32V1 grid drive problem

[KL7OF]

The 32 v has output of 80 watts on the 600 volt tap but the grid meter needle is hard over to the left..as soon as the transmitter is keyed......The grid bias measures -20V....should be -75V.....I measured bias at the center tap of the low volatge transformer and at the tube socket..same..Other voltages check good...Components are packed pretty tightly in this TX and it is hard to probe around in there .....I looking for any suggestions on where to look for the problem...Tubes all check good, High voltage and low voltage supplies are at proper values...What I don't understand is how the transmitter can have normal output with no grid drive showing....The meter checks out OK.....Any 32V experince out there?...Thanks...Steve

[AB2EZ]

Steve

If the bias voltage, measured between the center tap of the low voltage power supply transformer's secondary winding and ground, is -20V, the most likely cause of the problem would be a bad VR-75 bias voltage regulator tube or a bad 5Z4 low voltage rectifier tube.

Start your trouble shooting by unplugging the VR-75. If the measured grid bias voltage becomes more negative than -75V, then the VR-75 is the problem.

The final tube can still produce output power, even if the grid is not driven positive with respect to the cathode. I.e. even if there is no average grid current.

Stu

[N8ETQ]

   Yup, 

       What Stu said, There is some "Self Bias" but
it needs more to make it work right.

/Dan

[W7NGA]

Steve,

So, the VR-75 bias regulator isn't lit. First thing, of course, is to find out what is pulling the -75v down. I'd pull the 4D32 and third multiplier for starts. Since the grid metering is off the -75v I wouldn't investigate further until getting the bias regulated once again. I am sure you've tried all this but there aren't that many leakage paths on the -75v rail. I'd also check the capacitor on the -75v, and since I'm an old Tektronix guy, use a scope to insure the bias you are measuring is DC and not something quasi-sinusoidal with a far lower average value.

good luck ...

dan W7NGA
San Juan Island, Wa.

[AB2EZ]

I also suggest that you check the value of the 25W 7500 ohm voltage dropping resistor R303.

The (nominally) -75V bias supply circuit is an "interesting" design. It depends heavily on such things as the effective series resistance of the low voltage rectifier tube and the series resistance of the transformer secondary winding and the chokes.

The 7500 ohm resistor appears to run between a +250V source and the -75V bias bus. Therefore, it would nominally dissipate 325V x 325V / 7500 ohms = 14W. If the effective series resistance of the rectifier tube increased from its nominal value... that could easily cause the bias voltage to become less negative (or even positive).

A defective 5Z4... i.e. only one half of it is working or the filament emission is low... would cause this problem.

Stu

[KL7OF]

OK...First thanks for the suggestions....I pulled the 4D32, the vr75, and the multiplier tubes one at a time and all together....With the low voltage switch on, I show -21V with the positive lead of the VOM on the center tap of the low voltage transformer...The grid meter still shows hard over to the left (positive?) as soon as the low V is turned on.   I checked R303, R120, C141, R305 (the bias pot for the Mod tubes), R306, R119, L109 and all are good...That is it for the -75 volt bias line...unless I am missing something...The VR tube checks good but doesn't lite up....This transmitter has the 5Z4 replaced with a solid state bridge...I put a good 5R4 in there and nothing changed...(I don't have a 5Z4)....So ...still no joy...
     I read on the web that low grid drive could be a resistor associated with each tuning can in the multiplier circuits...??The resistor may be inside the can.... I haven't gone there yet..Doesn't make sense to me.   Thanks again...Steve

[KA2DZT]

Steve,

Have you checked the voltage at the VR tube.  Check the voltage on the dropping resistor feeding the VR tube,  should be 75 volts at the tube and higher voltage on the other side.  Those tubes only glow I think blue.  Also, check any coupling caps feeding the grid circuit from the previous stage.

Fred

[W3RSW]

open up the bias connector , pin 1 on p301 and j102. That eliminates all shorts on the RF last mult. and PA side.  

Measure P301 side.  Still -20 or -75?

R310, is another 7500 ohm 25 watt bleed along with R 309, 5000 ohms to gnd. or pin 5 of the VR-75 on my 32V2 diagram.

Measuring between those two resistors in my 32V2 yields 50vdc relay"on"
and 125 vdc T/R relay" off."  

I had some T/R relay problems once and had to rebuild the insulator tits on the notorious K301 relay. You said you checked R303, 7500 ohm 25 w.

Be sure pin 5 of the VR-75 and all incoming leads have a good ground and measure zero volts.

Are you really getting 240 volts at junction of last choke L302 and last 4uf
filter cap C301?  C301 may be sucking down current

Measure about every combination around that relay. Sleep on it. It'll jump out at you tomorrow.  Something other than defective tubes is drawing down current.  When you say tubes checked good, did you actually substitute
new tubes other than the one mentioned?

and about as tough as opening up cans, T302 we have to assume has a decent internal connection to the center tap, not hanging by a thread since your getting 80 watts output.  ... but I sometimes wonder.

