Kenwood TS-590 first mixer potential issue

[Chris Wilson]

I have used my TS-590 transceiver with no issues for several years. Recently it's been used as an exciter on 136kHz for feeding my big home brew amp. I used the 590's DRV out socket which gives 1mW into my pre amp then on into my main amp. All has seemed well until I decided to have a break from LF and go back on HF. When  used on LF no antennas have been connected at all to the 590. I have to assume its failure is coincidental, but thought I would mention all this.... The 590 was stone deaf on RX. It seemed to TX on HF into a dummy load OK.

I downloaded the repair manual from Kenwood's site and injected a signal of -110 dBm into the antenna socket, then realised my scope couldn't even see a signal so weak. In the end -42 dBm allowed a workable signal at 14.2 MHz to be seen on the scope and modulated at 1kHz it was JUST audible on the 590's speaker at max audio and RF gain.

I cautiously tried to follow the signal path with my scope, first according to the "Level Diagram" below, then as I homed in on things by following what to me is a very complicated schematic. In the second attachment below, the schematic, I see the signal at the input to the first mixer, but zilch at the output. The 590 uses dual IF frequencies depending on what band it's tuned to.

The signal, just looking relatively via the scope seems less at the input to the first mixer than at the output of the pre amp transistor Q236 on the Level page. Could this because the mixer has a short and is pulling the signal down, or have I misinterpreted what the fault may be? I hope there's enough info here for you to be able to understand what I have been doing! Thanks for any replies.

[Andy Watson]

The signal, just looking relatively via the scope seems less at the input to the first mixer than at the output of the pre amp transistor Q236 on the Level page. Could this because the mixer has a short and is pulling the signal down, or have I misinterpreted what the fault may be? I hope there's enough info here for you to be able to understand what I have been doing! Thanks for any replies.

Have you measured the DC voltages around the mixer - they should be balanced throughout - any imbalance would indicate possible transistor failure. When you looked at the output signal from the mixer did you allow for the fact that it will (should) be a different frequency. Also, did you check for signal level from the local oscillator - no L.O =  none to very little output from the mixer.

[Chris Wilson]

The mixer FET's are on the underside of the main board and I don't feel confident trying to power it up when the board is out of the case, even if it were possible to make all the interconnects reach, so short of tagging on test wires and reinstating the main board with flying leads I can't see how I could test the voltages. The LO seems OK, in fact it "kind of" has two as i added a GPS locked master oscillator board which is easily swapped back to the stock xtal controlled one by swapping jumpers. Both seem to show the issue, but I'll double check the output levels.

Checking the output level was just via my scope, but I did try and allow for a different mixed frequency by both manual time / div settings and "Autoset". it's a Philips / Fluke PM3380B Combiscope, (analogue and digital) at 100 MHz's. There seemed to be no output on my SA either, but I am very wary of blowing that up

Thanks for some more to go on there Andy, much appreciated.


EDIT, master reference oscillator seems OK, here's a scope shot Andy, cheers.

[Andy Watson]

The LO seems OK,

Yes, but is it reaching the mixer? On the schematic, does the signal path to the left lead directly to the antenna - if so, the mixer, being the first in-line, is a good candidate for being zapped by static - or other large signals
Looks like you might have to bite the bullet and remove the board.

[Chris Wilson]

Actually, I being dense here, looking again at the schematic I can access some of the terminals on the input and output transformers. I may be able to measure for the voltages that SHOULD be 12.1V and 7.19V.

I don't think I can access the 1.93V points on the gates though. Will measuring the accessible voltages tell me much?

The circuit is complex, to me, extremely complex, and I am unsure how direct a signal path there exists from the aerial socket straight to the mixer, there does seem to be a lot of stuff marked as "protection" dotted about though. I see no way of linking just to the schematics, only the whole service manual, which is at :

https://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=1&ved=0ahUKEwjKkLTT4s_MAhWFC8AKHSZcCIwQFgggMAA&url=http%3A%2F%2Fwww.radiomanual.info%2Fschemi%2FTS590S_serv.pdf&usg=AFQjCNFKN4_nE11JGYsIu02mgI6jkyHiBg&sig2=bOv39ld3lE-HzYufRBS75w&cad=rja

[Andy Watson]

! That is a complex beast!

Actually, I being dense here, looking again at the schematic I can access some of the terminals on the input and output transformers. I may be able to measure for the voltages that SHOULD be 12.1V and 7.19V.

I don't think I can access the 1.93V points on the gates though. Will measuring the accessible voltages tell me much?

If you can access most of the voltages you should be able to determine whether it will be necessary to remove the board - i.e. if they are unbalanced then you have no option but to remove the board to get at the transistors.

On the other hand, if the voltages are correct:
Are you sure the L.O. is reaching the mixer?
Are you certain that the mixer output is low? You could be loading it too much with your probe.
Are you certain that the receiver is in the correct mode to be using that mixer?

