IC-271H Repair Project

[ac8vk]

EEVBlog Repair Jedi(s):

The last fall I purchased a IC-271H at a local (Cincinnati) hamfest.
My goal with this rig is to explore weak signal VHF (find out if there are any QSOs to be had...)

Status:

- Rig has a IK2RND memory card installed, with a 05-2015 date
- 100% on FM (Tx works, Rx works - great signal reports)
- SSB Tx was intermittent and has since stopped working
- I have a complete copy of the ICOM service manual and schematics
- This project is a "step up" for me in terms of my repair skills - and that was part of goal in buying the radio (learning new maintenance / electronics skills)
- For test equipment, I have a 50Mhz O-scope, multi-meter, 20Mhz function generator, soldering stuff, and a Kenwood TH-F4 (that has SSB 2M Rx).

When I first set-up the rig, I was able to make 2 QSOs on SSB.

My goal is to repair the radio such that it works properly on all modes (SSB, CW and FM).

The style of construction of this radio has me a little intimidated - mostly the fact that there are so many  (delicate looking) wire interconnects between sub-systems, and the fact the ICOM really packed all of the components very tightly in the design.

So far I have:

1. Conducted a visual inspection of the radio internally (w/o removing any sub-assemblies).
2. Have located/identified the ICs and components that are described in section 5-2-1 "Audio Amplifier and Limiter Circuits)
(photos attached of those areas)

Everything looks OK - at least I don't see anything that looks burnt, poped, leaking, etc.

Note quite sure what my next steps are and I'm looking for some coaching.

Also, I am not quite sure what safety steps I need to take while working on a radio like this. The primary step I have taken so far, is using a 5A limited lab supply to power it while it is on the bench. ( I have access to a 30A supply also, but can't current limit that supply.)

Any ideas?

Thanks,
Erik / AC8VK

[mrz80]

You say FM TX/RX are working ok.  SSB TX is gone.  Does SSB RX work?  How about CW TX/RX?  A fuller picture of what works vs what doesn't can help you navigate the block diagram to at least get to a starting point for triage.

[ac8vk]

mrz80:

Thanks for the follow-up.

(1) FM Rx and Tx - tested on live QSOs - seems rock solid

(2) SSB Rx - Not may Qs to listen to, but if I tune the receiver to a frequency that I can transmit (in FM) from the HT I get a tone out of the receiver.

(3) SSB Tx - TX indicator light comes on, meter moves to 0 RF position, no RF power registering on external meter and no signal detected on nearby HT (with SSB Rx capability) (while of course I whistle into the mic with the mic gain turned up.)

- Tested using the PTT on mike and VOX (previously both of these worked)
- When is SSB mode, the Transmit switch has no effect either.

(4) CW Rx - FM signal from the HT is picked up and results in a CW tone.

(5) CW Tx - same results as SSB Tx - Testing was performed by keying the rig with the PTT and the Transmit switch.

My assumption is that my problem:

- Is in the Tx side of the rig.
- Before the point where the FM and SSB Tx functions combine.

In the block diagrams for the radio:

1. The audio paths for FM and SSB are the same though IC1 but split immediately after.
2. The FM signal proceeds thought a low-pass filter Q1 and into FM Modulator.
3. The SSB signal comes from IC1 through Q9 to IC2 (BFO Mixer)

I had a assumed this would be the space to look for this fault - not quite sure how to test if this is the area of the fault, or if this is a reasonable place to begin.

Thanks for any insights!
Erik

[tkamiya]

A proper way to repair this would be to set everything up for SSB transmit operation.  Feed an audio tone into the microphone jack, key up the mic, and follow the signal to the final.  Block diagram and circuit diagram is essential.  At one point, you will find signal is no longer present.  That is the spot with fault(s). 

As far as receiver testing/repair goes, you could probably use your function generator, select square wave, WATCH THE OUTPUT LEVEL!, and use harmonic that falls on 2 meter and a test signal.  Actual on-the-air signal will only confuse you.

My guess is either cold solder or bad relays.  ICOM of that era used lots of sub-miniture relays.  They were known to develop bad contacts.