[AB2EZ]

Steve

You indicate two symptoms:

Low measured bias voltage and the meter moves the wrong way when the meter switch is in the grid current position.

I think that this second symptom is the key to understanding the cause of both symptoms.

The meter is actually reading the voltage drop, from left to right, across the 6.2 ohm resistor R119. Current should normally move from left to right through R119 when the grid of the 4D32 is drawing current.

If the meter is pinning to the left, that means that current is flowing the wrong way (i.e. right-to-left) through R119. This implies that there is resistive path to ground (but not a short to ground) on the right side of R119 that is "pulling down" (making less negative) the bias voltage toward ground or there is leakage through 100pF grid input coupling capacitor C140

I suggest that you start by measuring the voltage between the grid of the 4D32 and the bias bus.  If it is zero volts or negative then there is no reverse current flowing through R120.  If it is positive, then try disconnecting one side of C140 to see if that fixes both problems. If it does, then replace C140 with a new 100pF capacitor. Otherwise, look for a leakage path to ground between the right side of R119 and the grid of the output tube. It could be a that bypass capacitor (C141) is leaky. Also check for dirt causing a leakage path to ground on the meter switch... where the wire from the right side of R119 connects to the meter switch.

Stu
 

OK...First thanks for the suggestions....I pulled the 4D32, the vr75, and the multiplier tubes one at a time and all together....With the low voltage switch on, I show -21V with the positive lead of the VOM on the center tap of the low voltage transformer...The grid meter still shows hard over to the left (positive?) as soon as the low V is turned on.   I checked R303, R120, C141, R305 (the bias pot for the Mod tubes), R306, R119, L109 and all are good...That is it for the -75 volt bias line...unless I am missing somemetetthing...The VR tube checks good but doesn't lite up....This transmitter has the 5Z4 replaced with a solid state bridge...I put a good 5R4 in there and nothing changed...(I don't have a 5Z4)....So ...still no joy...
     I read on the web that low grid drive could be a resistor associated with each tuning can in the multiplier circuits...??The resistor may be inside the can.... I haven't gone there yet..Doesn't make sense to me.   Thanks again...Steve

[KL7OF]

After unplugging P301/J102 and checking the voltage on pin 1 (it was -30V), I plugged it back in .....then I checked R309 and r310 ( the resistors on the relay coil circuit) they checked good....and the VR tube lit up and the TX started to function almost normally...The bias now measures -75 V  but the grid drive meter shows only 3ma grid current with the tx producing full power into the dummy on 40 meters...So the grid drive is still low....Might have been a dirty connection at the P301 plug....?  So some progress has been made but I'm not sure what the fault was, it just started working after I poked  around...Still looking for a little more grid drive....
    There is one of the tits on the relay that is moving around...looks like it needs to be glued down to make it better

[AB2EZ]

Steve

Pin 2 of that connector is in the path from the right side of R119 (6.2 ohms) to the meter switch.

Your observations/results strongly suggest that there was (maybe still is) some dirt or other material at pin 2 of P302. If the low resistance path had been on pin 2 of J102, then the bias would have immediately returned to -75V when you unplugged P302 from J102.

You might want to examine the area around pin 2 of P302 to see if there is some extraneous material still there.

Stu

[KL7OF]

PROGRESS:.....Thanks STU and RICK for steering me around...I still don't have the TX working the way I want but it is producing acceptable modulated output...I wound up replacing C142 (the 4D32 screen bypass cap) because while checking it I found that it was a HAMMY replacement that was poorly soldered....Checked C140  (OUT of circuit) good.....NOW have 6 -7 .ma grid drive that wiggles on the meter...better but, wiggling....??..when turning on the tx, upon applying the LV, the grid meter still goes hard left..However,......when the tx is keyed I now get 6-7 .ma grid drive but  when I go to standby/rx the grid goes hard left...   There is still something pulling the bias down......I'm cleaning the plug junctions, the relays, the tube sockets and the bolted grounds ..seems like a good idea  at this point on this TX...

Thanks again..Steve

[KL7OF]

I cleaned the  J102/P302 plug and connector and inspected it ...The TX now seems to be working normally......I do have a question however...When the Lv is turned on via the front toggle switch with the meter in the grid position, the meter goes hard left......Is this normal? It would seems to me that the meter is just showing the -75V bias that is on that line coming from the right side of T119 (6.2 ohm)...As soon as the tx is keyed, the grid drive shows 7 ma. (normal)..although the needle is not steady and moves back and forth about 1 ma.    Steve

[AB2EZ]

Steve

Wires from both sides of the 6.2 ohm resistor go to pin 3 on both halves of the meter switch

When the switch is in the "grid" position, the meter is measuring the voltage, from left to right, across the 6.2 ohm resistor. It should be zero or positive. I.e. grid current flows through the 6.2 ohm resistor, and into the grid pin of the 4D32. [With RF applied to the grid, the voltage at the bottom of 3300 ohm resistor (R120) is actually more negative than the bias supply voltage is].