You should be able to double-check the remainder of circuit by forcing-in the correct IF frequency post-mixer.

[AF6LJ]

You will notice there are two mixers..
One is for the main receiver path the second one is to accomodate the "Dual Watch" feature.
Does the radio receive in Dual watch mode?
NOTE;
That in dual watch there is a different mixer and LO used.

[Chris Wilson]

! That is a complex beast!

Actually, I being dense here, looking again at the schematic I can access some of the terminals on the input and output transformers. I may be able to measure for the voltages that SHOULD be 12.1V and 7.19V.

I don't think I can access the 1.93V points on the gates though. Will measuring the accessible voltages tell me much?

If you can access most of the voltages you should be able to determine whether it will be necessary to remove the board - i.e. if they are unbalanced then you have no option but to remove the board to get at the transistors.

On the other hand, if the voltages are correct:
Are you sure the L.O. is reaching the mixer?
Are you certain that the mixer output is low? You could be loading it too much with your probe.
Are you certain that the receiver is in the correct mode to be using that mixer?

You should be able to double-check the remainder of circuit by forcing-in the correct IF frequency post-mixer.

Can you tell me how to (safely) force in an external IF, and at what level? How would I tell if this brought the rest of the circuits to life? One thing that struck me as odd is if i apply my X1 scope probe to either ends od D461 to D464 or at least one end of D522 to D524 the audio noise increases tremendously, to something akin to the level it would emit at full AF and RF gain with background noise when operating normally. these diodes can be seen in the Levels pdf file in my first post. Does this signify only very low level signals are reaching this far due to a problem earlier in the signal path? Does it help? Thanks.

[AF6LJ]

! That is a complex beast!

Actually, I being dense here, looking again at the schematic I can access some of the terminals on the input and output transformers. I may be able to measure for the voltages that SHOULD be 12.1V and 7.19V.

I don't think I can access the 1.93V points on the gates though. Will measuring the accessible voltages tell me much?

If you can access most of the voltages you should be able to determine whether it will be necessary to remove the board - i.e. if they are unbalanced then you have no option but to remove the board to get at the transistors.

On the other hand, if the voltages are correct:
Are you sure the L.O. is reaching the mixer?
Are you certain that the mixer output is low? You could be loading it too much with your probe.
Are you certain that the receiver is in the correct mode to be using that mixer?

You should be able to double-check the remainder of circuit by forcing-in the correct IF frequency post-mixer.

Can you tell me how to (safely) force in an external IF, and at what level? How would I tell if this brought the rest of the circuits to life? One thing that struck me as odd is if i apply my X1 scope probe to either ends od D461 to D464 or at least one end of D522 to D524 the audio noise increases tremendously, to something akin to the level it would emit at full AF and RF gain with background noise when operating normally. these diodes can be seen in the Levels pdf file in my first post. Does this signify only very low level signals are reaching this far due to a problem earlier in the signal path? Does it help? Thanks.

First you need an RF signal generator.
couple the signal generator through a DC blocking cap use one of the leads of the cap as a probe to apply IF to the point in the circuit where you plan on injecting the signal.

[Andy Watson]

Can you tell me how to (safely) force in an external IF, and at what level? How would I tell if this brought the rest of the circuits to life? One thing that struck me as odd is if i apply my X1 scope probe to either ends od D461 to D464 or at least one end of D522 to D524 the audio noise increases tremendously, to something akin to the level it would emit at full AF and RF gain with background noise when operating normally. these diodes can be seen in the Levels pdf file in my first post. Does this signify only very low level signals are reaching this far due to a problem earlier in the signal path? Does it help? Thanks.

First you need an RF signal generator.
couple the signal generator through a DC blocking cap use one of the leads of the cap as a probe to apply IF to the point in the circuit where you plan on injecting the signal.

Yes, this ^^^ at a similar (give or take a few dB) level to that given on block diagram that you post above.

[Chris Wilson]

OK, I'll give it a go and report back. Thanks Andy / Sue and everyone who has posted here

[Chris Wilson]

OK, just got back to it after dinner. Talking 1st mixer now, set tuned to 14.2 MHz, injecting 14.2 MHz into the antenna socket at -33dBm

I refer to the schematic in my first post.

L262 is injecting a good signal, allowing for probe loading, at circa 25.3 MHz  I believe that's correct for the 1st LO injection.

On L264, either side of the centre tapped winding, I see about 12.7V

But on L261, on the mixer side of the 330R resistors I see no voltage.

If I check the 2nd mixer voltages are correct when tuned to a band that uses this one.

I am thinking one or more of the mixer FETS is shorted? Should I remove the main board and remove them and test them?

Hopefully I'm on the right track, although a downer is I see no place to buy 2SK174-4  FET's, apart from perhaps from Kenwood.

I chickened out about injecting the IF!

