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Radio Head Unit (EU 1992) - Repair and Capacitor Replacement

Joined
5 August 2014
Messages
202
Location
Germany
The radio that came with the three amplifiers was in a pretty bad shape. Capacitor leakage had even penetrated to the bottom of the board. The design of the 1992 EU radio utilizes several large capacitors on the lower PCB that can cause significant issues when leaking. Since the car spent about ten years in storage the electrolyte had plenty of time to do its damage.


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lower PCB, top view



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lower PCB, bottom view


As the owner already reported that it couldn't be turned on no attempt was made to try before removal of the old capacitors as well as cleaning and repair of the PCB. To get rid of the acid one of the flex cable connectors and the CD changer port was removed. The flex cable was stuck inside the connector so both connector and cable had to be replaced. Broken traces required two repair wires to be added and potentially more later.


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cleaned and repaired


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2nd repair wire

After installation of the new capacitors the radio came back to life - at least partially. It started and was able to tune to stations but audio output was muffled and needed a few minutes of warm-up. The CD changer was working but absolutely no sound even though it could be observed to arrive at the input pin.

Looking at the top of the board, the small Operational Amplifier (JR4560) near the CD changer port was heavily affected by capacitor acid and is responsible for connecting CD audio. It's no longer available as a direct replacement but an improved (and otherwise compatible) version (JR4580) is still sold, in a slightly narrower package (8-SOIC), though. The mounting pads of the old part are sufficiently large to work with the smaller package, too.


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new OP amp installed


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more repair wires


Getting the replacement installed required another repair wire and resulted in a working CD changer port but Audio was still subject to a warm up period. Poking around the PCB with an oscilloscope probe showed that the output amplifier's supply voltage was slowly changing and the muffled sound seemed to correlate. The voltage is created by a Z-diode in the acid affected area - a quick check with cooling spray and the soldering iron showed that the corresponding diode ZD807 strongly reacting to temperature changes.

After replacing it with a new Z-diode of 11.3 V the reference voltage returned back to 10 V and audio output was stable again. Unfortunately, a new issues became audible: Whenever muting was activated, the right channel made a popping noise. It took me until Saturday morning to find the reason in a defective capacitor on the equalizer board. It was introducing a DC offset that changed by 10 mV when the muting circuit was activated.


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the output amp reference voltage diode ZD807


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a working radio - back from the dead


A last issue with the tape deck seems to point to a defective motor. The speed control constantly switches between high and low speed creating a strange sound effect. Since the belt was fine and tape isn't planned to be used it was decided not to analyse it further. A replacement tape unit worked fine, though.

Quite a challenging but rewarding experience :smile:
 
Great write up! It's really nice to see an OEM unit back in operation like this(esp. a Honda ver in as good as shape as yours cosmetically). And now even better it's internally in just as great of shape! Thanks for sharing :listening_headphone
 
Thank you for the kind words. It wasn't easy but the owner is now using it together with a Bluetooth CD changer adapter which gives the system quite an acceptable sound (according to my experience).
My own EU radio got new capacitors, too but there was almost no damage. Hopefully I can complete the Gauge Cluster tester work before everything is installed back into the car for summer season.
 
Nice problem solving. As far as I can remember you are the first Prime member to return a functionally dead head unit to operational status.

For repairing / replacing damaged traces on circuit boards I like to use wire wrap wire. Wire wrap wire has a very thin insulation on it (typically Kynar) which is tough, very chemical resistant and has a fairly high temperature tolerance (105 C rated kynar will typically tolerate operation at 125 C). I use the stuff from Jonard Tools which is available in 24 - 30 gauge.

Wire Wrap Wire | Jonard

The insulation is coated directly on the wire so you don't that annoying shrink back on the insulation when you solder the wire in place. The insulating jacket is very thin which can make it easier to route route the jumper around on the circuit board. The down side to the wire is that the bonded / thin insulation can be difficult to strip off without damaging the actual conductor, particularly on the 30 gauge stuff that I have. It is also a little pricey.
 
