Valiant Making Progress

[ka4koe]

OK

Replaced the modulator tubes with matched pair of 6146 RCAs and their associated 22 ohm plate resistors. Getting good swing at audio gain, position 2 with both D104 and EV638. Audio with EV sounds better. The D104 sounds scratchy.

Need to replace input resistor with minimum 10 Mohm, or higher.

Insulated HV leads where we were experiencing some arcing.

Some hum on the audio, but we're right here in the same room with the cover off the radio.

Probably could put on air, but need to do some other things, like replace chernobyl resistor and calibrate VFO, which is about 80 KHz low on 75m band.

Philip

[ka4koe]

Next I will check carrier on the Flex bandscope in both CW and AM modes, and may post some screen captures later. I will tell you that the spectral display in AM mode has a lot of "grass" about 50 db down from the carrier peak.

Probably next step is to actually put her on the air and get some reports. I have a dow key relay ready to go with the Hammarlund. The Collins 388 will need a mod to energize the TR relay inside, so the 180 can be put into service without a lot of fuss.

Philip

[KM1H]

Much over 4.7M into a 12AX7 could be asking for instability. I settled for the 4.7M in a customers DX-100 and with a D-104 all reports were complimentary and no hum. The customer wanted the audio mods tailored to a D-104 and I wound up using a 12AT7 in order to get the gain pot from 2 to about 6 which smoothed out the response. Those with SDR rigs reported the pattern very clean at about a 8kc BW.

A EV-633 into a 47K right at the mike jack resulted in muffled audio and some hum reports. Most tests were with the TX up on its side and wide open.

There is also a problem with some mikes, especially dynamic,  when the mike and PTT ground is shared.

Carl

[ka4koe]

No waveforms tonight. Now I'm troubleshooting why it started blowing fuses when tuning up, going from PTT to MAN. Trying to address some intermittents in the ACC plug and VFO jumping as described by Tom Rauch.

On second thought, since I have no plans to use a SSB adapter, I'm going to strip off all the coils and caps on the accessory and hardwire everything. I will keep them, however, if there is a future owner.

While we are at it, is there any reason to keep all the same coils and bypass caps where the power wire enters the chassis?

Philip

[ka4koe]

May have found a potential source for the fuse blowing I've been experiencing with the Valiant out of the blue.....the nine pin accessory plug socket in the rear of the radio. Are we surprised my fellow old iron afficionados? There are several high voltage spots at the socket in close proximity that route power/audio here and yon. Looks like we may have had a potential short between pins 5 and 6; they were mighty close together and they are not solid....can wiggle a bit. Examining the handy dandy Valiant schematic, it appears there is B+ on pin 6 (600V) to the 6146 finals and pin 5 goes to the primary center tap of modulation transformer T3 (300V). Not good.

Fuses are a good thing. That accessory connector is a time bomb waiting to go off; potentially worse than the undersized power resistor in the VFO compartment.

[KA2DZT]

Philip,

I think you're not the first to have problems with those accessory sockets.  I remember posts about them in the past.  I think many have done away with the caps and coils that are on the pins.  That stuff was there to prevent TVI.  TVI is not much of a problem nowadays.

Fred

[Pete, WA2CWA]

They generally don't wiggle unless the Valiant is mounted on a shake table or you're diddling with them. Put an 9 pin plug into the socket; do any of the wired connections touch; yes/no; pull the plug out; do any of the wired connections touch, yes/no; if yes to either answer, separate and/or insulate the wired connections. That accessory socket type wiring is very typical in many transmitters of the 50's, 60's, and 70's.

[ka4koe]

Yes they wiggle and yes they touch; sorta like the Teletubbies in a bizarre, truly scary fashion!

Seriously though, the whole thing looks like a mess.

Philip

[ka4koe]

I may have found the problem, and its indeed in the accessory plug. Pin 2 goes to B+. The insulation on this pin was worn down and making contact with chassis ground. I cleaned the pin off and installed new heat shrink tubing as an insulator. There was enough play in this pin to cause issues.

I am going to finish off the work cleaning up the connections around the accessory plug/jack and then throw the "OH S***" switch SW8 and see what happens.

And, if this works, then we can get back to the issue of finding out why my audio has a lot of hum. I even bought some decent oscilloscope probes off Ebay for my Tek 468 to trace the audio chain and see if we have any ripple on the modulator tubes' bias/HV supply.

FEELEEP

[WA2OLZ]

Philip,

What does your Valiant show for HV and does it remain steady when modulated?

[ka4koe]

Nothing at the moment as its blowing fuses when throwing SW8. Didn't get anything accomplished this weekend due to mother's day and honey-do's.