[Andy Watson]

Can you measure the voltages on the mixer side of L262 (i.e. the gate voltage on the fets)? And whilst you're there, check the voltage on both ends of R268.
Edit: DOH!
R268 is a zero ohm link, so just the voltage at the junction of R273 and R274 will do.

[Chris Wilson]

Can you measure the voltages on the mixer side of L262 (i.e. the gate voltage on the fets)? And whilst you're there, check the voltage on both ends of R268.
Edit: DOH!
R268 is a zero ohm link, so just the voltage at the junction of R273 and R274 will do.

I had the board out, removed all four FETS from the 1st mixer and soldered tiny PTFE coated wires to each leg, and tested all four. WHAT a fiddly job, I hadn't realised just how tiny they were. Anyway, all tested OK on a Peak DCA55 tester. So I put them all back (even more fiddly...). I now have the board back in and the voltages you asked about are as follows.

Mixer side of L262 (gate voltages) only 13.2 mV on each

Junction of R273 and R274 only 5mV

[Andy Watson]

Mixer side of L262 (gate voltages) only 13.2 mV on each

Junction of R273 and R274 only 5mV

Can you confirm that there is the enabling voltage on the other side of R273? - should be 7.5V according to the schematic.

[Chris Wilson]

No, just been trying to work it out myself, there's negligible voltage there either. If I go to L302 on the centre tapped side Or at least there are three dots on the side I am checking, one going to Q301, , I am seeing 7.47 V and changing bands changes nothing there......

The base side of Q301 changes from 50mV on 14.2 MHz band to zero V on 50 MHz band. That's not right is it?

Thanks for staying with this

[Andy Watson]

No, just been trying to work it out myself, there's negligible voltage there either. If I go to L302 on the centre tapped side Or at least there are three dots on the side I am checking, one going to Q301, , I am seeing 7.47 V and changing bands changes nothing there......

The base side of Q301 changes from 50mV on 14.2 MHz band to zero V on 50 MHz band. That's not right is it?

Thanks for staying with this

You are looking for whatever provides the switching voltage to that line. It's difficult to follow since it disappears on to another sheet of the schematic as RX-B1. Before you leave this area, can you see C269, down to the right of the mixer, can you verify that it is ok - i.e. not shorting.

Edit: I think I've spotted the source of RXB-1 - half-way down the left side of page 133 - I'm looking at Q908. If C269 checks out ok, have a probe around the voltages on Q908.

[Chris Wilson]

I have had to pack up for tonight, it's 10.30 PM here and I have a 6.00 AM start in the morning to drop something off in the next county, but when I get back I'll be sure to check it. It's a can type electrolytic and I may be able to lift a leg, in case other stuff shows it as shorted. Thanks Andy, you have been very very helpful.

[AF6LJ]

This is why I have a strong dislike for manuals ported to PDF files.
This one I am not going to print up and tape together to figure out....
I have done it plenty of times for my own gear when a manual was not obtainable by any other means.

[Chris Wilson]

Sue, yes, especially to a beginner this pdf manual is a nightmare, I may do as you say you
have in the past and try and tape a section together on paper!

[AF6LJ]

Sue, yes, especially to a beginner this pdf manual is a nightmare, I may do as you say you
have in the past and try and tape a section together on paper!

If you are willing to pay for it; you may be able to purchase one from Kenwood.

[Chris Wilson]

You are looking for whatever provides the switching voltage to that line. It's difficult to follow since it disappears on to another sheet of the schematic as RX-B1. Before you leave this area, can you see C269, down to the right of the mixer, can you verify that it is ok - i.e. not shorting.

Edit: I think I've spotted the source of RXB-1 - half-way down the left side of page 133 - I'm looking at Q908. If C269 checks out ok, have a probe around the voltages on Q908.

FIXED!!!   Thank you most sincerely for bearing with me on this saga, you were right to direct my attention to the area of the schematic around Q908. There's a tiny 1mm long fuse in that area of the circuit, and whilst I am not sure if it was fully open there was certainly only mV on one side and the correct voltage on the other. Couold have been fully open, and the voltage was bleed through from components on the other side of the fuse. Rather than try and remove it I soldered a new one across the top of it. Why the hell they make sacrificial components like SM fuses so darned small I don't know... A few sizes bigger would make their replacement SO much easier.

The rig immediately came to life and "hears" down to -113 dBm fine from my sig gen, and works fine on RX and TX and on the bands I have tried. it's all buttoned up again and hopefully this isn't a warning it's of the age it's going to give trouble. I leave it standby, if not RX 24 hours a day, every day of the year. Often it is in RX mode proper. Not sure if permanently on is less stressful to such devices than switching on and off a lot...?

You and everyone else who has posted have been very patient, and I am glad to report your time has not been wasted, from my point of view, thanks again!!

[AF6LJ]

Good Catch.