The reverse grid current reading, in the absence of RF on the grid, indicates that there is still a resistive leakage path from the right side of the 6.2 ohm resistor to ground. When RF is applied, the self-biasing effect overcomes this... and the (grid) current flows in the correct direction (left-to-right) through the 6.2 ohm resistor.

The jumping grid current reading indicates that the leakage path resistance is changing from less resistance to more resistance.

Disconnect P302 from J102, again. Set the meter switch to a position that is not the grid current position.  Measure the resistance from pin 2 of J102 to ground. It should be essentially infinite. Measure the resistance from pin 2 of P302 to ground. It also should be essentially infinite.

If both readings are not essentially infinite (ohms), then try to find the resistive leakage path.

Stu

[N2DTS]

I do not think its normal on my 32V.
Does it act the same way in CW and phone?
Is the modulator bias/resting current acting normal?

I think a 32v that does not get much use can have issues with the relay contacts, the filter caps, and the adjustable caps in the multiplier section.

I got two 32V's for $90.00 a long time ago and was told one worked, one had a bad low voltage transformer, blew the fuse on power up.
Trans was good, one of those trimmers was shorted.
If there are any trimmers in the bias voltage buss check them out.

I also got an entire RF section off ebay as a spare that had a very cooked up mica loading cap, it was twice its normal size....

It seems like there is leakage to ground from the bias supply, normally grid drive makes grid current that moves the meter right, a bias supply drain after the meter shunt will move it left.
 

[KL7OF]

Stu....Following your very good instructions.....I determined that the leakage path was on the J102 side of the circuit....I found the culprit......C141..the PA grid bypass cap..A postage stamp mica that I  previously checked good for capacitance but did not check for leakage....clipped it out and clipped in a 600v orange drop and the meter behaves normally..... It is going to take a little while to get the new cap soldered in as there is limited space to work in that area....Transmitter working normally now ....no meter wiggle, no hard over to the left on startup.....Thanks to all who helped on this...   73    Steve

[N2DTS]

I have never seen an orange drop as a grid bypass...
Might have too much inductance, be sure to try 10 meters.
A disk would be what I would use...

[KL7OF]

tx functions normally on 10 meters...

[KA2DZT]

Steve,

Postage stamp caps should be the first thing to get rid of,  by this time most all of them are bad due to age.  No orange drops in RF circuits,  replace it, when you can, with a disc ceramic cap.  Disc ceramics are usually good even old ones.

Fred

[KA7WOC]

Steve
I assume the V1 is what you were running tonight.  Tough copy with band conditions and qrm.  Heard ya though.   
From the boatyard
woc

[W3RSW]

Now I have this tremendous urge to clean all my 32V's J's and P's. 
Happy you got it working.

[WD5JKO]

There are many types of orange drops. Some might be Mylar, and some Polypropylene. These days a polypropylene capacitor (be it green, orange, etc...) sometimes has multiple sections in parallel inside the case. Those designed for low ESR for use in switch mode power supplies, transient suppression usually are low inductance designs with very low ESR.

I have tried some polypropylene caps in RF circuits (0.005uf/500v) and they worked well. In one case in the mixer plate bypass of a CE 20A, the bypass capacitor has to handle the resonant RF current of that LC tank. If a Mica provided X gain, then a ceramic disk provided about 0.7X gain. Here the ceramic was inferior. So what the heck, I tried a polypropylene, and the gain shot up higher than with the mica, like 1.2X. I ended up putting about 1 ohm CC in series with the poly cap to normalize the gain to match the mica bypassed gain. In this circuit, too much gain means instability, and too little gain cuts back the grid drive to the PA tubes.

If the circuit impedance is high, the type of bypass is less critical.

Jim
Wd5JKO

[KA2DZT]

Jim,

Thanks for the info on the orange drops.  I guess the more modern ones may be OK for RF.  I was thinking about the old original ones.

I use silver micas wherever I can and ceramics for by-pass.

Fred

[KL7OF]

Now I have this tremendous urge to clean all my 32V's J's and P's. 
Happy you got it working.

That J/P connector is an octal crowded down in the corner of the chassis and is a bugger to get unplugged.....I had to make a hook tool to get ahold of the plug side and gently prise it apart.....Thanks Rick........Steve