[WQ9E]

With the transmitter unplugged let it sit for a few minutes and then short the HV line to ground just to make sure the filter caps are discharged.  Then remove the short and measure the resistance to ground from pin 1 or pin 4 of one of the HV rectifier sockets, if you measure significantly less than 40K trace the problem.  If the resistance is OK you should still do a test without the rectifiers in place (make sure the plate caps are well insulated from anything nearby) to see if the fuse still blows.  If it doesn't then the problem is still on the rectified side of the HV, possibly something arcing over.  If the fuse still blows with the rectifiers out of circuit I would temporarily disconnect the primary leads to the plate transformer to make sure the problem isn't there.

If there is something in the rectified/filtered side of the HV circuit causing the fuse to blow instantly you are putting a heavy load on the transformer and it may show its displeasure if this abuse continues.

[WA2OLZ]

Nothing at the moment as its blowing fuses when throwing SW8. Didn't get anything accomplished this weekend due to mother's day and honey-do's.

Makes perfect sense!

[VE3AJM]

You mentioned that the HV PS has been solid stated. I would check out those diodes and or SS plugins that you may have used. That audio feedback event that you've written about in another thread may have taken out the SS devices. Its wise to get a transmiiter like that going first with a minimum of modifications. Have it working in a fairly reliable way as stock as possible and then do some of the mods such as SSing the PS and do it step by step.

Did you add any sort of soft start when you SS'ed the HV PS or the other supplies? Inrush current issues? Check out the bypass caps at the top and bottom of the big RF choke for the final. They can fail/short.

Hopefully the modulation transformer hasn't developed an intermittancy. A replacement mod iron isn't an item that you can order up from AES.

Al VE3AJM

[ka4koe]

Yes, direct replacement Taylor 866AS from RF Parts. No current limiting added.

http://www.rfparts.com/catalogsearch/result/?q=866as

But, these were replaced after the oscillation event.

I've had intermittents that caused multiple problems (acc plug). I'm not planning on modding anything else at this time until I restore HV, which I think I've located.

If the mod iron is intermittent, then my options are likely very limited.

It is humming with the audio gain turned all the way off, but I've yet to track that down as I need to get the HV fixed first.

[ka4koe]

Yes, direct replacement Taylor 866AS from RF Parts. No current limiting added.

http://www.rfparts.com/catalogsearch/result/?q=866as

Looking at the HV on a scope would tell us if the rectumfliers are good or not, one would think?

I've had intermittents that caused multiple problems (acc plug). I'm not planning on modding anything else at this time until I restore HV, which I think I've located.

If the mod iron is intermittent, then my options are likely very limited.

It is humming with the audio gain turned all the way off, but I've yet to track that down as I need to get the HV fixed first.

[ka4koe]

Just got a quote from Hammond/Dahl. New modulation iron for the Valiant is a tad over 200 bux. Not obscene, but not cheap either.
Lets hope this isn't the problem.

Philip

[ka4koe]

HV rectifiers measure almost exactly 40 KOhms to ground.
Checked modulation transformer secondary between pins 2 and 3 and R was less than 100 ohms.

Philip

[Steve - K4HX]

HV rectifiers measure almost exactly 40 KOhms to ground.

??

[ka4koe]

HV rectifiers measure almost exactly 40 KOhms to ground.

??

up a bit

With the transmitter unplugged let it sit for a few minutes and then short the HV line to ground just to make sure the filter caps are discharged.  Then remove the short and measure the resistance to ground from pin 1 or pin 4 of one of the HV rectifier sockets, if you measure significantly less than 40K trace the problem.  If the resistance is OK you should still do a test without the rectifiers in place (make sure the plate caps are well insulated from anything nearby) to see if the fuse still blows.  If it doesn't then the problem is still on the rectified side of the HV, possibly something arcing over.  If the fuse still blows with the rectifiers out of circuit I would temporarily disconnect the primary leads to the plate transformer to make sure the problem isn't there.

[ka4koe]

OK, HV no longer blow fuses. Tx loading up fine into dummy load, 450 mils, 7.5 mA grid, positions CW, VFO.

Here is the waveform with my Flex bandscope. Looks like the grass starts growing around -33 dB from the carrier.

Hear hum on carrier, although given the proximity to the receiver, this may be normal. I wonder if part of my audio problems are due, in part, to the rinky dink connections on the audio plug/socket. Will check the audio tonight. I also chatted with Timtron on 7295 and absorbed a great deal of his wisdom in the process.

Philip

[KA2DZT]

What did you fix to stop the fuse from blowing??

[ka4koe]

Short B+ to ground on the ACC plug.

[KA2DZT]

A lot cheaper than the mod xfmr.  At least you're making some headway.

Fred

[ka4koe]

Fix one problem find another....

Got HV fixed. Tried rig on CW. Plate idle current pegs meter on key up.

Clamper circuit now?