathode volts on the El86's is bang on , the 1 meg (formerly 10 meg ) strapped across the coupling cap I suspect to be badly out of wack ,Ive also weeded out three duds from my 6au6 Pile ,two tubes out of about 7 pairs ruptured the fuse ,its not looking good ,and I need not be trying out any more dead short, dud tubes on a dc coupled instrument with transformers ,mind you the designers fuse choice is very good ,if theres more that a certain % unbalance ,bzzfffct, the 600ma fizzles and the transformers are saved .
I might try to use the logging function on the multimeter to capture the graph of the drift , its looking good now, all voltages are within spec , and equal on both sides of the circuit , Im getting -24.5V on the cathodes of the EL86's ,but that roughly correlates to the Ht volts Im using which is 250 v instead of 215 v the later version which specifies -19.5 volts .
I also note that in the manual ,if symetry is required an external balanced attenuator is recomended , I am able to get a 50:50 duty sine ,but its not at full output ,between about - 20 and -30 db , at full attenuation or max out more like 51.3% out of balance, I havent eyeballed the waveform yet :
New 1meg cap bypass resistors as well as 820K grid leak and .5uf cap all need to be replaced for proper sine wave opperation , possibly 22k and 68k in the oscillator too ,a little slip on the dial is acepatable if your using a freq counter to measure ,but you still want a proper sine ,with proper stabillity .
Things have settled down nicely after a prolonged burn in and re-adjust , without touching the capacitor plates only slipping the dial every range is within a fraction of 1 % frequency wise , I can set the unit on 600.00hz and it will happily hold frequency to 1 Hz for hours after the warm up period , Im still not getting the full 24 volts at 600khz but Im getting full power on all other ranges ,
There was a kind of spongeiness to the attenuator dial , I could only access the back section of the dual gang AllenBradley pot , a gentle swabbing with a q tip and iso on the track removed a lot of debris and loose carbon particles , just a quick wipe with the iso then a clean dry qtip to polish it up , the pot shaft might also do with a drop of light oil ,slightly sludgy oil sluggish feel to it ,Im fairly sure once thats done , its more or less sound .
Myself and the sender settled things amicably over the initial mix up in the add ,
He couldnt locate the original second item in the listing ,but he had a third 200cd ,late model ,let me have it for 25 euro's ,and he covered shipping .
I thought it would be fair ,since I dont engage positive or negative feedback on Ebay , to give the guy a mention here , he's got some interesting stuff ,some really nice radio gear , RCA , Halicrafters , Marconi ,Eddystone ,test gear and other bits
Thats the original add for the 200cd(s) ,
https://www.ebay.co.uk/itm/Hewlett-Packard-HP-Model-200CD-Wide-Range-Oscillator/162897514140
Room EQ Wizard definately deserves a mention here too , valuable tool in the fight for favourable harmonics 8)
There was a kind of spongeiness to the attenuator dial , I could only access the back section of the dual gang AllenBradley pot , a gentle swabbing with a q tip and iso on the track removed a lot of debris and loose carbon particles , just a quick wipe with the iso then a clean dry qtip to polish it up , the pot shaft might also do with a drop of light oil ,slightly sludgy oil sluggish feel to it ,Im fairly sure once thats done , its more or less sound .
Myself and the sender settled things amicably over the initial mix up in the add ,
He couldnt locate the original second item in the listing ,but he had a third 200cd ,late model ,let me have it for 25 euro's ,and he covered shipping .
I thought it would be fair ,since I dont engage positive or negative feedback on Ebay , to give the guy a mention here , he's got some interesting stuff ,some really nice radio gear , RCA , Halicrafters , Marconi ,Eddystone ,test gear and other bits
Thats the original add for the 200cd(s) ,
https://www.ebay.co.uk/itm/Hewlett-Packard-HP-Model-200CD-Wide-Range-Oscillator/162897514140
Room EQ Wizard definately deserves a mention here too , valuable tool in the fight for favourable harmonics 8)