That PVFD wire sounds interesting. It's not that popular in Europe but adding it to the next digikey order would be an option. A German shop offers something called "Teflon wire" which could be suitable, too.
 
The Jonard wire is meant for wire wrap applications which was a method of back plane interconnect wiring in custom / low production telecom and industrial applications popular in the '70s and '80s. I don't know how much new stuff is built with wire wrap which might explain the limited availability. As a matter of trivia, those header connectors with the square pins that everybody uses for breadboarding are actually wire wrap pins. The 90 deg corners on the pin grab the wire when it is wrapped. I expect that today 99% of the headers made with 'wire wrap' pins are not used for wire wrap.

PTFE / Teflon has an even higher temperature limit than Kynar. Last time I checked it also came with a bigger price tag.
 
Great work. Do you have an idea of how many hours expended so far?
At some point, the availability of spare parts like the radio may result in either intensive repairs like your effort, or replacement with modern systems, driven by desire for original versus cost.
 
You're certainly right - when the tester topic is done [MENTION=5430]drew[/MENTION] and me will continue on the radio project which could provide another way out of the dilemma :smile:
The radio took about 20 hours to fix but it's hardly a representable number. If the same issue is detected in another radio it would be much quicker to diagnose and repair.
 
You're certainly right - when the tester topic is done [MENTION=5430]drew[/MENTION] and me will continue on the radio project which could provide another way out of the dilemma :smile:
The radio took about 20 hours to fix but it's hardly a representable number. If the same issue is detected in another radio it would be much quicker to diagnose and repair.

That is a reasonable amount of time, probably close the break even point versus new retrofit. I had my radio repaired a long time ago, and other than a wonky fwd button it is still going strong. But it almost makes me wonder if a periodic inspection / repair makes sense?
 
From a technical point of view the radio has few "serviceable parts". The capacitors are an issue, of course but once replaced with quality parts that shouldn't be an issue anymore - same for the amplifiers. Cassette belts tend to fail at the same age as the capacitors (replacement is easy after getting the right belt) and the mechanical elements can use a drop of oil after such a long time. Considering all this, maybe not enough for a periodic check - just getting the caps done before they fail.
 
From a technical point of view the radio has few "serviceable parts". The capacitors are an issue, of course but once replaced with quality parts that shouldn't be an issue anymore - same for the amplifiers. Cassette belts tend to fail at the same age as the capacitors (replacement is easy after getting the right belt) and the mechanical elements can use a drop of oil after such a long time. Considering all this, maybe not enough for a periodic check - just getting the caps done before they fail.

That is the $64,000 question. I would like to hope all caps were replaced when I had my radio serviced. That was more than 15 years ago, and I don’t use my cassette players so the belt is a non issue. A thought from a PMS perspective might say that a capacitor failure could result in catastrophic damage and therefore would be worth an inspection (at least) at some periodic interval.
 
Technically you're right and a regular inspection would be good idea, unfortunately it's a little more tricky. The biggest issue is that capacitor leakage can be hard to trace. These are pictures from a radio I recently bought which was still working fine - no signs of any issues. Preventive cap replacement was on the list, of course so I opened it up and had a closer look:

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Looks fine, right?
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What's this?
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After removal

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Nothing to see at the large ones on the right but are they OK? Actually no, not at all:

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After removal
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After cleaning
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After re-capping

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Half of the trace under the smaller cap was already eaten away even though almost no trace of leakage was visible before removal. It's pretty difficult to detect leakage in its early stage before removal of the capacitors and the radio, especially the flex cables, don't like to be assembled too often (replacements are available, though).

My solution to these issues once and for all is to install Aluminium-Polymer or Tantal capacitors where possible. These are dry so they can't leak but should only be used where their higher electrical leakage is of no concern - e.g. power stabilization in the radio and amplifier (usually these are the big, leaky ones). A pricey but perfect choice for an ease of mind.

To summarize the topic for the radio, my suggestion would be to install dry caps instead of scheduling regular maintenance. If regular caps are installed it probably makes sense to have a look every few years but that would also be a good opportunity to replace them right away.
 
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