Kenwood Amateur Radios

Re: SM-220 scope question

mail@... (Mike) — Sun, 01 Jun 2025 09:38:05 -0700

I replaced all the oil/paper capacitors with metallised polypropylene, but that didn't fix the problem.

Then I realised that it only occurred on the higher sweep ranges, so I looked at the blanking pulse and horizontal deflection.

Here they are on the 100-1k range:

That looks OK so I repeated the measurement on the 10-100k range:

Aha! The blanking pulse amplitude is 10 volts lower and it doesn't align with the initial part of the flyback.

Now I need to find out which components are causing it.

Mike G8GYW

MONTHLY REMINDER--Our Amateur Radio Groups

@... — Sun, 01 Jun 2025 05:32:14 -0700

MONTHLY REMINDER--Our Amateur Radio Groups

Below are our amateur radio related groups. Some may be new enough there are only a few members. If one of those interests you, join it and give it some time to grow. It costs nothing and if there’s few members, it won’t be filling up your inbox while you wait for it to grow. 😊

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Re: TS50 high current draw

W4JFABob@... (Bob) — Fri, 30 May 2025 12:44:44 -0700

I found the problem, Q5. It was causing all the high current draw.

Now the million dollar question, where to get a replacement(s). Looking at Mouser, the MRF421 may be a replacement.

I'll be researching

Bob W4JFA

Re: TS50 high current draw

W4JFABob@... (Bob) — Thu, 29 May 2025 19:23:47 -0700

Lou, I found it and joined. It looks like the posts need to be approved B4 they go out.

Thanks for calling that to my attention.

Bob W4JFA

Re: TS50 high current draw

W4JFABob@... (Bob) — Thu, 29 May 2025 19:08:13 -0700

I didn't know there was one. I'll look now.

Thank you.

Bob W4JFA

Re: TS50 high current draw

loueverettsr@... (Lou WA5LOU) — Thu, 29 May 2025 18:52:51 -0700

Have you tried the TS-50 Group?

Lou WA5LOU

Re: TS50 high current draw

W4JFABob@... (Bob) — Thu, 29 May 2025 18:01:15 -0700

I have enuff test equipment. I only lack good unsoldering equipment. What I have has been fine 99% of the time but I just can't

get this transistor unsoldered. I'm pretty close to believing that the transistor leaks under the full 14 volts but does not leak under the

DVOM diode test. That's why I tried to unsolder the collector and test cuzz that would have answered my questions.

I do like to ask on groups like this cuzz you never know who may have had the same issue at one time.

Thanks for your help. I'll let you know.

Re: TS50 high current draw

gac@... (Gary Cook) — Thu, 29 May 2025 17:50:14 -0700

The transistor needs to be completely removed to properly test it. If it's still good, start testing everything around it. Keep in mind that if you don't have adequate test equipment (and knowledge), you can do a lot more damage trying to do it yourself.

Gary

On 5/29/2025 7:43 PM, Bob via groups.io wrote:

Re: TS50 high current draw

W4JFABob@... (Bob) — Thu, 29 May 2025 17:44:19 -0700

Oh I have, with the finger, it's Q6. I tried to get the collector unsoldered so I could test but I can't get it unsoldered. I have the electric solenoid

desoldering tool, I have solder braid, and the manual "clicker" type solder sucker but I can't get it off. I may have to find someone locally that

can unsolder that for me. My thoughts were if I remove only the collector and the current draw disappears, I found it.

Bob W4JFA

Re: TS50 high current draw

gac@... (Gary Cook) — Thu, 29 May 2025 17:22:40 -0700

It could be shorted, or another device may have opened, and current is taking an alternative route through the transistor. Remove the transistor, then apply power again.

Gary

On 5/29/2025 7:15 PM, Bob via groups.io wrote:

Re: TS50 high current draw

W4JFABob@... (Bob) — Thu, 29 May 2025 17:15:45 -0700

I've obviously missing something because I've checked everything I can and am not finding a bad part. Another symptom is the final transistors

especially Q6, gets hot. My guess the excessive current draw is related to this. Looking at the schematic, there's 12 volts going to the collector

all the time. Since the transistor gets hot, that tells me the current is going through the transistor from the collector to the base which is grounded

through resistors and the emitters are directly grounded.

Do you think the transistor(s) are leaking under the power supply 14 volts but not leaking with the small current the DVOM puts out on the diode test?

Thank you.

Bob W4JFA

Re: TS50 high current draw

gac@... (Gary Cook) — Thu, 29 May 2025 17:05:17 -0700

If you have a thermal camera, now's the time to use it. If not, start using your finger to find the overheating device.

Gary

On 5/29/2025 7:01 PM, Gary Cook via groups.io wrote:

Re: TS50 high current draw

gac@... (Gary Cook) — Thu, 29 May 2025 17:01:42 -0700

Start checking diodes, resistors and capacitors until you find it.

Gary

On 5/29/2025 6:56 PM, Bob via groups.io wrote:

TS50 high current draw

W4JFABob@... (Bob) — Thu, 29 May 2025 16:57:35 -0700

Hi all,

If anyone is familiar with the TS-50, I could use some advice.

The radio had no RF out. Was narrowed down to the final board.

It then started blowing fuses. It's not now and I don't know why however

it does draw 6 amps on receive and also draws 6 amps with the power off.

Checking the final transistors, in circuit, they test OK with a DVOM on diode

test. I read no shorts from collector to base/ground. Reverse polarity diode

tests good.

This should be a simple diagnosis so maybe I'm missing something obvious?

If anyone has any hints I sure would appreciate it.

73, Bob W4JFA

Re: SM-220 scope question

mail@... (Mike) — Fri, 23 May 2025 09:33:22 -0700

I found time to have a quick look at the waveforms.

Here is the output of the blanking pulse generator going into C135:

This is the other side of C135 where it connects to the CRT cathode:

Perhaps the capacitor is dead?

Mike G8GYW

Re: SM-220 scope question

mail@... (Mike) — Thu, 22 May 2025 13:22:12 -0700

Of course, but it will be a couple of weeks as I have a holiday coming up.

Mike G8GYW

Re: SM-220 scope question

W4JFABob@... (Bob) — Thu, 22 May 2025 13:05:43 -0700

Let us know how you do.

Bob W4JFA

Re: SM-220 scope question

mail@... (Mike) — Thu, 22 May 2025 11:04:34 -0700

I found the service manual which clearly states that the flyback should be blanked.

The circuit that creates the blanking pulse is just two transistors and a capacitor so it shouldn't be hard to fix.

Mike G8GYW

SM-220 scope question

mail@... (Mike) — Thu, 22 May 2025 05:05:57 -0700

I noticed recently that in oscilloscope mode the flyback is partially visible. Is this a feature or a fault, and if it's a fault can it be fixed?

Mike G8GYW

Re: TM-V71E LEFT HAND RADIO FAULT

mike@... (Mike Davidsohn G3ZCC) — Sun, 18 May 2025 15:17:44 -0700

Repeating the cable didn't help

Kind regards,

Mike

G3ZCC